CHAPTER 15

Aging, Dilution, and Filtration

David G. Taylor

CONTENTS

15.1 Introduction

15.1.1 Objectives of Aging and Finishing

15.1.2 Component Processes

15.2 Flavor Maturation

15.2.1 Introduction

15.2.2 Important Flavor Compounds

15.2.2.1 Diacetyl and 2,3-Pentanedione

15.2.2.2 Sulfur Compounds

15.2.2.3 Nonvolatile Flavor Maturation

15.2.2.4 Yeast Autolysis

15.3 Lagering and Secondary Fermentation (Kräusening)

15.3.1 Historical Lagering Practice

15.3.2 Kräusening

15.3.3 Lagering without Secondary Fermentation

15.4 Ale Maturation Processes

15.5 Beer Recovery

15.5.1 Economics

15.5.2 Quality of Recovered Beer

15.6 Clarification

15.6.1 Gravity Sedimentation

15.6.2 Finings

15.6.3 Centrifugation

15.6.4 Filtration

15.6.4.1 Filters

15.6.4.2 Cross-Flow Filtration

15.6.4.3 Sterile Filtration

15.6.4.4 Transfer to Packaging

15.7 Stabilization

15.7.1 Flavor Stability

15.7.2 Biological Stability

15.7.3 Physical Stability

15.7.4 Brewhouse Procedures and Filtration

15.7.5 Measurement of Haze

15.8 Carbonation

15.8.1 Basics of Beer Carbonation

15.8.2 Historical Carbonation

15.8.3 Modern Carbonation

15.9 Standardization

15.9.1 Introduction

15.9.2 Blending for Consistency

15.9.3 Process Additions and Process Additives to Beer

15.9.4 Addition of Modified Hop Extracts

15.9.5 High-Gravity Brewing

15.10 Summary

Acknowledgment

References

15.1 INTRODUCTION

15.1.1 Objectives of Aging and Finishing

In this chapter, aging refers to flavor maturation. At the end of fermentation, many undesirable flavors and aromas of a “green” character (called green because it often bears the aroma of green apples) or immature beer are present. The aging process reduces the levels of these undesirable compounds to produce a mature, drinkable beer.

This stage in the brewing process follows primary fermentation (details in Chapters 8 and 14) and involves all the processing required to produce the desired beer in the correct finished condition, ready for packaging. Finishing usually refers to the production of a brilliantly clear beverage after aging that will remain so until it is consumed. This usually requires removal of all suspended solids from the beer by sedimentation, filtration, and/or centrifugation. Further, if beer has been produced by brewing at high-gravity (to produce a higher alcohol content than that required in the final beer) then dilution (also called “cutting”) to the desired alcohol content or final gravity is necessary (details of high-gravity brewing are in Chapter 9).

These postfermentation processing stages are known variously by brewers around the world as: aging, maturation, lagering, conditioning, secondary fermentation, cold storage, and so on, and they represent the most variable processes within the various brewing procedures practiced by different brewers globally. Some will follow historical and traditional practices, whereas other brewers will employ “modern” techniques, although retaining at least some relationship to historical practice. The number of variations is large, not only in terms of procedures followed but also in relation to the time frames allocated to this postfermentation phase of the overall brewing process. However, excellent beers may be obtained from any one of the variable approaches, and nearly every brewing company has its own characteristic approach to beer maturation and processing; the various component processes are discussed as follows.

15.1.2 Component Processes

The components of postfermentation processing are:

1. Aging, lagering, or maturation (this is flavor maturation) (further details in Chapter 14)
2. Clarification
3. Stabilization
4. Carbonation
5. Standardization or blending (including dilution—see Chapter 9)
6. Packaging (details in Chapter 16).

Every process can be accomplished in a variety of ways, but each is independent and can be treated as a separate unit operation. A brief description of the unit processes is given here and in Table 15.1, with fuller discussions in subsequent sections.

Table 15.1 Unit Operations for Aging and Finishing

Unit Operation	Purpose	Equipment and Methods
Transfer	Yeast separation	Decant beer Centrifuge Filter
Aging (lagering, ruh storage)	Flavor maturation	Some yeast present for VDK control and management CO2 purge
Stabilization	Protect beer from:
	1.Oxidized flavor	Maintain yeast present Minimize O2 pick up Use CO2 during all transfers of beer Add antioxidants
	2.Biological haze and off-flavors	Pasteurization Sterile filtration
	3.Physical haze (chillproofing)	Tannic acid Proteolytic enzymes PVPP (polyvinylpolypyrrolidone) Silica gels
Clarification (filtration)	Removal of all suspended particles	Filters Centrifuges
Carbonation	Attain proper CO2 concentration	Traditional aging Pressurized fermentation and maturation CO2 injection
Standardization (blending)	Uniformity of packaged product Dilution	Tankage for transfers Blenders High-gravity beer adjustment (“cutting”)

In modern brewing practice, cold aging or lagering is the storage of beer for the purpose of flavor maturation. Because historical practice had additional functions, there are other meanings attached to these terms as will be explained in later sections of this chapter.

After aging, clarification is required to remove any remaining yeast and suspended inert particles formed during cold storage. At least one filtration step (sometimes more) is usually required before beer is suitable for packaging, if a clear, bright beer is desired. An exception to this is the production of traditional cask-conditioned ale (which by definition is not filtered or pasteurized and contains live yeast and residual fermentable sugars).

Stabilization refers to protecting the finished product from changes that may occur after packaging. These changes are: (a) flavor changes primarily due to oxidation, (b) nonmicrobiological haze caused by the formation of molecular complexes, and (c) biological haze produced by the growth of bacteria or yeast.

Carbonation is the process of adjusting the beer carbon dioxide (CO₂) level to a specified concentration. Carbonation by injection of CO₂ into beer is carried out as a replacement for the traditional increasing of the CO₂ level by cold secondary fermentation.

Blending or standardization is the process of mixing separate batches of beer to achieve uniformity of flavor or analytical characteristics, making process additions in order to bring the beer into line with accepted process and final product specifications (further details in Chapter 10). Where high-gravity brewing has been employed, blending with specially treated dilution water occurs in order to achieve the desired alcohol content and/or finished specific gravity (further details in Chapter 9).

Brewers often, these days, combine some of these operations (e.g., late fermentation and maturation) or change the order in which they are conducted. The possible variations are too numerous to detail here, but most brewers, for reasons of efficiency, economy, and product uniformity, attempt to combine some of the unit operations.

15.2 FLAVOR MATURATION

15.2.1 Introduction

Flavor maturation (also called lagering) is generally considered to be the most significant aging outcome. Successful flavor maturation is especially important in beers that are “lighter” in flavor because taste thresholds of objectionable flavors are lower in lighter beers. In heavier beers, the presence of more flavorful compounds will mask some objectionable flavors and aromas. It is important to emphasize that the later stages of fermentation and the beginning of maturation often overlap (see Chapter 14). Considerable research in the brewing industry has been devoted to an understanding of flavor maturation. In some cases, it can be described in terms of individual compounds that can be detected in wort and beer, permitting the brewer to rely on laboratory tests in addition to taste tests that determine the success of maturation. Taste tests can be unreliable and should be used with the knowledge that tasters vary in their sensitivity to different flavors (further details in Chapter 26). Therefore, most brewers supplement tasting with chemical tests and establish specification limits on objectionable flavor compounds. In-process beer must meet such specifications that satisfy taste requirements before being released to downstream processing and packaging (details in Chapters 3 and 16).

Because most of the important compounds discussed during flavor maturation are the result of yeast metabolism, the central role of consistent yeast growth during fermentation is again stressed. As discussed in Chapters 8 and 14, yeast growth is used in this chapter to refer to the increase in cell population during fermentation.

15.2.2 Important Flavor Compounds

15.2.2.1 Diacetyl and 2,3-Pentanedione

As already discussed briefly in Chapters 3 and 14, diacetyl and the homologous compound 2,3-pentanedione have flavor properties that are important to the brewer. Collectively, diacetyl and 2,3-pentanedione are called vicinal diketones (VDKs). Both compounds have a buttery (butterscotch) flavor generally considered objectionable in lighter-bodied lagers but sometimes desirable, to some degree, in ales and more full-bodied lagers. Diacetyl has a higher flavor impact than 2,3-pentanedione. The flavor threshold of diacetyl, and other flavor compounds as well, depends on the background flavor intensity of the beer, but it is readily detectable at about 0.1 mg/l and at much lower concentrations (approximately 20 μg/l) in very light-flavored lagers. It is thought that at sub-threshold levels, diacetyl contributes positively to palate fullness. Brewers may speak only of diacetyl, but both VDKs are important during maturation.

Research has elucidated the chemical reactions and biochemical pathways of these important compounds.^1–4 The precursor to diacetyl, α-acetolactate, is produced by yeast as it synthesizes the amino acids valine and leucine, which are needed for protein synthesis (Figure 15.1a). The α-acetolactate is transported out of the cell where it is decarboxylated nonenzymatically to diacetyl. This purely chemical step is the slowest or rate-limiting step and is accelerated at a higher temperature and a lower pH. The diacetyl is subsequently re-assimilated by the yeast and reduced enzymatically to butanediol via acetoin.^4–7 The importance of this step is that butanediol has virtually no impact on beer flavor. A similar series of reactions occurs for 2,3-pentanedione, and the precursor is α-acetohydroxy butyrate (Figure 15.1a).

Figure 15.1 Diacetyl metabolism in a fermenting wort. (a) Formation of diacetyl and 2,3-pentanedione during logarithmic and early stationary growth phases. (b) Conversion of diacetyl to acetoin and 2,3-butanediol late in the fermentation.

