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Yeast and Fermentation Fungi of the genus SACCHAROMYCES that have the ability to metabolize sugars into carbon dioxide and alcohol. Two species are used commonly in brewing: S. cerevisiae, or ale yeast; and S. uvarum (or S. carlsbergensis), or lager yeast. Other types are used in Belgian lambics and wheat beers. YEAST TYPES Ale yeast (S. cerevisiae) is called top-fermenting yeast since it forms colonies that are supported by the surface tension of the wort and create a thick, rich yeast head. Ale yeasts generally work in the 55 to 70 degree range. They will ferment out the four monosaccharides glucose, fructose, mannose, and galactose, the disaccharides sucrose and maltose, xylulose, the trisaccharide maltotriose, and partially ferment the trisaccharide raffinose. They do not ferment melibiose or lactose. Lager yeast (S. uvarum) is called bottom-fermenting yeast because the colonies don't have as great an ability to clump together, form a thinner, more tenuous head, and sediment out more readily to the bottom of the fermenter. Lager yeasts work best below 55 degrees. They ferment all the sugars that ale yeasts ferment, but also ferment the disaccharide melibiose and fully ferment the trisaccharide raffinose. They do not ferment lactose. Lager yeasts have two sub-groups: Frohberg, or powdery yeasts (Staubhefen), that fail to clump and remain in suspension in the wort; and Saaz or break yeasts (Bruchhefen) that flocculate readily to the bottom of the fermenter 2E Frohberg yeasts are strong attenuators and ferment isomaltose as well as maltose. Saaz yeasts are weak fermenters that reduce the extract slowly and do not ferment isomaltose. Belgian Lambics use an S. cerevisiae strain and three types not otherwise used in brewing: Brettanomyces bruxelliensis, B. lambicus, and Saccharomyces bayanus. South German wheat beers use a strain of Saccharomyces delbruckii, usually along with S. uvarum. FACTORS FOR YEAST HEALTH Yeast health is a function of six factors: temperature, wort pH, oxygen, nutrients, food, and overall health. Temperatures that are best for the particular strain will vary. Unless the temperature is above 110 degrees or too cold for the strain, the yeast will continue to reproduce and function. The best temperature for the beer desired will depend on the strain used. Wort pH should be in the 5.0-5.5 range for optimal yeast growth, although this will drop during fermentation to about 4.5. Oxygen is needed in the wort for the yeast to function properly during the early phases of its life cycle. This is best done by shaking the carboy vigorously before pitching the yeast. However, oxygen reintroduced into the beer after the start of visible fermentation is undesired, and will result in off-flavors. At this time, the yeast is living anaerobically. Nutrients are certain nitrogenous compounds and amino acids. These will be present in the wort, formed by the proteolytic enzymes during the protein rest for undermodified malts, and by the malt itself when fully modified. Trace elements are also present in the malt and from the hops. Food for yeast is sugar, ideally a proper balance of the sugar varieties. Popular strains of beer yeast will not develop properly in a wort that has large amounts of corn sugar (glucose). This will retard proper enzyme development and cause other problems further in the life cycle. Large amounts of corn sugar in yeast starters or beer should be avoided. Health is a critical factor in determining if the yeast will properly repro duce and produce the proper flavors in the beer. Weak yeast will not prope rly develop, and may flocculate prematurely or not at all, and display characteristics totally at a variance from those predicted for the strain in use. When in doubt, a known healthy pure culture pitched to the proper amounts is the best choice. LIFE CYCLE Yeasts have three distinct phases of life in wort. They are: Respiration; Fermentation; and Sedimentation. In the Respiration or Lag phase the yeast uses the oxygen in the wort to derive energy from both the wort and internal sources for reproduction. Internal energy stores of glycogen will be depleted as the yeast prepares for fermentation. The enzymes necessary for sugar metabolization are produced during this phase. These enzymes will be improperly developed if the wort has large amounts of corn sugar. Carbon dioxide is generated in this phase, as are flavor characteristics such as esters and diacetyls. No alcohol is produced during this phase. This phase will generally last for 24 hours after pitching or less. In the Fermentation phase the yeast continues to