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SINGLE CELL PROTEIN (SCP)

Single cell proteins or microbial proteins refers to the microbial cells or total protein extracted from pure microbial cell culture (monoculture) which can be used as protein supplement for humans or animals. The name was introduced by Prof.Scrimshow of MIT in 1967. Microorganisms like algae, fungi, yeast and bacteria, utilize inexpensive feed stock and wastes as sources of carbon and energy for growth to produce biomass, protein concentrate or amino acids. SCP may also be produced entirely independent of agricultural waste products through autotrophic growth.In addition to proteins, SCP contains other nutrients, such as lipids and vitamins. The proteins are called as single cell proteins because most of the microorganisms used as producers grow as single or filamentous individuals.It is estimated that about 25% of the world’s population currently suffers from hunger and malnutrition.Therefore, SCP deserves a serious consideration for its use as food or feed supplement. In addition to its utility as a nutritional supplement, SCP can also be used for the isolation of several compounds e.g. carbohydrates, fats, vitamins, minerals.. Pruteen was the first commercially produced SCP which was as animal feed. Pruteen was produced from bacteria Methylophilus methylotrophus cultured on methanol and had 72% protein content.

Raw Materials

• Production of SCP requires micro-organisms that serve as the protein source and the substrate that is biomass on which they grow.
• The biomass used can be plant biomass or organic biomass.
• The micro-organisms used belong to the group of algae, fungi and bacteria.

Microorganisms

Microbes used for production of single cell proteins include:

• Yeast
• Saccharomyces cerevisiae
• Pichia pastoris
• Candida utilis
• Torulopsis corallina
• Geotrichum candidum
• Fungi (Mycoprotein)
• Aspergillus oryzae
• Fusarium venenatum
• Sclerotium rolfsii
• Polyporus sp.
• Trichoderma sp.
• Scytalidium acidophilum
• Bacteria
• Pseudomonas fluroescens
• Aeromonas hydrophylla
• Lactobacillus sp.
• Algae
• Spirulina sp. (dietary supplement)
• Chlorella sp.

Biomass

Biomass plays a very important role in the production of SCP. Selection of biomass depends on the micro-organisms used for production. Algae are cultivated on sewage waters whereas yeast are cultivated on agro-industrial wastes. Algal Biomass: Algae grows autotropically. It requires low intensity of light, the temperature of 35-40°C and pH of 8.5-10.5 is maintained. It is cultivated in large trenches of sewage oxidation ponds. Bacterial and Fungal Biomass: Bacteria and Fungi can be grown easily on a wide range of substrates. They require a minimum temperature of 15-34°C and a pH of 5-7 to grow on substrate. Yeast Biomass: Yeast is cultivated on agro-industrial wastes such as molasses, starchy materials, fruit pulp, wood pulp etc. It requires a temperature of 30-34°C and pH of 3.5-4.5 for their growth. It also requires addition of inorganic acids and sulphur supplements in the form of salts.

SCP Production

Bioreactor

The process of SCP production from any microorganism or substrate using a bioreactor would have the following basic steps:

• The selection of certain microbial strain is very important, some of the criteria are:

1. Performance (growth rate, productivity, yield) on the specific. preferably low-cost substrates to be used.
2. Temperature and pH tolerance.
3. Oxygen requirements, heat generation during fermentation and foaming characteristics.
4. Growth morphology and genetic stability in the fermentation.
5. Ease of recovery, and requirements for further downstream processing
6. Structure and composition of the final product, in terms of protein.

• Fermentation: The reactions are carried out in the fermentor which is equipped with aerator, thermostat, pH. microbes are cultured in fed batch culture.
• Harvesting: When the colonies of microbes are fully developed,they are then harvested. The bulk of cells are removed the the fermentor by decantation.
• Post harvest treatment: After harvesting, the cells are subjected to a variety of processes. Post harvesting treatments includes steps like separation by centrifugation, washing, drying etc. Further processing is done which includes liberation of cell proteins by destruction of indigestible cell wall by mechanical,chemical or physical methods and reduction of nucleic acid content through chemical and enzymatic treatments.

In short, the steps involved for the production of SCP are:

1. Provision of a carbon source; it may need physical and/or chemical pretreatments.
2. Addition, to the carbon source, of sources of nitrogen, phosphorus and other nutrients needed to support optimal growth of the selected microorganism.
3. Prevention of contamination by maintaining sterile or hygienic conditions. The medium components may be heated or sterilized by filtration and fermentation equipments may be sterilized.
4. The selected microorganism is inoculated in a pure state.
5. SCP processes are highly aerobic (except those using algae). Therefore, adequate aeration must be provided. In addition, cooling is necessary as considerable heat is generated.
6. The microbial biomass is recovered from the medium.
7. Processing of the biomass for enhancing its usefulness and/or storability.

It has been calculated that 100 lbs of yeast will produce 250 tons of proteins in 24 hours, whereas a 1000 lbs of steer(castrated male bovine) will synthesize only 1 lb of protein in 24 hours and by consuming 12 to 20 lbs of plant proteins. Similarly, algae grown in ponds can produce 20 tons (dry weight) of protein, per acre, per year.

However, the main problem of SCP production is the relatively high cost in the downstream processing and marketing SCP as food. Using waste materials as substrate provides lower yield of biomass than using more defined substrates, many processes are required to increase the biomass concentrations including centrifugation, flotation, precipitation, coagulation and filtration, or the use of semi-permeable membranes. SCP's are treated with various chemicals to kill the microbes, increase the digestibility, and to reduce the nucleic acid content. These processes require extra costs which may exceed the costs for conventional food production. The removal or reduction of nucleic acid content of various SCP's is achieved with one of the following treatments:

• chemical treatment with NaOH;
• treatment of cells with 10% NaCl;
• thermal shock.

These methods aim to reduce the RNA content from about 7% to 1% which is considered within acceptable levels.

Advantages

• Rapid successions of generations.
• Cellular, molecular and genetic alterations cab be easily done.
• High protein content of 43-85% in dry mass.
• Broad spectrum of original raw material are used for production, which also includes waste products.
• Production is done in continuous cultures.
• Consistent quality, not dependent on climate.
• Low land requirements, economically beneficial.
• Utilization of solar energy

Disadvantages

• High content of nucleic acids leading to elevated levels of uric acid.
• Development of kidney stone and gout if consumed in high quality.
• Possibility for the presence of secondary toxic metabolites.
• Poor digestibility.
• Hypersensitivity skin reactions.

Applications

Used as protein supplemented food, it's also a source of vitamins, amino acids, minerals and crude fibers, Used in therapeutic and natural medicines, used in cosmetics, used as poultry and cattle feed.