| Applications of biotechnology to traditional fermented foods |
|VI. Human health, safety, and nutrition|
O. Paredes Lopez
Fermentation was one of the first methods used by Man to produce and preserve foods. Microbial fermentations have played an important role in food processing for thousands of years. Fermentations provide a way to preserve food products, to enhance nutritive value, to destroy undesirable factors, to make a safer product, to improve the appearance and taste of some foods, to salvage material otherwise not usable for human consumption, and to reduce the energy required for cooking (1). Preservation of foods by salting is an age-old practice; while preventing the growth of pathogenic microbes, it allows the development of harmless, halotolerant ones that produce desirable censorial changes in the substrate (2).
Traditional fermented foods may be divided into two broad categories: (a) submerged culture-fermentations (SCFs) and (b) solid substrate fermentations (SSFs). In SCFs microbial activity occurs at a relatively low biomass concentration in the liquid phase, while in SCFs microbial growth and product formation occur on the surfaces of solid substrates (3,4). Some examples of traditional fermented foods for SCFs are pulque and tesguino, soy sauce, fish sauce, kaffir beer, and palm and rice wines. Examples of SSFs are tempe, miso, pozol, oncom, and natto. One of the major characteristics that distinguishes SSFs from SCFs is that SSF processes usually occur at low-moisture contents (e.g., 10 to 20 percent), conditions under which water activity favors the development of filamentous fungi. However, for many indigenous fermentations the microbial interactions are complex and mixed fungal-bacterial, fungal-yeast, and yeast-bacterial combinations occur (5). These interactions play an important role in the nutritional, safety, and sensory characteristics of the end product (6).
EFFECT OF FERMENTATION ON NUTRITIONAL COMPOSITION
Changes in Proximate Composition and Soluble Components
During fermentation the microorganisms secrete hydrolytic enzymes into the substrate and assimilate some of the fatty acids, amino acids, and simple sugars thus liberated. These are converted into microbial structural components and secondary metabolites. Lactic acid fermentation is an ancient process whereby a varied group of bacteria ferment carbohydrates, producing lactic acid as the major end product. This type of fermentation is used for the production of dairy products, sauerkraut, bread, meat, and silage. In particular, traditional SSFs of legumes, cereals, and starchy substrates have been associated in many regions of the world with the activity of lactic acid bacteria (7); during fermentation lactic acid accumulates, with a concomitant increase in acidity and a decrease of dry matter yields. The higher pH values of fermented legumes, compared to other materials under similar conditions, have been attributed to their higher protein content (8,9).
It seems that the only fermented food showing significant changes in its crude composition is pozol. The fermentation mixture contains Agrobacterium azotophilum, which is capable of fixing nitrogen (10). Due to the crude methods of analysis, the proximate composition of foods does not change much during fermentation. However, there is almost always a high increase in the soluble fraction of a food during fermentation. The proteolytic activity of bacteria in traditional fermentations degrades complex proteins into simpler proteins, peptides, and amino acids. The bacteria used in natto fermentation cause substantial increases in the level of free amino acids and soluble carbohydrates. On the other hand, Rhizopus spp., used in the fermentation of various types of tempe, are highly hydrolytic, and outstanding increases in soluble fat, protein, and carbohydrate are observed. Free fatty acids, including the essential fatty acids, linoleic and linolenic acids, may increase in these indigenous fermented foods (11,12); this increase is thought to be of nutritional significance.
The increase in soluble solids is a nutritionally desirable event, as the food is effectively digested prior to consumption. In some cases the microorganisms are capable of producing pectinases and cellulases, softening the texture of the food and liberating sugars that would otherwise be unavailable to the human digestive system. Consequently, fermented foods are expected to be more digestible than their unfermented counterparts.
Changes in Composition of Amino Acids and Vitamins
Methionine, the limiting amino acid in legumes, has been reported to increase during tempe kedele production, and lysine, the limiting amino acid in cereals, increases during fermentation with Rhizopus spp. (1). During kocho production, an acidic fermentation, the essential amino acid content is considerably enhanced. On the other hand, during tape' ketan and enjera production, the levels of some essential amino acids fall, whereas others remain unchanged (11). In general, most traditional fermented foods exhibit slight changes in essential amino acids.
Interestingly, isolation of improved strains of Aspergillus niger for an SSF process allowed 200 to 300 percent lysine overproduction compared to the parent strain (5). However, it should be emphasized that bioavailability and balance of amino acids are more important than their total content. Hence, biological experiments to assess their nutritional value are warranted.
Traditional fermentations dramatically improve the vitamin content of a wide variety of substrates. Of all the foods investigated, only enjera showed a decline in vitamin content (1,13).
Changes in Unwanted Components
Unwanted components, such as physic acid, trypsin inhibitor, flatus factors, and lectins, may be present in high concentrations in several desirable foods. Phytic acid and trypsin inhibitor interfere with digestion by binding enzymes. Phytic acid may also bind minerals, reducing their bioavailability. Lectins are capable of binding to the intestinal wall and thus interfering with nutrient absorption. Presoaking and cooking of foods can reduce the levels of some, but not all, of these antinutritional factors. However, microorganisms have the capacity to hydrolyze them, reducing their levels even further (14). Hence, bacteria, yeasts, and fungi that degrade antinutrients at a fast rate and at early stages of fermentation need to be identified or developed (1).
Changes in Biological Value
Since fermentation increases the quantity of soluble proteins in foods, it may improve the amino acid profile, and because it reduces the levels of certain antinutritional factors that interfere with digestion, it would not be unreasonable to suggest that fermented foods will be more efficiently utilized by the human digestive system. Single- as well as mixed-culture fermentations of pearl millet by yeasts improve starch and protein digestibility (15). Enjera is one of the few traditional fermented foods that shows a decline in protein efficiency ratio (PER), probably due to a decline in the essential amino acid content (16). Also, increases in PER values of some indigenous fermented foods can be obtained by incorporating soybeans into cereal-based substrates.
SAFETY ASPECTS OF TRADITIONAL FERMENTED FOODS
Because many fermented foods are produced using fungi, the risk of mycotoxin contamination is high. During natural fermentations, food-poisoning flora and coliforms may also grow with the lactics. These microorganisms need to be eliminated to make fermented foods safe for consumption (16). Several factors contribute to the safety of fermented foods: (a) Soaking and cooking. Washing, soaking, and cooking treatments reduce the in situ microbial contaminants. (b) Salting. Various fermented foods are made with the addition of salt, which acts as a preservative. (c) Acid formation. Many indigenous fermentations are carried out by acid-producing microorganisms, where these organic acids (e.g., lactic, acetic, fumaric acids) act as preservatives or as bacteriostatic agents. An inhibitory pH for bacterial growth is considered to be 3.6 to 4.1. (d) Antibiotic production. Molds used in some traditional fermentations produce antimicrobial glycopeptides. (e) Low moisture content. In the case of SSF processes, the low water activity may be an important preservative factor. and (f) Reduction of aflatoxin by some microorganisms. Rhizopus and Neurospora species, among others, are reported to decrease aflatoxin content of contaminated substrates.
Despite these factors, it has been reported that the sanitary quality of some Oriental fermented foods is poor (17,18). Safe products are usually obtained when the following recommendations are observed: (a) appropriate soaking of the beans in acid at a low pH; (b) adequate cooking time; (c) using hygienic conditions during production, handling, and storage; and (d) good refrigeration of products (5°C) between production and consumption.
In summary, production of foods with high nutritional and sensory values, and free of microbiological health risks, is a key component of any policy aimed at upgrading the social role of traditional fermented foods in less developed countries.
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