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South Bay, Los Angeles County, California |
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The National Homebrewers Conference this past June had a number of experts on yeast, yeast management and yeast propagation for homebrewing. David Logsdon from Wyeast discussed Fermentation Characteristics and Beer Profiles. The quality and character of beer is highly dependent on the yeast strain, fermentation temperature and pitching rate. It is possible to get a complete fermentation done in 24 hours or less, and fermentations are often managed to attenuate in about 72 hours. Do we want to do this? David gave the short answer of maybe not, and then proceeded to discuss a number of issues relevant to yeast and fermentation. On the subject of fresh versus old yeast, David defines old yeast as anything older than 3 months (stored under refrigeration, of course). In his experience, he has found that old yeast give a lag time about 12 hours greater than that for fresh yeast, and has found that fresh yeast give a lag time of 12 hours from pitching. Including nutrients in the wort decreases this lag time to about 6 hours. The recommended amount of nutrient for homebrew is 1/2 teaspoon per pint of yeast slurry. David sees a problem pitching when the wort is too cold, since the fatty acids in the yeast cells are solidified at the lower temperatures and don't absorb nutrients and compounds easily. David presented a table comparing the character of a 1.048 OG ale as a function of the length of the primary fermentation:
Note that the length of the ferment is changed by changing the cell count and the amount of oxygen in the wort at pitching. A similar table compared the character of a 1.048 lager:
David's target for lagers is about 11 days for primary fermentation-he prefers the character of a beer made with the longer fermentations over that made with a shorter fermentation. His target cell count for ales is 1 million cells / ml minimum for fresh, healthy yeast, and 1 million cells / ml / Plato for stored or harvested yeast. These targets increase by 50% for lager fermentations. In discussing beer spoilage, David noted that yeast cells take about 2 hours to reproduce, while bacteria reproduce every 20 minutes. So even if the bacteria count in the chilled wort is significantly lower than the yeast cell count, it is possible that by about 4.5 hours after pitching the difference in growth rates results in only a 10-fold difference in cell counts. David does not believe that fast pitching is the cure to curb bacteria in wort-especially given that he prefers a slightly longer primary fermentation. Instead spoilage bacteria need to be minimized during the process. We need to ask ourselves where are we introducing these microorganisms: The Air? Incidental contact? Post-boil contact? We can conduct a wort stability test to qualitative assess the presence of unwanted microorganisms in the cooled wort. After chilling the wort, fill a sterile flask with about 200 ml of wort. Hold the flask at room temperature (25 to 35 C) for 72 hours, and look for the following characteristics: cloudiness, carbon dioxide production, and off odors (especially phenolic). The wort should remain stable without growth during this time, where stable is defined as clear with trub at the bottom. If any indications of contamination are observed, these organisms will be in your beer and yeast culture. On the subject of oxygen in the headspace of bottles, David looked at beer quality versus time in the bottle for filtered, commercial pale ale. He found that an oxygen level of 0 to 2 parts per million resulted in a beer that improved in quality over the first 6 to 7 weeks in the bottle before declining with age. At an oxygen level of 10 ppm, the quality rise was seen only in first 2 to 3 weeks, followed by a very rapid decline. The quality of the beer with 2 ppm oxygen at 16 weeks in age was roughly equal to the quality of the beer with 10 ppm oxygen at only 7 to 8 weeks in age. In order to minimize the oxygen in the headspace, many commercial bottlers will "crown on foam," in which a small jet of sterile water is shot into the neck of a filled bottle just before capping in order to intentionally cause the beer to foam and push the air out of the bottle. Since homebrewers usually don't have the means to do this, David advocates what would be a commercial over-fill when bottle conditioning-fill that bottle high. Finally, David gave his suggestions for producing balanced and complex beers: 1. Yeast Provide time to warm the yeast from cold storage; use multiple strains to increase complexity. Many commercial beers have been "cleaned up," often times reducing the use of multiple strains to just a single culture. Full Sail Maibock, though, is made with two strains that were selected from a blind panel judging. One strain is flocculent, the other powdery. The two strains are cultured separately and pitched together for great results. 2. Lag Time You can increase ester production by reducing yeast growth. The decreased yeast growth leaves fatty acids available for ester production. Allow the fermentation temperature to rise at the end of primary fermentation. For ales, start in the low to mid 60s, and come up to 68 to 72 F at the end. For lagers, start around 45 F, conduct most of the fermentation at 52 to 56 F, and then come up to 58 F. 3. Beer Spoilage Not prevented by a short lag; monitor your wort spoilage. 4. Grain Bill A greater variety of grains increases the complexity of the malt profile. Don't make ales too light (less than 1.050), and don't make lagers too strong (keep pils on the low side). 5. Hopping Schedule More frequent hopping during and after the boil increases the complexity. Use low alpha hops for best flavors without increasing astringency or becoming out of balance and too bitter. 6. Finishing and Conditioning Rack beer off the primary and handle it minimally to avoid oxidizing. Bottle with a minimum of headspace. Since trub accelerates the autolysis of yeast, get the beer off of the yeast within 7 to 10 days after fermentation ends.
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