Vigorously boiling the mash runoff produces several desirable effects: it destroys mash enzymes, sterilizes the wort, and stabilizes salts in solutions. It extracts hop resins, drives off kettle-harsh hop oils, and coagulates and precipitates unstable protein. Boiling also evaporates excess water, lowers the wort pH, and creates a stable medium for controlled fermentation by the culture yeast. Boiling may begin when enough wort has been collected to cover the bottom of the kettle.
Hops should be added to the kettle by being scattered over the surface of violently boiling wort. They may be added all at once, but more commonly they are meted out in portions throughout the boil. The actual sequence is determined by the hop character that is meant to be carried over into the finished beer.
Adding hops early on in the boil ensures greater utilization of bittering principles and a more complete precipitation of proteins, hop tannins, and hop particles. A sixty-to-ninety-minute boil succeeds in isomerizing 25 to 30 percent of the alpha resins and in bonding them to the wort as iso-alpha-acids. With pelletized hops, ruptured and better-exposed lupulin glands give greater utilization, even as high as 35 percent. This is the greatest percentage of hop bittering and preservative principles that is normally ever carried over into the finished beer. On the other hand, the bitterness derived from long boiling is coarser than that from a more moderate period; for this reason, it is usual to add only a fraction of the hops at the start of the boil.
Some of the hop polyphenols are transported into the ferment in combination with simple albumins, forming tiny substances-in-solution known as colloids. This colloidal matter is not significantly precipitated and is involved in forming the body and head of the finished beer. Because their surface area is disproportionately greater than their volume, colloids do not readily settle out of solution. Consequently, their contribution to the beer’s body is not offset by inherent instability, as is the case with noncolloidal protein.
It is common to add 5 to 15 percent of the hops at or before the onset of boiling to break the surface tension of the wort so that it does not throw up as voluminous a protein head and boil over. When the wort is the product of an infusion mash, it should be boiled vigorously for fifteen to thirty minutes before more of the hops are added to allow the boiling action to decompose and precipitate some of the proteins. If this is not accomplished before the hops are added, then hop polyphenols will combine with the coarse protein flocks and be precipitated out of solution, carrying hop resins with them.
Even an intense initial boil, however, does not eliminate the large proteins as effectively as do the processes of decoction mashing. Although the proteins can be precipitated, they cannot be dissolved into albumin, peptides, and amino acids, because all enzyme activity has been terminated by the boil.
When the wort is the product of a decoction mash, excessive complex proteins aren’t usually a problem. The several boilings and rests largely reduce or eliminate them. Decoction-mashed wort can therefore be hopped somewhat more conservatively than infusion-mashed wort, simply because the hops need not overcome a great amount of protein.
All of the aromatic hop character of the beer is lost during a long boil. The hops’ volatile essential oils and esters can be preserved by adding hops later in the boil. It is usual, in fact, to add the hops in two, three, or even four portions. Only lightly hopped beers that employ hops for their preservative contribution rather than for their flavor and aroma fully extract the entire quantity of hops during a sixty-minute or longer boil.
Beers that are heavily hopped in the beginning of the boil exhibit a cleaner kraeusen fermentation head and are more stable than beers hopped later, but the hop bitterness will be coarser and less pleasant. It is essential, however, that most of the hops should be vigorously boiled in the uncovered wort for forty-five minutes or more to efficiently isomerize alpha acid and precipitate tannin and proteins. Generally, a small portion of the bittering hops is added to the kettle with the first mash runoff. The largest part is added to boil for forty-five to sixty minutes. A smaller portion may be cast onto the wort fifteen to thirty minutes before the boil ends. Finishing hops, which give the beer a spicy hop flavor and bouquet, may be added within the last minutes of the boil, as the wort is struck from the kettle, or as an extract during fermentation.
The quantity of hops is determined by several factors: the desired bitterness level, the hop flavor and aromatic character in the finished beer, the alpha-acid content and condition of the hops, and the efficiency of the hop extraction.
