Jams, jellies, and preserves are protected by the high sugar content as well as by their acid content. The amount of heat required to can nonacid foods successfully is very high because of the need to destroy the thermophilic bacteria and the very tough bacterial spores. The chemical changes brought about by a certain group of microorganisms is called fermentation. This group is used to produce fermented foods and beverages. Sour milk and cream, buttermilk, and yogurt are produced by bacteria that change milk sugar into acid by fermentation.
Cheeses owe their various flavors almost entirely to different kinds of combinations of bacteria.
Corn is changed into silage by certain bacteria that can turn plant sugars into acids. Bacteria in the Chemical Industry Fermentation is also important to the chemical industry. To ensure a pure strain, a single parent bacterium is selected in the laboratory. From this cell the strain is grown in large quantities. A Lactobacillus bacterium makes lactic acid from corn starch or whey.