Investigating Yeast and Hop Interactions During The Fermentation of Beer

The brewing of beer requires four main ingredients: malt, water, hops, and yeast. During the brewing process, the brewer creates a malt-derived sugar water, called wort, that is inoculated with yeast, causing fermentation to create carbon dioxide and alcohol from the sugars. Traditionally hops are added to the wort during the boiling process in order to balance the sweetness with isomerized alpha-acids that create bitterness. Hops also contain many other flavor-active compounds that brewers are attempting to harness for the creation of aromatic and complex beers. These flavor-active compounds can become even more desirable when hops are added to active fermentation in the presence of Saccharomyces yeast, a process known as dry-hopping. During this dry-hopping process, aromatic compounds from the hops can undergo biotransformation with glycosides of the yeast to create unique flavors, but these dry-hops also contain enzymes that are biochemically changing more than just aromatic compounds. The enzymes in the hops are breaking down residual oligosaccharides in the beer that are meant to create a fuller bodied beverage, known as dextrins, and converting them into simple, more fermentable sugars that the yeast can easily assimilate. The yeast ferments the newly hydrolyzed sugars, adding excess alcohol and possibly yeast derived off-flavors to the beer, a process termed “hop creep”. In this series of studies, the interactions between yeast and hops during the dry-hopping and fermentation process of beer was examined, as well as factors contributing to the hop creep phenomenon. Hop creep has been extensively studied from the back end of the brewing process, the hop perspective; this work intends to look at the phenomenon from the front end of fermentation, the yeast perspective. In the first part of this study, a bench top analysis was repeated to determine contribution to dry-hop creep from different hop cultivars. Previous research had determined a statistical difference amongst cultivars, but was unable to be confirmed here, so research set out to develop a bench method for hop creep analysis that brewers could perform easily with little laboratory skills. Following method development, a screening of Saccharomyces yeasts from the UC Davis Phaff Yeast Culture Collection commenced in order to identify yeasts with properties advantageous to brewing. Yeasts were screened for their ethanol tolerance, carbohydrate metabolism, as well as nitrogen and amino acid assimilation, all of which are important qualities for the brewer. The screening was successful and expanded to full pilot scale at Anheuser-Busch InBev Research Pilot Brewery at the Robert Mondavi Institute. Six non-cerevisiae and non-pastorianus Saccharomyces yeasts that had not previously been used to create ales were used in fermentations in duplicate, with one fermenter in each set receiving 10 g/L of dry-hops during fermentation. Beers were measure for alcohol, real degree of fermentation (RDF), gravity, calories, pH, and yeast viability during fermentation, and sensory analysis was performed on finished beer. All yeasts displayed unique characteristics that may offer great potential to a complexly evolving and consumer driven beer market. In addition to the six yeasts described above, twenty-four more Saccharomyces strains chosen for their typical use in the production of alcoholic beverages were also used in fermentation at 40 L pilot scale, totaling thirty unique yeasts used in this study. Yeasts from the Phaff Collection, as well as three commercial suppliers, were aseptically propagated from single cultures with the necessary cell volume for these pilot scale fermentations. Again, they were fermented in duplicate, with one fermenter receiving a dry-hop of 10 g/L, and fermentation was monitored until gravity was deemed terminal and the beer had fully attenuated. RDF, alcohol, and pH were tracked as fermentation progressed; the previously determined bench method was also utilized to determine the amount of hop creep expected. No Saccharomyces yeasts in this study presented themselves for the effective mitigation of hop creep, but analysis of this manner has never been performed before, so much insight was gained. More research remains on a correlation of flocculation and dry-hop creep, amino acid and diacetyl analysis on dry-hopped in comparison to non-hopped fermentations, and secondary metabolites from the fermentation of beer with these unique yeasts, especially in the presence of hops