Yeast And Hop Aroma: The Interplay Of Aromatics

The perception of hop-derived flavour in beer is not well understood, but it is known that yeast strains influence the type of beer produced and its flavour profile. Yeast interacts with hop compounds to determine the final flavour of the beer. Yeast also alters the perception of hop flavour by biotransforming hop flavour precursors during fermentation, with different yeast strains producing beers with differing concentrations of hop-related volatile organic compounds.

The hop aroma in beer is primarily determined by the hop oil composition, which is divided into three categories: the hydrocarbon fraction, the oxygenated fraction, and the sulphur fraction. Some oils from the oxygenated fraction are odourless because they are bound to other molecules, but yeast-derived enzymes can cleave the conjugate to release these compounds and boost hop flavour and aroma.

The process of biotransformation shows a significant difference in terpene esters and terpenoid compounds regulated by yeast strain, which also contributes to the different sensory perception in the beer samples. Yeast's role in beer aroma is clear: during fermentation, yeast produces aromatic esters, giving the beer specific flavours.

Yeast strain impacts hop aroma

The yeast strain used in the fermentation process has a significant impact on the hop aroma of the final beer. Yeast interacts with hop compounds to determine the beer's flavour profile. Yeast strains alter the perception of hop flavour by biotransforming hop flavour precursors during fermentation. Different yeast strains produce different concentrations of hop-related volatile organic compounds (VOCs).

A study by Kumar et al. (2023) evaluated the influence of yeast strain on the sensory properties and volatile composition of beer. The study used a standard wort, late-hopped with New Zealand Motueka hops, and fermented it with one of twelve yeast strains. The beers were then evaluated using a free sorting sensory methodology, and their volatile organic compounds (VOCs) were assessed using gas chromatography mass spectrometry (GC/MS).

The study found that the yeast strain used had a significant impact on the sensory properties of the beer. For example, the beer fermented with SafLager W-34/70 yeast was associated with a hoppy flavour, while the WY1272 and OTA79 beers were sulfury, and the WB06 and WLP730 beers were perceived as spicy.

The volatile analysis also showed distinct differences in the VOC profiles of the beers fermented with different yeast strains. For instance, the beer made with WLP730, OTA29, SPH, and WB06 yeasts had the highest levels of 4-vinylguaiacol, which contributed to their spicy attribute. On the other hand, the beer made with W3470 had high levels of nerol, geraniol, and citronellol, which supported its sensory characterisation as being 'hoppy'.

The study concluded that yeast strain plays an important role in modulating hop flavour in beer and that further research is needed to understand how yeast strain and fermentation factors influence hop aroma to meet consumer demands.

Yeast's role in biotransformation

Yeast plays a crucial role in the biotransformation of hop aroma. Yeast can alter and boost hop-derived flavour compounds in beer. Yeast contains β-glucosidase enzymes, which can turn non-aromatic glycosides into aromatic terpenes. This process, called hydrolysis, releases aromatic terpenes from glycosides that were previously not volatile.

During fermentation, yeast produces aromatic esters, giving the beer specific flavours. Yeast also produces β-lyase, which can release fruit-flavoured thiols with low flavour thresholds. Floral and citrus-flavoured terpene alcohols can also be released or interconverted.

The biotransformation of monoterpenes by yeast during fermentation has been observed to convert monoterpene alcohols such as geraniol and linalool to a range of other terpenoid products via either isomerisation, reduction, or esterification.

The β-lyase activity of yeasts will be discussed in more detail in the following section. The release of volatile thiols from conjugated precursors during yeast fermentation has been directly linked to IRC7 expression. The resulting β-lyase enzyme is involved in sulphur metabolism and amino acid biosynthesis, and expression of the gene is linked to the nitrogen catabolite repression process.

Yeast's impact on dry-hopping

Yeast has a significant impact on the dry-hopping process and the resulting beer's aroma and flavour.

Yeast Strain and Enzyme Activity

Yeast strain selection is crucial when defining recipes, as different strains produce different esters and enzymes, which can affect the final beer's aroma and flavour. For example, yeast-derived enzymes can cleave non-volatile compounds in hops, releasing compounds that boost hop flavour and aroma.

