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Animal or plant tissues subjected to the action of microorganisms and/or enzymes to give desirable biochemical changes and significant modification of food quality are referred to as fermented foods (Campbell-Platt 1994). Fermentation is the oldest known form of food biotechnology; records of barley conversion to beer date back more than 5000 years (Borgstrom 1968). According to Steinkraus (1995), the traditional fermentation of foods serves several functions:
"1. Enrichment of the diet through development of a diversity
of flavors, aromas, and textures in food substrates
2. Preservation of substantial amounts of food through lactic acid, alcoholic, acetic acid, and alkaline fermentations
3. Enrichment of food substrates biologically with protein, essential amino acids, essential fatty acids, and vitamins
4. Detoxification during food fermentation processing
5. A decrease in cooking times and fuel requirements"
Aside from alcoholic fermentations and the production of yogurt and leavened bread, food fermentations continue to be important primarily in developing countries where the lack of resources limits the use of techniques such as vitamin enrichment of foods, and the use of energy and capital intensive processes for food preservation. The technology of producing many indigenous fermented foods from cereals remains a household art in these countries (Chaven and Kadam 1989). Prospects for applying advanced technologies to indigenous fermented foods (Wood 1994) and for the production of value-added additive products, such as colors, flavors, enzymes, antimicrobials, and health products (Cook 1994) during food fermentations have been reviewed.
Special mention should be made of the microbiological risk factors associated with fermented foods. The safety of fermented foods has been recently reviewed (Nout 1994). Cases of food-born infection, and intoxications due to microbial metabolites such as mycotoxins, ethyl carbamate, and biogenic amines have been reported in fermented foods. Major risk factors include the use of contaminated raw materials, lack of pasteurization, and use of poorly controlled fermentation conditions. On the other hand, non-toxigenic microorganisms can serve to antagonize pathogenic microorganisms and even degrade toxic substances such as mycotoxins (Nakazato et al. 1990) in fermented foods.
Indigenous Fermented Cereal Foods
Most bacterial fermentations produce lactic acids; while yeast fermentation results in alcohol production. Many of the indigenous fermentation products of cereals are valued for the taste and aroma active components produced and are used as seasonings and condiments. A summary of flavor compounds formed in such products was compiled by Chaven and Kadam (1989). A number of fermented products utilize cereals in combination with legumes, thus improving the overall protein quality of the fermented product. Cereals are deficient in lysine, but are rich in cystine and methionine. Legumes on the other hand are rich in lysine but deficient in sulfur containing amino acids. Thus, by combining cereals with legumes, the overall protein quality is improved. The Chinese concept of "fan" (rice) and "tsai"(other vegetables) for a balanced and interesting diet is seen throughout the world (Campbell-Platt 1994).
Importance and Benefits of Fermented Cereals
Fermented foods contribute to about one-third of the diet worldwide (Campbell-Platt 1994). Cereals are particularly important substrates for fermented foods in all parts of the world and are staples in the Indian subcontinent, in Asia, and in Africa. Fermentation causes changes in food quality indices including texture, flavor, appearance, nutrition and safety. The benefits of fermentation may include improvement in palatability and acceptability by developing improved flavours and textures; preservation through formation of acidulants, alcohol, and antibacterial compounds; enrichment of nutritive content by microbial synthesis of essential nutrients and improving digestibility of protein and carbohydrates; removal of antinutrients, natural toxicants and mycotoxins; and decreased cooking times.
The content and quality of cereal proteins may be improved by fermentation (Wang and Fields 1978; Cahvan et al. 1988). Natural fermentation of cereals increases their relative nutritive value and available lysine (Hamad and Fields 1979) (Fig. 4). Bacterial fermentations involving proteolytic activity are expected to increase the biological availability of essential amino acids more so than yeast fermentations which mainly degrade carbohydrates (Chaven and Kadam 1989). Starch and fiber tend to decrease during fermentation of cereals (El-Tinay et al. 1979). Although it would not be expected that fermentation would alter the mineral content of the product, the hydrolysis of chelating agents such as phytic acid during fermentation, improves the bioavailability of minerals. Changes in the vitamin content of cereals with fermentation vary according to the fermentation process, and the raw material used in the fermentation. B group vitamins generally show an increase on fermentation (Chavan et al. 1989) (Fig. 5). During the fermentation of maize or kaffircorn in the preparation of kaffir beer, thiamine levels are virtually unchanged, but riboflavin and niacin contents almost double (Steinkraus 1994).
Reddy and Pierson (1994), reviewed the effect of fermentation on antinutritional and toxic components in plant foods. Fermentation of corn meal and soybean-corn meal blends lowers flatus producing carbohydrates, trypsin inhibitor and phytates (Compreeda and Fields 1981; Chompreeda and Fields 1984). However, fermentation of cereals with fungi, such as Rhizopus oligosporus, has been reported to release bound trypsin inhibitor, thus increasing its activity (Wang et al. 1972). Fungal and lactic acid fermentations have also been reported to reduce aflatoxin B1, sometimes by opening of the lactone ring which results in complete detoxification (Nout 1994).
Another benefit of fermentation is that frequently the product does not require cooking or the heating time required for preparation is greatly reduced (Steinkraus 1994).