Brewers should be concerned about the concentration of precursors and whether or not sufficient yeast cells are present in suspension to remove VDKs when they are formed. The important concepts are: (a) that the VDK precursors are produced as a result of yeast growth relative to the wort valine and other amino acid concentrations; (b) the precursors are potential flavor-active VDKs; (c) conversion of precursors to VDKs is an extracellular chemical reaction that varies with temperature, pH, and so on; (d) these extracellular reactions are rate-limiting in the conversion of precursors and in the removal of VDKs from beer; and (e) yeast re-assimilates VDKs, and therefore needs to be present to reduce the VDKs as they are formed (Figure 15.1b).

The production of precursors continues throughout carbohydrate fermentation (Figure 15.2). In reality, the profile marked diacetyl in Figure 15.2 includes both diacetyl and its precursor because the chemical test generally converts the precursor to diacetyl. Thus, the figure shows the total potential diacetyl concentration in the final beer. Because precursor conversion to diacetyl is the rate-limiting step, with yeast present in the fermenting wort, the concentration of diacetyl is small compared to the precursor. Considerable potential for VDK formation remains after active fermentation because of the high concentration of remaining precursors. Regarding VDKs, the maturation process has two objectives: the spontaneous conversion of precursors to VDKs and their removal by yeast. Thus, to hasten the conversion of precursors to VDKs, the temperature in the fermentation vessel can be increased to about 18°C for a period of time after the completion of carbohydrate fermentation (sometimes called a “free rise”). Once the total precursors (potential VDKs) and VDKs fall below a specified level, the temperature can be reduced to stimulate yeast sedimentation. However, the higher temperature may lead to other off-flavors from nonvolatile yeast products or yeast autolysis.

Figure 15.2 Production of VDKs during fermentation. The graph shows the approximate relationship of the concentration of diacetyl and 2,3-pentanedione as they relate to yeast cell growth and specific gravity decline during fermentation.

Research on the metabolism of diacetyl during brewing fermentation has been ongoing since the 1960s,^{8, 9} and during the past two decades, a novel approach has been undertaken. The enzyme α-acetolactate decarboxylase catalyzes the following reaction:

α-acetolactate → acetoin + carbon dioxide

without the formation of the flavor-active diacetyl.

Alpha-acetolactate decarboxylase (ALDC) is not produced by brewer’s yeast strains. However, it is produced by another generally recognized as safe (GRAS) microorganism, Acetobacter aceti, which converts ethanol to acetic acid during the production of vinegar.⁶ ALDC has been isolated, purified, and added to a brewer’s wort fermentation and the total diacetyl concentration throughout the fermentation cycle determined (Figure 15.3).^{10, 11} Compared to an untreated control, little diacetyl is produced, and its concentration is directly related to the concentration of ALDC added to the fermentation. However, the ALDC must be added at the start of the fermentation. ALDC does not reduce diacetyl levels if it is added once the fermentation has commenced. In addition, wort fermentation rates with and without added ALDC are similar (Figure 15.4). It is worthy of note that commercial quantities of ALDC (called Maturex) are currently produced by a genetically modified strain of Bacillus subtilis, which has received the genetic coding for ALDC from a strain of Bacillus brevis.¹²

Figure 15.3 Effect of α-acetolactate decarboxylase enzyme (ADU) on diacetyl metabolism in fermenting wort.

Figure 15.4 Effect of α-acetolactate decarboxylase enzyme (ADU) on wort fermentation rate and extent.

Figure 15.5 Effect of α-acetolactate decarboxylase in an ALDC brewing yeast strain on diacetyl metabolism during wort fermentation.

The genetic coding for ALDC has also been cloned into brewing yeast strains. Wort fermentation trials with the ALDC yeast were conducted and compared to the same uncloned yeast strain as control. The diacetyl production and reduction profiles were profoundly different when compared to the uncloned control culture (Figure 15.5). Because of the presence of ALDC, the α-acetolactic acid was not spontaneously converted to diacetyl but to acetoin instead. Acetoin does not have the same flavor impact as the butterscotch aroma of diacetyl. The overall fermentation performance of some cloned strains can be adversely affected when compared to the uncloned strain.¹³ This was not the situation with the ALDC yeast (Figure 15.6).

Figure 15.6 Effect of α-acetolactate decarboxylase expression in an ALDC brewing yeast strain on overall fermentation rate during wort fermentation.

Table 15.2 Content of Hydrogen Sulfide (H₂S) and Copper in Maturing Beer Samples

	Before Electrolysis		After Electrolysis
	Copper (µg/L)	H2S (µg/L)	Copper (µg/L)	H2S (µg/L)
Beer A	32	4	69	Traces
Beer B	29	3	68	Traces

It has already been discussed in this chapter that the removal of diacetyl, 2,3-pentanedione, and other VDKs and their precursors is one of the major features of beer flavor maturation. It is also worth repeating (see Chapter 14) that this beer processing stage is usually batch, the most time-consuming stage during lager beer production. A number of continuous maturation systems have been studied.^{14, 15} The primary objective of these trials was to improve the efficiency of the maturation process, particularly to reduce the maturation time. One such continuous maturation process has been developed by the Finnish engineering company Cultor following basic research by Finland’s VTT laboratories and with the collaboration of Sinebrychoff and Bavaria breweries from Finland and the Netherlands, respectively. This process utilizes immobilized yeast cells for the accelerated maturation of beer.¹⁶

This process employs a proprietary carrier Spezyme^® (diethylaminoethyl [DEAE] cellulose) for the immobilization of yeast cells. These immobilized cells are formulated into a downflow packed bed continuous bioreactor through which a yeast slurry is recirculated. The principal advantage of this technology is high volumetric productivity corresponding to residence times of only a few hours. This maturation process employing purely physical processes is considered by some breweries as a more acceptable alternative from the consumer’s point of view, rather than the proposed employment of an α-acetolactate decarboxylase gene cloned into a brewing yeast strain, as has already been described.¹³

The system developed by Cultor and their associates has been operational on an industrial scale (maximum throughput 1 mhL maturing beer/year) in the Sinebrychoff Kerava Brewery located on the outskirts of Helsinki, Finland. Figure 15.7 provides a schematic of this maturation process.¹⁷ In order to achieve rapid reduction in diacetyl and other VDKs in the “green beer,” the yeast cells are removed from it by centrifugation, and the yeast-free beer is subjected to a heat treatment process (65ºC to 90ºC for 7 to 20 min). This heat treatment accelerates the nonenzymatic conversion of the diacetyl precursor, α-acetolactate, to diacetyl and acetoin. After cooling, the beer is then introduced into a packed bed column containing yeast cells immobilized on the DEAE cellulose particles. In this final stage, the yeast cells complete the conversion of the remaining diacetyl into acetoin; also, adsorption of beer staling carbonyls occurs. Although this process has significant merit, particularly regarding the process time that it saves, it adds another level of complexity to the maturation stage. In particular, the centrifugation and heating steps have to be conducted with considerable care. The ingress of oxygen must be prevented because the combination of heat and oxygen will exacerbate the development of stale beer flavors and heating residual yeast will introduce adverse flavors and physical instability to the maturing beer.

Figure 15.7 Two-hour continuous maturation system.

When the continuous maturation process was introduced into the Kerava Brewery in the late 1980s, the initial industrial unit operated at approximately 150 hL/h, and this corresponded to 1 mhL beer/annum. This capacity was gradually increased in the ensuing years, and the maturation process worked well with acceptable diacetyl management and a finished beer with good taste and stability.¹⁷ However, problems arose with increasing production volumes and brands and, as a result, fermenter residence times and flow rates became too long! If the Kerava Brewery needs to produce 2 mhL beer/annum, the continuous capacity should be tripled to approximately 500 hL/h. In addition, the installation of further high-performance centrifuges will be required. Also necessary will be holding facilities for heat treatment and larger immobilized yeast tank volumes in order to achieve a consistent two-hour continuous maturation process with short enough tank emptying times in order to take greater advantage of the process. This increased capacity and the need for greater brand flexibility has resulted in a move away from this continuous system and a return, in part, to batch maturation procedures.

15.2.2.2 Sulfur Compounds

Maturation is a process that also manages the levels of unwanted sulfur compounds (hydrogen sulfide [H₂S], sulfur dioxide [SO₂], thiols, etc.) that are produced by yeast during primary fermentation and that will have a negative effect on the flavor of finished beer^{18, 19} (further details later in this chapter). The presence of copper ions in fermented wort has had a positive effect on the reduction of these compounds.²⁰ The positive effect of copper in this regard has also been illustrated by its importance in the distilling industry.^{20, 21} The expanding application of stainless steel for the manufacture of brewing equipment, away from copper, has reduced the copper concentration in beer during recent decades. Copper ions can precipitate hydrogen sulfide and some other sulfur-containing compounds as insoluble copper sulfide. The resultant copper sulfide can then be filtered out of the maturing beer or adsorbed in the filter bed. The traditional use of copper vessels/pipes or copper plates during wort and beer production does not permit precise copper ion addition. Consequently, deliberate and careful treatment of beer with copper would be advisable. Copper electrolysis of maturing beer would appear to be a potential alternative.²² It should be noted that Cu⁺⁺ ions can have a negative effect on beer flavor stability. For example, Irwin et al.²³ reported that the rate of beer staling increased in the presence of small amounts of copper ions. As a transition metal ion, copper catalyzes the activation of molecular oxygen, which in turn can oxidize primary alcohols to beer-staling aldehydes. Nevertheless, the application of a copper electrode during maturation cannot be ignored. In a typical application, a copper electrolysis system reduced H₂S levels from 4 µg/L to nondetectable levels, while copper beer levels increased from 32 to 69 µg/L (Table 15.2). Further details in Chapter 20.

