CHAPTER 13

Secondary Fermentation

Whether or not a beer will be extensively lagered, a secondary fermentation in a closed fermenter allows for the slow reduction (conditioning) of the remaining fermentable extract. In the secondary fermentation stage of lagering, the beer is free from the flavor impairment of sedimented trub and degenerating yeast cells. Seven to twenty-one days may be required for the yeast to deplete the fermentable sugar left after the kraeusen period has ended. Traditional lagering entails holding the beer for a further two to seven weeks for clarification and stabilization.

During the secondary fermentation phase, the beer is slowly attenuated. It is slowly cooled to 33 to 37 degrees F (1 to 3 degrees C) — or to as low as 30 degrees F (-1 degree C) to settle dusty yeast — to allow the yeast to settle thoroughly and to inhibit the activity of any microorganisms possibly contaminating the ferment.

Because the potential risk of airborne contamination is great during the slow, cold ferment, the beer absolutely must be protected from contact with the atmosphere by being fermented in a closed vessel fitted with a fermentation lock. Contamination is otherwise a major risk at this point; yeast activity is slow, because the beer no longer contains abundant extract and nutrients, yet bacteria may be capable of significant dextrin, protein, or yeast-waste fermentation. Even though the pH is below their optimum, given a warm enough temperature even a few wild yeast or lactic-acid bacteria might rapidly propagate and ruin the beer.

Reproduction by the culture yeast will have entirely ceased during this stage of fermentation; further attenuation relies solely on the metabolic activity of the relatively few remaining yeast cells. It is imperative that conditions be conducive to the continued metabolism of the fermentable extract by these yeast cells and that they not be subjected to temperature shock. If the yeast culture needs regeneration, then an active starter culture or 5 percent kraeusen beer is added.

The duration of the secondary fermentation and the temperature at which it should be conducted are determined by the maltotriose and dextrin content of the post-kraeusen beer. If its reduction in density has naturally slowed and the hydrometer reading is still about one-third the value of the original wort reading, then the beer is rich in dextrins. It should be fermented out in the secondary at 33 to 41 degrees F (l to 5 degrees C), depending on the temperature preferences of the yeast strain, for at least fourteen days. When the post-kraeusen density is much less than one-third the value of the wort density, it indicates that the beer is lacking in dextrins and should undergo a secondary fermentation at 34 to 37 degreesF (1 to 3 degrees C) for not more than ten days before the temperature is reduced for lagering. Lagering times will also be shortened.

Long fermentations often darken the color of the beer. Consequently, when lagers are brewed for paleness, secondary fermentation may be carried out at higher temperatures (36 to 39 degrees F [2 to 4 degrees C], but not above 40 degrees F [5 degrees C]) over the shorter time period.

Since beer for draft need not be brewed for a long shelf life, it may also be fermented at higher temperatures (34 to 41 degrees F [1 to 5 degrees C]) and for a shorter period of time. When the hydrometer reading drops less than .2 °Plato/1 degree of gravity over a twenty-four-hour period and is within .4 °Plato/2 degrees of gravity of its anticipated terminal gravity, it can be assumed that there is just enough yeast and fermentable extract left in solution to support cask carbonation. The beer is racked into a keg or cask.

Lagering

A long, cold, post-fermentive rest is usually employed when the wort is from a decoction mash. It yields a more stable beer with a smoother flavor.

Lagering mellows harsh flavors by the combined effects of the falling rate of yeast metabolism, increased acidity, and low temperatures. Astringent tannins coagulate with haze-forming proteins, precipitating these and other, sulfurous compounds out of solution.

Yeast cells are not usually decomposed during lagering, but the culture becomes progressively dormant as fermentable extract (and to varying extents, glycogen reserves) are depleted. With the decline in available carbohydrates, the yeast reabsorb some of the esters and sulfur compounds from the beer.

Successful lagering requires that the beer not be subjected to temperature fluctuations or oxygenation. Oxygen in nearly fermented beer causes the irreversible formation of diacetyl and the oxidation of fusel alcohols and lipids. Where lagering temperatures are too warm, aldehyde formation is accelerated.

“Staling” aldehydes give beer stale, papery, cardboardy or sherrylike flavors. When higher temperatures decompose yeast cells, sulfury, stale, and soapy flavors arise.

When the kraeusen tradition is being followed, a lattice of beech chips is laid on the bottom of the secondary fermenter and covered with the nearly fermented, or ruh, beer. From 5 to 15 percent new beer at up to 39 degrees F (4 degrees C) is roused into it. Where tank construction permits pressurization, and the tank is fitted with some manner of pressure relief, it is common to lager the beer under .2 to 2 atmospheres (3 to 28 psi) of pressure, after the vessel is purged of the atmosphere in the headspace. This can be accomplished by various pressure-regulating arrangements.

The lagering period is determined by referring back to the mash program, the hydrometer reading of the cooled wort, and the primary fermentation time and temperature. Dextrinous beer from a decoction mash should undergo a secondary fermentation and lagering period of seven to twelve days at 33 to 34 degrees F (1 to 2 degrees C) for each 2 °Plato (SG 1008) of cooled-wort hydrometer reading (OG). Lighter beer, lacking dextrins, is usually held for only three to seven days for each 2 °Plato of the wort density. Very strong, kraeusened beer, on the other hand, may be lagered for six to eight months before it is bottled.

