The role of yeast in winemaking

I am coming down with bacteria at the moment, most of it inflicted on me by my family, including kefir from my wife, kimchi from my daughter and a host of other sauerkrauts, pickles and fermented foods. I blame David Chang, the Korean-American chef. And “fermentation fetishist” Sandor Katz, who appeared at Ballymaloe Litfest again this year. Apparently it is all very good for me.

My favourite microorganism (in this case a unicellular fungi) is Saccharomyces cerevisiae, the sugar-loving yeast largely responsible for bread, beer and, of course, wine.

Yeasts and other microbes will grow anywhere that conditions permit, including wineries and vineyards. Given the right temperature, they will usually ferment grape juice or must into wine. But they do a lot more than that.

According to Jamie Goode in his book Wine Science, at least 400 of the 1,000 or so volatile flavour compounds found in wine are actually produced by yeasts. Yet when did you last read a tasting note that mentioned yeasts alongside the usual grape varieties and oak treatments?

A few years ago I was presented with six wines made from sauvignon blanc by a producer. All had been made from the same grape juice, but each with a separate strain of yeast. The difference between them was remarkable.

The producer argued that many of today’s wines were the result not of climate, soil or any other quaint notions of terroir, but the yeast used in the fermentation. In particular, he said the classic lime zest and gooseberry aromas of Marlborough’s sauvignon blanc had less to do with the long, cool growing season than the strain of cultured yeast that was universally used at that time. The fact that these days the same wines are far more subtle and complex may prove his point.

Traditionally, yeasts present in the air began the fermentation with little human interference. These are referred to as indigenous, natural, native or local yeasts.

With a spontaneous fermentation, a variety of microbes (some good, some bad) fight to establish themselves and then die off allowing another to take over. More than 20 different strains of yeast can be present in freshly crushed must. The problem is that a natural fermentation takes longer, particularly at the start, and you don’t know how the fermentation is going to turn out. There is a risk of spoilage.

By using a commercially-produced cultured yeast, a winemaker can retain control of the entire process ensuring that the fermentation starts and finishes safely. Those in favour of cultured yeasts argue that they are essential to the production of quality wine on a consistent basis. A few spoiled batches of wine could lead to ruin for a producer.

That is not all cultured yeasts can do. There are hundreds of strains of Saccharomyces cerevisiae. The right yeast can endow your wine with specific characters; increasing particular flavour compounds, such as the aromas in your sauvignon blanc and even improve the wines' ability to flocculate (sparkle) if you are making sparkling wine.

There is an ideological divide between those who favour wild or cultured yeasts. It tends to be divided between old world and new. Opponents of cultured yeasts see them as completely unnatural. Others see wild yeasts as dangerous. Spontaneous fermentations tend to produce richer, more supple wines with a greater complexity. They also contribute to the individual character of the wine. The natural wine movement eschews the use of cultured yeasts. Increasingly larger wineries in Europe are using cultured yeasts while some producers in the new world have been experimenting with spontaneous ferments, releasing limited editions such as those below. Both sides, however, are very aware that yeasts play a vital role in deciding how our wine will taste.

Even after they die off, yeasts can play an important part in a wine’s development. The lees, or dead yeast cells, float to the top of the fermented wine; producers can leave the wine in contact with the lees, sometimes regularly stirring this back into the wine, to add extra complexity.

If permitted, flor, a specific strain of Saccharomyces cerevisiae, sometimes forms a film on the surface of wine, giving a special character to Manzanilla and fino sherry, as well as wines in the Jura and Tokaji regions.

In Champagne, the ageing of wines on their lees is a vital part of production. Some of the most highly sought-after cuvées, such as Bollinger RD, will spend many years on their lees before being disgorged and sold.