5.2.8.2 Vinegar
Vinegar is prepared by reducing the strength of acetic acid by diluting it with water. The completed vinegar must contain a minimum of 4.3 g acetic acid/100 ml (Woolfe, 1992). Recently, a new red vinegar was developed in Japan via fermentation of purple fleshed sweet potato cv. Ayamurasaki. The developed red vinegar had a higher antioxidant activity than white and black vinegars. The red vinegars contained some new compounds, probably derived from the purple fleshed sweet potato. A major component was isolated using preparative HPLC (High Performance Liquid Chromatography) and the chemical structure was determined to be 6-O-(E)-Caffeoyl-(2-0-β-d-glucopyranosyl)-α-d-glucopyranose (Caffeoylsophorose) by mass spectroscopy and NMR (Nuclear Magnetic Resonance). As the caffeoyl-sophorose shows a high antioxidant activity, it plays an important role in red vinegar like anthocyanins and other components (Terahara et al., 2003).
5.2.8.3 Soy Sauce
Soy sauce, a popular condiment used every day with Asian dishes, is traditionally prepared from a mixture of soybeans and wheat, fermented by moulds, especially Aspergillus oryzae or A. sojae, to give a dark brown salty liquid used as a flavouring agent. Sweet potato flour can replace wheat flour for soy sauce production (Ray and Ward, 2006).
5.2.8.4 Monosodium Glutamate (MSG)
MSG is an important flavour enhancer of a wide range of savoury foods. China is the largest producer and consumer of MSG in the world. The starch has first to be degraded to sugars, which are then converted by microorganisms such as Brevibac-terium glutamicum to glutamic acid. This is then converted to MSG salt (Jiang et al., 1993). China uses sweet potato starch as one of the raw materials for production of MSG. In Sichuan Province in China, it is the fifth-most important product from sweet potato, almost equal in tonnage to citric acid.
5.2.8.5 Microbial Polysaccharides (Gums)
Root crops bagasse (residues after starch extraction) serves as a substrate for production of microbial exopolysaccharides, which have a number of uses in brewing and food industries as a thickener and an emulsifier. Cassava bagasse and cassava roots have been used to produce microbial polysaccharides like pullulan (by Aureobasidum pullulans) and xanthan gums (by the bacterium, Xanthomonas campestris) and flour (Ray and Moorthy, 2007; Selbmann et al., 2002).
5.2.8.6 Biological Enrichment and Safety of Fermented Foods
Food fermentations, in general, raise the protein content or improve the balance of essential amino acids, or their availability has a direct curative effect (Steinkraus, 1997). Similarly, fermentation increases the vitamins such as thiamine, riboflavin, niacin and folic acid that can have a profound direct effect on the health of the consumers of such foods. Most of the food fermentations of roots and tubers are associated with LAB such as Lactobacillus, Leuconostoc, Streptococcus, etc. and yeasts (S. cerevisiae), which are commonly considered as “probiotics” (Agrawal, 2005). The probiotics concept has been defined by Fuller (1989) to mean “alive microbial food/feed supplements, which beneficially affect the host animal by improving its intestinal microbial balance”. Examples of health benefits associated with the consumption of probiotics include a decrease in rotavirus shedding in infants, reductions in antibiotic-associated diarrhoea, reduction in the incidence of childhood atopic eczema, and management of inflammatory bowel diseases such as Crohn’s disease (Kearney et al., 2008). Fermented foods like gari, fufu, lafun, etc., are considered as functional foods, as these are rich in dietary fibres, vitamins and essential minerals, as well as LA (Oyewole and Ogundele, 2001). Fermented foods generally have a very good safety record, even in the developing world, where the foods are manufactured by people without training in microbiology or chemistry. Cassava- or sweet potato-based fermented foods are prepared mostly in households of the African and Asian continents, in a comparatively unhygienic environment, as compared to the developed world. These are consumed by hundreds of millions of people every day, and yet they have a high safety record (Oyewole and Ogundele, 2001; Ray and Ward, 2006).
Foods enriched with β-carotene (precursor of vitamin A), lutein and anthocyanin (anti-oxidants) pigments and/or with vitamins, polyphenols, structural lipids and dietary fibres are also designated as functional foods (Agrawal, 2005), which have several health attributes like anti-aging, anti-cancer, anti-immunodilation, protection against cataracts, muscular degeneration and liver injury (Kaur and Kapoor, 2001). Fermented foods (p-carotene/anthocyanin rich lacto-pickles) and beverages like wine and lacto-juice prepared from purple-fleshed sweet potato can be designated as functional foods.
5.3 Summary and Future Perspectives
Fermented foods from roots and tubers provide and preserve vast quantities of nutritious health foods in a wide variety of flavours and textures, which enrich the human diet. Fermented foods from cassava, yams and sweet potato are part of the regular diets of people in Africa, Latin America and Asian continents. Suitable technology of microbial safety and packaging can facilitate their distribution to every corner of the world. This is likely to expand in the 21st century, when the world population reaches 8-12 billion.
The bulk of cassava in Africa and Latin America is bio-processed into fermented foods and food additives such as organic (acetic, citric, lactic, formic, propionic) acids, monosodium glutamate, etc. The fermented foods from cassava are gari, fufu, lafun, chickwanghe, agbelima, attieke and kivunde in Africa, tape in Asia, and cheese bread and coated peanuts in Latin America. Lactic acid bacteria and yeasts are the major group of microorganisms associated with cassava fermentation. In Africa and Latin America, root crops such as cassava are still the preferred food, and fermentation will probably remain the most important mode of processing of such crops into edible foods, feeds and food additives.
In China, sweet potato is a major crop, which is processed into many fermented food products like MSG and edible alcohols. Similarly, sweet potatoes can be fermented into soy sauce, vinegar, lacto-juices, lacto-pickles, yoghurt, wine, beer and sochu (an alcoholic drink produced in Japan), and yams into fermented flour. Up-scaling and commercialization of value-added products (wine, beer, vinegar, yoghurt, pickles, gari) from root crops via fermentation can improve the socio-economic status of several communities on the Asian and African continents. Viewing the production and nutritional quality of root crops throughout the world, there is ample scope for setting up industries for production of novel fermented foods from tropical roots and tubers.
References
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Achi, O.K. (1999) Quality attributes of fermented yam flour supplemented with processed soy flour. Plant Foods for Human Nutrition, 54: 151–158.
Achi, O.K. and Akubor, P.I. (2000) Microbiological characterization of yam fermentation for “Elubo” (yam flour) production. World Journal of Microbiology and Biotechnology, 16: 3–8.