The subject of sulfur compounds in brewing is broad and complex.²⁴ They are particularly important because of their very low flavor threshold and a flavor perception that is generally (but not always) objectionable. Important sulfur compounds result from yeast metabolism, but many present in beer come from malt and hops. Yeast requires sulfur-containing compounds for the synthesis of proteins. Sources of sulfur in wort for yeast metabolism are sulfate ions from water, and thiols and sulfides from raw materials, particularly sulfur-containing amino acids. Sulfur compounds in beer arise through a combination of raw material sources, processing conditions, yeast strain, metabolism and autolysis, and microbial contamination.

Three of the more important volatile compounds are hydrogen sulfide (H₂S), sulfur dioxide (SO₂), and dimethyl sulfide (DMS). Some DMS is formed during fermentation by the action of yeast on dimethyl sulfoxide produced in the kettle, although most DMS comes from the conversion of a precursor from malt (S-methylmethionine) during kettle boil.²⁵ DMS at low concentrations is believed to make a positive contribution to beer flavor.²⁶ At higher concentrations, DMS can have the objectionable aroma of cooked corn. Chapter 11 discusses in depth the origins and fate of DMS.

Hydrogen sulfide is a product of the transport of sulfide ions by yeast during the metabolism of sulfate ions and organic sulfur compounds.²⁷ Production of H₂S during fermentation is related to yeast growth. Hydrogen sulfide has the aroma of rotten eggs, and the elimination of this compound is usually accomplished by the purging action of CO₂ evolution. More H₂S is produced in lager fermentations than ales because the higher temperatures used in ale fermentations increase the volatility of the compound and the metabolic differences between ale and lager yeast strains (further details in Chapters 8 and 20).

Sulfur dioxide is also present in beer although usually in concentrations well below 10 mg/L, at which level it does not normally have a flavor impact in most beers. Higher concentrations of SO₂ are produced under conditions of low yeast growth and can be considered as beneficial for flavor stability as described later in Chapters 20 and 26.

15.2.2.3 Nonvolatile Flavor Maturation

Packaged beer contains low concentrations of amino acids, peptides, nucleotides, organic acids, inorganic phosphates, and other ions that contribute to the overall flavor of beer. Some nonvolatile compounds are products of raw materials, normal fermentation, and processing steps. Others are internal components of yeast cells released because of a change in cell permeability following fermentation. Free amino nitrogen, pH, phosphates, color, and invertase activity in beer all increase during storage.²⁸ It would be reasonable to assume that these increases are dependent on temperature, time, yeast strain, physiological condition of the yeast, and fermentation vessel geometry.

It is important to note that these changes in nonvolatile compounds are not necessarily undesirable. Nonvolatile compounds can contribute to palate fullness or mouthfeel, and they act synergistically with other flavor-active substances and contribute to the overall flavor quality of beer.

15.2.2.4 Yeast Autolysis

Yeast autolysis is not clearly defined, but the general use of the term refers to the dissolution of dead or moribund cells by their own enzymes. The autolysis products released into the beer result in a sharp, bitter taste and a yeasty aroma. Autolysis occurs under conditions of starvation and high temperature. Holding a fermentation vessel at higher temperature (above 18°C) in order to facilitate conversion of the VDK precursors is a condition that can lead to cell autolysis. It is important therefore not to permit yeast to remain in beer for long periods at higher temperatures—not longer than two to three days.

15.3 LAGERING AND SECONDARY FERMENTATION (KRÄUSENING)

The term lagering comes from the German verb lagern, which means to store, to age, to lay down. The use of this term in the brewing industry is often synonymous with aging and storage, and sometimes other terms that are a consequence of aging, such as maturation, conditioning, and secondary fermentation, are applied. The term lager beer follows from historical aging practices before the advent of refrigeration.

For clarity, the following definitions will be used in this chapter.

• Primary or main fermentation: The initial fermentation, during which most of the carbohydrates in the wort are assimilated. If no secondary fermentation is conducted, then all carbohydrates are assimilated during primary fermentation.
• Secondary fermentation: Fermentation subsequent to transfer of the beer from primary fermentation vessels with some yeast and fermentable carbohydrates present, during which the residual carbohydrates are assimilated. This process is usually performed at a reduced temperature. Secondary fermentation is not always employed in modern brewing practice.
• Kräusening: The addition of fermenting wort to the secondary fermentation. Kräusening is not often used in modern practice, but it is used during the production of some commercially important lager beers!
• Lagering: This is a nonspecific term applied to aging (or maturation) and other processing stages following primary fermentation. Historically, for many beers, lagering included a secondary fermentation followed by a long, cold storage period.
• Ruh storage: A practice that refers to cold aging of beer with some yeast present following the completion of fermentation.
• Maturation, aging, storage, conditioning: These terms are used interchangeably to refer to the maturing of beer flavor. A secondary fermentation may or may not be included.

15.3.1 Historical Lagering Practice

Historical lagering^{19, 29} was necessary because of the absence of refrigeration and the need to remove yeast and to control the level of carbonation in beer. Consequently, lager beer was usually brewed during colder months of the year and stored in iced caves for long periods of time. Cutting ice from local lakes for summer storage of beer was well known prior to the advent of refrigeration. For example, this originated in Bavaria by harvesting ice in the winter from rivers and lakes and was continued in North America by obtaining ice from the Great Lakes—from Lake Michigan near Milwaukee, for example.

Primary lager fermentation is usually conducted at or below 16°C. The resulting primary beer, containing about 1% fermentable extract, is then transferred to cold storage cellars along with some suspended yeast.²⁹ The yeast will assimilate any oxygen picked up during the transfer of primary beer into storage, thus mitigating potential oxidative beer flavor problems. Secondary fermentation of the remaining fermentable extract proceeds slowly while the beer gradually cools over several days. Because CO₂ is more soluble at lower temperatures, elevated levels of carbonation are obtained. Total storage time used to be approximately 50 days at 0°C. This long cold storage period allowed not only for the settling of the remaining yeast but also for the settling of haze-forming materials. Extended storage times have been promoted as giving superior flavor maturation and stability. Modern thinking is that long cold aging is unnecessary as long as the process provides for the elimination of VDKs, their precursors, and other compounds responsible for green beer and sulfur flavors in the immature beer.

Selection of a yeast strain with the appropriate flocculation characteristics was obviously important for a long aging process. With a powdery (nonflocculent) yeast, the transfer to storage and secondary fermentation would carry over too much yeast, secondary fermentation would occur too quickly, and the yeast would not settle sufficiently at the completion of fermentation. With a very flocculent strain, too little yeast would be carried into secondary fermentation, which would then not go to completion unless the yeast was agitated or roused.

15.3.2 Kräusening

Kräusen is a German term meaning “rocky head.” In brewing, the word refers to the appearance of the foam head in the primary fermentation vessel. When fermentation is most active, foam formation is greatest and the fermentation is said to be at “high kräusen.”

During the practice of kräusening, beer is transferred to the storage cellar after primary fermentation, usually with some residual fermentable extract remaining. Then a volume of high-kräusen beer, about 5% to 20% of the primary fermented beer volume,²⁹ is added to the tank. The secondary fermentation continues as in the traditional process, except more rapidly. The degree of secondary fermentation can be controlled by the amount of residual wort fermentable extract at the transfer and the amount and fermentable extract of the high kräusen added. In kräusening, a more flocculent strain can be used because the secondary fermentation is more vigorous than without kräusen. However, in some brewing systems that conduct kräusening, the same yeast strain is employed throughout the process.

Cooling may occur gradually during secondary fermentation or rapidly at the end of fermentation in order to promote yeast settling. The CO₂ produced helps to achieve the packaged beer carbonation level. However, the introduction of high kräusen adds more fermentable extract and produces more flavor compounds. Off-flavors such as H₂S and diacetyl also increase after being at low concentrations at the end of primary fermentation.¹⁹ Lengthy storage may then be required to reduce undesirable beer flavors to acceptable levels.

15.3.3 Lagering without Secondary Fermentation

Historically, beers using lagering employed shallow, open fermentation vessels for the primary fermentation stage but closed vessels for secondary fermentation in order to maximize carbonation. Modern equipment with refrigeration, carbonation, filtration, and so on obviates the need for secondary fermentation and a long, cold storage period. Modern brewing practice employs shortened fermentation and lagering times and uses rapid cooling after fermentation to stimulate yeast settling. Techniques for rapid fermentation accelerate the utilization of wort sugars. If the wort is fully attenuated during primary fermentation, there is no need for secondary fermentation per se, and the aging process is principally focused on flavor maturation and yeast settling.

As described earlier, one of the more important classes of compounds involved in flavor maturation are the VDKs. Because the rate of VDK precursor conversion is temperature dependent, elevating the temperature (also termed a “free rise”) can be used to hasten the conversion. Short, warm lagering has proven quite effective, with minimal deleterious effects on beer quality. This lagering should be accomplished in the presence of yeast by extending the residence time in the fermentation vessel at the upper temperature limit after the wort is fully attenuated. If there is a lack of sufficient suspended yeast, wort recirculation or “rousing” with a CO₂ purge can help.

With modern equipment, the use of separate vessels is unnecessary; and some unit operations may be combined in a vertical fermenter unitank operation. For example, after a predetermined attenuation limit has been reached, yeast (for reuse) can be removed from the bottom cone and the beer cooled for lagering. Periodic removal of more yeast may be beneficial during the lagering phase to prevent off-flavor development. There are compelling economic advantages for combining fermentation and aging in one tank.^{30, 31} Other advantages of this concept³⁰ are (a) fewer microbiological and foam retention problems because of reduced transfers; (b) more efficient yeast collection; (c) improved control of beer CO₂ levels, with the possibility of eliminating carbonation; and (d) better opportunities for automation. However, a major disadvantage of the unitank approach relates to the need for longer residence time in the single vessel due to the increased time required to cool beer to 0°C, or lower, within a single vessel, compared with the rapid chilling available through a plate heat exchanger on transfer from fermenter to a maturation vessel, without the risk of freezing the beer.