Reducing the temperature to near freezing several days after secondary fermentation falls off reduces lagering time; in fact, the decrease in the solubility of body-forming colloids at 30 to 33 degrees F (-1 to +l degree C) necessitates a briefer lager period.

Fining

Whether or not the beer is being lagered, fining improves its head retention, lacing, and clarity, and reduces aging times. It precipitates degenerated yeast cells, haze proteins, and tannins out of the beer. Gelatin and isinglass act as fining substances by enveloping suspended particles in their matrix, and gelatin further combines with tannic acid to form an insoluble precipitate. Either brewers’ gelatin, unflavored 95-percent-pure gelatin, or isinglass may be used, so long as it is dry, smooth, pale colored, and odor free. Finings spoil if they absorb moisture during storage.

Isinglass finings are made from the shedded air bladders of certain fish. They are even more subject to spoiling than gelatin finings, and have a more limited shelf life. Isinglass works quickly and is generally added to the beer only two or three days before the beer will be racked. Isinglass precipitates lipids and can dramatically improve head retention.

When the beer has fermented out, it is ready for fining. A reducing-sugar analysis should show less than 2 percent. The beer must be colder than 50 degrees F (10 degrees C) for gelatin to react with the ferment; the closer to freezing temperature the beer is, the more efficient the action of gelatin finings will be.

Isinglass finings act more rapidly than gelatin finings do under the same conditions, and far better at warm temperatures. Isinglass can be used at up to 60 degrees F (16 degrees C), although it performs better in colder beer.

Dissolving finings into beer, wort, or water must be intelligently handled so that the finings are completely liquefied. They need to be evenly dispersed into the aged beer in a sanitary fashion. Finings cannot combine with yeast, polyphenols, and albumins unless they come into intimate contact with them. To accomplish this, the finings must be diffused throughout the entire volume of beer.

Where the beer is to be filtered, insoluble polyvinylpyrrolidones, such as Polyclar, and polyamides, such as nylon, can be used to remove polyphenols, especially chill-haze anthocyanogens, by bonding to them. Other media used are diatomaceous earth (kieselguhr) and silica gel. Diatomaceous earth selectively removes particles of a certain and greater size and is the filtering agent most commonly used by small breweries. Silica gel absorbs large polypeptides and proteins, removing them from solution so they cannot combine with polyphenols.

If a strong hop aroma is desired in a lager, liquid hop extract may be added with the gelatin finings, or before other treatment. Even with British ales, hops for strong aroma are never usually added after fining. Hops, and especially extracts, when added with finings, can improve the clarification of beer from a well-mashed and boiled wort by increasing the polyphenols available for coprecipitation with albumin.

If the bottled beer lacks a good foam head or is thin, then the clarifying treatment should be reduced in future batches. Use of particular malts and brewing techniques precludes the need for clarifying.

Clarifying with Beech Chips

Beech or hazelnut chips one-eighth inch thick by one-half inch wide are laid on the bottom of the lagering vessel to form a loosely woven lattice. As the wood absorbs moisture, extract coats the many surfaces of the chips, bonding weak yeast cells to them and thus clearing the beer.

The chips are first soaked and then boiled for twelve to twenty-four hours in a sodium bicarbonate solution (11/2 pounds/gallon) before being rinsed. The process is repeated, sometimes substituting bisulfate of lime. Finally, the shavings are rinsed in a cold-hot-cold water cycle. The pH of the last rinse should be neutral, indicating that all of the bicarbonate has been washed from the chips. They may be washed and reused until they crack from age.

Real Terminal Extract

When the aged beer is ready to be bottled, its real-extract content can be determined by boiling off the alcohol from a measured volume of the beer, topping it up to its original volume with distilled water, and gauging its density with a hydrometer. The liquid pressure exerted by the fermented beer (sugar analysis less than 2 percent) is due to unfermentable dextrins, maltotetraose, and soluble nitrogen. Taking a hydrometer reading of a dealcoholized sample is the only means by which the “real extract” may be quantified with absolute accuracy, by removing the “apparent density” effect of alcohol from it.

The amount of unfermentable extract remaining in the finished beer is the direct result of the duration and effectiveness of the proteolytic, alpha-amylase and beta-amylase mash rests, the amount and nature of malt used, the effectiveness of protein/polyphenol bonding in the boil, and the ability of the yeast to ferment maltotriose and isomaltose. Typical bottled lagers have a real density of 3.0 to 5.0 °Plato (SG 1012 to 1020); richer types average 5.5 °Plato (SG 1022) to 6.5 °Plato (SG 1026). Bocks may range even higher. The real density of a fully fermented beer is generally 40 percent greater than its apparent density.

Full-bodied beers should show 45 to 60 percent real attenuation, light beers up to 70 percent. Apparent attenuation is usually 60 to 75 percent in the former case and up to 85 percent in the latter.

