SCP production

The principal interest in photosynthetic bacteria for thek appHcabiUty to SCP production (Table 1) has been in Japan, where Jiodobacter capsulatus has been used to treat industrial wastes in sewage ponds (40,41). The product has been evaluated as a protein supplement in laying hen rations for egg production with acceptable results (40). 

Large-scale SCP production processes for growing yeasts of the genus Candida from hydrocarbon substrates were developed by British Petroleum Co., Ltd. and Kanegafuchi Chemical Industry, Ltd. of Japan (57). However, the 100,000-t/yr capacity plants based on these processes, and constmcted in Sardinia and Italy, were abandoned because of regulatory agency questions regarding residual hydrocarbon contents of the products (2,3). 

Table 5. SCP Production Processes Based on Nonphotosynthetic Microorganisms...

Most of the bacteria, yeasts, molds, and higher fungi of interest for SCP production are deficient in methionine and must be supplemented with this amino acid to be suitable for animal feeding or human food appHcations. Also, lysine—arginine ratios should be adjusted in poultry rations in which yeast SCP is used (62). Human feeding studies have shown that only limited quantities of yeast such as Candida utilis can be added to food products without adverse effects on flavor (63). 

Nucleic acid contents of SCP products, which range up to 16% in bacteria and 6—11% in yeasts, must be reduced by processing so that intakes are less than 2 g/d to prevent kidney stone formation or gout. Adverse skin and gastrointestinal reactions have also been encountered as a result of human consumption of some SCP products (87). 

Single cell protein, normally called simply SCP, is the term used to describe microbial cells, or proteins from them, which are used as food (food for humans) or feed (food for farm animals or fish). Although the term micro-organisms covers viruses, bacteria, fungi, algae and protozoa, viruses and protozoa are not considered suitable for SCP production. 

You are now familiar with the major characteristics of organisms that are useful for SCP production, and the types of substrates on which they can be grown. We are now going to consider in detail the processes that have been developed. Some of these processes have been developed only as far as the pilot scale, and have not reached commercial operation. Others have reached full production scale but have subsequently failed, for a variety of reasons. These have been included as well as the successes, as they show you the variety in the technology of SCP production, and also show how economic and political factors influence the success and failure of processes. These processes might also become useful and economic some time in the future. Emphasis will be put on the technology involved in the fermentation and down-stream processing of each process. 

Eukaryotic algae (Chlorophyceae) of the genera Chforella and Scenedesmus have been used for SCP production. Several types of cultivation systems have been considered, depending on the substrate used and whether the SCP is intended for use as food or feed. 

Various wastes available as carbon substrates for SCP productions are listed in column A. Match each waste with a suitable organism (perhaps more than one) from column B. For each organism select the most appropriate production system from column C. 

For SCP production continuous rather than batch cultures are preferred because ... 

Item 1 is the major factor in choosing continuous systems rather than batch systems for improved SCP production. Economics are improved by lower capital cost for the bioreactor (a conomica major equipment cost, see Section 4.10) and by a higher output rate. Item 3 leads to greater control of product quality. 

Detailed economics of individual industrial processes, including SCP processes, are usually regarded as confidential, out of fear that publication may lend advantage to competitors. In addition, economy of scale rule generally applies (that is as the production capacity increases, the cost of the product decreases), so that direct comparisons can only be made between systems of similar capacity. Some economic data on SCP processes have been published and are presented in the Resource Material at the end of this chapter. You should appreciate that the data are outdated by more than a decade, during which time substrate costs will have varied relative to each other, and technology will have improved. This means that the comparative costs presented in Table 4.13, for example, may not be now as presented there. Nevertheless the data presented do provide an outline of the economics of SCP production. The processes referred to in the Resource Material are not necessarily those mentioned in the text and so you may find some differences in detail. 

The tables that follow give the costs of various SCP production processes in comparative rather than in actual form. To see what this means examine Table 4.9. The production cost of raw materials for yeasts grown on n-alkanes is given as 58.5. This means that tire cost of raw materials accounts for 58.5% of the total production costs of this process. The same cost for bacteria grown on methanol is 73.8. This means that in this case 73.8% of the total production cost is accounted for by raw materials. This does not mean that the actual cost of raw materials for tire methanol process is more titan that for the n-alkanes process, as the total costs of the two processes are not necessarily similar. 

The most significant production cost in SCP production is the cost of raw materials,... 

In biochemical engineering processes, measurement of dissolved oxygen (DO) is essential. The production of SCP may reach a steady-state condition by keeping the DO level constant, while the viable protein is continuously harvested. The concentration of protein is proportional to oxygen uptake rate. Control of DO would lead us to achieve steady SCP production. Variation of DO may affect retention time and other process variables such as substrate and product concentrations, retention time, dilution rate and aeration rate. Microbial activities are monitored by the oxygen uptake rate from the supplied ah or oxygen. 

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