Yeast is the magic that causes fermentation and transforms grape sugars into alcohol. You must have read that many times, and I have certainly written it, and that is usually yeast’s last mention. But yeast plays a large part in how a finished wine tastes. Most wineries nowadays start their fermentation with yeasts bought from commercial companies. It comes freeze-dried in large blocks sealed in foil. Winemakers consult catalogues that describe the features that different strains of yeast produce.

Until recently, before it was possible to buy dried yeasts, wineries waited for wine to spontaneously ferment from wild yeasts borne in the air and living on the skins of grapes. Colonies of yeasts live in the fabric of wineries and every vintage they ferment the wine, giving a consistent flavour to the wines year after year.

The yeast companies collected specimens of yeast from wineries with reputations for producing typical wines of the region, bred them and packaged them. Thus a winemaker wanting to make a Chardonnay in the style of Burgundy, France, can buy yeasts collected from that region.

I read of a recent expedition to collect new yeasts. Avola in the south-east corner of the Italian island of Sicily was chosen because winemakers there still use traditional ‘wild’ yeast fermentations in their open stone tanks. Scientists collected 900 yeast samples and found more than 200 different strains. Of these they selected 209 for study in detail.

The scientists were looking for suitability for commercial use including how vigorous a fermentation the yeast produced, because few wineries want their fermentation to last for months which can happen with wild yeasts. They wanted the yeast to be robust, not to be quick to die in adverse conditions, and to produce minimal sulphites.

This produced 14 yeasts which went on to the next stage, which was to make wine. They made 16 wines, 14 fermented with the trial yeasts, plus one from a commercial packet and one they let ferment spontaneously.

After three months in bottle the 16 wines were tasted by a panel of wine experts who rated wines for look, smell and taste. The spontaneous ferment scored lowest, but six wines made with the experimental yeasts scored higher than the wine made with the commercially produced yeast. These six wines were aged further and tasted again. The yeasts that made the best three wines were chosen for commercial production. However two of these yeasts didn’t take well to being freeze-dried. That left just one yeast which, luckily, was the one that had produced the highest scoring wine.

This yeast, named NDA21, is now being made commercial available, and recommend for red wines made from Merlot, Cabernet Sauvignon, Shiraz and Nero d’Avola. The yeast is said to produce wine with intense colour, harmonious fruity wines with persistent spicy notes and with a short fermentation time.

This yeast is now being made commercial available, recommended for red wines made from Merlot, Cabernet Sauvignon and Shiraz. The yeast is said to produce wine with intense colour, harmonious fruity wines with persistent spicy notes and with a short fermentation time.

As consumers we rarely acknowledge the part yeast plays in the taste of our wines, but it is a vital tool in the winemakers armoury.

Note: For technical details of yeast NDA21 see www.warrenchem.co.za/images/products/wine/techdatasheets/FT%20NDA%2021%20EN.pdf

Peter F May is the author of Marilyn Merlot and the Naked Grape: Odd Wines from Around the World which features more than 100 wine labels and the stories behind them, and PINOTAGE: Behind the Legends of South Africa’s Own Wine which tells the story behind the Pinotage wine and grape, also available for the Kindle, Nook and iPad.

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