Production of Yeast Biomass

In Primary Metabolites, much has been discussed about the use of yeasts in fermentation since centuries (see Alcohols). Consumption of baker's yeast (S. cerevisiae) as food in Germany during World War I increased its importance. Since then, rapid development took place in biotechnological applications of S. cerevisiae, as far as culture development, process optimization and scale up of products are concerned. World production of yeast biomass is of the order of 0.4 million metric tonnes per annum including 0.2 million tonnes baker's yeast alone.

Yeasts synthesize amino acids from inorganic acids and sulphur supplemented in the form of salts. They get carbon and energy sources from the organic wastes, e.g. molasses, starchy materials, milk whey, fruit pulp, wood pulp and sulphite liquor.

A comparative composition of nutrients is given in Table 18.3. It is obvious that biomass of S. cerevisiae produced on sugarcane molasses differs from that of bear. Yield of yeast biomass is greatly affected by many factors similar to bacteria. Yield corresponds to growth nutrients, organic wastes, temperature, culture, oxygen, etc. Yield of yeasts is given in parentheses in Table 18.2. Bennett et al. (1969) have given the typical equations for the growth of yeasts on carbohydrates or hydrocarbons:

Carbohydrates:
8n CHO + 0.8 nO₂ + 0.19n NH₄ + trace elements n(CH_1.7O₉₅N_0.12 Ash) + 0.8n CO₂+1.3 nH₂O + 80,000n KCal.

Hydrocarbons:
2nCH₂+2nO₂+0.19n NH₄+ trace elements n(CH_1.7O_0.5N_0.19 Ash) + nCO₂ + 1.5 H₂O + 200,000n KCal.

Fig. 18.2. Outline of treatment and recovery of yeast cells.

Factors Affecting the Yield of Yeast Biomass
Like bacteria, growth and yield of yeasts are also affected by the following factors: (i) organic substrate and nitrogen ratio (optimum C : N ratio favoring high protein content should be between 7:1 and 10:1); (ii) pH of nutrient medium (pH should be in the range of 3.5 to 4.5 to minimize growth of bacterial contaminates); (iii) temperature (it differs from organism to organism). Most yeasts have specific growth rate in the range of 30°C to 34°C. Some strains also grow in the range of 40-45°C; (iv) oxygen (for growth on carbohydrates), O₂ required should be 1 g/g of dried cells, and for growth on n-alkanes it should be about 2 g/g dried cells); (v) maintenance of sterile condition through out the process and (vi) suitable strain of yeast.

Recovery of Yeast Biomass
Yeast cells are small in size (5-8 m), the density of which reaches to 1.1 g/ml. Post-fermentation treatment of food yeast is shown in Fig. 18.2. Yeast cells are recovered by decantation-centrifugation (including washing) drying treatment methods. After washing undesirable traces of medium are removed which are again recycled for economic reasons. As a result of final harvesting by rotary vacuum filter a cake containing 20-40 per cent dry matter is obtained which is then dried to get a product of 6-10 per cent water content (Riviere, 1977).