Advantages and disadvantages of employing different groups of microorganisms are given below:
Autotrophic nature of algae makes these organisms ideal and economical. Spirulina meseima, chlorella, Microoclanum, Euglena and Senedesmus are some of the organisms being exploited. General method of processing algal SCP is illustrated in Fig. 16.1.
It illustrates methods of upstream and downstream process of algal protein production and flow chart of Spirulina protein production is given in Fig 16.2.
However, these organisms have advantages with certain limitations.
1. Growth with simple inorganic substances without the need for energy and carbon source.
2. Some algae utilize atmospheric nitrogen.
3. SCP production can be carried out in large open pools, natural lakes or ponds and do not require aseptic conditions.
4. Collection or separation procedures are easier.
5. Lysine content of algae is high.
The limitations of these groups of organisms include:
1. Algal growth rate is slower than that of fungal growth.
2. Biological values of algal proteins are less than that of bacteria and yeasts.
3. Algal proteins are deficient in S-amino acids.
4. Poor mammalian digestibility of algal protein except ruminant.
5. Relatively high concentration of unsaturated fatty acids, which cause adverse effect on animals.
Fungi in view of their ubiquitous nature, rapid growth on variety of waste substrates, are receiving greater attention in the exploitation as a source of SCP. At present mycoproteins of human food are being met from mushrooms, penicillium notatum, Fusarium and Trichoderma harzianum.
Merits and demerits of fungi as SCP organisms are:
1. Grow on wide range of substrates as they produce wide range of enzymes.
2. Can be recovered by single filtration which reduces the processing costs.
3. Because of filamentous nature of the product, it can be used as textured food.
1. Growth rate of fungi are slower than those of yeasts and bacteria.
2. Protein content is lower.
3. Fungal proteins are deficient in sulphur containing amino acids.
4. The digestibility of fungal protein is problematic.
5. Fungi are unknown entity with regard to human nutrition and toxicology.
In fact the concept of SCP is developed from yeasts. They are not only ubiquitous and most familiar to man as a components of beverage or bakery products. Three types of food yeasts used extensively are brewer yeasts (Saccharomyces sp), bakers yeast (S. cervisiae) and dried yeast (Candida utilis and Torula thermophila). Flow scheme for production of bakers yeast is given in Fig. 16.3.
Dried yeast is a byproduct of beverage fermentation is being used Candida utilis was employed as food during World War I and II by Germans as meat substitute for army, prisoners of war and civilians. They have several good qualities to choose as SCP.
1. Grown on any type of carbohydrate.
2. Sufficient quantities of essential amino acids (Lysine, tryptophan and threonine).
3. Good source of vitamin B, D and E.
4. Low nucleic acid content.
5. Larger size makes SCP separation easy.
6. They are with lower toxicity potential.
7. Greater acceptance by consumers.
These organisms are reported to contain low concentration of sulphur containing amino acids.
Bacteria which are microscopic, fast growing and versatile in nutrient requirement are rich in proteins. Hence, many bacteria such as species of Bacillus are being exploited for SCP.
Several advantages of bacterial SCP are:
1. Efficient in conversion of substrate carbon to cell mass,
2. Higher protein conversion efficiency,
3. Lower oxygen requirement,
4. The proteins of bacteria have fairly good amino acid profile. Similarly they are rich in sulphur containing amino acids.
They are beset with some limitations such as:
1. Only limited number bacterial species are exploited for food purpose,
2. Small size of the bacteria makes the separation of protein difficult,
3. In view of their growth under limited oxygen condition, they emit unpleasant odour which may make it unacceptable, and
4. Contamination with pathogenic species.
5. Mixed Cultures:
Mixed cultures proved to be more efficient in SCP the than monoculture in view of following the reasons:
1. Better utilization of a complex carbon source as carbon source.
2. Removal of organic carbon excreted by one component microorganisms.
(a) Removal of toxic compounds effecting one component microorganisms.
(b) Increase of the total biomass yield from the primary carbon source.
(c) Elimination of foam.
(d) Increase the growth rate of one component microorganism.
(e) Enable water recycling.
(f) Resistance to perturbation in culture environment.
(g) Resistance to contamination by bacteria, yeast, fungi and phages.
(h) Lower maintenance energy requirement.
3. Attainment of desired protein composition.
However, in choosing a substrate, consideration must be given to:
1. The cost of the substrate.
2. Biomass yield generation.
3. Oxygen requirement during fermentation.
4. Heat produced and level of fermenter cooling requires.
5. Downstream processing costs.