Single Cell Protein - Yeast

Yeast is another source of Single Cell Protein, and have been produced since a long time ago. In World War I, Torula yeast (Candida utilis) was produced in Germany and used in soups and sausages. Nowadays, the pet food industry is a major outlet of microbial biomass. The dog, cat. fish feed is supplemented with yeasts, it will make the product more palatable to the animals. Use of yeast as food seasoning is commonly found in vegetarians diet, Torula yeast has been commercially used for this purpose, an example of this product is Hickory Smoked Dried Torula Yeast. Yeast has some advantages among other SCP sources, such as:

Food seasoning made from Torula yeast

1. Easy to harvest because of their size (larger than bacteria)
2. High level of malic acid content
3. High lysine content
4. Can grow at acidic pH
5. Long history of traditional use
   
   

This nutritious microbe unfortunately has few disadvantages that have to be taken as consideration, such as:

1. Lower growth rates compared to bacteria
2. Lower protein content than bacteria (45-65%)
3. Lower methionine content than bacteria, solved by the addition of methionine in the final product.

Yeasts propagated for food purpose (food yeast) and to feed animals (fodder yeast). The mass cultivation of yeast for use as food is to compensate the dietary inadequacies of cheap food materials, especially in the regions where human malnutrition is chronic. Secondly, there is great emphasis and interest on lowering BOD of the effluents from industrial plants. Thus several processes to manufacture yeast from industrial and agricultural wastes have been undertaken. This converts wastes into products of value, and at the same time prevents environmental pollution. Few examples of yeasts that often used in SCP production are Candida utilis (Torula),  Saccharomyces cerevisiae, Candida lipolytica, and many others.

Yeast is a unicellular organism

Food and fodder yeast is also manufactured from waste materials, such as wood shavings, sawdust, strain, corn cobs, other agricultural wastes. Yeasts are capable of assimilating diverse source of carbon. 

• The asporogenous yeast, Candida utilis, is commonly used because of it’s ability to assimilate various carbon sources, including xylose, and a wide range of organic acids. Torula are grown on sulphite-liquor, from pulp and paper mills. After removal of wood fibres for paper, the waste sulphite liquor contains valuable wood sugar. Yields of up to 50% of total reducing sugar consumed in term of dry torula are obtainable.

• The fat producing yeast, Rhodotorula gracilis, undergoes normal cellular growth during log phase in a medium containing adequate nitrogen, however, at the onset of nitrogen starvation, this organism accumulates large amount of lipid within its cell.

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  Yeast as dietary supplement

  The production of baker’s yeast (Saccharomyces cerevisiae) is the largest domestic use of microorganism for food purpose. The strain of yeast is carefully selected for its capacity to produce abundant gas quickly, its viability during ordinary storage, and its ability to produce desirable flavour. During manufacturing, the strain is inoculated into a medium which contains molasses and corn-steep liquor as source of carbon, nitrogen and mineral salts. temperature 25-26 C. aerated during incubation period.
  At the end of incubation, yeast cells are removed from the fermented medium by centrifugation, washed and mixed with starch or corn meal, and then pressed into cake form. Yeast cakes must be kept cool to preserve the cells and prevent spoilage by other microorganisms. They may also be dried (can remain for several months). Yeasts are rich in vitamins and in most of the essential amino acids required by man and animals.
  
  
  Baker’s yeast production commences with propagation of a starter culture, which originates from pure freeze-dried sample or agar-medium culture. Yeast cells are transferred to small liquid culture flasks then on to larger intermediate vessels before being used to inoculate the large production fermenters. Overall, this may involve up to 8 scale-up stages to produce the necessary final inoculum volume.
  
  

  

  Molasses can be used as a substrate

  
  

  Medium for the production normally contains molasses, which may be pretreated with acid to remove sulphides and heated to precipitate protein. Molasses is often deficient in certain amino acids and supplements of biotin and pantothenic acid are usually necessary. Ammonium salts or urea may be added as additional N sources. Along with orthophosphate and other mineral ions and the pH is adjusted to 4.0-4.4
  
  
  The main objective of this process is to generate a high yield of biomass that exhibits an optimal balance of properties, including a high fermenting activity and good storage properties. Aerobic fermentation favours a high biomass yield, as approx. 50% of the available carbon can be potentially converted into biomass. The maximum theoretical growth yield is 0.54 gig whereas under anaerobic condition this value is reduced to 0.12 gig.

 

Industrial aspect

Cellulose agricultural byproducts are potential carbohydrate sources for growing yeast, but this substrate requires preliminary acid hydrolisis of the cellulose OR use of cellulolytic microorganisms or their enzymes for hydrolysis.

Problem 1 : Present fermentation procedure for producing yeast utilizes liquid hydrocarbon. The yeast removes most of the paraffinic portion, leaving behind aromatic fraction and some of the more highly branched paraffins. This aromatic fraction is difficult to separate completely from the cells at harvesting procedure. However, this must be accomplished because of the potential carcinogenic activity.

Problem 2 : The protein content of several yeast is low on one or more of the essential amino acids. However, this can be solved by fortifying the product with amino acids obtained from bacterial fermentation.

Commercial products

Yeast as animal feed supplement

• Toprina - C. lipolytica and C. tropicals - For 12 years TOPRINA was marketed as a replacement for fish meal in high protein feeds and as a replacement for skimmed milk powder in milk replacers. The substrate used for Toprina production is C12-C20 alkanes from petroleum industries. in the late 1950s British Petroleum built a 16000ton/year plant in France and a 4000ton/year plant in England. In 1977 the SCP production was stopped due to the increase in oil prices. The price of soya was more competitive.

• Torutein – Candida utilis/Torula - The substrate used for this yeasts is ethanol, although it is quite expensive for a substrate. The process comes from the Amoco Company in the US utilising a food grade yeast: "Torula". The product is sold by the name "TORUTEIN" and government clearances have been obtained to market Torutein in Canada and Sweden. The yeast is about 52% protein and due to its relatively low Methionine level has a PER of about 1.7. The PER of wheat from1.1 to 2.0. Torutein is being marketed as a flavour enhancer of high nutritional value, and a replacement for meat, milk and egg protein. However it is not very successful in the United States since soya which is plentiful and cheap can serve as an alternative or substitute to meat and egg diets.

Yeasts	Substrate	Production
Candida utilis (Torula Yeast)	Confectionary	England
	Ethanol	USA
	Sulphite Liquor	Europe, USA, Russia
C. intermedia	Whey	Vienna
C. krusei (+ Lactobacillus bulgarius)	Whey	Kiel
C. lipolytica	n-alkanes (C10 - C23) + ammonia	Russia
Kluyveromyces fragilis	Whey	France
Saccharomyces cerevisiae	Molasses	Most countries

 

By : Adrian T.

References

1. Srivastava, M.L., (2008), Fermentation Technology, Alpha Science International, Oxford
2. White, J. (1954) Yeast Technology, Chapman & Hall, London 
3. Chen, S. L. and Chinger, M. (1985) Production of Baker´s Yeast. In Comprehensive Biotechnology (H. W. Blanch, S. Drew and D. I. C. Wang Eds.) Vol. 3, p. 429, Pergamon, Oxford
4. Gellissen, G. (2000) Heterologous protein production in methylotrophic yeasts. Appl. Microbiol. Biotechnol. 54: 741
5. Trehan, K., (1990), Biotechnology, New Age International, New Delhi