Yeast plays a significant role in our foods and indispensable role in all alcoholic beverages. Without yeast there would be no alcoholic beverage, and yet relatively the general public knows little about yeast.
Yeast is a single-celled fungal organism that occurs naturally on the skin of grapes, and in the air, which is called ambient yeast. Dumping grape pomace in the vineyard can modify ambient yeast cultures.
In a sugar-containing liquid and in the presence of oxygen, yeasts convert sugar to alcohol and carbon dioxide.
Romans knew about yeast and studied it. They called it saccaromyces, which means, “sugar fungus” in Latin. Cerevisiae (of beer) completes the general Latin name saccaromyces cerevisiae.
Yeast needs a carbon source, good supply of nitrogen, a few minerals and traces of other substances or vitamins to function. The common carbon source is sugar, amino acids supply the nitrogen, and practically all the fruit juices provide the rest.
Brewers use saccaromyces cerevisiae and its many variants, as do bakers, but winemakers prefer saccaromyces bayanus,- uvarum, – pastorianus, which are more suitable for fruit juices e.g grape juice and now more and more professionals like using cultivated eyast strains to avoid problems.
Wild yeasts (s.Hansenula, – Klockera, – Candido, and – torulopsis die when the alcohol level reaches approximately five per cent ABV, and are less tolerant to sulphur dioxide) exist on the skin of grapes, and start fermentation, then s.Metchinkowia, Pichia, take over and continue the process. Eventually s.cerevisiae start to work. For these reasons winemakers prefer using cultivated yeasts which are more efficient in converting sugar to alcohol and rarely, if ever, cause a “stuck fermentation”.
Once all the sugar has been converted, the yeast dies and floats either to the surface of the liquid or to the bottom. S- Brettanomyces, – Kluyvoromyces, and – schizosaccaromyces may exist in the fermented juice, and all in sufficient quantities may change the smell and flavour of the wine unfavourably.
Yeast can be cultivated in laboratories and “tamed” to function differently, i.e in some cases the strain dies at low alcohol concentrations (3 – 5 per cent ABV), in others it can live up to (10 – 12 per cent ABV), and in yet other cases it can live up to 15 – 16 ABV and very seldom to 20.
Aside from brewing, wine making and distilling, saccaromyces species are used in baking.
There are hundreds of yeast strains, and many are cultivated in laboratories to suit different uses.
A yeast cell is composed of walls of cellulose surrounding living matter “protoplasma”. It reproduces during the fermentation process, and when properly extracted, can be used for another batch as it is done in sour mash whiskey in
the USA, or some breweries.
Dr. L. Pasteur unfolded the science behind yeast in the 19th century, although it was used for millennia before his research revealed how it functions.
In brewing, two distinct strains of yeast are used – for warm (top) fermentation of ale and wheat beers, and for cold fermentation in lager production. All brewing yeasts are called saccaromyces cerevisiae, although there exist hundreds of strains.
In some wineries the natural (ambient) yeast on the grape skin is used (these days rarely), but most winemakers prefer cultivated yeast strains specifically bred to achieve consistent and desirable results.
Wild yeast changes the flavour of wine and behaves unpredictably, sometimes creating a “stuck” fermentation, which is highly undesirable. Some flavours of wild yeast are captivating, others unusual and disappointing.
Some yeast strains lend themselves more to cool fermentation and preserve the natural fruit flavours, while others (mostly for red wine) thrive in red wine and like high temperatures (25 – 30 C).
Winemakers like to use specially developed strains for their base wines, most of which have been extracted in the champagne region in France approximately 140 Kilometres east and north of Paris. They preserve fragrance and purity of fruit.
In Germany, several strains were developed for riesling and other indigenous or hybrid German grapes.
Today research in yeast has become a major scientific endeavour by food manufacturers, large brewing organizations, and wineries.