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Yeasts are microscopic unicellular fungi, which are non-motile round or elliptical. Yeasts are larger than bacteria in size. Yeasts reproduce by the process of “budding” and therefore known as true yeast. The bud when fully mature, breaks away from the mother cell and becomes independent and repeats the process of multiplication.
Yeasts require less moisture and acidic pH to grow and do not grow in alkaline medium. Yeasts grow under moderate temperature (25-30°C) in sugar solution of low concentration. Most of the yeasts usually do not grow in media containing more than 65 per cent of sugar or 0.5 per cent acetic acid. Heating at 60°C for a few minutes is sufficient to destroy most species of yeasts. Boiling destroys yeast cells and spores effectively.
Some yeasts are very useful in making bread, beer, wine, vinegar and many other fermented products. Yeasts are responsible for fermentation of fruits and fruit products. Yeasts are undesirable when they grow on fruits, juices, squashes, sherbet, honey etc. and spoil the appearance, taste, texture and wholesomeness of fruits and fruit products. During active fermentation, yeast can be recognized by formation of bubbles of carbon dioxide gas on the surface of the product. Some of yeasts which grow on fruits are Saccharomyces, Candida etc. Saccharomyces generally spoils low sugar products whereas Candida is responsible for spoilage of high acid foods, salty foods and butter etc.
Some yeasts which are like true yeast but do not form spores are known as pseudo-yeasts. This type of yeast is not suitable for fermentation as they produce off-flavours and cloudiness.
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