15.4 ALE MATURATION PROCESSES

Ale processing usually includes both warm maturation and cold maturation stages. Because of higher temperatures employed during primary fermentation (up to 25°C), residence time in a fermentation vessel is much shorter than for lager production. However, it is not uncommon to carry out warm ale maturation in the fermentation vessel, by cooling in the vessel to 13°C to 15°C and holding for about 48 hours (to reduce diacetyl and complete fermentation), before transferring for cold maturation. Alternatively, ales can be transferred to designated maturation vessels to release fermenters for subsequent fermentations. If the ale is to be chilled and filtered for packaging into kegs and small packs (bottle or can), then cold maturation is carried out at 0°C or below, and treatment is comparable to lager production in terms of clarification and stabilization.

For a traditional cask-conditioned beer, processing is significantly different because the maturation process is completed in the finished container—the cask. In recent years, there has been a resurgence of interest in cask-conditioned ales, not only in the United Kingdom but among craft brewers in many countries, especially the United States.

The essential features of the traditional ale process are as follows: beer after warm maturation is cooled to about 10°C and transferred either directly into a cask or, more usually, into a holding vessel (called a “racking back”). Normally, the yeast count is in the range of 0.25 to 2.0 × 10⁶ cells/mL. The brewer will tend to hold the yeast culture in the fermenter until skimming (or suction) and/or sedimentation. Some yeast sedimentation during racking may help in this respect. Alternatively, a more modern approach is to remove yeast by centrifugation, either totally or adding back—to the fermented wort—an appropriate small amount of fresh pitching yeast or by a small volume of beer containing yeast bypassing the centrifuge.

Another requisite is for adequate fermentable extract to be present to allow some secondary fermentation to occur in order to augment the level of dissolved CO₂. To provide sufficient fermentable extract, the brewer may add fermentable syrup (specific gravity [SG] 1.150) to the beer either in a fermenter, racking vessel, or directly to the cask. The addition of extract is timed in order that some fermentation will occur in the cask located in the dispense cellar (set to 11°C to 13°C) to bring the beer into correct “condition” in terms of CO₂ content. The syrup may be sucrose, inverted sucrose, or a mixture of a cereal starch hydrolysate, plus inverted sucrose (details in Chapter 6). Some coloring matter in the form of caramel is often present in this “priming sugar.” In many beers, the primings serve not only as a substrate for secondary fermentation but also as a beer sweetener.

Other additions may be made to the cask beer such as dry hops, aromatic hop pellets, or hop oil. These add an extra bouquet to the beer that is derived from the hydrocarbon and oxygenated fractions of the essential hop oils (details of hop additions in Chapter 7).

Finally, there is the treatment of beer with finings, either in the racking back or directly into the cask at racking or a few days later (see Section 15.6). The addition of auxiliary and isinglass finings will achieve the necessary clarification during dispense. When isinglass finally slowly sediments in the beer, the yeast cells form compact flocs that settle into the base of the cask. Disturbance of these flocs leads to the beer becoming turbid again but re-clarification is usually possible on two or even three occasions without fresh finings being added.

15.5 BEER RECOVERY

15.5.1 Economics

During normal production, beer is lost in the yeast culture, in spent filter aids and in tank bottoms. Yeast cropped by skimming or in connection with beer decantation will have significant solids in the slurry. The beer in such a slurry, sometimes called barm, may be over 50% beer by weight. It is estimated that up to 2% of total beer output is held in the collected yeast.³² Tank bottoms may contain 2% to 7% solids. The recovery of barm beer from these sources may be economically advantageous. The recovery of beer also reduces biological oxygen demand (BOD) and chemical oxygen demand (COD) in brewery effluent, thereby reducing sewer charges—an additional cost savings and an environmental positive.

Beer can be recovered from these various sources in several ways. In some cases, yeast strain differences may play a role in the selection of suitable equipment. Methods include centrifuges, membranes, or diaphragm filter presses, and other types of filters³³ (discussed later in this chapter and in Chapter 11).

15.5.2 Quality of Recovered Beer

The microbiological stability of the recovered beer is extremely important. Proper equipment, piping, and so on, must be chosen so that acceptable cleanliness can be maintained. Minimum residence time for feedstocks and recovered filtrates is essential for microbiological stability. A flash pasteurization step for the recovered beer is sometimes necessary to obtain a quality filtrate for blending.

A second factor pertinent to the quality of recovered beer is its dissolved oxygen (DO) content. Processing should be carried out under conditions such as an anaerobic environment. It is nearly impossible, however, to make transfers without some oxygen pickup. The introduction of any oxygen will contribute to beer flavor deterioration. Blending recovered beer at a low percentage into the primary beer will help minimize the adverse effects of oxidation.³³

A third, major consideration is the clarity of the recovered beer. The requirement for clarity, after recovery, depends upon subsequent processing and blending. If the recovered beer is added to primary production beer during transfer to aging, further clarification occurs downstream. If the recovered beer is added later in the process, it may be necessary to filter it before pasteurization and blending.

Other properties of recovered beer that are likely to vary are its color, pH, and flavor. Color and pH changes are not generally significant because of subsequent blending. Particular attention should be paid to the flavor of any recovered beer. Flavor changes may be caused by yeast autolysis or the recovery processing steps. Maintaining the temperature of the feedstock below 5°C will help minimize flavor changes.

The recovered beer can be blended into normal production beer at any convenient step in the operation. However, the actual choice may depend on the configuration of fermentation and aging equipment, the number and types of beer being brewed, and so on. The final extract of the recovered beer may dictate the point of blending, and the recovered beer will have low carbonation. In any case, the brewer must determine a maximum percentage of recovered beer to blend into production beer. Normal practice is to use not more than 10%, and taste-testing of blended beers gives additional confidence in the use of recovered beer.

15.6 CLARIFICATION

At the completion of aging, the beer contains some yeast, colloidal particles of protein-polyphenol complexes, and other insoluble material that was precipitated out of solution by the low pH and the cold temperature during aging. If a brilliant, clear beer is desired, the clarification must remove these substances before beer packaging can be carried out. Four basic clarification techniques are used either separately or in combination: (a) sedimentation, (b) use of finings, (c) centrifugation, and (d) filtration.

15.6.1 Gravity Sedimentation

This is the simplest method for achieving clarity and was the only method available prior to the development of centrifuges and filters. Historically, the chilling of fermented beer to about 0°C for long periods promoted the sedimentation of yeast and other particles. However, despite its simplicity, caution is needed because yeast autolysis occurs readily, especially if the temperature of the packed yeast mass begins to increase.²⁹ With clarification by sedimentation, beer losses are relatively large and cleanup of tank bottoms is costly.

The rate at which suspended matter will sediment is governed by Stoke’s Law, which is:

Vg=d2(ρp−ρ1)g18μ

where V_g = terminal settling velocity

d = particle diameter

ρ_p = particle density

ρ_l = liquid density

µ = liquid viscosity

g = gravitational constant.

The essential features are:

• Larger particles settle much more quickly.
• Denser particles settle more rapidly.
• Liquids that are less dense and less viscous permit more rapid settling.
• Stoke’s law describes the rate at which a sphere falls through a liquid, and this indicates the parameters that influence the rate of beer clarification. Clearly, the factors that the brewer can influence that will have the greatest effect on time to clarify are:
• Height of the settling tank—the smaller the sedimentation distance, the faster the rate of clarification. Consequently, beer will clarify more quickly in shallow, horizontal tanks than in tall cylindro-conical vessels.
• Particle diameter (d), which can be increased by causing particles to
• agglomerate, for instance by the use of finings.
• Gravitational force (g), which can be increased by centrifugation.

15.6.2 Finings

Although good clarity can be obtained from simple sedimentation, better results can be obtained in less time by using fining agents, such as isinglass finings and auxiliary finings (either silicate- or alginate-based).

Because of the chemical structure of collagen, isinglass finings (produced by chemically treating the swim bladders of certain tropical fish) carry a net positive charge and interact with yeast cells, which are negatively charged, and with negatively charged proteins.²⁸ Negatively charged proteins have been implicated in haze formation.³⁴

Auxiliary finings usually comprise alginates (such as carrageenan) or silicates and are negatively charged. They therefore flocculate certain protein and other materials that are positively charged. They are usually used in combination (but added separately) with isinglass finings.

Consequently, removal of all these compounds improves beer physical stability. However, finings can increase the volume of tank bottoms and also increase tank cleanup costs and beer losses, although their use improves subsequent filtration.

The combined use of isinglass and auxiliary finings is usually the only treatment involved in clarifying traditional cask-conditioned ales, although some ale brewers will use centrifugation to control yeast levels prior to fining (see Section 15.4). Other clarifying agents include tannic acid, silica gels, and clays.²⁹ The preparation and use of all these agents, including finings, is detailed in Chapters 10 and 11.

15.6.3 Centrifugation

Centrifugation represents the most spectacular measure available to increase the rate of sedimentation. In addition, the design of centrifuges (by insertion of trays or stacks of discs) is highly effective at minimizing the depth in which the particles have to settle (Figure 15.8).³⁵

Figure 15.8 (a) The disc stack centrifuge (5 hL/h) and (b) the disc stack.

