The Role of Microorganisms in Food
Blue-green algae of the genus Spirulina have been harvested from ponds and eaten for centuries by the ancient Aztecs in Mexico and Africans in the region of Lake Chad. Mushrooms, the fruiting bodies of microorganisms that live on decaying lignocellulosic compounds in soil, are highly prized as food by nearly all human societies, as well as by many animals, including insects. Fermentation plays several roles: (1) enrichment of the human diet through development of a wide diversity of flavors, aromas, and textures in food; (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. Protein content is often increased, as for example in Malaysian tape ketan and tape ketella by utilization of the carbohydrates, lowering their percentage and raising the percentage of protein in the food. Protein quality is also increased by the synthesis of essential amino acids such as lysine, first limiting amino acid in rice. In the Malaysian tape fermentation the content of lysine is raised, improving its protein quality. In the Indian idli fementation, it has been reported that methionine, the first limiting amino acid in many legumes, is increased from 10.6 to 60 percent. Highly polished rice is deficient in thiamine (vitamin B1), and consumption can lead to beriberi, a disease characterized by muscular weakness. In the Malaysian tape fermentation, thiamine content is raised to that of the original unpolished rice. In the Indonesian tempeh fermentation the content of riboflavin doubles, niacin increases seven-fold, and vitamin B12, which generally absent in vegetarian foods, is synthesized. In the African kafir beer fermentation, riboflavin doubles and niacin/nicotinic acid concentration nearly doubles. Mexican pulque, the oldest alcoholic beverage on the American continent, contains thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, and biotin that are of particular importance to the low income children of Mexico. There is much hunger, starvation, and malnutrition in parts of the world today, and the world population is predicted to reach eight to twelve billion by the year 2050. As world population increases, the supply of meat and other animal products available per person is likely to decrease. A large, capable research institute in England has developed a process in which edible mold mycelium is grown and used to provide protein and texture for meat analogues (substitutes) for the human diet.
Microbial protein can also be extracted from cells, and then concentrated, isolated, and spun or extruded to make meat substitutes. Although this would appear to be very advanced technology, the Indonesians for centuries have overgrown soaked, partially cooked soybean cotyledons with the mold Rhizopus oligosporus (as mentioned above), which knits the soybean cotyledons into a firm cake that can be sliced and deep-fat fried or used in chunks as a substitute for meat in soups. The protein content rivals that of meat and the cost is very low, within the means of the average Indonesian. Also, the microorganisms involved enrich the food with vitamin B12, increase niacin by a factor of seven, and double the riboflavin content. Among plants, the grasses are the most efficient fixers and utilizers of carbon dioxide, producing sugars, starches, and cellulose; they are also synthesizers of protein, using nitrogen from the soil. Grasses can double their cell-mass in two to three weeks. A 1000 kg harvest of grass can be repeated every two to three weeks. However, yeasts are much more efficient in this regard. A yeast (1000 kg) grown in tanks on limited land space can produce 168,000 kg of cells containing 84,000 kg of protein every two weeks. Bacteria are even more efficient: whereas yeasts can double their cell mass in about two hours, some bacteria can double their cell mass in twenty minutes. Still, 1000 kg of yeast growing in a suitable fermentor can produce 1000 kg of new cells for harvesting every two hours, with a daily production of 12,000 kg of cells containing 50 percent or 6000 kg of protein. (Molds generally grow more slowly, doubling their cell mass in four to six hours.) Since the protein content of bacterial cells may reach 80 percent (compared with 40 to 45 percent in soybeans, for example), there is no method of producing protein that can compete with microbial cells. Except for algae, microbes require energy sources such as sugars, starches, cellulose, or hydrocarbons—all derived originally from the sun's radiation. But they can utilize energy sources that humans cannot digest, such as cellulose and lignocellulose found in straw. As described earlier, mushrooms are a good example of such microorganisms: they produce delicious, edible food directly from straw and sugarcane bagasse. Only about twenty-five species of more than two thousand edible fungi are widely accepted as human food. The four most important mushrooms are the commonly cultivated white mushroom or button mushroom (Agaricus campestris), the black forest mushroom shiitake (Lentinus edodes), the straw mushroom (Volvariella volvacea), and the oyster mushroom (Pleurotus ostreatus).