Graduate Thesis Or Dissertation
 

The Biotransformation of Hop Aroma: An Exploration into Yeasts' Ability to Release Polyfunctional Thiols in Beer

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0k225k30r

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  • The biotransformation of hop aroma via yeast is a prominent topic in the brewing industry with considerable interest placed on how yeast can enhance volatile thiol production during wort fermentation. Three potent thiols are known contributors to tropical qualities in wine and beer aroma, specifically 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-methyl-4-mercaptopentan-2-one (4MMP). As consumer demand for tropical aromas in hop-forward beer styles continues to rise, so does the need to understand and manipulate the mechanisms responsible for the expression of these aromatic thiols in beer. The ability of yeast to release cysteine- and glutathione-conjugated thiol precursors has been well explored in wine literature, with the key gene IRC7 being the center of current research, because it encodes a carbon-sulfur beta-lyase (CSL). This enzyme is responsible for cleaving odorless, cysteinylated thiol precursors found in grape must releasing the highly odor active unconjugated form (polyfunctional/free thiols). However, this activity is poorly understood in wort fermentation and there is a gap of knowledge between the two industries regarding how beta-lyase activity plays a role in finished beer aroma. Given the vast pool of odorless precursors that have been identified in malt and hops (the two primary raw materials used to make beer), there is an unexplored, and potentially underutilized, resource that has yet to be fully examined scientifically. This thesis explored two avenues for exploiting these polyfunctional thiols in beer. The first body of work examined 10 commercial yeast strains, one of which was Saccharomyces pastorianus and nine which were Saccharomyces cerevisiae, of which one was previously established as a high thiol producing wine specific strain. All strains were added to wort produced on a pilot scale (~1.5hL) with a single malt, single hop recipe using Cascade hops which contained high levels of precursors and low levels of free thiols. Separately, yeast-peptone-dextrose media was hopped at the same rate as the wort fermentations with Cascade hops and 1 L bench scale fermentations were performed using the same yeast. The results of this study identified 3MH to be the most prominent thiol, with values increasing from 50 ng/L in wort up to 215 ng/L in finished beer, and 38 ng/L in pre-fermentation media up to 168 ng/L post-fermentation media. However, there were many differences observed in terms of relative thiol precursor and free thiol values when comparing wort to YPD fermentations, such as the complete absence of 3MHA in YPD media samples. It is hypothesized that two potential sources of variation that influence the release of bound thiols are amino acid composition and pH of the substrate, with a potential correlation of suppressed thiol release as pH and amino acid levels increase. This study also provides evidence of thiol x fermentation compound interactions, which may influence the organoleptic properties more than previously suspected. The second body of work focused on genetically modified (GM) yeast designed to release polyfunctional thiols. By inserting highly active CSL encoded from genes derived from Citrobacter spp. and alcohol acetyltransferase enzymes (AAT) derived from Malus pumila and Escherichia coli, 3MH concentrations were increased 73-fold compared to the parent strain and >130 fold the sensory detection thresholds in beer. Additionally, significant increases in 3MH and 3MHA levels were observed across all four GM strains without alterations to fermentation performance and finished beer chemistry, as compared to the parent strain. This work highlights the successful ability to genetically alter yeast strains to promote thiol expression in beer. Both studies advance our understanding of beta-lyase activity’s role in releasing polyfunctional thiols in beer and set a framework for future experimentation that is imperative in further understanding biotransformation of hop aroma.
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