Microbes and food 3. Food producers
Yeast photomicrograph
Picture 3.2d Yeast used in bread making.

3.2b Yeast fermentations
Yeasts are simple, single celled fungi which are widely found in the environment. They can break down sugars into ethanol (alcohol) and carbon dioxide. This ability is harnessed to make bread, beer, wine and many other alcoholic drinks.

Yeasts also contribute to the development and flavour of some fermented milk, vegetable and meat products.

Bread rising animation
Picture 3.2e bread rising time lapse


Yeast contributes to the texture and taste of bread. A dough is made by mixing yeast with flour, salt and water. The yeast ferments sugars in the mixture to make alcohol and bubbles of carbon dioxide. The gas gets trapped in the sticky proteins of the dough and causes it to rise. The alcohol is converted to compounds which give the bread flavour as it is cooked.

Beers in a supermarket
Picture 3.2f Beers on sale in the supermarket.

Alcoholic drinks

Any fruit or vegetable containing fermentable carbohydrates can be used to produce alcoholic drinks.

Beer is made from cereals (usually barley, but sometimes wheat) by a complex process known as brewing.

Wine is made from fruit juice – usually crushed grapes.

Cider is made from apples and perry from pears.

Yeast carries out the fermentation and other microbes contribute to the flavour and aroma of alcoholic drinks as they mature. Most drinks are pasteurised before they are packaged to prevent spoilage. The yeast residues may be recycled or used as food flavourings or supplements (yeast extract). Marmite is a by-product of beer manufacture.

Spirits like whisky and brandy are made by distilling alcoholic liquors.