Hop acids have limited solubility and ability to isomerize, which lessen with increasing wort gravity. Usual lager hop rates are approximately .2 to .4 ounces of hops per gallon of cooled wort, but may be as low as .15 ounces or as high as .75 ounces per gallon, depending on hop quality, the alpha acidity of the hops, the beer type and its density. Contrary to what might be expected, hop acids become less soluble as wort density increases. Kettle-hop rates may or may not be increased to balance the terminal density of some beers; rather, finishing-hop rates may be increased so that hop flavor, not bitterness, balances the sweetness.
As soon as all of the sparging runoff has been brought to a full boil, the wort’s extract content and volume can be measured. With the hops fully submerged, correct the volume to 60 or 68 degrees F (15.56 or 20 degrees C). Correct the volume to 60 degrees F by multiplying volume at full boil by .959, to 68 degrees F by .960 (the displacement of wort by the hops is insignificant). These figures can be used to establish the evaporation rate necessary to evaporate the wort to the desired volume and concentration within the prescribed parameters of the boil. The usual evaporation rate in an uncovered boil is 10 percent per hour, although the actual rate depends upon the wort’s surface area, surface tension, kettle geometry, the amount of energy applied, ventilation, and the ambient atmospheric pressure.
It may happen that the brewer will need to proceed with a boil that will yield less wort than is needed to satisfy fermenting, priming, topping-up, or kraeusen and yeast-culturing requirements. If a correction must be made, then the volume is generally allowed to vary from what was expected. Too great or too small a volume of sweet wort is of less concern than the correct density. When extract-poor malt, inefficient mashing or sparging, or miscalculation results in a wide disparity between the density that was expected and what occurs, a lighter-density beer must be accepted or the extract content increased with malt-extract or wort, should any be available.
Never boil for less than the prescribed time. The kettle may be partly covered for part of the boil to control evaporation, but the wort must be vigorously boiled, uncovered, for at least the final thirty minutes to drive off harsh, volatile kettle-hop and malt oils, sulphur compounds, ketones, and esters.
Never simmer the wort in lieu of a vigorous boil. Efficient hop-resin isomerization, albumin/resin bonding, and protein/tannin precipitation is achieved only through the agitation of the boil. In fact, a violent boil has the greatest influence on the stabilization of the wort. If movement cannot be induced by the circulation of thermal currents in the wort (heating the kettle asymmetrically improves circulation), then agitation becomes increasingly important. Oxygenation improves flocculation, but at the unacceptable cost of oxidizing and discoloring the wort. Aeration of the mash, wort, or beer at any time except after wort cooling should be avoided.
Once adjustments to the volume of the wort have been made, the pH of the boil should be checked in a sample cooled to 68 degrees F (20 degrees C). Optimum protein flocculation occurs at above pH 5.5, but an initial pH of 5.2 to 5.5 is more appropriate to satisfy the other pH requirements of wort boiling and fermentation. Corrections to the wort acidity should be made with acid or calcium carbonate. Lower pH values produce fewer, smaller flocks; below pH 5.0, the protein does not coagulate. Whenever the pH is less than optimal, agitation and movement within the kettle become increasingly important to flock size.
The pH of the wort drops during boiling as calcium phosphate is precipitated out of solution (sodium and potassium phosphate are unaffected by boiling); usual pH reduction is approximately 0.2 for a sweet wort of 5.5, and .3 for a pH of 5.8.
Samples periodically taken from the wort and viewed in a glass container should reveal the progressive flocculation of albuminous protein with hop tannin (polyphenols). Invisible in suspension, they first appear as a mist of tiny flakes that cloud the wort soon after boiling commences. The rolling motion of the boil causes the malt proteins to collide with and adhere to the sticky hop polyphenols. The particles rapidly coagulate into a much smaller number of larger flocks one-eighth inch across, roughly composed of 50 to 60 percent protein, 20 to 30 percent polyphenols, 15 to 20 percent resins, and 2 to 3 percent ash. Upon resting, these large flocks should readily precipitate, leaving the sample brilliantly clear.
