Single Cell Protein

 Single Cell Protein

Single Cell Protein (SCP) are protein derived from microorganisms.  The biomass or protein extract from pure or mixed cultures of algae, yeasts, fungi or bacteria is used as an ingredient or a substitute for protein-rich foods.  This is suitable for human consumption and as animal feeds.

It emerged in the 1950s and 1960s as an alternate and unconventional source of food to bridge the ‘food gap’ between the industrialized and the less industrialized parts of the world, especially as a protein source.

SCP has a number of attractive features:

·         It is not subject to weather variations and can be produced throughout year

·         Microorganisms have a much more rapid growth than plants or animals.

·         Waste products can be turned into SCP.

Disadvantages of SCP

·         Lack of expertise and/or the financial resources to develop fermentation industries in developing countries, where protein malnutrition exists

·         Microorganisms contain high levels of RNA, consumption of which lead to uric acid accumulation, kidney stone formation and gout.

During the First World War, Saccharomyces cerevisiae, were grown on a molasses-ammonium medium. Geotrichum lactis, Endomyces vernalis, and Candida utilis were grown for food.

A wide variety of substrates are used for SCP production such as hydrocarbons, alcohols, and wastes from various sources.

Hydrocarbons

a. Aliphatic hydrocarbons are assimilated by strains of yeasts while other classes of hydrocarbons, including aromatics are not efficiently assimilated.

b. n-Alkanes of chain length shorter than n-nonane are not assimilated and Yield factors increase with increasing chain length.

c. Unsaturated compounds are degraded less readily than saturated ones and branched chain compounds are degraded less readily than straight chain compounds.

Among the gaseous hydrocarbons, methane has been most widely studied as a source of SCP and propane and butane are also studied.   Single cell protein production from methane use continuous cultures and a mixed population of microorganisms and the advantages are higher growth rate, higher yield coefficient, greater resistance to contaminations and a reduction in foaming.

The four-organism mixture is a fast growing mixture - the unnamed methane bacterium utilizes methane and produces methanol, Hyphomicrobium utilizes the methanol and Flavobacterium and Acinetobacter remove waste products.

The major source of liquid hydrocarbons is crude petroleum which is highly variable in composition. The petroleum hydrocarbons which are used to grow SCP are diesel oil, gas oil, fuel oil, n-alkanes (C10 - C30 and C14 – C18, C11 – C18, C10 - C18) n-hexadecane n-dodecane.

Due to the crude oil price rise, the use of crude oil as a substrate for SCP is on a decline.

Methanol is suitable as a substrate for SCP for the following reasons:

(a) it is highly soluble in water

(b) the explosion hazard of methanol is less

(c) it is readily available in a wide range of hydrocarbon sources

(d) it can be readily purified

(e) it requires less oxygen than methane for metabolism by micro-organisms

(f) it is not utilized by many organisms.

Several companies in Italy, West Germany, Norway, Sweden, Israel, the United Kingdom, and the United use methanol as a SCP substrate. Example is Imperial Chemical Industries (ICI) in UK which use the bacterium, Methylophilus methylotropha to produce ‘Pruteen’ using the loop fermentor.

Hansenula, Pichia, Torulopsis and Candida grow on methanol.

Ethanol can be utilized by many bacteria and yeasts and as a substrate for SCP, it is used by yeasts. Ethanol has the following advantages:

(a) It is like methanol, highly miscible with water

(b) Can be more safely stored and transported

(c) Unlike methanol, it is non-toxic it can be more easily handled

(d) Ethanol is partially oxidized, requires less oxygen

The major disadvantage in using ethanol for SCP production is that it is expensive

Candida utilis. Hansenula anomala, Acinetobacter caloaceticum grow using ethanol

Waste Products

Due to the hike in petroleum prices, substrates derived from plants which are renewable are used as substrate for SCP.

(i) Plant/wood wastes: corn cobs, plant stems, leaves, stalks, husks, etc  are cellulose containing materials. Pretreatment such as ball-milling, acid, alkali, sodium chlorate or liquid ammonia treatment is needed to make cellulose susceptible to fermentation and lignin must be broken down. 

(ii) Starch-wastes: Starch-containing wastes from rice, potatoes, or cassava industry can be used for SCP production. Starch hydrolysis is relatively easy.  In Symba Process developed by the Swedish Sugar Corporation, two yeasts are used symbiotically - Endomycopsis fibuligera hydrolyses starch to glucose and maltose and Candida utilis utilizes these sugars.

(iii) Dairy wastes: Whey, by-product of diary industry is liquid rich in lactose.  Saccharomyces fragilis is grown in it to produce either SCP or alcohol.

(iv) Wastes from chemical industries: C. lipolytica or Trichosporon cutaneum can be used for SCP production in oxanone water, a waste mixture of organic acids from the copralactam used for the manufacture of nylon.

