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What’s yeast got to do with it?

By Nick Race, Mudbrick Cellar Door Team

Ever wondered how you turn water into wine? Well, the first thing you’re going to need is some yeast…

Wine was first discovered by accident back in ancient times, when someone left some grapes in a vessel for a few days. When they returned they found that juice had taken on pleasant flavour and if you drank enough of it, it gave you a bit of a buzz. We now know this to be alcoholic fermentation, caused by a micro-organism interacting with sugar in the grape to produce alcohol. That micro-organism is yeast and it’s a member of the fungus kingdom. Yeast also produces another by-product and that’s carbon dioxide, but more on that later.

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Close up of Saccharomyces cerevisiae yeast

Winemakers have long understood this process and have picked grapes at their ripest to maximise the sugar content – because yeast loves sugar! Yeast occurs naturally on the grapes skins and is ever present in the air of the vineyards and in the winery. By crushing the grapes you expose the sugary juice to the yeast and fermentation starts (known as wild fermentation).

The most common yeast used in winemaking is Saccharomyces cerevisiae (also present when brewing beers or baking cakes), which even produces killer toxins to ensure it becomes the dominant species!! The yeast eats the sugar until there is nothing left, at which point it starves and dies. The riper the grape, the more sugar, the more alcoholic a wine. When all the sugar is consumed you make a dry wine. By killing the yeast early (either by chilling or adding sulphur) you produce a sweeter wine, which is also less alcoholic.

Even in death, yeast can be a winemakers friend. Dead yeast cells form a sediment (called the lees) which if left in the wine imparts flavours such as biscuit, nuttiness and even brioche. Known as autolysis, the best example is champagne.

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Methode Traditionnelle in progress…

Called Méthode Traditionnelle (or the traditional method) in New Zealand or Champagne in Champagne, the wine undergoes two alcoholic fermentations. First a still, dry white is made in the normal manner; then it placed in a bottle along with a little sugar (The maximum amount allowed today is 12 grams of sugar per litre for a Champagne Brut) and a little yeast and sealed.

The yeast then goes to work on the sugar producing extra alcohol and more importantly, carbon dioxide (the bubbles!). Once all of the sugar is consumed the yeast dies and forms lees in the bottle; if left undisturbed for a year or so those wonderful autolytic notes found in premium Champagne are produced. The lees are then removed during the corking process.

There are other yeasts in the winery such as Brettanomyces, but that’s a whole different story…

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super close up of Brettanomyces bruxellensis yeast