Although less high profile than it was 25-30 years ago, there is still interest in the cultivation of microbes specifically as foodstuffs, rather than as agents in the production of other products, which is how we have encountered them in this book. The term 'single cell protein' was coined to describe these products, which were based on diverse bacteria and yeasts, growing on a range of carbon sources (Table 17.1).

Only one product has survived in substantial quantity to this day, Quorn™. It is a joint venture between two major British companies and has been marketed as a meat substitute since 1984.

The organism, Fusarium venenatum, is grown at 30°C in rigorously sterile conditions in air lift (pressure cycle) fermenters. The liquid medium flows continuously into the fermenter (the residence time is 5-6 h), and the conditions are highly aerobic, with the compressed air serving both as nutrient and as the vehicle for agitation.

Carbon source is glucose produced by the hydrolysis of corn starch, and ammonium salts are included as the nitrogen source. The pH is maintained at 4.5-7.0 and iron, manganese, potassium, calcium, magnesium, cobalt, copper and biotin are added. Unlike the other products considered in this book, the cells themselves are really all that impact on the properties of the finished product in the present instance. The medium composition is relevant only

Table 17.1 Some single cell protein processes.

Substrate Organism







Molasses Starch


Sulphite waste liquor Whey

Alcaligenes, Cellulomonas

Candida utilis, Acinetobacter calcoaceticus

Fusarium venenatum

Candida tropicalis, Yarrowia lipolytica

Methylococcus capsulatus

Methylomonas clara, Methylophilus methylotrophus, Pichia pastoris Candida utilis

Saccharomyces cerevisiae, Saccharomycopsis fibuligera/Candida utilis Candida utilis Candida utilis

Candida intermedia, Candida krusei, Candida pintolepesii, Candida utilis, Kluyveromyces lactis, Kluyveromyces marxianus, Lactobacillus bulgaricus insofar as it impacts the yield and properties of the organism per se and has no role to play, for instance, in determining final product flavour or appearance. The continuous fermentation system will be re-established every 1000 h. After fermentation, the cell suspension is heat-shocked to reduce the extent of development of RNA degradation products, the presence of which will otherwise elevate the risk of gout in those partaking of the foodstuff. Heating is at 64°C to eliminate the enzymes that convert RNA to nucleotides.

The cell suspension is harvested by centrifugation and the hyphae mixed with binding agents and flavourants and heated to cause a gelling of the binder and a linking of the hyphae.

The product is some 45% protein, 14% fat and 26% fibre by dry weight. It is 11% protein, 3% available carbohydrate, 6% fibre, 3% fat, 2% ash and 75% water by wet weight. It is sold in a variety of commercial forms, for example, pieces and minced.

Nutritionally, it stacks up very well against other foods. It possesses a complete complement of essential amino acids and is a particularly good source of threonine, which tends to be the limiting amino acid in meat. Quorn has little saturated fat and has a favourable ratio of polyunsaturated to saturated fatty acids when compared with beef and chicken. It is devoid of cholesterol and is low in calories. It possesses significant levels of fibre in the form of chitin and j-glucan from the Fusarium cell walls. It contains the breadth of B vitamins, with the exception of B12. Finally it is devoid of phytic acid, and so tends not to interfere with metal uptake from the diet.

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