Assessing the Stability of Polyfunctional Thiols in Hops and Beer Throughout the Brewing Process
Restricted (Penn State Only)
- Author:
- Naziemiec, Magdalena
- Graduate Program:
- Food Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 26, 2022
- Committee Members:
- Ryan Elias, Co-Chair & Dissertation Advisor
Ramaswamy Anantheswaran, Co-Chair & Dissertation Advisor
Daniel Ciolkosz, Outside Unit & Field Member
Helene Hopfer, Major Field Member
Robert Roberts, Program Head/Chair - Keywords:
- hops
beer
thiols
polyfunctional thiols
copper
vacuum drying
redox reactions - Abstract:
- Hops (Humulus lupulus L.) are one of the four main ingredients in beer, along with water, malted barley, and yeast. Hops are a vital component to beer as they impart flavor, aroma, microbial stability, and oxidative stability to the final product. While hops have been prized for their bittering compounds such as alpha acids and aroma compounds such as terpene hydrocarbons, it has only been within the past two decades that polyfunctional thiols have been discovered and studied in hops. Polyfunctional thiols such as 4-methyl-4-methylpentan-2-one (4MMP), 3-mercaptohexanol (3MH), and 3-mercaptohexyl acetate (3MHA) occur in small concentrations (ng/L), but due to their low aroma thresholds, they contribute powerful aromas with citrus and tropical notes. With the growing demand for beers with these desirable aromas and the low concentrations with which polyfunctional thiols occur in hops and beer, it is important to study these compounds to learn how to best preserve them. Preservation of polyfunctional thiols begins with the hop harvest, as hops are convectively dried to maintain chemical and microbial stability. While there are studies exploring the impact of hop drying conditions on α-acids and terpene hydrocarbons, there are currently no studies evaluating the impact of drying on polyfunctional thiols. Additionally, the negative impacts of copper on beer stability and polyfunctional thiols has been shown. Indirectly, copper in beers can lead to the formation of reactive oxygen species and radicals, which can react with polyfunctional thiols. Directly, copper (Cu(II)) can oxidize polyfunctional thiols and form odorless disulfides. While it has been shown that the presence of copper in hop growing and brewing could reduce the concentration of polyfunctional thiols, recent studies suggest that beer may contain organic compounds that can bind and remove free copper, thus protecting against loss of polyfunctional thiols. In this work, the stability of polyfunctional thiols was evaluated from post-harvest drying to extraction into beer in order to determine the conditions at which polyfunctional thiols can be protected from loss. It is hypothesized that conditions such as vacuum drying after harvest, the presence of dipeptides in beer, and dry hopping in the presence of yeast can reduce the loss of polyfunctional thiols due to a reduction in thermal degradation and volatilization, or protection from reacting with copper. It was found that hops dried under vacuum for long periods of time had less 4MMP than hops dried without vacuum for similar or longer periods of time. When the dipeptide Gly-His was added to Cu(II)-containing wort and beer, 4MMP and 3MH were protected from loss but 3MHA was not. Electron paramagnetic resonance (EPR) analysis showed that lower concentrations of 3MHA were required to reduce Cu(II) to Cu(I) compared to 4MMP or 3MH. Dry hopping finished beers with or without the presence of yeast or exogenous Cu(II) showed that the presence of yeast resulted in more 3MH and 3MHA, but the addition of Cu(II) did not impact final thiol concentrations. The outcomes of these studies will benefit hop growers and brewers, both of which are interested in preserving the polyfunctional thiols in hops in order to impart those compounds into beer, resulting in a higher quality product.