Everyone knows that to make wine you need to ferment the grapes, and given you can’t get an alcoholic fermentation without the presence of yeast, it is fair to say then that while ‘yeast’ may not sound an appealing topic it is a hugely relevant one when it comes to winemaking! As without yeast there would be no alcohol!
What is yeast?
Yeast is a naturally occurring single-celled microorganism that is part of the fungus family. The pre-eminent fermentation yeast used for wine is Saccharomyces Cerevisiae but within this ‘species’ are hundreds of different strains which contribute subtle (and sometimes not so subtle) characteristics to the wine during the winemaking process.
Did you know? The scientific name Saccharomyces means “sugar-fungus” in Greek. Yes, yeasts do love sugar!
Before we look at the different types of yeast and their impact on the fermentation process and wine produced, I thought it would be good to so a quick 101 on what fermentation actually is.
The fermentation process explained (simply)
Fermentation is the breakdown of sugars to release carbon dioxide, alcohol and energy by the anaerobic metabolism of yeast. (Grapes contain naturally very high levels of sugars which the yeast consumes). Much of the ‘energy’ produced is given off as heat – which is why fermentation tanks in wineries are often hot!
Fermentation stops naturally either once all the sugars have been consumed or when the rising alcohol levels in the wine kill off the yeasts.
At this stage the dead yeast cells drop to the bottom of the tank where they form “lees” (yeast cells, grape skin etc). Mostly the wine is racked off these lees but sometimes they remain on them for a period of ageing.
How yeast affects the wine
Apart from allowing wine to be made in the first place, it has a number of other uses:
Texture and flavour: Bi-products of the fermentation include Glycerol (smoothness and body) and aroma esters. Different yeast strains react differently and produce different esters.
Also while we are on the subject of flavour. Remember those dead yeast cells I mentioned? Well these “lees” add both texture and complexity to wines due to the autolysis of these dead yeast cells. Sound revolting? Perhaps, but the flavours contributed include bread, brioche and biscuit notes.
Did you know? When you see the term “aged on lees” on bottles, this indicates the wine has remained in contact with the lees for a period of time to contribute additional flavours and structure to the wine. Muscadet sur Lie is a good example. Note this also happens in Champagne.
Another term you might come across is “Lees stirring” (aka Bâttonage for those of you who speak French) which is a process used a lot particularly with Chardonnay. This also has the effect of adding a creamy texture and depth to the wine.
Types of Fermentation
Because of the variations in the different yeast strains, winemakers have a myriad of options available to them when it comes to deciding on the fermentation. These are generally broken down into two ‘types’: “Uninoculated” or “Inoculated”.
Uninoculated fermentation – is how wine was made before the science allowed for yeasts to be ‘cultured’. I have already noted that yeast is a naturally occurring organism that can be found everywhere: in vineyards, on grape skins, in the winery, on winery equipment and many other places. Numerous species of these ‘indigenous’ yeasts if left to their own devices, will begin to ferment grape juice naturally (often using multiple strains). Although they do have a tendency to die off around the 4% ABV mark when the stronger population of Saccharomyces Cerevisiae yeasts (not present in the vineyard but usually lurking somewhere in the winery!) take over. The main thing to note here is the yeasts are naturally occurring.
Inoculated fermentation – these use ‘cultured’ yeasts grown from samples taken from vineyards around the globe. These commercially active dried yeasts produced from cultures of different strains offer the winemaker lots of choice and control whether to help accentuate a flavour or to more easily control the fermentation rate.
For example, a winemaker producing a Sauvignon Blanc may look to accentuate the lifted gooseberry, passionfruit characteristics and use a yeast which produces these type of aroma compounds. In the case of traditional method sparkling wines (second fermentation in the bottle), yeasts that are low-foaming may be selected by winemakers (they don’t want exploding bottles after all!).
Which is better? In all honesty there are pros and cons of both and there is no doubting that exceptional wines are made using both techniques. To my mind, using uninoculated yeasts gives a wine something different: a complexity and individuality not found so easily in wines produced with commercial yeasts. It is more how nature intended if you like. However, there are negatives too: These wines can be prone to stuck fermentation, and the yeasts can contribute to off-flavours but generally speaking, most of the wines I enjoy the most, have used uninoculated yeasts.
What to look for on the label
If you spot any of the following terms; “Wild”, “Indigenous”, “Natural” or “Spontaneous” fermentation it means that the wine was fermented using uninoculated yeasts.
Wines to try
Allow my indulgence here, these are two of my favourite wines and winemakers:
Greywacke’s ‘Wild Sauvignon’ fermented for over six months with lees stirring, is just about the most characterful, moreish, food-worthy New Zealand Sauvignon Blanc you can find.
Similarly, Millton’s Riverpoint Vineyard Viognier is simply stunning. Biodynamic ,vegan and an indigenous fermentation. What more could you ask for!
Ok, that wraps up “Y”, and now finally… drum roll….onto “Z” for Zinfandel.