There are three potential drawbacks to the use of centrifuges that must be borne in mind in order to maintain beer integrity³⁶:

1. The beer may be subject to shearing.
2. Beer temperature rises during the clarification.
3. Centrifugation is rarely sufficient to clarify beer completely and is normally followed by a filter.

For a centrifuge to remove yeast and other particles, the beer must have a sufficient residence time in the machine for cells and flocs to fall through the average path length under the applied “g” force. Clearly, modern centrifuges are easily capable of achieving this. When smaller particle size is considered, the residence time (and therefore the pumping rate) or centrifuge speed become more critical. Disc bowl centrifuges are ideally designed for beer treatment. These machines contain several disc plates onto which the deposited particles collect and then slide into the solids holding area (Figure 15.9).³⁵

Figure 15.9 Flow pattern in a disc stack centrifuge.

Cleaning is via discharge slots opened by a vertically movable annular piston operated by pressurized water. The ejection system is very rapid—less than 0.1 second.

A solid content of greater than 355 v/v can be handled, and discharge is usually greater than 25% dry matter.

Both design and operation of the centrifuge are key to ensuring the following:

• Maximum beer yield
• Minimum oxygen pickup
• Minimum temperature increase
• Minimum damage to yeast cells.^{35, 37}

The design of the centrifuge and the manner of pumping can have a major influence on beer integrity, and it is imperative that beer is fed into the machine in a gentle manner in order to ensure that there is little or no damage to yeast cells^{35, 37} (further details in Chapters 9 and 14). Beer must be kept separate from the atmosphere, by the use of hydro-hermetic or sophisticated mechanical seals, in order to achieve minimal oxygen pickup.

The rate of beer feed into the centrifuge should be matched to the capacity of the machine and the solid load in the beer. Usually, higher yeast counts are encountered at the start and end of beer transfers and so, beer is fed to the centrifuge slowly at first until a consistent feed is obtained and then the pumping rate can be increased. Again, the feed rate is reduced toward the end of the transfer. Turbidity meters connected to divert can be included to avoid “slugs” of yeast entering the centrifuge. Modern centrifuges also include a self-sensing system to detect when exit beer haze starts to increase and thus initiate a bowl discharge sequence. The rate of solid discharge influences both beer quality and product losses in that an infrequent discharge allows yeast cell damage to occur (risking poorer beer quality and increased filtration difficulty), whereas a too frequent discharge increases losses.³⁶

15.6.4 Filtration

Filtration generally refers to beer clarification through several stages to produce a crystal-clear product. The purpose is to remove suspended nonbiological materials and residual yeast, which would otherwise cause the beer to be hazy. The particle size of suspended material in beer is 0.5 to 4 μm.³⁸ Particle size information is necessary for the brewer to set filtration parameters.

The mechanisms of filtration can be classified into three types: (1) surface filtration, (2) depth filtration through mechanical entrapment of particles, and (3) depth filtration through adsorption of particles. Surface filtration means that particles are blocked at the surface of the filtration medium because the particles are larger than the pores in the medium. During depth filtration, particles pass into the filtration matrix, and they are either mechanically trapped in the pores or adsorbed onto the surface of the internal pores of the filtration medium.

Filtration may be used at two or more stages after aging, depending on the particulars of process operations. The terminology for various filtrations differs from brewery to brewery. The first or primary filtration stage removes the bulk of the yeast and suspended material. The second stage produces a brilliantly clear beer. The addition of stabilization agents occurs before primary filtration, and they are substantially removed by the filter. Primary filters are almost always powder filters. A turbidity sensor can be installed at the outlet of the filter to monitor filter performance.

In modern operations, the primary filtration may be of sufficiently high performance and the only filtration step required, especially if centrifugation has been used upstream to reduce yeast levels. However, if a second stage is required, polish or final filtration will remove any additional suspended solids resulting from lagering at cold temperatures and any adsorbents added to aid stabilization. These final finishing steps are generally preceded by a final beer-cooling operation to encourage precipitation and ensure that the beer reaches the bright beer tanks at the proper temperature.

Polish filtration may consist of two separate filters. After a first filter, trap filters may be used as an immediate final stage to guard against any breakthrough from the upstream filter, not to perform further filtration. Trap filters are usually membrane filters. There should be no further addition of any substance to the beer stream after the last filter as the introduction of unfiltered liquids may prove harmful to the clarity (and possibly the flavor) of packaged beer. Sterile filtration³⁹ to remove bacteria present in the beer will be described in the Section 15.6.4.3 considering this parameter and in Chapter 17.

15.6.4.1 Filters

Currently, the most popular beer filters that will be discussed use powders or filter aids. The materials for powder filtration include kieselguhr (diatomaceous earth [DE]) and perlite (volcanic silicate). Also, these systems use sheets, cartridges, and membranes, which will be described later. Finally, the use of “cross-flow” filtration systems will be outlined, which are being employed with increasing frequency in modern brewing operations. Filters are used not only to clarify beer but to clarify wort, recover wort from separated trub, and recover beer from tank bottoms.

DE is usually calcined after mining in order to eliminate organic matter. The high porosity of the diatom skeletons is ideal for filter beds as the liquid passes through the bed while the suspended particles cannot pass through. DE is supplied in a variety of grades, from which brewers can choose, to accomplish the clarification objectives. The different grades have particle size distributions that affect filter flow rates, filter bed permeability, the degree of filtration (coarse to fine), and so on.

Perlite is an ore of volcanic rock containing silica. When crushed and heated, perlite expands to become a light, fluffy powder and is suitable as a filter aid. The expanded perlite is milled and graded, producing filter aids with a range of permeabilities. For any filter aid, the important properties are as follows: good permeability to keep the pressure drop low across the filter and good wetting to ensure uniform dispersion and bed formation. Filters that use powders are sometimes called DE or kieselguhr filters.

Filters that use filter aids (powders) operate on the principle of building a bed or cake of powder on a septum or filter screen. The porous bed creates a surface that traps suspended solids, thus removing them from the beer. Normally, the filter septum is precoated with a filter aid in advance of the beer filtration run. This precoat forms the base layer for the bed. The rough beer to be filtered is dosed with more filter aid, called body feed, at a concentration based on the solids content to be removed. The use of body feed helps to achieve the goal of maximum filter throughput. As beer is run through the filter, the bed increases in thickness because of the body feed, thereby maintaining bed permeability. Various grades of powders are used, depending on the filtration performance desired and beer to be filtered. For example, primary and polish filtrations will use a different grade and thickness of precoat. Body feed may not be required in polish filtration as it is in primary filtration. The different types of DE filters that follow are simply different implementations of these principles.

Filters are operated until the differential pressure rises beyond a designated point, which requires the flow rate to be reduced, or to the point when the bed depth reaches a thickness that bridges the spaces between the septa in the filter. Filter systems are designed to function within a specific range of pressures. An excessive differential pressure can cause: (a) the filter leaves to collapse, (b) the filter shell to burst, or (c) the pumps to fail as they are not sized to operate with the increased energy needed to maintain the flow rate.

The relationship between filter operational parameters is:

Δ P = µ V × L/ß

where Δ P is the differential pressure, V is the specific flow rate (flow rate of beer per unit filter area), µ is the beer viscosity, L is the bed depth, and ß is the bed permeability. The specific flow rate will depend on the differential pressure or pressure drop across the filter bed. The differential pressure is directly proportional to the specific flow rate, beer viscosity, and bed depth and is inversely proportional to the bed permeability. Therefore, at a constant flow rate and viscosity, filter performance depends on the ratio of the bed depth to its permeability. The DE is added to the beer (body feed) being filtered in an effort to maintain permeability as filtered particles from the beer reduce the bed permeability. However, this ratio inevitably gradually increases, causing the differential pressure to rise to an unacceptable level and resulting in the end of the filter run.

Plate and frame filters are one traditional type used in the industry. They comprise a series of parallel plates covered with filter sheets used to support the filter bed. The frames between the plates control the bed depth. Different numbers of plates can be used, depending on requirements. Most filters allow beer to pass through both sides of the plates, thus doubling the surface area per plate.

Leaf filters consist of a series of circular, stainless steel leaves as perforated support plates. The leaf configuration can be horizontal or vertical. The leaves in horizontal filters have a stainless steel woven septum to support the bed, while vertical filters use the septum on both sides. Their operation is quite similar to plate and frame filters.

The candle filter is of a different design entirely, although the filtration principle is the same. The candles can be porous ceramic but are usually perforated or fluted, stainless steel tubes covered or surrounded by a stainless steel support of various types. This rigid septum is easier to clean than the filter leaves used in the powder filters. There is also an operational advantage. The beer is fed to the outside of the candles and the filtrate collected through the inside. The circular design means that the increase in bed thickness during operation is less than other filters, and the pressure drop increase occurs at a slower rate. The ceramic filter can be used for the sterile filtration of beer.³⁹

Sheet filters are similar in design to plate and frame filters. Whereas the sheet used in powder filters acts as a septum to hold the precoat, in the sheet filter, the sheet acts as the filtration medium. The sheet is usually made of cellulose impregnated with DE. Other materials can be added to achieve both the desired liquid permeability and solids retention. These filters have wide applicability but are generally used after a primary DE filter because they do not have the capacity of the powder filters. They are also suitable for sterile filtration. Most filters of this type can be easily backwashed, and several runs can be made before replacing the sheets. Two disadvantages are the high labor costs of handling the sheets and the lack of automation. Sheet filters are often used for keg beer.

A related type of filter is the traditional pulp filter. The cellulose and cotton fibers are formed into circular pads and joined face to face. The filter can be used for primary filtration or later filtration stages and is suitable for sterile filtration.⁴⁰ The pads can be reused by washing the material after dispersing the pad fibers in water. Because the pads are reusable, disposal problems are considerably reduced. The use of these filters is very labor intensive.

