The preparation of fermented foods is a practice with a very long history. Already thousands of years ago, for instance—as reported in the bible—wine and cheese were being made. In those times, hardly anything was known about the underlying mechanisms; however, many interesting products evolved. All of them were the result of spontaneous fermentations, essentially resulting in products with an extended shelf life and (often) flavorful characteristics. The extension of shelf life was obviously a major reason for fermentation practices. Later on, after the discovery the causal agents for the fermentation processes, these microorganisms were isolated and used in order to be able to produce products with a consistent quality. This led to the practice of using industrial starter cultures and gave the manufacturer more control over the technological, safety, and consistent quality aspects of their food products.
Many fermented products exist, and for each of them cultures with specific characteristics are involved. For instance, in the fermentation of meat, combinations of various bacterial species are used. In almost all cases, lactic acid bacteria are present in these starter cultures, with Lactobacillus sakei, Lb. curvatus, Lb. plantarum, Pediococcuspentasaceus, and P. acidilactici as most important species. Also, Micrococcus varians, Staphylococcus car-nosus, and S. xylosus species are used. These microorganisms are able to grow under low pH (4.5-5.0) conditions, low water activities (Aw 0.9-0.95), anaerobic conditions, and in the presence of salt (2-3%). Not only do these organisms cause a decline in pH, but they also contribute to the typical flavor of fermented meat products.
A number of fermented meat products also exhibit a fermentation on the surface, resulting from the growth of yeasts (Debaromyces hansenii, Candida famata, and C. utilis) and molds (Penicillium chrysogenum and P. nalgiovense). The surface flora usually adds to the flavor formation through specific lipolytic and proteolytic activities.
For the production of bread and sourdoughs, yeast (Saccharomyces cerevisiae) and lactic acid bacteria (e.g., Lactobacillus sanfransiscensis, Lb. amylovorum) are employed. The required gas formation in these products largely is due to the sugar fermentation of the yeast cells, although also some heterofermentative lactobacilli can add to this. The fermentation of sourdough is more complex because it requires the interaction of various microorganisms. The various flavor components formed in these products, such as aldehydes, alcohols, and pyrols, are partly the result of biochemical conversions by the starter cultures but also result from the baking process itself (1,2).
The fermentation of vegetables, such as olives, cabbage, and pickles, belongs to the least developed practices. The production methods are still rather simple: basically, lactic acid bacteria such as Lb. plantarum, Leuconostoc spp, Pediococcus, and Lactococcus are used. The acid formation results in extended shelf life, and also flavor formation by the microorganisms adds to the overall quality of these products. For the fermentation of soy, which is a very common and traditional process in east Asia, various mold species are used, such as Rhizopus oligosporus and R. oryzae. These fermentations are often solid-state fermentations, for which the raw material is inoculated with microorganisms that grow at the surface of the raw material, making it essentially an aerobic fermentation process. For some of the products, there is an extensive proteolysis, leading to products like soy sauce, whereas in other fermentations (e.g., tempeh) only limited proteolysis occurs (3).
Wine and beer are fermented with various yeast strains (S. cerevisiae). In some wines, also Leuconostoc oenos is used. In these cases, the main pathway related to preservation is the formation of alcohol from sugar of the grain or the grapes, respectively. Again, as in other fermentation processes, many other reactions occur that contribute to the final sensory quality of the products and that are in fact even more important for the final quality. In the preparation of malt for beer brewing, microorganisms are also employed in order to lower the pH and to reduce the outgrowth of spoilage organisms (e.g., certain molds) (4,5).
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