Biotransformation

Yeast can also act on hop oils through a natural process called biotransformation, where one organic compound can chemically change into another. For instance, in dry hopping, fermentation can alter some oil compounds, like terpenes, into more volatile and flavour-active ones. Brewers often cite the transformation of geraniol to beta-citronellol as an example.

Yeast Hydrolysis

Another pathway involves an enzymatic reaction where active yeast hydrolyses glycoside, a non-aromatic type of sugar molecule, into two components: digestible sugars and terpenoids, a highly aromatic class of organic chemicals. This process, called hydrolysis, releases aromatic terpenes from glycosides that were previously not volatile.

Yeast and Hop Creep

Yeast is one of the main causes of hop creep, a common term referring to the over-attenuation of dry-hopped beer. Refermentation appears to occur when diastase enzymes in hops are introduced to unfermentable carbohydrates in the presence of active yeast. Hops carry amylolytic enzymes that convert long-chain, unfermentable dextrins into fermentable sugars, and any live yeast in suspension acts on those sugars to cause a gradual secondary fermentation.

Yeast and Dry-Hopping Timing

The timing of dry-hopping can also affect the beer's aroma and flavour. For instance, in a Cloudwater Brew experiment, a beer dry-hopped during primary fermentation was described as "catty, funky and herbal, but with nice tropical notes," while a beer dry-hopped afterward was described as "lemonade-citrusy and full of additional orange peel, orange blossom, honey and lime notes."

Yeast strain and hop oil composition

The perception of hop-derived flavour in beer is not well understood, but it is known that different yeast strains and fermentation parameters have an effect on the perceived hop aroma. Yeast activity during fermentation modifies hop compounds, which results in a diverse range of hop flavours in beer, including resinous, fruity, citrusy, floral, and spicy attributes.

The monoterpene alcohols geraniol, nerol, citronellol, and linalool are important aroma-active terpenoids found in beer. Geraniol and nerol contribute a rose-like, floral, and citrusy flavour, while citronellol and linalool are found at low levels in hop essential oil but at greater abundance in beer.

Citronellol acetate, nerol acetate, and geranyl acetate are terpene acetate esters that are not naturally found in hops but are formed during fermentation by yeast esterase activity. These compounds contribute floral, fruity, pear, apple, and green flavours to beer.

The biotransformation of terpenes by yeast during fermentation involves a complex series of reaction pathways, which leads to distinct patterns of hop-derived terpenoid compounds in beer as a function of yeast strain. The levels of monoterpene alcohols vary across different yeast strains, although there may be no differences in the abundance of linalool. The occurrence of terpene acetate esters does not correlate with the abundance of fermentation esters or monoterpene alcohols.

In conclusion, the yeast strain and fermentation factors influence hop aroma in beer, and understanding this relationship can help brewers control beer flavour to meet consumer demands.

Yeast and hop glycosides

The aglycone, the molecule bound to the sugar in a glycoside, can be any functional group, and in hops, these molecules are often aromatic. Aglycones can be freed from glycosides through a low pH, and this process is influenced by the pH of the beer. A lower pH increases the amount of certain compounds, such as the ketone damascenone and dimethyl trisulfide.

Beta-glucosidase is the enzyme required to break aglycones from glucosides, and it is produced by many plants, bacteria, and fungi, including some yeasts. Certain isolates of both Brettanomyces and Saccharomyces yeasts have been shown to produce beta-glucosidase. However, this trait is rare and not all isolates of the same species are consistent.

In beer, beta-glucosidase-positive yeasts can increase the levels of certain compounds, enhancing the flavour and aroma. For example, a beta-glucosidase-positive Brettanomyces yeast was found to increase the levels of benzaldehyde, linalool, and eugenol in cherry beers, resulting in a significant difference in flavour compared to cherry beers that were not exposed to the enzyme.

When it comes to dry hopping, there are differing opinions on whether the presence of yeast in suspension reduces hop aroma. Some argue that dry hopping with minimal yeast in suspension is best, as hop oils will cling to the yeast and be removed when they flocculate. Others find that dry hopping near the end of fermentation when there is still some yeast activity does not affect hop aroma. Additionally, the presence of yeast during dry hopping can help prevent oxidation of the beer.

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