Bottling

Naturally carbonated beer must be racked off its sediment into a sterile, closed container and mixed with a quantity of actively fermenting kraeusen beer or priming solution sufficient to produce the desired bottle pressure (see table 23). When corn sugar (dextrose) is used to carbonate the bottled beer, it should be made up into a solution; dry primings are not recommended. High-quality dextrose should be the only sugar employed for bottling.

Whether kraeusen beer or sugar fuels bottle carbonation, measurements need to be exact, and the solution mixed with the aged beer very thoroughly, without splashing, or inconsistent carbonation results. The character of the finished beer is greatly influenced by the degree of carbonation.

Mixing the kraeusen beer or priming solution into the aged beer should not be done in an aerating fashion but should raise a foam head, indicating CO₂-- release. This aids in the prevention of oxidation and contamination of the beer at bottling by forming a protective blanket of carbonic gas above the beer and driving some atmosphere from the head space above.

The bottles into which the beer is siphoned must be biologically as well as physically clean. Commercially, bottles are washed with 3 percent caustic soda, rinsed, then sterilized at above 170 degrees F (77 degrees C), drained, and rinsed. Clean bottles may be “pasteurized” by soaking them for at least thirty minutes in clean water at above 140 degrees F (60 degrees C) or placing them wet in an oven at 200 degrees F (93 degrees C) for twenty minutes before they are inverted to dry and be inspected.

Any type of bottle may be used, so long as it can be sealed and can withstand the pressure of bottle fermentation. Carbonation in excess of three atmospheres requires the use of a heavy-gauge bottle. Under carefully controlled conditions, thin-walled, “nonreturnable” bottles are sufficient when brewing lager beer of normal carbonation.

Each bottle should be filled to within at least three-quarters of an inch of the top and be left to rest, loosely capped, for several minutes before the caps are secured. This allows air trapped in the neck space to be driven off by the release of carbonic gas. Oxygen that is not displaced may be scavenged by the yeast, but it is possible that where there is a significant amount of air it may be absorbed into the beer and diminish its stability and mar its flavor. Oxygenation at bottling from splashing the beer or from trapping air in the neck space can be a problem as serious as contamination.

The bottled beer should be held at 50 degrees F (10 degrees C) or above for several days to allow fermentation to be established within the bottle before lowering the temperature. Temperature reduction should be gradual, not exceeding 5 degrees F (3 degrees C) daily. Bottle conditioning should take place away from direct sunlight, and the bottles should not be subjected to major temperature fluctuations.

The beer should be aged in the bottle for an absolute minimum of ten to fourteen days, and preferably thirty days, before serving. Lagered, bottle-conditioned beers usually keep for at least several months. There is an optimum storage period for every beer, when chemical changes within the bottle produce the best taste and aroma, and this should dictate the length of the bottle-aging period.

Imbibing

Handcrafted beers are not usually consumed with the same carelessness as a can of beer at a ball game. They are brewed to be drunk at the right temperature, with discernment and appreciation for the subtler aspects of their character. When bottle-conditioned beer is ready for drinking, it should be carefully poured so that the yeast sediment is not disturbed; a good yeast strain cakes solidly on the bottom of the bottle.

A beer is judged by its flavor, aroma, body, head, color, and clarity. All of these values are subjective, and various standards depend on the type of beer being brewed, the balance of one characteristic against another, and the predisposition and preference of the consumer.

Areas for taste

Color, clarity, head formation and retention, and aroma can be assessed before the beer is even tasted. The aroma is usually sampled by swirling the glass and holding it under the nose. The cleanliness and sharpness of the hop bouquet, the balance of roastiness or maltiness, and the intensity and character of fermentation products are the major criteria by which aroma is judged. Aroma may be characterized as being ethereal, aromatic, floral, spicy, malty, roasty, nutty, yeasty, fruity, vegetative, sulfury, buttery, phenolic, solventy, or musty.

Flavor is evaluated by the first sensation, the taste, then by the aftertaste once the beer is swallowed. The body of beer is judged by its fullness and texture, and is closely related to “mouthfeel.” The palate fullness of beer is mostly due to the low molecular weight albumins and dextrins carried over to the finished beer, and its viscosity.

Taste perceptions are very generally classified as sweet, sour, bitter, or salty. Sweetness is produced by unfermentable sugars and the breakdown of simple dextrins by saliva. Sourness (acidity) is directly affected by the pH of the beer, and below pH 4.0 it becomes both noticeable and in most cases, objectionable. Bitterness is due to the iso-alpha acids from the hops, and from roasted malt. Saltiness is formed by the mineral content of the wort and generally complements bitterness and accentuates dryness.

Because our taste perceptions are highly sophisticated, we can differentiate between far more flavor characteristics than the four basic tastes can account for. Olfactory perceptions greatly affect flavor assessment. Beer flavor is likely to be expressed using olfactory terms such as malty, roasty, caramel, grainy, hoppy, vegetative, medicinal, metallic, sulphury, skunky, burnt, nutty, yeasty, buttery, and fruity.

In the final analysis, the character of the beer should cater to the preferences of the ultimate consumer.