Single Cell Protein - Bacteria

Many species of bacteria have been investigated for use in single cell protein production. Methylophilus Methylotrophus has a generation time of about 2 hours and is usually and mainly used in animal feed as bacteria, in general produce a more favorable protein composition than yeast or fungi. Therefore the large quantities of SCP animal feed can be produced using bacteria. Among the characteristics that make bacteria suitable for this application include:

• Their rapid growth
• Their short generation times; most can double their cell mass in 20 minutes to 2 hours.
• Capable of growing on a variety of raw materials, ranging from carbohydrates such as starch and sugars, to gaseous and liquid hydrocarbons such as methane and petroleum fractions, to petrochemicals such as methanol and ethanol.
• Suitable nitrogen sources for bacterial growth include ammonia, ammonium salts, urea, nitrates, and the organic nitrogen in wastes.
• A mineral nutrient supplement must be added to the bacterial culture medium to furnish nutrients that may not be present in natural waters in concentrations sufficient to support growth.

 

Single cell protein integrated in Animal Feed

Cultivation of Bacteria (Methylophilus Methylotrophus)

Bacterial species used for single cell protein production should: 

• Grow best in slightly acid to neutral pHs in the range 5 to 7. 
• Tolerate temperatures in the range 35 to 45 degrees Celsius as heat is released during bacterial growth. 
• Be non-pathogenic for animal, human or plants depending on who the end product is intended for. 

Production of SCP from Methylophilus Methylotrophus

• Methanol is used as substrate. 2 metric tons of methanol yield around 1 metric ton of dry single cell protein. Methanol has several advantages over methane and many other carbon sources, particularly because it is completely miscible with water and is available in a very pure form.
• In addition to the carbon source, nutrient sources which can provide phosphorus, nitrogen, calcium and potassium must be provided in order to support optimal growth.
• Prevention of contamination of the medium and the plant can be done simply; by maintaining sterile and hygienic conditions. The medium components may be heated or sterilized by filtration. The circulating air and the gaseous components of the medium (NH₃, CO₂) are
• sterilized by passing through specialized filters. It is a common practice to sterilize other components by passing through steam. In addition to the components required for the fermentation and medium components the fermentation equipment is also sterilized. Filter-sterilized compressed air is used for both agitation and oxygenation. 
• Production of the microorganism; the desired micro organism has to be produced in a sufficient quantity in medium best suited for its growth, then inoculated into a medium similar to the fermentation medium and finally introduced into the main fermentation broth. At each step the purity of the culture has to be checked and can be done either by Gram staining or plating of the inoculated medium on to microbiological media. Fermentation was performed at pH 6.5 – 6.9 and 34-37^oC. Aeration is necessary for most of SCP production.
• Recovery of the microbial biomass from the spent medium: Methods available for concentrating single cell proteins include filtration, precipitation, coagulation, centrifugation and the use of semi permeable membranes.  Bacterial cells are recovered by flocculation which is promoted by acid and heat shock. Centrifugation is an effective concentration method which requires a high velocity rotor because of the microscopic size and low specific gravity and density of the cells and viscosity of the medium.
• Single cell protein must be dried to about 10% moisture, or condensed and has to be acidified to prevent spoilage from occurring or can alternatively be fed shortly after being produced. In addition to acidification for preservation purposes, it is also necessary to decrease the concentration of nucleic acid. Some of the methods employed for reducing nucleic acid content in microbial biomass are, alkane hydrolysis, chemical extraction, and activation of endogenous nucleases during final stage of microbial biomass production.
• The resulting product is the dried biomass of bacteria which appears pinkish-white, is odourless, tasteless and non-toxic. It consists of about 65-75% protein of balanced amino acid compilation.

Organic Carbon + Nitrogen + Mineral Nutrients + Oxygen -> Single Cell Protein + Carbon 

                                                                                                              Dioxide + Water + Heat

ICI 'Pruteen' plant for manufacturing single cell protein

However the general public thinks that all bacteria are harmful and produce diseases, therefore an extensive education program is required to remove this mistaken belief and to make the public accept bacterial protein. Although the cost of producing SCP form bacteria is relatively high due to the following factors:

• Bacterial cells have small size and low density, which makes harvesting from the fermented medium difficult and costly
• Bacterial cells have high nucleic acid content relative to yeast and fungi. To decrease the nucleic acid level additional processing step has to be introduced and this increases the cost.

References


Diaz, L.F., Savage, G.M., Eggerth, L.L., Rosenberg, L. & Centre, U.I.E.T., CalRecovery & Inc (2005) Solid waste management, Volume 1, UNEP/Earthprint. Available at: http://books.google.com/books?id=5gyfohtKFtEC&pgis=1


Marx, J.L. (1989) A Revolution in biotechnology, Cambridge University Press. Available at: http://books.google.com/books?id=yU89JSZsS30C&pgis=1

Nasseri, A., Rasoul-Amini, S., Morowvat, M. & Ghasemi, Y. (2010) Single Cell Protein: Production and Process. American Journal of Food Technology, 6, p.pp.103-116. Available at: http://scialert.net/fulltext/?doi=ajft.2011.103.116&org=10

Waites, M.J. (2001) Industrial microbiology, Wiley-Blackwell. Available at: http://books.google.com/books?id=9vB0gz775MYC&pgis=1


By Kishan Vaghela