BIOTECHNOLOGY - V. H. Gerasymenko - 2006

Part II. Special Biotechnologies

Chapter 21. BIOTECHNOLOGY OF PROTEIN PRODUCTION

21.2. MICROORGANISMS AS PROTEIN PRODUCERS

Yeasts are utilized in various microbiological processes and represent highly promising protein producers. Various Yeast strains are employed to obtain Single-Cell Protein (SCP) biomass, with cultures of the genera Candida, Torulopsis, and Saccharomyces being the most common. These genera are capable of utilizing hexoses, pentoses, and organic acids alike. During production, they rapidly adapt to toxic and inhibitory substances, exhibiting optimal growth at pH 4.2–4.4.

Yeast production relies on readily available and inexpensive raw Materials. Traditional carbon substrates for yeast cultivation include molasses, wood hydrolysates, whey, as well as starch industry and agricultural wastes. Non-traditional raw materials encompass petroleum distillates, n-alkanes, synthetic ethanol and methanol, among others.

As protein producers, yeasts possess several advantages over other microorganisms: their industrial cultivation Methods are well-established; yeast Cells are larger than bacterial cells, which facilitates Separation; and the nutritional value and hygienic quality of the biomass have been thoroughly studied.

The nutrient content of yeast biomass depends on the yeast species, cultivation conditions, and substrate. Protein content ranges from 40 to 60% of the dry biomass weight. In terms of protein quality, yeasts—much like other microorganisms—significantly outperform plant-based feeds and are comparable to animal-derived Proteins. For instance, fodder yeasts contain 5 times more protein than barley (including 10 times more Lysine, 5 times more Methionine, and 3 times more Tryptophan).

What makes fodder yeasts particularly valuable is their complex of B-group Vitamins, which play a key role in carbohydrate breakdown and energy release. In terms of vitamin content, yeasts surpass all protein feeds of PLANT AND ANIMAL origin.

Alongside their positive attributes, yeasts also exhibit certain drawbacks. Yeast protein is deficient in Sulfur-Containing Amino Acids (methionine content is 2–3 times lower than in meat protein) and contains a relatively high concentration (3–6%) of Nucleic Acids. Furthermore, yeast cells feature a rigid Cell wall that hinders the access of digestive Enzymes to intracellular nutrients, necessitating preliminary cell disintegration (rupture).

Ammonium salts or nitrates serve as the nitrogen source for yeasts, while carbon is derived from various organic substrates. The conversion of inorganic nitrogen into protein and the assimilation of diverse organic substrates as carbon sources are the core processes that have driven the large-scale industrial production of yeast protein.

Non-pathogenic Bacteria. As protein producers, bacteria attract attention due to their exceptionally high growth rate: on average, they grow about 4 times faster than yeasts and nearly 30 times faster than Algae. Additional advantages include a higher protein content compared to other microorganisms (60–83% of dry matter), as well as higher levels of the amino acids methionine and cystine relative to yeasts. Furthermore, bacterial cell walls are more easily disrupted.

The primary drawbacks of bacteria include a high nucleic acid level in the protein (up to 25%), the presence of cellular components whose safety for animals is questionable (such as cyclopropane and multi-branched Fatty acids, poly-β-hydroxybutyric acid, α-amino acids, etc.), a lower biomass yield coefficient, and a relatively complex harvesting process from the culture medium due to their small cell size.

Molds are undemanding regarding cultivation conditions and can thrive across various ranges of acidity, Temperature, and osmotic pressure. A distinctive feature of microscopic Fungi is their ability to synthesize a complex of hydrolytic enzymes capable of utilizing complex polyoses and Lignin, which remain largely inaccessible to other microorganisms. This enables the direct transformation of Cellulose- and starch-bearing substrates into protein, positioning microscopic fungi as promising producers of microbial protein derived from agricultural and forestry wastes, as well as certain food industry effluents.

Fungi of the genera Aspergillus, Fusarium, Trichoderma, and others are utilized as producers of protein, amino acids, vitamins, and other BIOLOGICALLY ACTIVE SUBSTANCES.

Fungal protein products offer several distinct advantages: they contain Aromatic Compounds that impart a pleasant odor; the filamentous nature of fungal protein ensures relatively easy biomass separation during industrial Processing; The amino acid profile features higher levels of sulfur-containing amino acids, closely approximating that of meat; fungal protein contains negligible amounts (1–4%) of nucleic acids; and the fungal cell wall is thin, allowing the protein to be easily digested without prior Treatment. They are highly promising as producers for obtaining fodder proteins, and fungal preparations are used to enhance the digestibility of roughage and enrich it with various biologically active substances.

The disadvantages of fungi include a comparatively low protein content, which varies widely (20–60% of dry matter), and relatively slow growth—the biomass doubling time for fungi is 4–16 hours, compared to 2–3 hours for yeasts.

Microalgae. Unicellular algae are phototrophic microorganisms that require distinct growth conditions compared to other protein-producing microorganisms. Their growth depends on light, an adequate supply of carbon dioxide and minerals, and a specific temperature regime, while the process itself requires substantial volumes of Water.

Among the vast array of unicellular algae species, the most suitable producers are green protococcal algae of the genera Chlorella and Scenedesmus, as well as the blue-green spiral alga Spirulina. Spirulina is approximately 100 times larger than Chlorella and Scenedesmus, measuring up to 500 μm. It thrives in alkaline environments at pH 9–11, whereas Chlorella and Scenedesmus prefer near-neutral environments. Protococcal algae possess a tough cellulosic cell wall, which is absent in Spirulina.

Under artificial conditions, microalgae are cultivated primarily on mineral media, though they also grow well in wastewater, manure slurry, and saline, fresh, or alkaline water bodies (in the case of Spirulina).

Microalgae belong to organisms characterized by active Biosynthesis of Proteins, vitamins, lipid-like compounds, and other biologically active substances. The protein content in green algae ranges from 45–55%, and reaches 60% in Spirulina. Microalgal protein contains all 10 Essential Amino Acids. In terms of these amino acids, it is comparable to the protein of fodder yeast and soybean. Conversely, algal protein has a low content of sulfur-containing amino acids, while Spirulina protein also exhibits a low lysine content and a relatively high concentration of nucleic acids.

Global experience indicates that algal biomass surpasses soybean meal in protein content (40–65% versus 35–40%, respectively), but lags behind soybean in fat content (2–15% versus 15–20%, respectively). However, the amino acid balance of algal biomass protein is lower compared to soybean protein. The protein digestibility of protococcal algae fluctuates between 45–46% due to their rigid cell wall, whereas for Spirulina it reaches 70%.

Microalgal biomass is rich in carotene (1500 mg/kg of dry biomass, which is 7–9 times higher than in top-grade alfalfa grass meal) and contains an Abundance of vitamins, macroelements, and microelements.

The chemical composition of microalgae is labile and largely determined by their cultivation conditions. By adjusting the nutrient medium composition and other parameters (such as temperature and illumination), the protein content in microalgae can be increased from 8 to 60%, CARBOHYDRATES from 6 to 37%, and Lipids from 5 to 85%.

Among microalgae, Spirulina is considered more promising for protein production, as its biomass contains an average of 65% protein, although the protein is somewhat deficient in lysine.