MICROBIAL BIOTECHNOLOGY: SCOPE, TECHNIQUES CONTD
The term single-cell protein, or SCP, describes the protein-rich cell mass derived from microorganisms grown on a large scale for either animal or human consumption. SCP has a high content of protein containing all the essential amino acids. Microorganisms are an excellent source of SCP because of their rapid growth rate, their ability to use very inexpensive raw materials as carbon sources, and the uniquely high efficiency, expressed as grams of protein produced per kilogram of raw material, with which they transform these carbon sources to protein. In spite of these advantages,only one SCP product approved for human consumption has reached the market. This product is “mycoprotein,” the processed cell mass preparation from the filamentous fungus Fusarium venenatum. We consider here the positive nutritional properties of this product and examine the many concerns that needed to be examined and addressed before this product gained regulatory approvals. The source organism,F. venenatum strain PTA-2684, was cultured
from a soil sample obtained from Buckingham shire, United Kingdom. Marlow Foods Ltd. chose this strain
of F. venenatum from more than 3000 organisms obtained from around the world. The manufacturing process for mycoprotein is designed to ensure the absence of undesirable constituents of fungal cells from the final product .
F. venenatum is grown with aeration under steady-state conditions maintained by continuous feed of nutrient medium and concomitant removal of the culture. These fermentation conditions were chosen to prevent the production of the highly toxic mycotoxins. Fusarium species produce trichothecene and fusarin mycotoxins when growth is limited by nutrient limitation, a high ratio of carbon to nitrogen nutrients, low oxygen tension, or the lack of a micro-nutrient. To prevent mycotoxin synthesis, the production strain is grown at a high rate without any nutritional limitations. The culture is supplied with a nutritionally balanced, chemically defined fermentation medium, with glucose as the sole carbon source. The medium is provided at a rate that allows the cells to grow at a specific rate of at least 0.17 per hour. To monitor the levels of mycotoxins, the final product is analyzed for these compounds by high-performance liquid chromatography with mass spectrometric detection. The detection limits per kilogram wet weight of product are 2µg for
individual trichothecenes and 5µg for fusarin mycotoxins. With these sensitivity levels, no mycotoxins are detected in the final product.
Rapidly growing bacterial and fungal cells are rich in RNA. RNA in the diet is broken down into purines and pyrimidines. Purines are converted to uric acid and add the serum uric acid derived from the metabolism
of endogenous purines. Elevated uric acid increases the risk of developing out and kidney stones in susceptible individuals. To address this problem, a United Nations Protein Advisory Group recommended in 1972 that SCPs intended for human consumption provide no more than 2 g of RNA per day. The fermentation broth containing the fungal biomass removed from the fermentor is rapidly by injection of steam.
The rapid heating process kills the cells, with concomitant degradation of RNA. The fermentation
broth is subsequently separated from the cell mass by centrifugation, and the RNA degradation products are discarded with the supernatant. These steps reduce the content of RNA in the cell mass from about 10% in viable cells to about 0.5% to a maximum of 2% in mycoprotein on a dry weight basis. With estimated limits of dietary intake of mycoprotein of 17 to 33 g/person/day on a dry weight basis, the intake of RNA from consumption of mycoprotein would range from 0.35 to 0.7 g/person/day, well below the level recommended by the United Nations Protein Advisory Group. Animal studies have shown that mycoprotein does
not cause chronic toxicity, is not a reproductive toxicant, is not a teratogen, and is not carcinogenic. It does
not interfere with the absorption of calcium, iron, or other essential inorganic nutrients. Marlow Foods Ltd.reported that mycoprotein is much less allergenic in humans than are many commonly consumed foods,
such as those containing shellfish or peanuts. Anecdotal reports hint at higher numbers of adverse reactions.
Mycoprotein has been commercially available in the United Kingdom since 1985, in other countries in
Europe since 1991, and in the United States since 2002. Products marketed in Europe include meat-free burgers and fillets and prepared meals, such as stir-fries, curries, and pasta dishes, in which mycoprotein is the central component. In the United Kingdom and Europe, the acceptance of mycoprotein as a meat substitute in a wide variety of foods has been significant,with a reported 15 million customers. The story of mycoprotein illustrates the long road of regulatory approvals and customer acceptance that a new SCP product must travel.
NOTE: Mycotoxins are synthesized by Fusarium species as well as by members of other
genera of filamentous fungi, such as Aspergillus and Penicillium. Mycotoxins are products of fungal secondary metabolism. Thus, they are not essential to the energy producing or biosynthetic metabolism of the fungus, or to fungal reproduction. Rather, under growth-limiting or stress conditions, they appear to give the fungus an advantage over other fungi and bacteria with which it may be competing. Mycotoxins are nearly all cytotoxic. They disrupt cell membranes and interfere with protein, RNA, and DNA synthesis. Their toxicity extends beyond microorganisms to the cells of higher plants and animals, including humans.
Fusarium species produce different classes of mycotoxins, trichothecenes and fusarins. Deoxynivalenol, also known as vomitoxin, is one of about 150 related trichothecene compounds that are formed by a number of species of Fusarium and some other fungi. Deoxynivalenol is nearly always formed before harvest when crops are invaded by certain species of Fusarium closely related to Fusarium venenatum. These Fusarium species are important plant pathogens that cause heat blight in wheat. Deoxynivalenol is heat stable and persists in stored grain.
Cited By Kamal Singh Khadka/ Shailendra Parajuli
Msc Microbiology, TU.
Assistant Professor In PU, PBTPC,PNC, LA, NA.
Fig: Fusarium Spp