Nutritional Value of SCP

Nowadays, considerable information is available on the composition of microbial cells e.g. protein, amino acid, vitamin, and minerals (Litchfield, 1979). Commercial value of SCP depends on their nutritional performance and nevertheless, it has to be evaluated to the prevalent feed protein. SCPs either from alkanes or methanols, are characterized by good content and balance in essential amino acids (Senez, 1986).

Composition of growth medium governs the protein and lipid contents of microorganisms. Yeasts, moulds and higher fungi have higher cellular lipid content and lower nitrogen and protein contents, when grown in media having high amount of available carbon as energy source and low nitrogen (Litchfield, 1979).

Ignoring a few extreme values, the mean crude protein in dry matter of algae and yeasts, on conventional substrates, lies between 50 and 60 per cent, for alkane yeasts between 55 and 65 per cent, and for bacteria about 80 per cent. A high content of nucleic acid free protein is extremely important for the economic efficiency of the procedure in SCP production. Because of high protein and fat contents, the contribution of carbohydrates to the nutritional value of SCP is not of prime importance.

The crude ash content is determined in particular by the nutrient salts of the fermentation medium (Roth, 1982). Estimation of crude protein is based on total nitrogen which is multiplied by the factor 6.25. The protein content of microorganisms computed in this manner does not give the exact figure of protein content, as in the estimation of total nitrogen, the value of nucleic acid is also included which is somewhat erroneous.

The most important measure of nutritional value is the actual performance of SCP products as determined in feeding studies. The determinants of the utility of SCP product for application as food for human beings and feed for animals differ. For human beings, protein digestibility and protein efficiency ratio (PER), biological value or net protein utilization (NPU), determined in rats, are the parameters for food application, whereas for animals, metabolizable energy, protein digestibility and feed conversion ratio (weight of ration consumer/weight gain) are the measures or performance in broiler, chickens, swine and calves (and egg laying in hens).

Digestibility (D) is the percentage of total nitrogen consumed, which is absorbed through the alimentary tract. It is calculated as below:

D =	Ni - Fn
	Ni

where,
Ni =  nitrogen ingested from SCP.
Fn =  nitrogen content in faeces after feeding SCP

Biological value (B) is the percentage of total nitrogen assimilated which is retained by the body, taking into account the simultaneous loss of endogenous nitrogen through excretion in urine. This is expressed by the following formula :

B =	Ni - ( Fn + Un )	x 100
	(Ni - Fn)

where,
Un = nitrogen content in urine after feeding SCP.

Protein efficiency is the proportion of nitrogen retained when protein under test is fed compared with that retained when a reference protein (e.g. egg albumin) is fed (Riviere, 1977).

Nutritional values of SCP product are given in Table 18.3 which indicates that protein digestibility range from good to very good and is true for bacteria and yeasts growth on unconventional substrates (Roth, 1982).

However, there are certain problems which warrant the use of SCP products as human foods such as (i) high content of nucleic acid leading to development of kidney stone and gout if consumed in high quantity, (ii) possibility for the presence of toxic secondary metabolites and (iii) poor digestibility and stimulation of gastrointestinal and skin reactions (Litchfield, 1979).

Table 18.3. Nutritional value of food protein.
Source : A, Senez(1986); B, Roth (1982); * Digestibility determined by pig feeding; NPU = Net protein utilization; NPV = Net protein value