Cartridge or membrane filters are generally much smaller and serve as sterile filters and as trap filters that catch breakthroughs of DE occurring upstream. The filter medium is usually a membrane produced from polymeric synthetic materials, for example, Nylon 66 or cellulose esters. With manufactured materials, the membrane can be constructed to a desired permeability and mechanical strength with a large surface area. These systems are generally economical and easy to maintain.

15.6.4.2 Cross-Flow Filtration

All the filtration systems described so far operate on the principle of “dead-end” filtration, that is the flow of beer containing particles is directly against (at right angles to) the filter septum. In cross-flow filtration,⁴¹ the direction of flow is over the surface of the filter medium such that clear beer flows through the membrane pores and particles are retained on the surface. A schematic comparison of “dead-end” and “cross-flow” principles is presented in Figure 15.10.

Figure 15.10 A schematic comparison of “dead-end” and “cross-flow” filtration principles.

The operating principles of a cross-flow filter system require the liquid to be filtered, under pressure, and over a fine pore membrane. The filtrate passes through the membrane and all particles are thereby removed while the retentate is constantly re-circulated and becomes increasingly more concentrated, although fresh feedstock is continuingly fed into the process. Because pumping is required, an in-line chiller is necessary in this re-circulation loop to ensure low temperatures are maintained. During filtration, the membrane becomes progressively blocked, filtration then stops, and the membrane is rinsed with water and then washed with acid and caustic detergents. Modern working systems use hollow fiber polyethersulfone (PES), or ceramic membranes, at 0.65 µm. They are arranged in bundles, mounted in stainless steel housings, and are approximately 1 meter long.

For many years, cross-flow filters were typically used for beer recovery separation, but recently, many brewers have been installing full-scale, multi-module systems for mainstream beer filtration. These systems may or may not include high-speed centrifuges upstream of the filter. One major advantage of these modern membrane systems relates to the elimination of a need for filter powders, with particular emphasis on the avoidance of disposal issues and costs.⁴¹

A typical cross-flow installation is presented schematically in Figure 15.11.

The operational details of such a plant are as follows:

Figure 15.11 A schematic representation of a typical cross-flow filtration installation.

• During forward filtration, the beer is first transferred from the storage vessels to the concentrate tank, where beer is supplied to the membranes via a feed pump.
• Once a set pressure has been achieved, the re-circulation pump starts and pumps the beer in a loop to induce high velocities in the fibers.
• The filtrate valve then opens and filtration starts.
• To prevent a solid concentration buildup in the loop, the bleed valve is opened and some of the beer in the loop is bled back to the concentrate tank.
• During filtration, the amount of suspended solids increases in the concentrate tank.
• To reduce the amount of solids in the system, the concentrate tank normally is purged.
• During filtration, pressure increases slowly with time as the inevitable membrane fouling takes place.
• To partially restore the membrane flux, a backwash is conducted.
• The multi-module design allows for one bank of modules to be cleaned without disrupting forward flow. The clean module will then come back on stream to allow another bank of modules to be cleaned.

15.6.4.3 Sterile Filtration

Sterile beer filtration is defined as an operation that produces sterile beer ready for packaging with no subsequent pasteurization. As discussed previously, several filter types are suitable for this task (i.e., sheet, membrane, ceramic candle, and pulp filters). The type of filter selected for sterile filtration will depend on the brewer’s requirements and on appropriate features, such as throughput, ease of maintenance, cleaning, and sterilization.

Whichever filter type is used, it must be preceded by at least one other filter that can remove all of the colloidal load, including chillproofing agents, and reduce the yeast count, preferably to near zero. Typically, the bulk of suspended particles is removed with a DE filter, followed by a DE sheet or cartridge filter that will remove residual material sufficiently to reach a haze specification prior to the sterile filter. The sterile filter acts as the final trap for yeast and bacteria.

Sterile filters are not absolute filters. Therefore, the brewer will set a specification for the maximum concentration of bacteria in sterile-filtered beer. As it is possible for a single beer-spoiling bacterium in a bottle or can to spoil the beer, there is a need to balance the risk of spoilage against filter practicality and throughput.

Suitable microbiological sampling and methodology is needed to measure adherence to specifications. Rapid microbiological assay methods are of particular importance to reduce the quantity of product awaiting release to packaging or packaged goods waiting for shipping (details in Chapter 17).

To determine if the filtration system will allow microbiological specifications to be met, the brewer must measure the efficiency of the system for removal of microorganisms from beer. This is also called challenge or integrity testing. For example, for a specific yeast/bacterial load in the beer, there is a measured reduction in that load in the beer filtrate. Beer or water is seeded at a known concentration with a beer-spoiling bacterium. During standardized filtration conditions, beer filtrates are collected and plated. The ratio of colony-forming units before filtration to that after filtration, that is, the change in microbiological load across the filter, is called the log reduction value and is expressed as a logarithm. For example, a ratio of 1×10⁹ means the system has a log reduction value of 9. A log reduction value of 8 to 9 is required of a filter system if it is to be useful as a sterile filtration system. Filter media for sterile filtration, particularly sheets and membranes, will have specific log reduction values that help the brewer optimize the system. Filters must be tested with appropriate bacteria. The brewer should select beer-spoiling bacteria common to the brewery in order to obtain a practical measure of their filter integrity.

Based on the filter-system log reduction value and the filtered beer specification, the incoming beer may present a greater microbiological load than that for which the filter is designed. In such cases, sanitation procedures further upstream in the process must be addressed. The sterile filter cannot be expected to remedy poor microbiological practices upstream. In the end, instilling a proper attitude toward sanitation and care with regard to producing sterile beer is invaluable to reducing microbiological problems.

15.6.4.4 Transfer to Packaging

During the finishing steps of aged beer transfer to packaging, a major concern is to minimize oxygen pickup. Chillproofing, dilution, carbonation, and final filtration steps generally occur in a continuous sequence leading to package release tanks. These operations are separated by transfer tanks and connected by pipes. Minimizing oxygen pickup in these vessels and piping/pumping systems is important because it is difficult to correct a package release tank that has a high concentration of dissolved oxygen without other negative effects on beer quality (e.g., foam stability and beer clarity).

To minimize oxygen pickup, filter feed, surge, and transfer tanks should be purged with CO₂ or packed with carbonated water prior to use. For filtration, oxygen in the filter precoat and body feed in makeup tanks is reduced by CO₂ purging for a sufficient period of time. Using deaerated water for makeup is also positive.

A critical area to reduce oxygen pickup is in the filters themselves, and they are usually opened to the atmosphere for cleaning. Even when a closed filter is purged free of the filtration medium, sluicing with water that has not been deaerated presents a risk because the filter may require CO₂ purging. Deaerated water can also be used to purge transfer lines. Account must be made for beer dilution from water remaining in filters, tanks, and transfer lines.

If packaged beer is not pasteurized, the transfer of sterile beer to packaging presents additional challenges because the contact of sterile beer with any surface presents an opportunity for contamination. Generally, it is advantageous to dedicate specific transfer and package release tanks for sterile-filtered beer. This reduces the possibility of contamination of tanks and transfer lines from beer that is not sterile. A number of dedicated tanks must be chosen to buffer the sterile filter output with packaging requirements. To prevent contamination of sterile beer further downstream, the use of dedicated bottle and can packaging lines is advantageous.

15.7 STABILIZATION

The stabilization of beers may refer to flavor and/or microbiological stability, although stabilization also refers to its physical characteristics such as foam and haze formation.

15.7.1 Flavor Stability

Chemical reactions continue to occur after beer is packaged. Many of the changes that lead to stale beer flavor are caused by chemical oxidation. Flavor stabilization generally refers to the protection of beer from oxidative changes. In early research, the cardboard flavor of stale beer was attributed to trans-2-nonenal.^42–45 Furfural and related compounds have also been identified in stale beer.⁴³ Stale off-flavors are generally attributed to the oxidation of higher alcohols to aldehydes by melanoidins, but there are many more chemical routes participating in the staling of beer. The topic of flavor stability is far ranging and complex⁴⁴ and will not be discussed here in detail. Only two important factors, SO₂ and oxygen,⁴⁵ which have a clearly established effect on flavor stability, will be mentioned here, but they are considered in greater detail in Chapters 20 and 24.

Sulfur dioxide, in the form of bisulfite ions, protects beer against oxidative flavors in two ways. It reacts with oxygen, eliminating it from beer and the potential oxidation of beer components. It also complexes with aldehydes, which have stale flavors, rendering these complexes flavor inactive. Although the complex of bisulfite and unsaturated aldehydes is irreversible, the complex with saturated aldehydes is reversible as other chemical species compete for the bisulfate.⁴⁵ Sulfur dioxide in beer occurs naturally during fermentation and is increased under conditions of slow yeast growth. In addition to naturally occurring SO₂, one can add antioxidants to beer. Potassium/sodium metabisulfite or forms of ascorbic acid are sometimes added after fermentation as reducing agents to counteract oxidative changes and, in addition, metabisulfite binds to carbonyl staling compounds.^{46, 47}

Excluding oxygen from beer is an important step in enhancing flavor stability. Because yeast is an excellent oxygen scavenger, once it is removed, any subsequent oxygen in beer has the potential to oxidize the beer. Therefore, flavor stability is enhanced by excluding oxygen from the beer during aging and finishing operations after the yeast is removed.⁴⁷ The use of CO₂ to pack tanks and for beer transfer reduces the possibility of air pickup. Flavor stability may also be enhanced by proper handling of wort in the brewhouse, by minimizing oxygen pickup during mashing, lautering, and wort cooling, and so on⁴⁴ (further details of the occurrence and use of sulfur dioxide in beer are in Chapter 24).

