|Production of Bacterial and Actinomycetous Biomass
Bacteria are widely used as a source of single cell protein because of their short life cycle (20-30 minutes) and capacity to utilize a wide range of organic substrates as a source of energy. Actinomycetes also utilize these renewable sources as they have more or less same generation rate as bacteria.
Since the establishment of British Petroleum in 1960, a significant progress is made in the production of microbial products by using gaseous and liquid hydrocarbons and chemicals derived from them, for example, methanol, ethanol, etc. The Shell Research Limited, U.K. conducted research on pilot plant scale process for the production of bacterial SCP from methane by using Methylococcus capsulatus or mixed culture of Pseudomonas sp., Hyphomicrobium sp., Acinetobacter sp. and Flavobacterium (Litchfield, 1979). Several processes for the production of bacterial SCP from gaseous hydrocarbons have been developed at Kyowa Hakko Kogyo Company, Limited in Japan. Brevibacterium ketoglutamicum ATCC No. 15587 was able to utilize methane, ethane, propane, n-butane, iso-butane, propylene, butylene or mixture of these hydrocarbons (Tanaka et al. 1972). A pilot scale process for the production of Achromobacter delvacvate from diesel oil is developed at the Chinese Petroleum Corporation, Taiwan.
Streptomyces. sp. is capable of growing on methanol. Theromonospora fusca, a thermophilic species, degrades 60-65 per cent paper mill fines resulting in 30 per cent protein product. Nowadays, cellulose degrading thermophilic actinomycetes offer a great opportunity to yield SCP from cellulosic wastes.
Method of Production
Roth (1982) has described the following steps for the production of bacterial biomass : (i) supply of a nutrient substrate; (ii) formulation of a suitable medium; (iii) multiplication of microorganisms through fermentation, (iv) separation of cellular substances from the left over medium; and (v) further treatment to kill and dry the bacterial biomass.
Imperial Chemical Industry (ICI) is a world's leader in biotechnology, as far as the production of a bacterial biomass, pruteen, from Methylophilus methylotrophus is concerned. Pruteen was produced on methanol but it served as high grade protein for animal feed. It contains 72 per cent protein, 86 per cent total lipid and an amino acid profile high in lysin and methionine. The conversion of methanol to SCP by M. methylotrophus is represented by the following equation :
OH + 0.23 NH3
+ 0.72 CO2
+ 2.94 H2
Production of SCP by other bacteria and actinomycetes is shown in Table 18.1.
Factors Affecting Biomass Production
Growth of bacteria and actinomycetes are affected by many factors, as a result production of biomass on a given substrate significantly changes. These are : (i)
suitable strain of bacterial culture ; (ii)
genetic stability of bacterial strain; (iii)
absence of bacteriophage; (iv)
suitable pH 5-7 of growth medium; (v)
temperature (15-35°C according to strain); (vi)
oxygen/agitation to create aerobic condition; (vii)
organic substrate and nitrogen concentrations. Optimum C:N ratio which favors high protein contents in cells and inhibits accumulation of lipid is 10:1; and (viii)
maintenance of sterile conditions throughout the growth.
Like algal biomass, there are many problems related to the recovery of bacterial cells as they are very small and have cell density in the order of 10-20 g/liter. Centrifugation cost is also high. Processes for cell recovery have been devised in such a way that could cut down the cost. Many pilot plants use flocculants and many have set up decanter type centrifuge. For example, Hoechst (Germany) has developed a device for separation of Methylomonas clara
from methanol containing culture medium which is based on electrochemical, coagulation and centrifugation. The cells are washed and spent medium is again treated by conventional treatment process as it contains inorganic salts and small amount of cells. Cell biomass is then spray-dried (Faust et al,