As the end of the prescribed boil approaches, samples taken and force-cooled to below 50 degrees F (10 degrees C) are examined. The wort that showed clear when it was hot should cloud slowly as it cools, as previously invisible coagulum loses its solubility in the cooler solution. This cold break should settle, again leaving the wort clear, bright, and sparkling.
The wort must be boiled past a positive cold break in the sample, and flavoring hops should not be added until after the break has been achieved. It is important that the break samples be evaluated; however, boiling should not be extended beyond the recommended time even when the break is poor. A scarcity of flocculum in a well-agitated, strong boil at the proper pH may be caused by malt of poor quality or by either an excessively long or insufficient albumin rest. In the first case, almost all the albumin has been reduced to amino acids or retained in the spent grain, and the beer can be expected to be thin. In the latter case, the protein is too complex to coagulate, and the beer will lack stability and be prone to serious oxidation and taste impairment.
In any case, no correction in the kettle is possible if temperature, pH, and movement of the wort are all satisfactory. The boiling should not be extended unless it is subsequent to a pH or temperature adjustment to the wort.
If a satisfactory break cannot be established because proteolysis has been insufficient, the only recourse is to rack the beer off its sediment several times during fermentation and lagering to separate it from proteins in the trub, and to chill it or tightly filter it before packaging. Even so, the beer may form a chill haze.
Finishing hops are usually the very finest hops, chosen for their flavor and aromatics. Generally they are only a fraction of the quantity of kettle hops employed. Fragrant hops are broken up and added to the kettle or the hop back, or an extraction of their hop oils is infused into the cooled wort or fermented beer.
The later in the brewing cycle that finishing hops are added, the greater their bouquet will be. Flavoring hops are commonly added ten or fifteen minutes before the end of the boil for lager beer, so that humulene, carophyllene, and their oxidation products are effectively extracted by exposure to the boiling-hot wort. Late hops contribute little bitterness to the beer and only subdued aroma, but they give the beer a crisp hop flavor. Some aroma hops may be added as the wort is filtered through the hop bed.
Later addition of hops is made only when a distinctive hop aroma is desired. Traditionally, whole hops are added to British ales, even up to the point of packaging, but a hop extract is more appropriate for lager styles. A hop extract can be made by steeping aroma hops for ten minutes or more at pH 5.5 or above in four fluid ounces of wort per each half ounce of hops. An extract is usually added to the wort post-primary, so that none of it is lost in the hop and trub residue and the aroma is not scrubbed out during primary fermentation. Extracts give a cleaner kraeusen head than adding loose or bagged hops post-kettle and present less risk of contamination. The aromatic character of an extract varies substantially from that achieved by dry-hopping; boiling drives off some volatile essential oils (myrecene, thioesters) while extracting others (humulene, carophyllene). Overall, aroma from an extract is milder, spicier, and less grassy/weedy than that derived by dry-hopping.
The hop nose and flavor characteristic of most lagers is obtained by adding loose hops to the wort at or shortly before the conclusion of the boil. Even the very hoppy character of some lagers is attributable to liberal kettle finishing-hop rates rather than to dry-hopping.
Hop nose and flavor are matters of personal preference; finishing-hop rates may be adjusted to suit the brewer’s preference, as well as to reflect the aromatic quality of the hops being used.
