It is little more than 100 years ago that Louis Pasteur proved that yeast cells cause fermentation.

Bread leaven was known long before the birth of Christ, and ale yeast too. Throughout the centuries it was used without knowing how it worked or even what it was. Around 1800 it was generally thought that fermentation was a purely chemical process. Some, however, thought that it was due to spon?taneous generation.

It wasn't until 1835 that a Frenchman, de la Tour, and a German, Schwann, discovered separately that beer and wine yeasts were living spherical organisms able to reproduce themselves and that their presence was essential for fermentation. De la Tour, studying drops of beer under a microscope, saw the yeast cells forming buds that grew, parted from the parent buds and soon formed buds them?selves. Schwann discovered that the cells needed nitrogenous matter as well as a sugar solution in which to thrive. He called them 'zuckerpilz' which simply means sugar fungi. Shortly afterwards Von Meyer, another scientist studying yeast cells, sug?gested the name 'saccharomyces' as a generic name for sugar fungi and this remains today.

A catalyst

To isolate the cause of fermentation was not the same as understanding how it worked. It was only known that yeast cells were always present during fermenta?tion and that they flourished and multiplied. The Swedish chemist, Jons Berzelius, had recognised that some substances, simply by their presence, caused other substances to react, without themselves being changed in anyway. He described such a substance as a catalyst, a name he derived from the Greek word 'Katalysis', meaning 'dissolution'. In 1839 Berzelius suggested that yeast was a catalyst since by its very presence in a sugar solution it caused a reduction of the large sugar molecules into ethyl alcohol and carbon dioxide in almost equal pro?portions.

In 1857 after a series of experiments based on his own thinking, Pasteur published an article on lactic fermentation. Further experiments proved that if a yeast which had been cultured in a germ-free medium was introduced into a beer wort that had been sterilised fermentation was quickly started. He was soon able to proved beyond doubt that the yeast cell alone caused alcoholic fermentation.

Yeast Enzymes

Other scientists, notably Jorgensen and Hansen in Denmark, developed these researches and later it was discovered that the yeast cell secreted a number of enzymes which were the actual catalysts that caused fermentation. Invertase split the sucrose molecule into glucose and fructose and then a whole series of different enzymes, called the apo-zymase complex for simplicity, carried on the complicated process of reduction, first to one chemical, then to another and so on throughout at least 15 changes, until ethyl alcohol and carbon dioxide were formed.

Once the yeast cell was known, scientists soon discovered many different varieties and sub-varieties of yeasts amounting in all to nearly 2,000. Only a very few are of interest to the winemaker and brewer. These are:

The yeasts we use

SACCHAROMYCES CEREVISIAE: the circular yeast cell used for fermenting bread doughs and beer worts. The word cerevisiae probably came from the noun cerevisia, a Gaulish word meaning beer. A Spanish word, cerveza, also means beer. The word was in use at least 200 years before the birth of Christ and well before the Romans came into contact with the Gauls from whom they probably took the word we use today.

SACCHAROMYCES CARLSBERGENSIS : a bottom fermenting yeast that works best at a low temperature and is used in the making of lager beer. It was named after the founder of the famous Carlsberg brewery in Denmark.