Microbiology of Food Fermentations, Second Edition
by Carl S. Pederson
The AVI Publishing Company, Westport, Connecticut; 1979
The changes that occur during fermentation are the result of the activity of enzymes, which speed up chemical reactions. There are 3 sources of enzymes in food:
- microorganisms involved in fermentation (bacteria, yeasts, and molds)
- enzymes native to the food itself (e.g. enzymes responsible for curing tea leaves)
- microbial flora on the unfermented food (dependent on type of food, environment, and handling. usually present in low numbers)
Good fermentation is usually produced by select microorganisms with possible involvement of native enzymes.
Most fermented foods rely on a few highly specialized species of microorganisms, usually lactic acid- and acetic acid-producing bacteria, alcohol-producing yeasts, and some molds. The specialized microorganisms usually do not alter much of the nutritional or caloric value of the fermenting product besides breaking down its carbohydrates, hence their value in food preparation, preservation, and human nutrition.
Natural sequence of fermentation
Most fermentations are the product of several types of microorganism, acting in sequence, not just a single type.
Bacteria are the smallest of the microorganisms (less than 1 micrometer – µm) and so have the greatest surface area and absorb nutrients most quickly from the surroundings. For this reason, they usually dominate the early stages of fermentation. Small species, such as those in genera Streptococcus and Leuconostoc will grow and ferment more rapidly than larger species such as those of genus Lactobacillus.
Yeasts are larger than bacteria (4-7µm), and molds are are the largest of the fermenting microorganisms, and so they generally follow bacterial fermentation in sequence.
Bacteria
Bacteria perform many important functions in food fermentation, including the breaking down of sugars, most importantly into lactic acid and acetic acid, but also sometimes including ethanol, carbon dioxide, and mannitol. Some bacteria produce primarily one product (homofermentative), while others produce several products (heterofermentative).
Important genera of bacteria used in food fermentation include:
Lactic acid bacteria break down sugar into lactic acid, which inhibits the growth of many other undesirable microorganisms, hence their preservative effect. They are microaerophilic, meaning they require oxygen for growth, but in lower levels than the quantity of oxygen present in the atmosphere. Because they do not rely on oxygen, they are inefficient and do not get as much energy out of the food as aerobic organisms, thus they must consume a lot of sugar to get energy. This inefficiency is beneficial to mankind, since it means the bacteria must be very active in order to survive, which leads to the production of lots of lactic acid, which preserves food.
Common types of lactic acid bacteria (by increasing amount of lactic acid produced):
Acetic acid bacteria require oxygen in order to grow and produce acetic acid (i.e. vinegar) from ethyl alcohol.
Yeasts
Yeasts also break down sugars. They require oxygen for optimal growth but not for fermentation. In the presence of oxygen, they break down sugars into carbon dioxide and water. In the absence of oxygen, they break down sugars into carbon dioxide and ethyl alcohol.
The alcohol and carbon dioxide produced by fermentative yeasts suppresses the growth of other detrimental bacterias and yeasts that may otherwise spoil the food, hence its use in preservation of food. Yeasts are especially important in leavening of dough, production of alcoholic beverages, and well as the synthesis of vitamin B.
There are many type variations in yeasts that differ slightly in their metabolisms. Some yeasts are entirely aerobic, while others are intermediate with varying respiratory and fermentative metabolisms.
Some yeasts commonly used in food production:
Molds
Molds have the greatest array of enzymes, are aerobic (i.e. require oxygen), and will grow on most foods. They are capable of rapidly decomposing woody plant materials, and are important in the production of many foods (Roquefort, Camembert, soy sauce, tempeh, etc).
Some molds important in food fermentation are:
Contact
Subscribe
Oct 29, 2009 
1 Comment 
birch, compote, fermentation, juice, olives, sap, supermarket