Kilning, which is the drying operation in hop cultivation, is a processing step that can have a tremendous impact on hop quality. Historically, hops have been dried at temperatures ranging from 140-155°F. In the last decade, growers have moved toward kilning hops at lower temperatures averaging between 125-140°F in the belief that it increases the quality of highly popular aroma hops. Low-temperature drying is thought to preserve volatile aroma compounds that high temperature drying might drive off. Aroma hops have become very popular in hop-forward beer and are used in large quantities during dry hopping, which is the addition of hops during or after fermentation. There is evidence that during dry-hopping, enzymes within or on hops can break down the remaining unfermentable dextrins in beer to fermentable sugars which in turn are fermented by yeast. This slow, secondary fermentation due to dry-hopping is colloquially known as "hop creep" and it is becoming an increasingly problematic issue for brewers. There is speculation that when the industry moved to lower kilning temperatures, these enzymes became more of an issue because they were no longer deactivated during kilning.
There exists little published information in the public domain regarding kilning effects on hop quality. There is some evidence that temperature could impact hops' aroma quality, but there were no investigations of the impact on brewing quality. This study was performed to understand how different kilning temperatures influence the brewing quality of major aroma cultivars by carrying out commercial-scale drying experiments and evaluating the enzyme potential and brewing quality of the resultant hops.
The results from this work provide compelling evidence that the dextrin reducing enzymes in hops are temperature sensitive. For both Amarillo® and Simcoe® hop varieties, kilning temperature had a significant effect on hop enzyme activity. Kilning hops at higher temperatures reduces hops' enzymatic power, which in turn decreases their ability to break down residual dextrins during dry-hopping and thus reduces the effects of hop creep. However, there exists significant field-to-field variation in the baseline level of hop enzyme activity whereby some fields produced hops that are inherently more enzymatically active than others. In these cases, the reduction in enzyme activity that occurred during kilning did not significantly reduce their hop creep potential. These results offer further insight on how the kilning operation influences hop quality.