Single-cell protein (SCP) typically refers to sources of mixed protein extracted from pure or mixed cultures of algae, yeasts, fungi or bacteria (grown on agricultural wastes) used as a substitute for protein-rich foods, in human and animal feeds.
Since 2500 BC, yeasts have been used in bread and beverage production. In 1781, processes for preparing highly concentrated forms of yeast were established.
"Food from oil"
In the 1960s, researchers at British Petroleum developed what they called "proteins-from-oil process": a technology for producing single-cell protein by yeast fed by waxy n-paraffins, a product produced by oil refineries. Initial research work was done by Alfred Champagnat at BP's Lavera Oil Refinery in France; a small pilot plant there started operations in March in 1963, and the same construction of the second pilot plant, at Grangemouth Oil Refinery in Britain, was authorized.
The "food from oil" idea became quite popular by the 1970s, with Champagnat being awarded the UNESCO Science Prize in 1976, and paraffin-fed yeast facilities being built in a number of countries. The primary use of the product was as poultry and cattle feed.
The Soviets were particularly enthusiastic, opening large "BVK" (belkovo-vitaminny kontsentrat, i.e., "protein-vitamin concentrate") plants next to their oil refineries in Kstovo (1973) and Kirishi (1974). The Soviet Ministry of Microbiological Industry had eight plants of this kind by 1989, when, pressured by the environmentalist movements, the government decided to close them down, or convert to some other microbiological processes.
Single-cell proteins develop when microbes ferment waste materials (including wood, straw, cannery, and food-processing wastes, residues from alcohol production, hydrocarbons, or human and animal excreta). The problem with extracting single-cell proteins from the wastes is the dilution and cost. They are found in very low concentrations, usually less than 5%. Engineers have developed ways to increase the concentrations including centrifugation, flotation, precipitation, coagulation, and filtration, or the use of semi-permeable membranes.
The single-cell protein must be dehydrated to approximately 10% moisture content and/or acidified to aid in storage and prevent spoilage. The methods to increase the concentrations to adequate levels and the de-watering process require equipment that is expensive and not always suitable for small-scale operations. It is economically prudent to feed the product locally and soon after it is produced.
Microbes employed include yeasts (Saccharomyces cerevisiae, Pichia pastoris, Candida utilis=Torulopsis and Geotrichum candidum (=Oidium lactis)), other fungi (Aspergillus oryzae, Fusarium venenatum, Sclerotium rolfsii, Polyporus and Trichoderma), bacteria (Rhodopseudomonas capsulata). and algae (Chlorella and Spirulina). Typical yields of 43 to 56%, with protein contents of 44% to 60%.
The fungus Scytalidium acidophilum grows at below pH 1, offering advantages of:
- low-cost aseptic conditions
- avoiding over 100-fold dilution of the acidic hydrolysates to pH values needed for other microbes
- after the biomass is harvested, the acids can be reused.
Product Safety and Quality
Microbial biomass has a high nucleic acid content, and levels must be limited in the diets of monogastric animals to <50 g per day. Ingestion of purine compounds arising from RNA breakdown leads to increased plasma levels of uric acid, which can cause gout and kidney stones. Uric acid can be converted to allantoin, which is excreted in urine. Nucleic acid removal is not necessary from animal feeds but is from human foods. A temperature hold at 64°C inactivates fungal proteases and allows RNases to hydrolyse RNA with release of nucleotides from cell to culture broth.
Advantages of Production
Large-scale production of microbial biomass has many advantages over the traditional methods for producing proteins for food or feed.
- Microorganisms have a high rate of multiplication and, hence, rapid succession of generations (algae: 2–6 hours, yeast: 1–3 hours, bacteria: 0.5–2 hours)
- They can be easily genetically modified for varying the amino acid composition.
- A very high protein content 43–85% in the dry mass.
- They can utilize a broad spectrum of raw materials as carbon sources, which include even waste products. Thus, they help in the removal of pollutants also.
- Strains with high yield and good composition can be selected or produce relatively easily.
- Microbial biomass production occurs in continuous cultures and the quality is consistent, since the growth is independent of seasonal and climatic variations.
- Land requirements is low and is ecologically beneficial.
- A high solar-energy-conversion efficiency per unit area.
- Solar energy conversion efficiency can be maximized and yield can be enhanced by easy regulation of physical and nutritional factors.
- Algal culture can be done in space that is normally unused and so there is no need to compete for land.
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- National Research Council (U.S.). Board on Science and Technology for International Development (1983). Workshop on Single-Cell Protein: summary report, Jakarta, Indonesia, February 1–5, 1983. National Academy Press. p. 40.
- Soviet Plant to Convert Oil to Protein for Feed; Use of Yeast Involved, By THEODORE SHABAD. the New York Times, November 10, 1973.
- RusVinyl – Summary of Social Issues (EBRD)
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- KIRISHI: A GREEN SUCCESS STORY? (Johnson's Russia List, Dec. 19, 2002)
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