The considerations raised in the previous section have meant that SSF technology has been used for many centuries. Some examples of traditional SSF processes are:
• tempe, which involves the cultivation of the fungus Rhizopus oligosporus on cooked soybeans. The fungal mycelium binds the soybeans into a compact cake, which is then fried and eaten as a meat substitute. This fermented food is quite popular in Indonesia;
• the koji step of soy sauce manufacture, which involves the cultivation of the fungus Aspergillus oryzae on cooked soybeans. During the initial SSF process of 2 to 3 days, the fungal mycelium not only covers the beans but also secretes a mixture of enzymes into them. The fermented beans are then transferred into brine, in which, over a period of several months, the enzymes slowly degrade the soybeans, leaving a dark brown sauce.
• ang-kak, or "red rice", which involves the cultivation of the fungus Monascus purpureus on cooked rice. The fungus produces a dark red pigment. At the end of the fermentation the red fermented rice is dried and ground, with the powder being used as a coloring agent in cooking.
Beyond this, over the last three decades, there has been an upsurge in interest in SSF technology, with research being undertaken into the production of a myriad of different products, including:
• enzymes such as amylases, proteases, lipases, pectinases, tannases, cellulases, and rennet;
• aromas and flavor compounds;
• "small organics" such as ethanol, oxalic acid, citric acid, and lactic acid;
• gibberellic acid (a plant growth hormone);
• protein-enriched agricultural residues for use as animal feeds;
• animal feeds with reduced levels of toxins or with improved digestibility;
• antibiotics, such as penicillin and oxytetracycline;
• biological control agents, including bioinsecticides and bioherbicides;
• spore inocula (such as spore inoculum of Penicillium roqueforti for blue cheese production).
There is also research into the use of microorganisms growing in SSF conditions to mediate processes such as:
• decolorization of dyes;
These processes commonly use waste products or byproducts of agriculture and food processing, selected as appropriate to favor growth of the producing organism and formation of the desired product. Such wastes and byproducts include wheat bran, rice bran, oil-press cakes, apple pomace, grape pomace, banana peels, citrus peels, wheat straw, rice straw, coffee pulp, citrus pulp, sugar beet pulp, coffee husk, and sugar beet molasses. Sometimes higher-value agricultural and food materials are used, such as granular milk curds, fodder beets, rice, and cassava meal. Recently there has also been some interest in the use of inert supports impregnated with nutrient solutions; at times natural inert supports such as sugar cane bagasse have been used, at other times artificial supports have been used, such as polyurethane foam cubes.
Note that the list presented above highlights only a small proportion of the overall activity in the development of SSF processes. Various reviews have been published on the applications of SSF, including details of the organisms and substrates used and the current chapter does not intend to repeat the information presented in these reviews. Readers with further interest should consult the reference section at the end of the chapter.
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