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Renewable
Carbon
Sources - The renewable carbon sources include,
(i) CO2,
(ii) molasses,
(iii) whey,
(iv) cellulose hydrolysates,
(v) starch hydrolysates,
(vi) industrial effluents and
(vii) cellulosic wastes.
SCP production using these substrates is briefly described below.
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1. Carbon Dioxide (CO2). This substrate is utilized by algae, which derive the required energy from sunlight. Spirulina is grown on a commercial scale by Sosa
Texcoco Co., Mexico, producing up to 5 tons dry biomass per day. The blue-green alga is cultivated in 1 m deep ponds and is harvested by filtration, followed by vacuum filtration and is dried using drums. Chlorella is commercially grown in Japan and Taiwan to get about 15,000 tons/yr dry biomass from many small production units.
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2. Molasses. This substrate is used for alcoholic fermentation from which yeast biomass is obtained as a by-product. Traditionally, the baker's yeast (S. cerevisiae) and Torula yeast (c. utilise) are produced on this substrate. About 200,000 tons dry weight of S. cerevisiae is produced each year, which is used as inoculum for dough fermentation in bakeries.
3. Whey. Whey is the liquid portion of milk remaining after the curd is separated during cheese production. On dry weight basis, whey contains 70% lactose, 9-14% protein and 9% ash. About 75 million tons of whey is produced each year of which over 50% is released as effluent, which necessitates treatments to avoid pollution. Yeasts (Kluyveromyces fragilis, Candida krusei in combination with Lactobacillus bulgaricus. and Candida intermedia) are the most widely used for-commercial scale production of SCP from whey. For example, Fromageries Le Bel, France produces about 2,300 tons of SCP/ yr using K. fragilis; the SCP is being used as human food supplement for about 2 decades.
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4. Cellulose Hydrolysate. Cellulase obtained from fungi like Trichoderma viride has been used to hydrolyse cellulase and to produce glucose. The main advantage of this approach is, the large quantities of cheap substrate. But the chief problems are
(i) difficult and expensive process for complete hydrolysis of lignocellulose,
(ii) the amount of pretreatment necessary, and
(iii) the extent of sugar destruction and by-product formation during chemical hydrolysis. The best utilization of cellulosic substrates appears to be for mushroom production and for semisolid fermentation systems (item 7).
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5. Starch Hydrolysate. This rather expensive substrate is used for SCP production using Fusarium graminearum. The SCP has a fibrous, meat-like texture, is marketed as ‘Mycoprotein', is approved for human use in U.K., and food products having Mycoprotein are being marketed. The fungus grows at 30°C on a variety of mono- and oligosaccharides, is recovered by vacuum filtration, and the recovered biomass is held at 60°C for 20 min to activate the native RNase to reduce the RNA content of SCP to about 1 % of dry weight.
6. Industrial Effluents. Effluents from many industries e.g., breweries, distilleries, confectionery industries, potato and canning industries, sulphite liquor from wood pulp mills, etc., contain large amounts of carbohydrates and other organic compounds, which can be used for SCP production. Some of these are already being used for the purpose. For example, Basett Ltd., U.K. uses C. utilise to treat 140,000 1 confectionery effluent/day by continuous fermentation and produces 1.5 tons of dry yeast/day. In addition, the biochemical oxygen demand (BOD) of the effluent is reduced by 81 %.
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Sulphite liquor is widely used for SCP production in North America, Europe and erstwhile Soviet Union. The S02 is first removed from the liquor, pH is adjusted to 4.5 and inoculation is done with the selected microorganism. The most advanced process is based on the fungus Paecilomyces varioti, which is being used on an industrial scale in Finland. The fungus utilizes both pentoses and hexoses, and the acetic acid present in the liquor. It is easily recovered by filtration; the SCP has 52-57% crude protein, and is used as animal feed.
7. Cellulosic Wastes. A new approach to use agricultural and forestry cellulosic wastes, e.g, straw, bagasse, sawdust, etc., for SCP production uses semisolid fermentation. The cellulosic material is pretreated thermally and chemically, and is then fermented with the fungus Chaetomium cellulolyticum. The fungus is reported to degrade the cellulosic wastes in very short time, e.g., 4 hr. This promising approach may facilitate an efficient and economic utilization of cellulosic wastes for SCP production.
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