reproduce, but no oxygen is used. The free oxygen will be scrubbed out by the CO2. During this phas e the yeast reproduces to maintain an optimal population throughout the wort. Here is where the sugars are converted to alcohols. The yeast will stay in suspension long enough to attenuate the wort to the desired degree. Often hydrogen sulfide (H2S) is produced during this stage, but it will normally be carried out of the wort by the CO2. Diacetyl and other fusel alcohols that are normal products of fermentation will be reduced or eliminated during this phase. This phase will include both Low Kraeusen and High Kraeusen, and the pH will drop to about 4.5-4.8, which signals the start of the sedimentation phase. In the Sedimentation phase the yeast realizes that the energy stores and food are near depletion. The cells go dormant and settle to the bottom of the fermenter. Glycogen is produced, which is used to maintain the cell durrng dormancy, as well as to provide a new energy source for initial activity if roused into fresh wort. Pitching rates should be on the order of .5 to .6 fluid ounce of slurry from culture per gallon of wort, or 2.5-3.0 ounces per five gallon batch. This is approximately 10-12 grams of roused packet yeast (in water prior to pitching) per five gallons. HISTORY Mitcherlich in 1841 discovered that yeast was necessary for fermentation, but it was not until Pasteur that the fermentation pathway that leads to alcohol production was discovered (zymase enzyme). Gabriel Sedlmayr and Anton Dreher in 1841 effectively started bottom fermentation, using mixed strains of predominantly bottom fermenting yeasts evolved by the Munich process of cold-conditioning beer (lagering) in caves, particularly over the summer. As S. uvarum is related to certain yeasts used in the wines that were made in Munich in those days, it is likely that their "contamination" of beers, combined with the cold-conditioning, resulted in their becoming dominant in the Munich beer yeast strains. Single-cell yeasts were not isolated until 1883, by Emil Hanson of the Carlsberg Brewery in Copenhagen. Refrigerated fermentation of cold-tolerant yeasts began in the 1880s with von Linde and the advent of mechanical refrigeration. FERMENTATION Fermentation is generally divided into two stages: Primary fermentation and Secondary fermentation. Primary fermentation refers to the initial, active phase when the yeast is forming the kraeusen head. During this stage the yeast is actively fermenting the malt sugars to alcohol and carbon dioxide. Primary fermentation i s generally over when the kraeusen head disappears. At this time about one half to two-thirds of the fermentables have been converted. Primary fermentation will generally take anywhere from three to ten days, depending on the yeast strain, the fermentation temperature, and the dextrin content of the wort. Secondary fermentation allows the slow reduction of the remaining fermentable sugars. This may take from seven to twenty-one days, depending on the yeast strain and the amount of fermentable sugars in the wort. Generally, ales will require a shorter period of secondary fermentation than lagers. During this period the yeast will begin to flocculate as the last of the sugars are used up. It is important that no oxygen be allowed into the beer at this time. Most brewers recommend racking the beer to a secondary fermentation vessel at the end of the primary fermentation. Fermentation should proceed at the proper temperature for the yeast strain and style of beer being brewed. Sudden variations in temperature in excess of 6 degrees F. per day must be avoided during fermentation. Strong light should also be excluded from fermenting vessels (especially glass carboys). Gas locks should be of the type that will not permit lock water to be sucked down into the carboy. Locks should not permit a great pressure to build up in the carboy. If using a blowout technique (recommended), make sure that the blowout hose is sanitized before installing it on the carboy. Suspend it above the catch bucket, so that there is no possibility of blowout being sucked back into the carboy should the carboy be cooled. The hose should also be big enough that pelletized hops will not cause it to foul, resulting in a blown cork and kraeusen foam on the floor. Never fasten or seal the cork to the carboy! PROBLEMS WITH FERMENTATION include the following: underpitching; overpitching; improper temperature; rapid temperature variations; light-struck; excessive fermentation time, stuck fermentation Pitching is the act of adding the active yeast starter to the chilled and aerated beer wort. Underpitching will