15.7.2 Biological Stability

Microorganisms can contribute to flavor and physical instability. Certain bacteria (e.g., Lactobacillus sp. and Pediococcus sp.) and wild yeasts of the Saccharomyces and Hansenula genera can spoil beer by producing undesirable flavor compounds, such as VDKs, lactic acid, and phenolic-off-flavors (POFs). Generally, brewers conduct microbiological tests specifically for beer spoilage microorganisms. Microorganisms can also grow and form a haze by increasing their concentration. Proper pasteurization ensures biological stability, but this requires the heating of beer, which can accelerate potential oxidative flavor changes and overall beer instability. Biological stability can be achieved by sterile filtration in which microorganisms are removed by special filtration systems. Although sterile beer can be produced by available filtration technology, contamination is still possible during filling and keg operations. In fact, aseptic filling is more difficult than producing beer with a pasteurization step! Further details regarding beer biological stability are discussed in Chapters 17 and 20.

15.7.3 Physical Stability

Colloidal or nonmicrobiological haze is a result of the precipitation of insoluble complexes formed from beer constituents or from yeast cell wall components.⁴⁸ The general components are known, but the mechanisms of interaction and complexation are still not well understood (for details, see References 51–55 and Chapter 20). There are two types of physical haze: chill haze, which appears when the beer is chilled to below freezing but redissolves upon re-warming, and permanent haze, which never fully redissolves under any conditions. Beer that contains permanent haze remains cloudy and may develop a sediment.

Studies have shown that several chemical species are present in haze material. The major component appears to be proteinaceous material in the size range of 1,000 to 40,000 daltons.⁵¹ Other components of colloidal haze are polyphenols, and to a lesser extent, metal ions and polysaccharides.^{52, 53} It is generally believed that complex proteinaceous compounds and polyphenols become associated through hydrophobic and hydrogen bonds involving proline residues of proteinaceous compounds.^49–51 The presence of oxygen may play a role in polymerizing the phenolic constituents. Those portions of the proteins and polyphenols that contribute to haze formation are referred to as the haze-active fractions.

As haze consists of insoluble protein-polyphenolic complexes, preventative measures can be directed at one or both of these classes of soluble compounds. Because various methods are employed to “chillproof” beer, as physical stabilization is commonly called, treatment with proteolytic enzymes, use of protein precipitants (such as tannic acid), adsorption of proteins by silica xero- and hydro-gels,^{54, 55} adsorption of polyphenols by polyvinylpolypyrrolidone⁵⁶ (PVPP); or modern treatments, available involving combinations of silica xerogels with PVPP, is conducted. Chillproofing strategies are summarized in Figure 15.12 and are described further in Chapters 10 and 20.

Figure 15.12 Chillproofing strategies.

15.7.4 Brewhouse Procedures and Filtration

Some remedial measures can be taken during brewing in order to improve beer physical stability and reduce the need for stabilization. However, removal of proteins and polyphenols must be conducted carefully as both contribute to the character of beer—both its flavor and physical characteristics.⁵³

Selection of malt with lower soluble nitrogen, good modification, and high diastatic power can lower the beer proteinaceous content. Proteolysis during mashing tends to reduce wort protein content, although this reduction of high-molecular-weight protein in the mash may be due to a precipitation mechanism rather than an enzymatic one.³⁴ When adjuncts are used, especially corn (maize) and/or rice, the wort protein level is reduced proportionally. Lower mash pH reduces the solubility of polyphenols. The last runnings from sparging can be high in polyphenol content if the sparge water pH is not carefully controlled.^{57, 58}

Proper adjustment of wort boiling helps control the concentrations of both polypeptides and polyphenols. A long and vigorous boil helps coagulate the complexes, and the presence of oxygen will aid the oxidation of polyphenols. However, oxidation of compounds that are important for flavor stability may also occur. A good hot break, along with efficient wort clarification, enhances beer physical stability.

15.7.5 Measurement of Haze

The measurement of beer haze or turbidity is based on the principle of nephelometry in which light reflected from particles in solution is measured. The angle of reflection is usually 90°, although smaller forward-scattering angles are also useful.^59–61 The measurement of turbidity depends upon the color of the incident light and upon the size and shape of the light-scattering particles. Calibration of instruments specifically designed for nephelometry depends greatly on stable particle standards. Formazin is usually employed. However, more expensive chemically polymerized (Latex) spheres can be employed.⁶¹ Use of analytical instruments is further complicated by imperfections in the measuring cells. Beer haze determined in bottles introduces large random errors, whereas the use of optical cells is time-consuming. Additional complications arise because different instruments produce varying results on the same samples and differ in their responses to particles of different sizes.⁶² In-line turbidity meters are often difficult to correlate with laboratory instruments.

It is also possible to rate beer haze visually by comparing the sample with standards usually based on different concentrations of formazin (American Society of Brewing Chemists [ASBC] Methods of Analysis: Beer-27, A).⁶² With this method, there is difficulty in obtaining agreement between individuals on the level of turbidity in a particular sample. At best, this method is qualitative!

Another element of confusion is that different measurement units are used, for example, the ASBC and the European Brewery Convention (EBC) use different units of measurement. The major problem, however, lies in trying to quantify the human perception of hazy beer by quantifying particles with a range of sizes and shapes and other light-scattering characteristics—the correlation is not always good.

15.8 CARBONATION

15.8.1 Basics of Beer Carbonation

Carbon dioxide solubility in beer is usually measured in volumes of CO₂ per volume of beer at standard temperature and pressure. This means that 1 volume of CO₂ is equal to 0.196% CO₂ by weight or 0.4 kg CO₂/hL (0.92 lb CO₂/bbl).³⁸ Typical American lagers contain 2.5 to 2.8 volumes of CO₂. Ales are generally lower (both small pack and draft). Because beer contains 1.2 to 1.7 volumes of CO₂ after a normal, nonpressurized fermentation,⁶² another 1 volume (or about 0.5 kg/hL [1 lb/bbl]) must be added before packaging. Considering that other uses for CO₂ in the process consume CO₂, it is generally economical to recover excess CO₂ from fermentation. In some breweries, losses together with requirements may exceed recovery and, as a consequence, CO₂ must be purchased; although under careful conditions breweries can be self-sufficient. This is particularly the case if the beer is not canned (details in Chapter 16). The recovery of CO₂ and its purity requirements are discussed in Chapter 19.

The amount of CO₂ in solution depends on Henry's Law. This law states that the amount of a gas dissolved in a liquid is proportional to the concentration of the gas in the headspace. Therefore, the CO₂ concentration in the beer can be elevated by increasing the head pressure of CO₂. Temperature changes the CO₂ solubility—a temperature increase leads to a decrease in solubility. Therefore, the desired CO₂ concentration can be achieved by establishing the temperature and pressure at appropriate settings. Because the solubilities of gases are independent of each other (Henry’s Law), the level of carbonation has no influence on oxygen pickup as the product moves through the brewing process.

The time required to achieve a desired CO₂ concentration depends on physical factors. Finer bubbles have greater surface area per unit weight and dissolve more rapidly than larger bubbles. Moreover, finer bubbles rise more slowly. The longer it takes for bubbles to rise through a tank, the more time there is for CO₂ to become dissolved in the beer. Therefore, carbonation stones are designed to form a fine mist of bubbles. If the headspace is filled with CO₂, a larger headspace–liquid interface area will shorten the carbonation time. The solution of CO₂ also slows as equilibration is approached.

Pressure and temperature relationships to CO₂ concentrations are used to establish a tank concentration. Measurement of CO₂ in tanks can be achieved with a sensor separated from the liquid by a membrane permeable to gases. A common alternative to sensors is the Zahm-Hartung method.⁶³ A metal bottle is filled under controlled temperature and pressure. After establishing equilibrium with the headspace, the temperature and pressure are read and converted by means of a table (Methods of Analysis: Beer-13, A)⁶¹ to volume CO₂. Corrections can be made for oxygen and nitrogen to improve accuracy. In tall tanks, the CO₂ concentration will be higher at tank bottom because of a greater hydrostatic head.

15.8.2 Historical Carbonation

Before there existed equipment and methods for recovering, purifying, and reusing CO₂, beer was carbonated by kräusening and low-temperature secondary fermentation, which increased the CO₂ concentration. Consequently, “historical carbonation” depended on retaining CO₂ rather than reintroducing it. An alternative approach to retaining natural carbonation is to conduct a secondary fermentation and close the tank at an appropriate time. An overpressure of about 1 atmosphere (1 bar g) should yield about 2.7 volumes of CO₂ in beer.³⁸ These approaches make it difficult to precisely control the CO₂ concentration batch to batch. Also, allowing the pressure to rise during fermentation may affect yeast growth and change a beer’s flavor characteristics.

15.8.3 Modern Carbonation

Carbonation can now be achieved by in-line injection or by in-tank carbonation. In-tank carbonation usually involves the introduction of CO₂ through a carbonation stone at the bottom of the tank. The purpose of the stone is to form fine CO₂ bubbles, which readily dissolve in the beer. Another reason for carbonating in tanks is that oxygen and objectionable aromas can be purged out of the beer if the tank can remain open to the atmosphere during the early part of the procedure. The tank is then closed and carbonation commences. In-line injection can occur whenever beer is transferred. However, it cannot occur upstream from DE filtration because CO₂ bubbles would disturb the filter bed. The process of in-line CO₂ injection is governed by three factors that influence solubility: (a) low temperature, (b) high pressure, and (c) a fine dispersal of the gas as micro-bubbles. Generally, all three factors are observed when designing a carbonation facility and there are several designs, such as the ones that follow.

