BIOTECHNOLOGY - V. H. Gerasymenko - 2006

Part II. Special Biotechnologies

Chapter 21. BIOTECHNOLOGY OF PROTEIN PRODUCTION

21.6. PRODUCTION OF MICROBIAL PROTEIN FROM LOWER ALCOHOLS: METHANOL AND ETHANOL

Methanol, a product of methane oxidation, is considered the most promising substrate for microbial protein production. Large-scale methanol production relies not only on natural gas methane, whose reserves are limited, but also on biogas methane derived from renewable plant biomass.

The main advantages of this substrate are its high purity, good Water solubility, and high volatility, which facilitates the easy removal of residues from the final product. Furthermore, microbial biomass produced on methanol is free of undesirable impurities, thus eliminating the purification stage in the technological process. Disadvantages of methanol as a substrate include its flammability, the potential to form explosive mixtures with air at concentrations ranging from 6 to 35 vol. %, and its high toxicity, which requires strict adherence to safety regulations during handling.

Both Yeast and bacterial strains serve as producer microorganisms. Commonly used Yeasts include Candida boidinii, Hansenula polymorpha, and Pichia pastoris, whereas bacterial cultures include Methylomonas clara, Pseudomonas rosea, and others.

A critical factor when using methanol is the Selection of an efficient producer strain. Research has shown that Bacteria assimilate methanol with a higher microbial biomass yield compared to yeasts; however, yeasts offer several technological advantages over bacteria. The initial oxidation of methanol in yeasts

is catalyzed by an oxidase reaction, whereas in bacteria it is catalyzed by a dehydrogenase. Combining the advantages of both microorganisms within a single Cell using Introduction/32.html">Genetic Engineering techniques represents a promising approach. To achieve this, Recombinant DNA technology is employed to transfer the Gene encoding the bacterial methanol dehydrogenase enzyme—which catalyzes the initial methanol oxidation—from a bacterial cell into a yeast cell, subsequently establishing conditions for its expression.

Another promising approach is The Use of mixed cultures, where some strains consume only methyl alcohol while others utilize the remaining substrates and metabolic byproducts.

Fodder yeast produced on methanol (meprin) contains 56–62 % crude protein, whereas bacterial biomass contains 70–74 %.

Ethanol as a substrate for microbial protein production offers several advantages: low toxicity, good water solubility, fairly high volatility, minimal impurities, and the ease with which it is assimilated by virtually all microorganisms. In addition, because the alcohol molecule contains oxygen, the demand for oxygen or air is lower, resulting in reduced heat generation and lower aeration requirements. Nevertheless, ethanol is a highly flammable liquid that forms explosive mixtures with air at concentrations ranging from 3 to 20 vol. %.

The global market price of purified ethanol is twice that of methanol, yet ethanol exhibits exceptionally high bioconversion efficiency. Up to 880 g of yeast biomass can be obtained from 1 kg of ethanol, compared to up to 440 g from 1 kg of methanol. Ethanol-based production yields high-quality protein suitable for human consumption.

Microorganisms utilized as protein producers on ethyl alcohol (as the sole carbon source) include yeasts (Candida utilis, Saccharomyces lambica, Hansenula anomala) and bacteria (Acinetobacter calcoaceticus), although the latter are of lesser practical importance.

Yeast-based Single-Cell Protein produced on ethanol (eprin) contains 60–62 % crude protein and is exceptionally rich in Lysine (up to 7 %). A drawback of this protein is its low content of Sulfur-Containing Amino Acids, primarily Methionine, which reduces its biological value.