In this article we will discuss about the renewable and non-renewable sources of substrates used for SCP production.
1. Fossil Carbon (Non-Renewable) Sources:
The various substrates 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:
(i) Gaseous Hydrocarbons:
C1 to C4 gaseous hydrocarbons have been used for SCP production. Methane (C1) 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 3 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 upto pilot plant scale.
(ii) Liquid Hydrocarbons:
Saturated, straight-chain hydrocarbons, called n-alkanes, constitute 0-30% of crude oil. C5 to C8 n-alkancs are liquid at room temperature, but are generally toxic to cells due to their solvent action. Generally, n alkanes having 9 to 18 carbons (C9 – C18) are used as SCP substrates.
These n-alkanes are insoluble in water; a micro-emulsion is formed due to the combined actions of agitation in the bioreactor and the surfactants produced by microorganisms. The hydrocarbons are taken up by cells and oxidised; thus the process requires very high O2. Many bacteria, actinomycetes, yeast and moulds are able to use liquid hydrocarbons.
Gas Oil:
Gas oil, in combination with ammonia and mineral nutrients, was used in air-lift fermentors to produce Candida yeast by British Petroleum. But the project was given up mainly due to complicated recovery procedures.
Pure n-alkanes :
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. The process was being reportedly used in the erstwhile Soviet Union for industrial SCP production.
(iii) 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 (Table 37.3), 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 in a specially designed ‘pressure cycle’ fermentor (the fermenter is designed to achieve very efficient substrate mixing and O2 transfer).
The cells are harvested by flocculation and floatation to give about 10% solids; the biomass is then subjected to centrifugation to remove water and is then air-dried. The SCP contains 71% protein, is marketed as ‘Pruteen’ and is used as a milk substitute in calf feeding. The bacterium M. methylotrophus had its NH3– assimilation modified by genetic engineering; this has significantly increased its biomass production.
(iv) 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 utilis (Torula yeast) in a 5.000 tons/yr capacity plant.
2. Renewable Carbon Sources:
The renewable carbon sources include:
(i) CO2.
(ii) Molasses,
(iii) Whey,
(iv) Cellulose hydro-lysates,
(v) Starch hydro-lysates,
(vi) Industrial effluents and
(vii) Cellulosic wastes.
SCP production using these substrates is briefly described below:
(i) Carbon Dioxide (CO2):
This substrate is utilized by algae which derive the required energy from sunlight. Spirulina is grown on a commercial scale by Sosa Texcoco Co., Mexico, producing upto 5 tons dry biomass per day.
The blue-green alga is cultivated in 1 m deep ponds and is harvested by filtration, followed by vaccum filtration and is dried using drums. Chlorella is commercially grown in Japan and Taiwan to get about 15,000 tons/yr dry biomass from many small production units.
The algae cultivation is more or less like crop cultivation, and offers the following advantages over the latter:
(i) Year-round cultivation,
(ii) More efficient harvesting of sunlight,
(iii) Optimal utilization of mineral nutrients,
(iv) Cultivation in seawater and alkaline lakes, and
(v) Very high yields, e.g., 50 tons/ha/yr Spirulina biomass.
The disadvantages are:
(i) Dependence on climate, including sunlight,
(ii) Utilizes considerable area of land and
(iii) Is attacked by animals, weeds, and pathogens.
(ii) Molasses:
This substrate is used for alcoholic fermentation from which yeast biomass is obtained as a by-product. Traditionally the baker’s yeast (Saccharomyces cerevisiae) and Torula yeast (Candida utilis) are produced on this substrate. About 200,000 tons dry weight of S. cerevisiae is produced each year which is used as inoculum for dough fermentation in bakeries.
(iii) Whey:
Whey is the liquid portion of milk remaining after the curd is separated during cheese production. On dry weight basis whey contains 70% lactose, 9-14% protein and 9% ash. About 75 million tons of whey is produced each year of which over 50% is released as effluent which necessitates treatments to avoid pollution.
Yeasts (Kluyveromyces fragilis, Candida krusei in combination with Lactobacillus bulgaricus , and Candida inter-media) are the most widely used for commercial scale production of SCP from whey. For example, Fromageries Le Bel, France produces about 2,300 tons of SCP/yr using K. fragilis; the SCP is being used as human food supplement for about 2 decades.
(iv) Cellulose Hydrolysate:
Cellulase obtained from fungi like Trichoderma viride has been used to hydrolyse cellulose and to produce glucose. The main advantage of this approach is the large quantities of cheap substrate.
But the chief problems are:
(i) Difficult and expensive process for complete hydrolysis of lignocellulose,
(ii) The amount of pre-treatment necessary, and
(iii) The extent of sugar destruction and by-product formation during chemical hydrolysis.
The best utilization of cellulosic substrates appears to be for mushroom production and for semisolid fermentation systems.
(v) Starch Hydrolysate:
This rather expensive substrate is used for SCP production using Fusarium graminearum. The SCP has a fibrous, meat-like texture, is marketed as ‘Mycoprotein’, is approved for human use in U.K., and food products having Mycoprotein are being marketed.
The fungus grows at 30°C on a variety of mono- and oligosaccharides, is recovered by vaccum filtration, and the recovered biomass is held at 60°C for 20 min to activate the native RNase (this reduces the RNA content of SCP from 10% to about 1% of dry weight).
(vi) Industrial Effluents:
Effluents from many industries e.g., breweries, distilleries, confectionery industries, potato and canning industries, sulphite liquor from wood pulp mills etc., contain large amounts of carbohydrates and other organic compounds which can be used for SCP production. Some of these are already being used for the purpose.
For example, Basett Ltd., U.K. uses Candida utilis to treat 140,000 1/confectionery effluent/day by continuous fermentation and produces 1.5 tons of dry yeast/day. In addition, the Biochemical Oxygen Demand (BOD) of the effluent is reduced by 81%.
Sulphite liquor is widely used for SCP production in North America, Europe and erstwhile Soviet Union. The SO2 is first removed from the liquor, pH is adjusted to 4.5 and inoculation is done with the selected microorganism. The most advanced process is based on the fungus Paecilomyces varioti which is being used on an industrial scale in Finland.
The fungus utilizes both pentose and hexose sugars as well as the acetic acid present in the liquor. It is easily recovered by filtration; the SCP has 52-57% crude protein, and is used as animal feed.
(vii) Cellulosic Wastes:
A new approach to use agricultural and forestry cellulosic wastes, e.g., straw, bagasse, sawdust etc., for SCP production uses semi-solid fermentation.
The cellulosic material is pretreated thermally and chemically, and is then fermented with the fungus Chaetomium cellulolyticum. The fungus is reported to degrade the cellulosic wastes in very short time, e.g., 4 hr. This promising approach may facilitate an efficient and economic utilization of cellulosic wastes for SCP production.