1. Venturi carbonator: CO₂ is injected in the low pressure zone of the venturi and the gas dissolves as the pressure increases, and the flow velocity reduces because of the increase in the pipe diameter. The venturi is made, for example, from a section of 100 mm diameter pipe, which has been “flattened,” producing a section having a reduced flow area. As beer passes through the venturi, the velocity increases, and there is a corresponding decrease in pressure. CO₂ is injected into the beer at the low pressure zone. Absorption is aided by the high degree of turbulence in this zone and by the increase in pressure, which occurs in the diffuser section. In this section, the area increases; consequently, there is a reduction in velocity and an increase in pressure.
2. Nozzles: Many breweries still prefer the simplicity of a simple injection nozzle for carbonation. The gas is forced through the fine hole at the end of a nozzle, and the result is a fine dispersal of bubbles in the liquid. Nozzles have an advantage over sintered diffusers in that they are more easily cleaned.
3. Membrane contactors: A modern approach for carbonation uses membrane contactors. These are typically fabricated with hydrophobic, hollow-fiber, microporous membranes, usually polypropylene with a pore size of 0.05 μm.⁶³

    These systems diffuse CO₂ into the liquid on a microscopic level. This produces a much more controlled level of carbonation in the end product and uses less CO₂ gas.

When selecting the point in the process to inject the CO₂, consideration is given to the factors that promote gas absorption; that is, low temperature and high pressure. A static mixer can be incorporated after the injection point to create turbulence, which will aid the CO₂ being dissolved in the beer.

For carbonating beer, the selection of an injection point is a compromise. Temperature and turbulence-mixing is often the choice! For example, if injection occurs immediately before a beer chiller, the turbulence through the chiller will increase the transfer rate, and the decrease in temperature, which occurs through the chiller, will also assist the gas going into solution.

Modern systems incorporate automatic gas injection control by means of a CO₂ in-line sensor, the signal from which is used to vary the gas flow by a standard process control loop.

15.9 STANDARDIZATION

15.9.1 Introduction

This postfermentation stage, consisting of aging/maturation/conditioning/filtration and so on, represents the one phase in the brewing process that permits some flexibility in terms of residence time—once the beer has been chilled to 0°C or lower. Most brewers will specify a minimum storage time for this stage, but it can be relaxed to holding beer for longer periods because this constitutes the phase where inventory and logistic alterations can be accommodated. It is far better to hold beer at this stage rather than postfiltration because bright tank residence times, prior to packaging, should usually be minimized.

This is also the stage at which adjustments of various parameters (e.g., dilution, carbonation, extract, etc.) can be made in order to achieve product uniformity. Further, process additives that are explicit to the particular beer specification, including flavor additions, are most ideally made during aging.

15.9.2 Blending for Consistency

Blending or standardization refers to the mixing of different batches of beer to achieve product uniformity. Generally, blending is conducted to achieve an exact alcohol concentration, specific gravity, or original gravity. Blending can also be conducted to achieve uniformity in other parameters, for example, bitterness units, color, or sweetness.

Occasionally, blending is used to attenuate an unwanted flavor note. For example, if a fermentation problem led to a high diacetyl concentration or a noticeable sulfury character, the beer could be blended with normal product in an attempt to dilute the objectionable flavor. Blending guidelines are established by brewers to prevent noticeable deviations from flavor uniformity.

15.9.3 Process Additions and Process Additives to Beer

At this stage in the process, there can be many additions to the beer, examples include the following:

• Priming sugar or syrups, usually to make the beer sweeter or to add more body but sometimes to provide extra sugar as a substrate in order to promote fermentation during conditioning or “krausening.”
• Color in the form of brewer’s caramel or colored malt extracts to adjust the beer color or to impart flavor.
• Hop extracts to adjust bitterness, add hop flavor, or enhance foam stability (details in Chapters 7, 10, and 20).
• “Exotic” flavorants and flavor extracts (e.g., fruits, herbs, spices, chocolate), for products such as fruit beers, coolers, and so on.
• Acid or alkali to adjust beer pH.
• Gases, including CO₂ and N₂.
• Foaming or heading agents; foam formation and retention are helped by relatively high viscosity as the liquid between the bubbles is less able to drain. Consequently, the use of carbohydrates, such as alginates (propylene glycol alginate—PGA) or gums, assist foam retention. Also reduced hop extracts have a similar effect (details in Chapter 7).
• Recovered beer can be blended at this point.
• Dilution water, for alcohol adjustment of high-gravity brewed beer (details in Chapter 9).⁶²

In addition, various process additives (discussed in Chapter 10) can be included, such as clarification agents (finings, tannic acid, etc.), stabilization agents (silica gels, PVPP, enzymes, etc.), and process aids (enzymes such as glucoamylase and ß-glucanase, proteinase, wood chips, anti-oxidant treatments, etc.).

15.9.4 Addition of Modified Hop Extracts

If modified hop extracts are used wholly or partially to replace the kettle addition of hops, they can be added to beer upon transfer into aging. These extracts can be preisomerized hop extracts, which contain iso-alpha acids (isohumulone and its homologues), or reduced hop extracts. Further details on hop extracts can be found in Chapter 7. The advantages for adding modified hop extracts to fermented beer include the following:

• Better utilization (efficiency) of hop-bittering acids (iso-alpha acids added) compared to adding hops to the kettle. More iso-alpha acids survive in the beer because hop loss during fermentation is eliminated.
• Kettle-hopping losses are eliminated, producing beer with more uniform international bitterness units (IBUs).
• Degradation of iso-acid-acids during kettle boil is prevented.
• Better control of IBU in beers. (IBU measurements can be conducted during aging and further extract added if the IBU is below specification.)
• Total replacement of conventional hops by reduced hop extracts gives protection against light when packaged in flint, green glass, or plastic bottles (lightstruck protection details in Chapter 7).
• Hop oil fractions and emulsions or even dry hops to add essential hop oil characters, in a variety of formats.
• Reduced hop compounds, especially tetra-hydro-isohumulone (added at 2 to 3 mg/L) will enhance beer foam (Chapter 7).

15.9.5 High-Gravity Brewing

The need to adjust the beer’s alcohol concentration as a result of high-gravity brewing (advantages and disadvantages of this process are discussed in Chapter 9)⁶⁴ means that additional equipment must be obtained and that the dilution water must be treated appropriately before it can be added to the beer. This water must be filtered, sterilized, deaerated, and carbonated prior to use (see Chapter 4 for details about the quality and processing requirements of dilution water). As already noted, the CO₂ used for carbonation of diluent water must be free of oxygen and other volatile compounds, which could modify the beer flavor.⁶⁴

Purification of water is usually conducted with a carbon filter. Deaeration can be achieved by several methods (see Chapters 4 and 9). By adding plate heat exchangers and carbonation equipment, a system can be constructed that produces purified, sterilized, chilled, deaerated, and carbonated water. Finally, accurate metering of this water into the beer is important for product consistency.

Beer is adjusted to the concentration it would normally have if high-gravity brewing were not used. This concentration is called the original gravity (OG), which refers to the solids (^oPlato) of the wort from which the beer was produced, whether it was the true wort gravity or not. The original gravity is related to the “heaviness” of the beer. After dilution, it is useful to know the equivalent wort gravity of the diluted beer in order to maintain product consistency in terms of its concentration.

The original gravity calculation is detailed in Methods of Analysis: Beer-6, A⁶¹

OG=2.0665A+E100+1.0665A×100

where A is the wt % alcohol and E is the real extract in wt % in the diluted beer.

Dilution calculations can be based on measuring precise alcohol, original gravity concentrations, and so on. To maximize brewing capacity, the dilution step should take place as late as possible in the process. However, this means that beer losses are increased if they occur before dilution.⁶⁴ With any blending, whether for dilution of high-gravity brews or for flavor uniformity, sufficient time must be allowed to attain a homogeneous product. Quality checks on the blend are a necessity before its release to packaging.

15.10 SUMMARY

It is apparent that this phase of postfermentation processing is the most variable of all the key stages of the brewing process. A number of possible variations are available in terms of timing and details of procedures, with different brewers, reflecting a broad spectrum of brewing philosophies. However, all the component features discussed in this chapter may be incorporated—to varying extents and in various details, including flavor maturation (aging), clarification, stabilization, dilution, carbonation, and blending. All of these features must be conducted in the most appropriate manner to prepare the beer for packaging (details in Chapter 16) and are designed to provide beer with optimal stability. This is in terms of both flavor and physical (colloidal) stability, throughout the shelf life of the packaged beer. Most brewers, however, will regard this stage as being the best point for final process adjustments to be made in order to achieve appropriate product specifications both in terms of chemical and flavor analyses.

It is usual that this phase is regarded as the most appropriate for balancing inventory requirements. Modifying the length of time of processing here by just a few hours or even days is much less of a risk than having to extend the storage time of beer postfiltration in bright beer tanks!

Overall, there is a spectrum of aging policies from extended cold fermentation with classical protracted cold conditioning to accelerated flavor maturation (represented primarily by diacetyl management and other parameters) by incorporating a diacetyl stand (or “rest/free rise”) into the final phase of fermentation, followed by virtually complete removal of yeast (by centrifugation or flocculation) plus cold storage (at 0ºC or lower) of just a few days to effect stabilization prior to filtration. Indeed, it is possible to identify the various aging techniques between these two options, which can be regarded as the most likely opposite extremes. Brewers may adopt either approach within a multiplicity of possible variations. Whatever approach is selected, the ultimate target will be to follow the most appropriate course for a specific brewery and/or product in terms of effective overall cost control, balanced by optimal consistency of beer quality, stability, and drinkability.

ACKNOWLEDGMENT

Some parts of this chapter were based on the chapter by James H. Munroe in the second edition of this book.

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