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Fossil Carbon Sources - The various substrate in this group are,

(i) gaseous hydrocarbons,

(ii) liquid hydrocarbons,

(iii) methanol and

(iv) ethanol; use of these for SCP production is briefly described below.

1. Gaseous Hydrocarbons. C1 to C4 gaseous hydrocarbons have been used for SCP production. Methane (Cl) has been extensively studied, and is highly desirable as it is available in high purity from natural gas, which in some parts of the world is burnt at the oil wells, e.g., Bombay High.

In addition, it is readily removed from the fermentation medium and supports high productivity in continuous processes. Methane is utilized by certain bacteria, e.g., Pseudomonas methanica, Methanomonas methanica, the thermophilic Methylococcus capsulatus, and Pseudomonas methanitrificans; the last bacterium utilizes methane as well as fixes atmospheric nitrogen.

But use of methane for SCP production presents three important problems:

(i) substrate limitation either due to methane or oxygen,

(ii) heat generation necessitating efficient cooling, and

(iii) hazards of explosion when over 12% O2 (v/v) is used.

These and some other associated problems have limited the activities only up to pilot plant scale.

2. Liquid Hydrocarbons. Saturated, straight chain hydrocarbons, called n alkanes, constitute 0-30% of crude oil. C5 to C8 n-alkanes is liquid at room temperature, but is generally toxic to cells due to their solvent action. Generally, n alkanes having 9 to 18 carbons (C9-C18) are used as SCP substrates.

Many bacteria, Actinomycetes, yeast and moulds are able to use liquid hydrocarbons. Pure n alkanes (>97.5%), having 10 to 23 carbons, was used by British
Petroleum to culture Candida lipolytica in a continuous process; the medium consisted of n alkanes, minerals and ammonia. The n-alkanes were almost completely utilized, biomass recovery was easier and cheaper; and the SCP having about 60% crude protein was marketed as 'Toprina'.

The SCP was comparable to soybean meal and fishmeal, and was cleared after nutritional and toxicological tests. However, large scale production was discontinued mainly due to legal difficulties and/or public protests.

3. Methanol. A very efficient chemical conversion of methane yields methanol; it can also be produced from coal, gas oil, wood, naphtha, etc. Methanol is fully water soluble, is used by many bacteria, and there is little danger of explosion (compare with methane). A highly successful process by Imperial Chemical Industries Ltd. (ICI) uses the bacterium Methylophilus methylotrophus for a continuous production process at 35-40°C. The SCP contains 71 % protein, is marketed as 'Pruteen', and is used as a milk substitute in calf feeding.

4. Ethanol. Ethanol can be obtained from ethylene by catalytic addition of a water molecule, or from organic substrates by alcoholic fermentation. It offers advantages similar to methanol, and is even more acceptable as a substrate for SCP production for human use.

Several bacteria, yeasts and mycelial fungi utilize ethanol. Amco foods, U.S.A. produces food grade SCP by growing Candida utilise (Torula yeast) in a 5,000 tons/yr capacity plant.
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