Fermentation basics

Hello all,

Now then, I just wanted to explain fermentation in a very simple way so that you can understand what it means and what happens in the wine during the process. We use yeast in winemaking because, basically, they like to eat sugar and convert it into alcohol. During this chemical reaction, heat is produced as well as Carbon Dioxide.

Yeasts are simple little beasts. They ‘eat’ sugar (which is their energy source) and Nitrogen (which they use to help make copies of themselves – i.e reproduce). The Nitrogen comes from the grapes themselves. The level of Nitrogen in the grapes depends on how fertile the soil is.

As the yeast eat the sugar in the grape juice, the environment that they are swimming around in becomes more and more alcoholic. The more alcohol there is in the wine (there will be more alcohol, the closer we get to the end of fermentation), the more difficult it is for the yeast to survive as alcohol is toxic to them. This means that winemakers have to monitor wine very carefully during fermentation, checking that there is no sugar left before the yeast eventually die.


We can protect the yeast by making sure the fermentation temperature stays below 30ºC (the yeast will start to struggle at higher temperatures and will start to die if the temperature reaches above 40ºC). In addition, when you ferment the wine at lower temperatures, you will preserve more of the fruity aromas which are quite delicate and can disappear when the temperature gets too high. For this reason, I will be trying to maintain the temperature of the fermentation at around 25ºC.


We also need to make sure that the yeast has enough food. They have enough sugar in the grape juice (this mix of grapes and juice is often referred to as ‘must’ in winemaking) but what about Nitrogen? If the yeast don’t have enough food, or become too hot, they get stressed and start to give off an unpleasant smell reminiscent of rotten eggs. Nobody wants rotten eggs in their wine so sometimes we need to add yeast food in the form of Nitrogen to the fermentation.

Part of the reason why the grapes at this vineyard have such a concentration of flavours is because the soil is so poor and infertile. However, this lack of nutrients in the soil, means that it is quite likely that there will not be enough food (Nitrogen) for the yeast present in just the grapes. So, although I haven’t done yet, I will probably have to feed the yeast. This means, I will be tasting and smelling the wine every day and, at the first sign of any stress, I will add some yeast food. The yeast food is essentially dead yeast cells which will break down into the wine/juice and provide food (Nitrogen) for the living yeast.

Wild Yeast

There are many different species of yeast living on grape skins, so if you wanted to, you could squish some grapes, stick them in a bucket and one or more of the yeast species would start to ferment the juice. The problem is that not all yeast species are strong enough to ferment all the sugar in the juice. This can mean a fermentation stopping early and you are then left with a low alcohol, sweet wine. Also, some of the yeast species can produce vinegar and other unpleasant flavours in addition to the alcohol.

If you do just let any old yeast start fermenting the grapes, the effects can be great because you get a combination of different fermentation “styles” from the different yeasts which can result in a complex wine. BUT, the effects can also be terrible, leaving a half finished wine that only tastes okay on your chips.

A happy medium

I like the idea of “wild ferments” but I don’t want to make vinegar so I am using a combination of two yeast species which I have bought. I know exactly what yeast I am adding to the wine and what effect it will have. However, because I am first adding a different species than usual (this species can only survive in about 10% alcohol) and another more classic species to finish the wine off (this classic species can ferment wines up to 16% alcohol should you wish to), I will be getting more complexity (i.e a combination of aromas and flavours produced by different species of yeasts working in tandem rather than competing with each other) in the wine without any vinegary headaches.

Ok, that was a long blog, but I think it contained some pretty important information. Let me know if you want me to develop any of those points further….

Thanks and Adéu


3 thoughts on “Fermentation basics

  1. Pingback: The white wine | The Cunning Anchovy

  2. Tot,

    Sounds like its all going to plan. Looking forward to getting a bottle or 3 when its ready.

    Best of luck with the wine adventure, hope its a success.



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