Winemaking yeasts and bacteria
By Ian Hickinbotham
Yeast cells are everywhere, even in our hair (if we have any). Our most common yeast is the baker's used for making our daily bread.
Starch of essential flour is converted to sugar by natural enzymes so the added yeast can 'ferment' it into alcohol and gas. Sadly, all the alcohol goes up the chimney during the baking, but the carbon dioxide wonderfully produces the 'holes' commonly seen in a loaf of bread – and the lightness of texture.
Unleavened bread is made without yeast – the 'leaven' – so it does not 'rise' during the process. 'Sourdough' is made using particular bacteria, which have been found to produce a similar physical effect as yeast while the 'sour' part of the definition means the lactic acid produced during the process. They are about an eighth of the size of yeasts and produce far less gas, which we perceive as the smaller 'holes' in sour-dough bread. We can only see both yeasts and bacteria with microscopes.
The yeasts used by the winemaker are again different from those of the baker. But, importantly, contrary to some current writing about wine, they are natural. (In fact, there is no such thing as 'artificial' or 'synthetic' yeasts as some 'experts' profess.)
Wine yeasts are so natural they are on the outside of all berries. As fruit ripens, it secretes a wax onto the outside of its skin to which local micro-flora attach. Of course, it is most obvious on dark coloured fruit, like Shiraz or Pinot Noir, the black grapes, and we define it as 'the bloom': we might carefully wipe it from a plum before eating it.
Oenologists have researched yeasts in all winemaking countries and some are chosen for specific wines. Thus for fortified wines (like Port and Sherry – as we used to call them), yeasts that make the most alcohol are usually preferred, while others actually subscribe desirable flavour, as in the making of Riesling (though it is a controversial achievement).
Then there are 'undesirable' yeasts, like the current scourge, Brettanomyces, commonly referred to as 'Brett'. Seems it is present in most cellars and afflicted wines develop a 'metallic' smell and taste, but a qualifying view is – a little can enhance smell and taste – slightly.
Truth be told, the older the winery the more diverse the yeast population as it progressively increases on fixtures in and around the cellar. A consequence is that a real mixture of yeasts ferment the vintage, so some microbiologists question the authenticity of the currently fashionable claims about 'wild' yeasts on labels.
Bacteria are also used for winemaking. 'Used' is hardly the correct word, because they have contributed to natural dry wine quality for thousands or years: in fact the Roman saying about the wine moving in sympathy with the advent of Spring is often quoted in the trade. The expression refers to the secondary fermentation that Nature intends for dry wine – the gentle 'fermenting' of harsh-tasting malic acid into soft-tasting lactic acid by naturally occurring bacteria.
Ian Hickinbotham, one of the most innovative and influential oenologists in Australia over his 50 year career, is the author of Australian Plonky (see related review below).
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