(v) Miscellaneous substrates: Molasses, the by-product of the sugar industry is used for production of SCP.  Coffee wastes, coconut wastes, palm-oil wastes, citrus waste, etc can also be used.

Microorganisms used in SCP production

Organisms to be used in SCP production should have the following properties:

(a) Absence of pathogenicity and toxicity

(b) Protein quality and content should be high

(c) Digestibility and organoleptic qualities

(d) Must grow rapidly in a cheap, easily available medium.

(e) Adaptability to unusual environmental conditions such as low pH conditions or high temperature

The heterotrophic microorganisms currently used are bacteria and fungi.  Protozoa are not used in SCP production. The gaseous hydrocarbons are used by bacteria and liquid hydrocarbons and alcohols are utilized by both bacteria and yeasts. Cellulose in peanut shells, carob beans, spoiled fruits, corn and pea wastes, sugarcane bagasse, palm, cassava wastes are used to make SCP using Trichoderma sp., Glicladium sp., Geotrichum sp., Fusarium, and Aspergilus.  Fungi are lower in RNA content and are easily harvested.

Autotrophic organisms such as photosynthetic bacteria and algae are used as SCP. The disadvantage with photosynthetic bacteria is that they require anaerobic conditions for photosynthesis which is difficult to provide and maintain.

Algae have high protein concentration, greater than soya bean and dietary energy from algae is higher than that of sugar beet, corn and potato.  For high algal yields carbon dioxide is supplied to algae growing in day light.  Where saline water rich in bicarbonates is available, supplementation with CO2 is not necessary.  Effluents from sewage treatment are ideal for growing algae for animal feed, where the algae should be heat-treated to avoid any possibility of pathogen transmission.

Algal cultivation is easier and it is highly digestible by ruminants and other animals.

Microbes employed include:

Yeast - Saccharomyces cerevisiae, Pichia pastoris, Candida utilis, Torulopsis coralline, Geotrichum candidum

Fungi - Aspergillus oryzae, Fusarium venenatum, Sclerotium rolfsii, Polyporus, Trichoderma

Bacteria - Methanomones sp., Methylococius capsulatus, Pseudononas sp., Flavobacterium sp. Arthrobacter simplex, Nocardia paraffinica, Nocardia paraffinica, Rhodobacter capsulatus, Rhodopseudomonas glatinosa

Algae – Spirulina, Chlorella

Concerns of using SCP

Due to the novelty of SCP as food receives strong opposition especially in Japan and Italy where the government is concerned with the possibility of the presence of carcinogenic compounds in petroleum-grown SCP, content of nucleic acid in SCP, the polycyclic aromatic hydrocarbons and the presence of n-paraffins, etc

Protein Advisory Group (PAG) formed by WHO in 1955 concluded that low levels of residual alkanes, the presence of odd-number fatty acids, or polycyclic hydrocarbons derived from petroleum do not possess a danger in terms of carcinogenicity or toxicity. They also developed guidelines for the production and nutritional and safety standards of SCP for human consumption.  These include microbiological examination for pathogens and toxin producers, chemical analyses for heavy metals, nucleic acid content, presence of hydrocarbons, safety tests on animals and protein quality studies.

Another problem associated with SCP is the nucleic acid content.  When nucleic acid is eaten by man, it is broken up by nucleases present in the pancreatic juice, and converted into nucleosides by intestinal juices. Guanine and adenine are converted to uric acid. As a result, when foods rich in nucleic acid are consumed in large amounts, uric acid level increases in blood plasma resulting in its deposition in various tissues in the body including the kidneys and the joints and kidney stones and gout may result.

Various methods for the removal of nucleic acids from SCP are

(a) Growth and cell physiology method: The RNA content of cell is dependent on growth rate.  The growth rate is reduced to reduce nucleic acid.

(b) Extraction with chemicals: Dilute bases such as NaOH or KOH will hydrolyze RNA easily. Hot 10% sodium chloride may also be used to extract RNA and the protein may then be extracted, purified and concentrated.

(c) Use of pancreatic juice: RNAase from bovine pancreatic juice, which is heat-stable, can be used to hydrolyze yeast RNA at 80°C.   At this temperature the cells are more permeable.

(d) Activation of endogenous RNAase: The RNAase of the organism is activated by heat-shock or by chemicals to reduce the RNA content of yeasts.

The nutritional value of SCP depends on the composition of the microbial cells used, especially their protein, amino acid, vitamin, and mineral contents. SCP derived from bacteria and yeasts is deficient in methionine. Glycine and methionine are sometimes deficient in molds. These can be improved by supplementation with small amounts of animal proteins.

 

References

1. Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers