Devin Tani, White Labs, San Diego, CA, U.S.A.
Yeast, Fermentation, and Microbiology I
Thursday, October 12
There has been a debate over what is the best practice of dry hopping to produce the most ideal flavor and aroma. Choosing the best method to introduce hops as well determining when to dry hop is in question. Some methods can also introduce unwanted off-flavors, such as diacetyl. Diacetyl, part of a family of vicinal diketones (VDK), is a byproduct of yeast’s natural fermentation of amino acids such as valine. It is considered an off-flavor, creating the presence of an oily, slick mouthfeel as well as a butter popcorn aroma. To reduce diacetyl in beer, a maturation phase is needed. This occurs when the fermentation temperature is raised slightly to ensure all the diacetyl and precursor, acetolactate, is reabsorbed by the yeast. Without a maturation phase, cold crashing the yeast out of solution can leave high levels of precursor around. Acetolactate can quickly be converted to diacetyl through a chemical reaction of oxidation. This oxidation can also come into play with different dry-hopping techniques. From this experiment, a trend on when and how to dry hop to reduce diacetyl production will be identified, as well as a conclusion whether an increase in diacetyl is from precursor conversion or through yeast refermentation. With the help of others, multiple trials with different hops as well as different dry-hopping techniques, such as how the hops are introduced, pounds per barrel, and number of additions, will be notated. The exact time of when to dry hop, such as before reaching terminal gravity or after a slight cold crash, is another a variable worth considering as well as if the yeast already partook in a diacetyl rest or maturation phase. The limiting variables being monitored quantitatively over time will include the amount of yeast in suspension, temperature, pH, specific gravity, and total and as-is diacetyl. With both total and as-is diacetyl (ASBC Beer 25-E) being analyzed through gas chromatography, the amount of precursor can be calculated. By monitoring precursor, it can be determined whether more diacetyl has formed from the yeast or if acetolactate has changed into diacetyl. Samples will be taken before dry hopping and every day up to cold crashing the beer. With so many variables being monitored, data will be analyzed through each constant to identify a trend in which dry-hopping method is deemed most effective for controlling diacetyl production.
Devin Tani received a B.S. degree in chemistry from the University of California, Irvine, as well as a Professional Certificate in Brewing from the University of California, San Diego Extension. He currently functions as the analytical laboratory specialist in White Lab’s analytical laboratory.