Need for Additional Research
Some advantages of traditional fermentations are that they are labor-intensive, integrated into village life, familiar, utilize locally produced raw materials, inexpensive, have barter potential and the subtle variations resulting, add interest and tradition to local consumers. From this perspective, research leading to new fermentation technologies should be sensitive to social and economic factors in developing countries. Rapid displacement of traditional foodstuffs in developing countries with technologies developed in more affluent countries may result in centralised production, distribution problems, less local involvement in food processing, less employment in some areas, less nutritionally adequate substitutions in raw materials, displacement of traditional arts, loss of unique local know-how, dependence on importation of equipment and materials, initially require the use of outside consultants, and may otherwise not meet local needs as fully as traditional fermented products. On the other hand, indigenous fermentations may have a number of problems, i.e., they are uncontrolled and often unhygenic, labor intensive, seen as primitive by some people, are normally not integrated into the economic mainstream, difficult to tax, have limited export potential (Wood 1994) and in some cases, the impact on nutritive value and safety is questionable.
Specific microflora involved with indigenous fermentations is, in many cases, not known at this time. Specific information on microflora appears to be lacking for several indigenous fermented cereal products. The microbiology of many of these fermentations is undoubtedly quite complex. Many indigenous cereal fermentations involve the combined action of bacteria, yeast and fungi. Some microflora may participate in parallel while others may participate in a sequential manner with a changing dominant flora during the course of the fermentation. The specific microflora involved may vary somewhat from village to village and from family to family within the same village. The identification of specific microflora involved is needed to amplify and control such positive factors as the excretion of lysine by strains of Lactobacillus plantarum (Newman and Sands 1984) and the metabolic detoxification of mycotoxins by Rhizopus oryzae (Nout 1994); as well as to minimize or prevent negative factors such as growth and metabolism of pathogenic and toxinogenic bacteria, e.g., bongrek acid and toxoflavin formation by Pseudomonas cocovenenans (Ko 1985). Identifying and providing a practical means of using appropriate starter cultures is advantageous due to the competitive role of microorganisms and their metabolites in preventing growth and metabolism of unwanted microorganisms. A strong starter may reduce fermentation times, minimise dry matter losses, avoid contamination with pathogenic and toxigenic bacteria and molds, and minimize the risk of incidental microflora causing off-flavor, etc. According to Nout (1994) optimization of starter cultures may be achieved by either conventional selection and mutation, or by recombinant-DNA techniques to result in increased levels of safety. Relatively litle is known of the contribution of microflora to the formation of desired flavor notes during such fermentations. Genes for flavor and other beneficial enzymes that come from incidental microflora may be incorporated into starter bacteria to facilitate more subtle and ancillary aspects of the fermentation along with primary events such as lactic acid production, thus preserving the distinctive nature of products made in different regions.
The contribution of specific enzymes to indigenous cereal fermentations is perhaps even less understood than that of microorganisms. It is likely that there is considerable synergy between complimentary enzymes from the cereal itself and from the microorganisms. One known example of this is the reduction of phytates resulting from 6-phytases of cereal origin and 3-phytases of microbial origin (Reddy and Pierson 1994). Another is the synergy of cereal enzymes and yeast in bread making (Fox and Mulvihill, 1982). It is interesting that fermentation of cereals (e.g. breadmaking and brewing) in the Western world was adversely affected in some ways by the introduction of modern dehydration and storage techniques that minimized fungal contamination and incipient germination. Partial germination of cereal in the field and contamination with otherwise innocuous fungal contaminants contribute enzymes, notably a-amylase and proteases, that aid these fermentations. Today, essentially all beer production and continuous breadmaking in the West is achieved with the aid of added enzymes (Tucker and Woods 1995). A similar situation may occur in developing countries, i.e. as improvements in cereal handling are introduced to minimise postharvest losses and mycotoxin formation, the otherwise improved crop may be less suitable in some ways for traditional fermentations. Hence, basic information is need on the contribution of cereal enzymes and other constituents to indigenous fermentations. With this information in hand, consideration can then be given to use of enzyme supplements and other additives to improve the rate and quality of fermentations.
Another consideration for future research is the contribution of the aforementioned enzyme inhibitors in cereal fermentations. In addition to their already discussed significance as antinutrients in the finished product; protease, amylase and other enzyme inhibitors are expected to influence the rate and extent of important bioconversions that occur during indigenous fermentations. The concentration and spectrum of enzyme inhibitors varies considerably between cereal cultivars (Izquirdo-Pulido et al. 1994). Not withstanding the benefits of the continual introduction of new cereal varieties (Meikle and Scarisbrick 1994), given the genes for enzyme inhibitors are part of the defensive system of plants against insects and other pests, it is possible that introduction of "improved" cereal cultivars in developing countries may adversely affect the utility of cereals for indigenous fermentation. For this reason, basic research on the participation of cereal enzyme inhibitors in the process may provide useful insights on the need for including tests for inhibitors prior to introducing new varieties in areas that extensively utilise cereal fermentations to produce staple foodstuffs.
As pointed out by Wood (1994), there is a possible backlash if consumers in developing countries abandon traditional fermented foods for "smart," sophisticated products popularized in Europe and America. For example, the replacement of indigenous fermented cereal drinks with cola beverages could have a significant negative impact on daily nutrition of many consumers in developing countries. Study of traditional fermentations will undoubtedly yield new information that will expand our global knowledge of science and impact technology throughout the world. Thus, basic research of indigenous cereal fermentations will lead to "inward" as well as "outward" technology transfer.
Traditional fermentations are likely to remain an important part of global food supply; many may evolve into fermentations involving the use of starter cultures, enzyme additives and controlled environmental conditions, and others may benefit from genetic modification of the cereal or starter bacteria.
Further research should be directed towards identifying the benefits and risks associated with specific indigenous fermented cereals; elucidating the contributions of microorganisms, enzymes and other cereal constituents in the fermentation process; and developing starter cultures, unique microbial strains for nutritive improvement and detoxification, and testing of new cereal varieties for their suitability as fermentation substrates.
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