The following points highlight the top four microorganisms used for SCP production. The microorganisms are: 1. Algae 2. Filamentous Fungi 3. Yeasts 4. Bacteria.
Microorganism # 1. Algae:
Members of the genera Chlorella, Scenedesmus and Spirulina are generally grown in ponds/tanks. They use CO2 and sunlight as substrate which are without any cost.
Generally, the limiting factor in their large scale production is illumination. Algal SCP has about 60% crude protein, which is generally good in amino acid composition except for some deficiency in sulphur-containing amino acids (Table 37.2). They are suitable for animal feed as protein-rich supplement.
The disadvantages of algal SCP are:
(i) Rich chlorophyll content which is not suitable for human use,
(ii) Serious problems when Chlorella and Scenedesmus are used in human diet (Spirulina is more suited tor human use),
(iii) Low cell density, e.g., 1-2 g dry weight/l,
(iv) Serious risk of contamination, and
(v) Costly recovery methods for unicellular algae (Spirulina harvested by filtration or simply by skimming).
Microorganism # 2. Filamentous Fungi:
Filamentous fungi have been used to produce SCP mainly from polysaccharide hydro-lysates, e.g., starch hydro-lysates, sulphate liquor from wood pulp industries etc. These are usually grown as submerged cultures in which they grow as yeast-like cells, in filamentous form or in pellets.
They have crude protein content of 50-55%; the protein is low in S-containing amino acids, but otherwise is excellent in amino acid composition (Table 37.2). The recovery of filamentous and pellet-forms is rather easy by filtration.
The problems associated with fungi are:
(i) Slower growth rates than bacteria and yeast, but some micro-fungi may be comparable to yeast,
(ii) Contamination by yeast may be frequent if sterility is not maintained, while that by bacteria can be minimised by keeping the pH of broth below 5,
(iii) They have high nucleic acid content (upto 15% RNA) which must be reduced, and
(iv) The strains have to be thoroughly evaluated for mycotoxin production. However, a substantial part of the N may be present in chitin, a component of the cell wall. ,
Microorganism # 3. Yeasts:
Members of Saccharomyces, Candida and Torulopsis have been widely studied for SCP production and those of the first two genera are used for some commercial processes (Table 37.1) using various substrates. The SCP has 55-60% crude protein which has good amino acid balance except for a deficiency in S- containing amino acids.
It is usually very rich in B-group of vitamins. The SCP is used both for human food and animal feed supplementation. The risk of bacterial contamination is low and recovery by continuous centrifugation is easy (Table 37.2).
The difficulties in use of yeasts as SCP are:
(i) Slower growth rates than fastest growing bacteria,
(ii) High nucleic acid content (upto 15%) which needs to be reduced, and
(iii) Methionine supplementation may be done to overcome S-containing amino acid deficiency of its proteins.
Microorganism # 4. Bacteria:
A large number of bacterial species have been evaluated for SCP production:
(i) Using a wide variety of substrates (Table 37.3);
(ii) Some of these are used for production at commercial scales, e.g., Methylophilus methylotrophus using methanol (Table 37.1).
SCP has very high crude protein (over 80%) of good amino acid composition, although in some cases a small deficit of sulphur-containing amino acids may be encountered.
The disadvantages of using bacteria for SCP production are:
(i) High nucleic acid, especially, RNA content (ca. 20%) which must be reduced,
(ii) Sterility must be maintained during the production process since pH of cultures is kept between 5 and 7,
(iii) Risk of contamination by pathogenic bacteria is considerable,
(iv) Recovery of cells is problematic, and
(v) Careful evaluation for endotoxin production is essential particularly when gram-negative bacteria are used.