MICROBIOLOGY Textbook - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.5. FERMENTATIONS

9.5.7. Butyric Acid Fermentation

Butyric acid Fermentation proceeds under strictly anaerobic conditions via the Hexose diphosphate pathway down to Pyruvate. A characteristic feature of butyric acid fermentation is the Condensation reaction of two acetyl-CoA molecules (i.e., C₂ + C₂ = C₄) mediated by the enzyme carboligase, yielding acetoacetyl-CoA, which is subsequently reduced to butyric acid (Fig. 33).

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Fig. 33. Pathway of butyric acid fermentation.

Enzymes involved in fermentation: F₁ — carboligase; F₂ — hydroxybutyryl-CoA dehydrogenase; F₃ — crotonase; F₄ — butyryl-CoA dehydrogenase; F₅ — CoA transferase

Typical agents of butyric acid fermentation are Bacteria of the genus Clostridium, specifically C. butyricum and C. pasteurianum.

Clostridia are rod-shaped, Gram-positive bacteria belonging to the family Bacillaceae. Most species are motile by means of peritrichous flagella. As the Cells age, they lose motility, accumulate the reserve material granulose (a starch-like polysaccharide), and initiate sporulation. Clostridia form oval or round endospores whose diameter exceeds that of The Cell. When the spore is located in the center of the cell, the latter assumes a spindle-like shape (fusiform); when situated at the pole, the cell resembles a drumstick or tennis racket (see Fig. 10). Clostridial spores are quite thermoresistant.

Clostridia are strict anaerobes devoid of Hemoproteins (Cytochromes, catalase). Their optimal growth Temperature ranges from 30 to 40 °C. Alongside mesophilic clostridia, thermophilic species also occur, having a temperature optimum of 60–75 °C, notably C. thermoaceticum. Like most members of the family Bacillaceae, clostridia are capable of growing only in a neutral or slightly alkaline environment. Their undesirable proliferation in food products can be completely suppressed by medium acidification (fermentation of cabbage and cucumbers, souring of minced meat in dry-cured sausages, pickling of vegetables and mushrooms).

Based on their ability to utilize various substrates, clostridia can be divided into the following groups:

✵ saccharolytic — breaking down primarily Polysaccharides or sugars. This group includes C. butyricum, C. acetobutylicum, etc.;

✵ proteolytic — breaking down Proteins, peptones, and Amino Acids. This group comprises C. putrificum, C. sporogenes, C. histolyticum;

✵ purinolytic — capable of degrading Purines and Pyrimidines. This group includes bacteria of the species C. acidiurici.

During butyric acid fermentation, pyruvate derived from glucose is cleaved to yield carbon dioxide, acetyl-CoA, and reduced ferredoxin (an FeS protein). The latter transfers electrons to protons, giving rise to another metabolite of butyric acid fermentation — hydrogen. Condensation of two acetyl-CoA molecules leads to The formation of acetoacetyl-CoA, which is then reduced to β-hydroxybutyryl-CoA. The elimination of Water from β-hydroxybutyryl-CoA produces a compound with a double carbon bond, crotonyl-CoA. It is reduced in a subsequent reaction to form butyryl-CoA. The transfer of coenzyme A from butyryl-CoA to acetate results in the Formation of the primary metabolite, butyric acid.

The presence and proliferation of butyric acid bacteria in food products is highly undesirable. Due to the excessive gas production during carbohydrate fermentation, defects such as late gas blowing in cheese and can Swelling (bombage) occur. The accumulation of butyric acid leads to a rancid taste and a sharp, unpleasant odor in the product.

In the microbiological industry, butyric acid fermentation is utilized to produce butyric acid, which serves as a base for synthesizing various esters. Butyric acid esters have a pleasant aroma and are widely used as flavoring agents in the perfumery, confectionery, and soft drink industries.