result in excessive contamination. The wort is in its most vulnerable state after cooling and prior to high-kraeusen blowout. Sufficient yeast must be pitched to bring about high-kraeusen blowout within 24 hours, and ideally less than 12 hours after cooling. This time between pitching and high-kraeusen is called "lag time", and the smaller the lag time the better. Underpitching will give any contamination a chance to establish itself in the beer, resulting in DMS, diacetyl, cooked vegetable, and similar off-flavors. Another result may be a delayed reaction in the bottled beer, resulting in overcarbonation from a seemingly normally-carbonated beer left for two or three months. Overpitching will generally result in "yeast bite", a noticeable yeasty taste in the beer. Re-using all of the slurry from the bottom of a primary fermenter in a new batch will usually overpitch the batch. Overpitching results in rapid consumption of the limited amounts of dissolved oxygen, simple sugars, and yeast nutrients. This will result in yeast autolysis, or self-digestion, and will result in yeasty, sulfur-like flavors. The optimal amount of yeast is about 0.4-0.6 ounce of pasty, thick yeast sediment per gallon of wort, or about 2.0 to 3.0 ounces per gallon. For dry yeasts this translates to no less than 12 grams of yeast, activated before pitching by dissolving in 90 degree F sanitized water (not wort or sugar wter) for about 20 minutes. For cultures, a starter of at least 16 oz of wort (for five gallons) should be prepared and allowed to go into high kraeusen before pitching into the wort. Using less than this amount (such as directly from a bagged culture package) will underpitch the batch and result in an excessively long lag time. Improper temperature will result in a totally different beer from what is intended. Too high a fermentation temperature for lager yeasts will, in most cases, result in fruity-estery tastes that are uncharacteristic of lagers (unless you are making a steam beer with a heat-tolerant lager yeast). Too cold a temperature with an ale yeast can cause it to stop working and flocculate out prematurely, resulting in a stuck fermentation, with accompanying diacetyl, acetaldehyde, and excessively sweet tastes. Rapid temperature fluctuations will often result in production of DMS and oher sulfur-like compounds, particularly with certain lager strains. It can also disrupt the yeast life cycle, resulting in premature flocculation and mutant yeast growth. Temperature changes should be limited to no more than six degrees (Fahrenheit) per 24 hour period once the yeast has established itself in the wort. Light can destroy a fermenting beer in the same manner as it can destroy bottled beer, by causing a breakdown of hop oils to undesirable sulfur compounds. This will result in a skunky taste and aroma. Keep glass carboys away from strong sources of light at all times. Leaving the beer in contact with the yeast sediment and trub for any extended length of time is undesirable. Such trub can contribute many off flavors such as medicinal, sulfury, diacetyl, and astringency. Yeast autolysis (decay) can also provide food for any stray bacteria present in the beer and permit it to grow. While racking to a secondary is generally not necessary for most ales due to the short fermentation time, it is a good idea to carefully rack to a clean secondary any beers destined to be secondary-fermented or cold-lagered. Stuck fermentations are generally a result of either premature flocculationof the yeast before it has a chance to properly attenuate the wort, or, in the case of high-gravity beers, a lack of alcohol tolerance in the yeast strain which causes it to die out and stop working before the fermentables in the wort have been consumed. "Rousing" the yeast, which can be done by gently shaking the sealed carboy, may sometimes result in the fermentation resuming sufficiently to complete the attenuation. Rousing must not result in additional oxygen being dissolved in the beer at this stage. Another me thod is to add more fresh yeast, in the form of a starter culture. This is the best method of completing the fermentation of a high-alcohol beer started with a normal yeast strain. Often times a fermentation is considered "stuck" when, in reality, all the fermentables have been consumed. A beer made with a high dextrine content may only attenuate to around 40% - 45% of the starting gravity, with the high end gravity a result of the unfermentable dextrine content of the beer. No amount of rousing or repitching will result in a lower final gravity of such a beer. Home |
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