In our previous article on beneficial micro-organisms, we described the role of lactic acid bacteria in food production. Yeast is another microbe that has made an important contribution to our tables.

Yeast is a tiny plant-like microbe used to make bread dough rise and alcoholic drinks such as wine, cider and beer. It is in itself a very nutritious food, being rich in protein and vitamins of the B group. Yeast residues from brewing are processed to provide spreads and ingredients to a wide variety of products, from snack foods to processed meats.

Yeast has been used in making bread and alcoholic drinks for thousands of years. Since the dawn of humanity, grains have been our most important food. Our ancestors noticed that grinding and baking grain made it more palatable. Today, flat breads, tortillas and matzos are still made using this technique.

The first combination of bread ingredients and yeast probably happened by accident. Similarly, since wild yeast naturally occurs on grape skins and grains, wine and beer were probably the result of grape juice or gruel (a mixture of grain and water) not being consumed immediately. It is possible that the first raised bread was made when an alcoholic drink was accidentally added to flat bread dough.

Over the centuries, making bread became an art. It took some time, however, for science to catch up. In 1676, a Dutch lens maker named Anton Leewenhoek developed the first microscope. This invention led to the identification of microbes, including yeast.

In 1859 Louis Pasteur, the father of modern microbiology, discovered how yeast works. By feeding on sugars derived from the starch in flour, yeast produces carbon dioxide. This gas expands the gluten proteins in the flour and causes the dough to rise. Scientists now cultivate strains of baker's yeast for their ability to make dough rise and produce loaves of good height, texture and flavor.

Alcoholic drinks no doubt enhanced our ancestors' meagre diets. Wine was being made more than 6000 years ago in the Middle East. A fermented drink described in Babylonian writings was brewed by soaking barley or wheat in water, letting it germinate in the sun and then boiling and fermenting the results. The Sumerians had names for fifteen different kinds of beers. Alcoholic beverages are now available throughout the world in many different forms. Starting materials include fruits, grain and honey and various strains of yeast that are used to ferment the sugars present in alcohol. As with baker's yeast, various methods are used to improve the yeast strains used in these fermentations. A particular goal is to increase the alcohol tolerance of yeast.

Breweries produce large amounts of yeast and while some of it is used to inoculate the next brew, much of it is used to make yeast extract spreads and food additives. The yeast is mixed with salt and heated to 50°C, which causes the cells to burst. The cell contents are separated and mixed with vegetable extracts to make the spread, while the residual material is used for animal feed. Yeast extracts are also widely used in foods where a meaty flavour is required, such as soups and sauces, frozen foods, hamburgers, sausages and potato crisps.

FOOD TODAY 07/2000