At the end of the recommended boiling period, the wort should be at its desired volume and concentration (both corrected to the reference temperature, 60 or 68 degrees F [15.56 or 20 degrees C]). The hot wort may simply be siphoned off its hop and trub residue, but this causes an unreasonable amount of extract to be lost. Where pellets are used, the wort is generally whirlpooled for several minutes, settled until it is clear (generally ten to fifteen minutes), and then run off from a side outlet. Otherwise, it is more efficient to strain the wort through a loose bed of hops, two inches thick, in a large strainer (hop back) or on a false bottom (for example, perforated with sixteenth-inch holes on eighth-inch centers, or slots .062 inches wide covering 30 percent of the surface). The wort may be recycled, very slowly at first, to settle the hops, and returned to the liquid above the filter bed until it runs clear.
In all cases, the wort should be run off or filtered through the hops before it cools below 170 degrees F (77 degrees C). The first clear runoff may immediately be force-cooled and mixed with the yeast starter to facilitate adaptation of the yeast upon pitching. When all of the clear wort has been run off, the hops can be slightly sparged with up to eight fluid ounces of boiling water per ounce of hops, or until the density of the runoff drops below 5 °Plato (SG 1020). The extract still retained by the hops is insignificant — never attempt to press or wring out the last of it. Great care should be taken to see that only clear runoff is taken for cooling and fermentation.
The clear runoff must be quickly cooled to separate the cold break trub from the wort. Fast cooling is essential; the more slowly the wort cools, the more protein and tannin is trapped in suspension, giving rise to chill haze and harsh aftertastes in the beer. The cold break is generally 10 to 20 percent of the volume of the hot-break sediment, and much less coarse.
Cooling in lager breweries traditionally took place in shallow, open coolships to present maximum surface for air cooling. Better flocculation is achieved, however, by force-cooling the wort and employing a deeper settling tank, closely covered against contamination. Below 145 degrees F (63 degrees C), great care must be taken to prevent contamination of the wort by airborne wild yeast and bacteria or unsterilized equipment. The wort should be force-cooled to below 50 degrees F (10 degrees C) to secure the maximum break. Complete precipitation of tannin/proteins — and thus brilliantly clear beer — is achieved by cooling the wort until it becomes slushy, but cooling to 39 to 43 degrees F (4 to 6 degrees C) before racking the beer off of its settlement is generally sufficient.
Since boiling the wort drives its oxygen out of solution, it must be aerated to force oxygen back in. Yeast require considerable (4 to 14 ppm) molecular oxygen during respiration; without it, they cannot reproduce. Cells that survive an oxygen-starved respiratory phase taint the ferment with abnormal, estery flavors. Their lag phase is characteristically shortened, reproduction is limited, and their fermentation is sluggish. Oxygen starvation produces “petite mutants,” which ferment weakly and often incompletely, giving a peculiar and cloying diacetyl taste and other off-flavors.
In an oxygenated wort, the yeast splits the sugar molecule in such a fashion that it produces more CO2 than alcohol. The carbonic gas rising to the surface quickly forms a blanket above the ferment, which insulates it from airborne infection. It may also carry with it a film of debris that can be readily skimmed from the head during the kraeusen stage of fermentation.
Aeration by rousing the wort when it is hot saturates the wort more completely than does aeration of the cooled wort. The risk of airborne contamination is less while the wort is above 145 degrees F (63 degrees C), and aeration of the hot wort causes some of the oxygen to combine with protein fractions, improving the cold break. It would seem that the wort should be aerated when hot, but oxidative polymerization of polyphenols to tannins and oxidation of wort constituents create very objectionable flavors. The color darkens when the hot wort is aerated and flavor suffers irreversible oxidation damage. Aerating the cooled wort (at 60 degrees F [16 degrees C] or below) is always preferred to aerating hot wort, and it yields satisfactory dissolved oxygen (up to 8 ppm). It is essential that the air or oxygen be sterile to preclude contamination of the extract.
If a settling tank is being employed, the cold break should be well settled before the wort is racked into the fermenter. The pH of the wort should be 5.0 to 5.5. With infusion-mashed and ale worts, a pH of 5.0 to 5.2 is usual, but for lager beers a cooled-wort pH of 5.3 to 5.5 is still considered normal.