Yeast: the unsung heroes!

As a young 16 year old I remember working in the cold, damp warehouse of a recycled clothing store. Frequently I would find myself muttering frustratedly under my breath that I was certain the store’s customers would never give me any credit for my work and that the more glamorous store assistants would be the recipients of their gratitude. I think that if our beloved yeast could speak they would vent similar frustrations. So often we wax lyrical about the beauty of soil, terroir, vineyard management and climate but less often do we give yeast their fare share of our appreciation.

Before this post gets too technical it’s probably best to get back to basics: what exactly is a yeast?

Yeast is a microscopic fungus consisting of single oval cells that reproduce by budding and are capable of converting sugar into both alcohol and carbon dioxide. Not only is yeast responsible for that boozy wine enjoyment that we all know and love, it also spits out the carbon dioxide which causes your favourite bread to rise.

There are a wide range of yeasts which have been identified as being involved in wine fermentation. However, the primary yeast associated, either as a result of inoculation (the introduction of non-indigenous yeast population to the must) or as an indigenous population in the winery, is Saccharomyces cerevisiae. Saccharomyces is a workhorse of a yeast which is able to survive relatively high levels of alcohol. Whether a winemaker chooses to inoculate or not it is almost unheard of that Saccharomyces initiates the fermentation, this tends to be the role of Kloeckera apiculata. These yeasts are found in the vineyard on the grapes and get to work as soon as the grape is exposed to any level of crushing. Kloeckera tends to become overwhelmed at relatively low levels of alcohol, at which point Saccharomyces takes over and completes the fermentation.

We all know that yeast is responsible for producing alcohol right? We also know that yeast is responsible for the secondary fermentation which gives us our beloved Champagne. So what’s so special about yeast that I would want to dedicate an entire blog post to it?

Have you ever drunk grape juice? It tastes nothing like wine! Well, guess who you have to thank for that …

Almost all of the aromas that we associate with wine are byproducts of fermentation. Grapes (particularly their skins) contain many of the precursors required to produce an array of aromas. These precursors; however, are often bound to sugars in an aromatically inactive form. These bound precursors are freed and introduced to the must by yeast during fermentation in excess of their equilibrium concentrations thus leading to their related aromas being prominent in the resulting wine. Over time, once freed, these compounds interact (through acid hydrolysis) with the acid medium of wine and either change, disappear or form completely new aromatic compounds. You could view yeast as being the liberator of all things aromatically delicious!

At this point it is worth briefly skipping over some of the primary aromatic compounds which yeast react with and/or create during fermentation as these particular compounds form the building blocks of a wines aromatic profile.

Esters, unlike other primary aroma compounds in wine, are not found in grapes and are created during fermentation as a result (put very simply) of the reaction between alcohols (created by yeast) and components of the grapes. Esters are a diverse group of aroma compounds found in wine and are primarily responsible for that fresh fruity kick possessed by many young wines. Banana, pineapple, citrus and floral aromas are just some of the associated aromatic profiles relating to the presence of esters. These esters are somewhat quickly hydrolysed and over time relinquish, hence the lack of fresh fruity aromas in many older wines.

Pyrazines, Terpenes, Thiols and Norisoprenoids – The precursors to this stinky bunch are all found in the grape itself and are freed by yeast during fermentation. They are responsible for the herbaceous notes in Sauvignon Blanc, the Blackcurrant in Bordeaux, the Eucalyptus in Shiraz and much, much more (including petrol in Riesling which I have covered in a recent blog) Even once yeast are dead they can still contribute to the overall flavour profile of a wine (as seen in Champagne, a process known as autolysis) This addition of flavour comes as a result of enzymes breaking down the cell wall of the yeast and releasing mannoproteins, and polysaccharides, in to the wine. This process of autolysis is often related to a more dense, rich and creamy mouthfeel and in particular a nutty almond note (this could be related to benzaldehyde, most aldehydes disappear with ageing, benzaldehyde does not and this could be due to it’s relationship with the polysaccharides released by the dead lees)

Please don’t be fooled by this short, sharp introduction to wine flavour, there is much more to the topic of aromatics in wine. The topic is much more complex than I have thus far portrayed and I will cover the aforementioned compounds in more detail in future posts, for now the basics is all I want to cover: let’s call it yeast 101.

I’ve already mentioned inoculation in passing and this term can certainly make things sound a little mad scientist, but inoculation can in fact be a crucial aspect of winemaking. Whilst many winemakers do opt to utilise indigenous yeasts, and with the growth of the natural wine movement I have no doubt that this will become a buzzword for the funky and fashionable. But there is also a perfectly good case for inoculating. Scientists have been able to breed certain strains of yeast that winemakers can use to control various aspects of winemaking including sulphur production, glycerol for mouthfeel, pronunciation of various aroma compounds and length of fermentation just to name a few.

Whilst many winemakers may argue that their indigenous yeasts represent their terroir, their typicity and their style there remains a valid argument to be made for inoculating. Not only can inoculating introduce a degree of control to the process but it may also offer winemakers a selection of tools which could assist them in combatting wine faults (such as excessive sulphur in wine from a single plot) and also assist in potentially revealing their wines ‘full’ expression (a touchy subject I know)

As wine science becomes more comprehensive, faculties like UC Davies are beginning to look at the factors which play a role in the efficacy of yeast during fermentation. Some of the factors which are known to play a role in yeast efficacy include the pre-fermentation preparation of the yeast, introduction of enzymes post and intra-fermentation, pH level, temperature, nutrient availability, nutrient inoculation and availability of precursors (non-exhaustive list) As more work is done to understand these factors it is likely that winemakers will be able to more comprehensively control fermentation and not only ensure quality and avoid fault but also manipulate the expression of their wine which may present particular benefits to regions which fall victim to climate change.

I hope this post has provided a somewhat insightful overview of what yeast are, the role they play in winemaking and what affects their efficacy during fermentation. If you have any interesting facts to share about yeast I would love to hear them in the comments below.

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