Rathfinny Wine Estate

YEAST: indigenous vs commercial

The majority of must/juice from our recent harvest has completed alcoholic fermentation. Now, we wait for our malolactic bacteria culture to be ready, so that we may use it to inoculate all the tanks so that they undergo malolactic fermentation, to soften the acids, which we expect to be completed early in the New Year.

We are often asked why we use commercial yeast rather than just letting indigenous (or native) yeasts naturally do the work. There are various reasons why we use commercial yeast.

Provided commercial yeast are used as recommended, they will ferment one hundred per cent of the sugars in the juice. These selected yeast strains are known for their resistance to SO2 and alcohol, which means that they aren’t killed by the conditions they create. We add SO2 during the pressing stage to clean the juice from native yeast and to prevent oxidation. It is crucial to know that the yeast won’t die in such media.

As these yeast metabolise sugar and convert it into alcohol (which is toxic to them), it is also good to know that they are resistant to high levels of ethanol. If yeast die before a wine is done fermenting (stuck fermentation, which is a common phenomenon in winemaking) it can leave the door wide open for off-flavours created by dying yeast and/or bacteria taking over the media.

Finally, selected commercial yeast do not produce high levels of volatile acidity (VA). Volatile acidity is a by-product of yeast activity, but found in high concentrations turns into a defect. It is made for the most part of acetic acid (you know, vinegar!).

One other benefit with selected yeast is that they do not produce too much foam when going through fermentation. In an overfilled tank, a fair amount of wine could be lost if it’s foaming excessively.

Natural yeast do not have these beneficial attributes. The way these yeast ferment is more random. Some might work well, some might produce too much VA, some might die half-way through fermentation because they are intolerant to even low levels of ethanol. This is all very unpredictable and quite scary for winemakers, especially for those who work with substantial volumes and/or elite brands. When working with natural yeast, it is extremely difficult to predict, if it’s even possible, which strain is going to lead the fermentation, so the winemaker is taking a risk in trying to manage the unknown

People often wonder if, with commercial yeast, we get the same terroir effect that we would get with indigenous yeast. Purists might say that wine should be left to do its own thing with the native yeast, and that this method, in theory, would create a wine that truly reflects its terroir. I find it difficult to let things happen spontaneously when you are responsible for thousands of litres of wine that might go bad because of one decision that may sound good in theory, but has lots of potential for a troublesome outcome.

Nowadays, there are lots of different yeast strains available commercially, including some that are designed to get the best out of nice, ripe grapes by increasing the production of aromatic compounds specific to different varieties. Personally, I feel that any perceived benefit of natural fermentations, which present an enormous risk, might be just that: perceived. Naturally fermented wines (assuming fermentation was able to complete) are not necessarily obvious to spot when comparing to an inoculated wine from the same vineyard.




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