Graduate Thesis Or Dissertation


Dry-Hop Induced Refermentation: An Overview and an Investigation of Agronomic Influences on Hop Diastatic Potential Public Deposited

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  • Beer brewing, broadly speaking, consists of two stages, a hot side and a cold side. The hot side occurs in the brewhouse and consists of steeping malted grain in hot water to extract sugars, separating the steeping liquid (wort) from the solids, and boiling the wort with an addition of hops. After boiling, any solid precipitate is separated and the wort is passed through a heat exchanger on its way to the cold side, or the cellar. In the cellar, yeast is added to the chilled wort and allowed to ferment until an alcoholic beverage is produced. Dry-hopping is the practice of adding hops to beer in the cellar in order to extract desirable volatile compounds that provide unique hoppy flavors and aromas. With the rise in popularity of the IPA, the brewing technique of dry-hopping is increasingly being applied to beers of many different styles. As dry-hopping becomes more prevalent, so too does the phenomenon of dry-hop-induced refermentation, also known colloquially in the American brewing industry as “hop creep”.The phenomenon of beer starting to ferment again with the addition of hops to a maturation vessel was widely known, at least in theUK, as early as 1893, and is likely the source of the term “dry-hopping.” Adding hops to British casks of beer was observedto result in a secondary fermentation that would ‘dry’ the beer out by consuming unfermented sugars and dextrin and make the beer more effervescent. In the modern age, this re-fermentation is less desirable, as it has been observed to lead to unwanted and unexpected increased a beer’s final alcohol content, and if it occurs within a package to unsafe package overpressurization which could lead to package failure and consumer injury. We now know that the action of hop creep is caused by hop-associated dextrin-hydrolyzing enzymes breaking down unfermentable dextrins into their component sugars, which are fermentable. Residual yeast in the beer will then ferment those sugars, producing alcohol, CO2, and other secondary metabolites, including alpha-acetolactate which breaks down into diacetyl. The potential diastatic activity of a given lot of hops varies widely, and has been observed to differ between hop varieties, harvest years, farms, and even between fields within a single farm. This makes predicting hop creep difficult for breweries that produce dry-hopped beer. There is no handbook of cellar practices that can be used to mitigate the refermentation, and no standard published method for testing the diastatic potential of a given lot of hops that brewers can rely on, so brewers are left to figure it out on their own. To evaluate the feasibility of a potential method for rapidly determining the diastatic potential of a given lot of hops using tools found in most breweries, a series of dry-hopped pilot-scale 2 hL fermentations were prepared, and benchtop-scale dry-hopped fermentations were carried out alongside them. The benchtop fermentations were monitored daily and compared at various time points to the terminal gravity of the pilot scale brews. This yielded a promising 72-hour forced fermentation protocol that could allow breweries to rapidly assess the diastatic potential of hops. Additionally, the source of the diastatic enzymes from the hops are not well understood. There is as yet no literature addressing why hops of a single variety, from the same farm, but from different fields or harvest years, should have substantially different diastatic potential. There is evidence from other crops that fertilization practice can influence diastatic power and protein production. To assess a potential link between agricultural practice and hop diastatic potential, three different hop varieties (Mosaic®, Simcoe® and Strata®) grown commercially at a number of different locations within the Willamette Valley, Oregon were harvested and analyzed for diastatic power. From each field, farm management, weather, and soil data were collected, and the hopswere analyzed for enzyme activity. Analysis of the data demonstrated a potential link between farm management practice and hop diastatic potential, with interesting implications that fungal infection may play a role in hop diastatic potential.
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