BIOTECHNOLOGY - V. H. Gerasymenko - 2006

Part II. Special Biotechnologies

Chapter 21. BIOTECHNOLOGY OF PROTEIN PRODUCTION

21.9. OBTAINING HIGH-PROTEIN FEED SUPPLEMENTS FROM RENEWABLE RAW MATERIALS

21.9.2. Solid-state fermentation of plant-based raw materials

Submerged culture Fermentation remains the primary method of microbiological synthesis today. Solid-state fermentation serves as a viable alternative approach in microbial technology.

Solid-state fermentation refers to microbiological processes that take place not in an aqueous phase, but within a mass of slightly moistened substrate. The optimal moisture content depends on The properties of the substrate and the producing microorganism, typically ranging from 25% to 75%. Under these conditions, the aqueous phase is represented solely by thin films On the surface of the substrate particles and Water sorbed by the substrate. Thus, Microbial growth in solid-state fermentation occurs at the solid substrate–water–air interface and, in some cases, inside the substrate particles.

Starch- or Cellulose-containing raw Materials are utilized as substrates. Among the producing microorganisms that thrive under solid-state fermentation conditions, filamentous Fungi and macromycetes in mycelial form are most commonly used, occasionally alongside mixed cultures of fungi and Yeasts, while pure Yeast cultures are typically employed for fermenting starchy substrates.

Solid-state fermentation has been practiced for centuries in the Far East and Indochina to prepare traditional national dishes. Europeans became acquainted with this technology in the early 20th century, and by the 1950s it was widespread before being largely supplanted by submerged fermentation. Today, solid-state fermentation is regaining prominence due to several compelling reasons.

First, using water-insoluble substrates (plant raw materials) significantly complicates submerged culture fermentation, requiring additional operations for substrate loading and product discharge, and rendering continuous processes nearly impossible. Second, it is an extensive, and therefore cost-effective, technological approach. Third, certain processes have been shown to proceed many times more intensively under solid-state fermentation conditions.

The simplest technological variants of solid-state fermentation include compost heaps and surface mold cultivation. Derivatives of these Methods form The basis of solid-state fermentation types utilized in both laboratory practice and industrial production.

1. Surface solid-state fermentation (the so-called "thin-layer" method), in which the microbial culture grows in a substrate layer 3–7 cm thick within a chamber where the required Temperature and air humidity are maintained. Large aluminum trays with areas of up to several square meters or dedicated cultivation chambers serve as bioreactors (Fig. 21.2).

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Fig. 21.2. Bioreactor (cultivation chamber) for solid-state surface cultivation

(according to N. S. Egorov et al., 1987):

the substrate (straw) rests on a metal mesh; exhaust gas leaves the apparatus through Pores in the lid (1); Glass wool (2) forms a filter that prevents foreign microflora from entering the bioreactor.

2. Submerged solid-state fermentation in a mixed bed (the so-called "deep-layer" method). In this approach, the microbial culture grows throughout the entire mass of the substrate, achieved via appropriate aeration (ventilation) techniques.

3. Solid-state fermentation in a mixed and aerated substrate mass. The substrate may be homogeneous (semi-liquid manure) or consist of suspended solid particles (representing a transitional variant between solid-phase and liquid-phase processes). Bioreactors equipped with low-speed mixers are typically used. Agitation in the presence of a high solids content can damage biological objects, which is particularly detrimental to filamentous fungi. Gentle mixing is achieved by using screw agitators or rotating drum bioreactors.

Interest in solid-state processes is driven by several advantages over liquid-medium processes:

1) they require lower capital and operating costs;

2) The Nature of the substrate facilitates product Separation and purification;

3) the low water content in the substrate inhibits contamination of the producer culture by foreign microflora;

4) unlike liquid-phase processes, solid-state processes do not generate large volumes of wastewater—a potential source of environmental pollution—requiring discharge.

Solid-state cultivation also presents certain drawbacks. Most solid-state processes lack agitation, and microbial growth occurs via colonization: as they multiply, microorganisms spread outward from the inoculation points throughout the entire volume. Consequently, certain zones within the substrate bulk become overcrowded with Cells, leading to localized nutrient depletion while a significant portion of the substrate remains untouched. Controlling aeration efficiency across different substrate zones, as well as maintaining temperature and moisture levels, remains an unresolved challenge.

The product obtained after solid-state fermentation contains residual starting material enriched with 6–22% protein, depending on the substrate composition. For instance, solid-state fermentation in a mixed and aerated bed using Aspergillus niger or yeast strains on granulated starchy substrates yielded a product containing 17–20% protein. Alkali-treated sawdust fermented in a thin layer contained up to 12% protein, whereas untreated sawdust yielded up to 6–8%.

Solid-state fermentation as a method for improving the Nutritional Value of roughage has broad Prospects in Ukraine as a foundation for developing low-capacity feed Supplement production technology tailored to small farms and livestock enterprises. Focusing on inexpensive local raw materials, such as straw and various agricultural and industrial wastes, this technology can become one of the primary pathways for producing animal protein. This will help accelerate the reform of the agrarian sector of our country's economy.

Whey, an affordable byproduct of cheesemaking, can also be used as a substrate for yeast production. In France, back in 1979, three plants with an annual production capacity of 6,000 tonnes were producing protein for animal feed and human consumption.