From Vol. 3, No. 11, April 2020
Merriam-Webster dictionary defines biotransformation as “the transformation of chemical compounds in a living system.” Pretty simple, and most often applied to a process that occurs within the human body. Basically, you start with a molecule (or molecules) and end up with another (or many others). Here are four beer-related examples, all of them transformations that result from the interaction of hops and yeast.
– Chemical modification of terpenes. The most obvious example of a high impact one is the reduction of geraniol to citronellol (which has a citrus, lime-like aroma). Terpenes make up about 80 percent of the essential oil in hops. Little of that oil survives the boil. Waiting to add hops after boiling, whether in the kettle or fermentation tank, preserves the compounds within, and at the same time gives yeast the opportunity to create new ones. (The longer version.)
– Conversion of organic acids into esters. During hop storage, branch chain acids increase as hops age. During fermentation, yeast may convert these acids, producing fruity ester compounds.
– Release of thiols. Most research has been focused on free thiols and identifying varieties that may contain them. They play a key role in creating tropical aromas. Thiols have made Citra and Mosaic, both rich in free thiols, hops stars. Other varieties may have relatively high levels of bound thiols, which are released by yeast. For instance, Calypso has plenty of bound 3MPH and 4MMP, but little in the way of free.
– Cleavage of glycosides. Glycosides consist of two parts, one a carbohydrate molecule and the other a nonsugar component called an aglycone. Yeast may cleave glycosides and free this aromatic component.
Quite honestly, I may have errored on the side of oversimplified. In each case, results are dependent on the hop variety and the yeast strain.