Microorganisms and Metabolism

"We can readily see that fermentations occupy a special place in the series of chemical and physical phenomena. What gives to fermentations certain exceptional characters, of which we are only now beginning to suspect the causes, is the mode of life in the minute plants designated under the generic name of ferments, a mode of life which is essentially different from that of other vegetables, and from which result phenomena equally exceptional throughout the whole range of the chemistry of living beings."

From The Physiological Theory of Fermentation by Louis Pasteur, 1879

When one considers the wide variety of fermented food products consumed around the world, it is not surprising that their manufacture requires a diverse array of microorganisms. Although lactic acid-producing bacteria and ethanol-producing yeasts are certainly the most frequently used organisms in fermented foods, there are many other bacteria, yeast, and fungi that contribute essential flavor, texture, appearance, and other functional properties to the finished foods. In most cases, more than one organism or group of organisms is involved in the fermentation.

For example, in the manufacture of Swiss-type cheeses, thermophilic lactic acid bacteria from two different genera are required to ferment lactose, produce lactic acid, and acidify the cheese to pH 5.2, a task that takes about eighteen hours. Weeks later, another organism, Propionibacterium freudenreichii subsp. sher-mani, begins to grow in the cheese, producing other organic acids, along with the carbon dioxide that eventually forms the holes or eyes that are characteristic of Swiss cheese.

Even for those fermented foods in which only a single organism is responsible for performing the fermentation, other organisms may still play inadvertent but important supporting roles. Thus, tempeh, a fermented food product popular in Indonesia, is made by inoculating soybeans with the fungal organism Rhizopus oligosporus. The manufacturing process lends itself, however, to chance contamination with other microorganisms, including bacteria that synthesize Vitamin B12, making tempeh a good source of a nutrient that might otherwise be absent in the diet of individuals who consume this product.

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