In my last article I wrote about the difference between the in-your-face gooseberry and elderflower flavour of New Zealand Sauvignon, compared to the subtle and hard-to-describe French Sauvignons of the Loire and Bordeaux. Different terroirs produce different wines, I suggested. But what if it’s not the terroir at all but the yeast?

Yeast is what converts the sugars in grape juice to alcohol. But that’s not all they do, or wine would taste like alcoholic grape juice. They also convert chemicals in the juice to the flavours that make it into wine. Traditionally, the yeasts used were those that occurred naturally on the skin of the grape or in the cellar. But now most winemakers use cultured yeast. These are still natural yeasts (not genetically modified) that have been selected because they produce a specific effect that the winemaker wants.

As an example, let’s stick with New Zealand Sauvignon. The most commonly used yeast strain is Zymaflore VL3,X5. It increases the concentration of thiol compounds, which are what give NZ Sauvignon that incredible flavour. And if the winemaker doesn’t want her wine to be too tart, she might add Uvaferm SVG, which will reduce the malic acid content of the wine. And it’s not just Sauvignon. CY3079 will bring out the ‘hazelnut and brioche’ flavour of Chardonnay. Rhone 4600 will increase the apricot-like flavour of Viognier. And so on. And it’s not just about flavour. Enoferm Syrah will increase the amount of glycerol in Syrah, giving the wine more body in the mouth.

What do we think about all this? Winemakers who use cultured yeasts are extraordinarily reticent on this issue, as though they fear they will be accused of manufacturing flavours that wouldn’t be there naturally. Some traditionalists rely on naturally occurring yeast, at least to start the fermentation process, claiming that this yeast is part of the terroir that makes their wine unique. However, science does not support this idea. The strains of yeast found in a cellar or vineyard one year will not necessarily be there the next. Relying on indigenous yeasts that just happen to be there leaves a lot to chance.

The thing to understand is that you can’t take Sauvignon grape juice from the Loire, ferment it with Zymoflore VL3,X5 and make NZ Sauvignon. Cultivated yeasts can only work on the chemicals that are already in the grape juice. They enhance the real flavour that is naturally there. They reveal what might otherwise just be potential. I’m all for it.

I am indebted to Benjamin Lewin MW for the technical detail contained in his article ‘Do you know what’s flavouring your wine?’ in ‘Decanter’ July 2014.

Andrew Polmear