The following points highlight the eight commonly occurring fermentations. The fermentations are: 1. Alcohol (Ethanol) Fermentation 2. Lactic Acid Fermentation 3. Butyric Acid (and Butanol) Fermentation 4. Formic Acid Fermentation 5. Mixed Acid Fermentation 6. Butanediol Fermentation 7. Propionic Acid Fermentation 8. Mixed Amino Acid Fermentation.
1. Alcohol (Ethanol) Fermentation:
Alcohol (ethanol) fermentation is carried out by yeasts (e.g., Saccharomyces cerevisiae) and by relatively few bacteria (e.g., Zymomonas). These microorganisms ferment hexose sugar (glucose) to ethanol and CO2. Yeast ferments glucose to ethanol via the glycolytic pathway, whereas Zymomonas employs the Enter-Doudoroff pathway.
i. Alcohol (ethanol) fermentation by yeast:
Yeasts (Saccharomyces cerevisiae) ferment glucose to ethanol via glycolytic pathway (glycolysis). In this fermentation the glucose is converted to pyruvate via various steps of glycolysis. The pyruvate is then decarboxylated to acetaldehydc by enzyme pyruvate decarboxylase, which is then reduced to ethanol by enzyme alcohol dehydrogenase with NADH as the electron donor (Fig. 26.6).
ii. Alcohol (ethanol) fermentation by Zymomonas:
Zymomonas is a large, gram-negative, rod-shaped bacterium that carries out a vigorous fermentation of sugars to ethanol. Zymomonas is a common microorganism involved in alcoholic fermentation of various plant saps. It occupies a position in the fermented beverage industry in many tropical areas of South and Central America, Africa and Asia similar to that of Saccharomyces cerevisiae (Yeast) in North America and Europe.
Although Zymomonas is rarely the sole organism involved in alcoholic fermentations, it is often the dominant organism and is probably responsible for the production of most of the ethanol in these beverages. Unlike yeast that ferments glucose to ethanol via glycolytic pathway (glycolysis), Zymomonas uses the Enter-Doudoroff pathway of glucose breakdown to pyruvate of glucose breakdown to ethanol (Fig. 26.7).
2. Lactic Acid Fermentation:
Lactic acid fermentation is carried out by lactic acid bacteria, a group of gram-positive bacilli and cocci bacteria which are usually aerotolerant anaerobes, i.e., they are not sensitive to O2 and can grow anaerobically in its presence as well as in its absence. Most of lactic acid bacteria obtain energy only from the metabolism of sugars (glucose) and hence are usually restricted to sugar containing habitats.
Lactic acid bacteria ferment sugar to lactic acid as a sole or major fermentation product. Members of this group lack porphyrins and cytochromes, do not carry out electron transport chain/oxidative phosporylation, and hence obtain energy only by substrate-level phosphorylation. Lactic acid bacteria consist of two subgroups differentiated on the basis of the nature of the products they form from the fermentation of sugars.
One subgroup is called homo fermentative, whereas the other subgroup is called heterofermentative. The differences observed in the fermentation products of the two subgroups are determined by the presence or absence of the enzyme aldolase, the key enzyme in glycolysis.
i. Homolactic fermentation:
Homolactic fermentation (Fig. 26.8) is carried out by the homofermentative subgroup of lactic acid bacteria constituted of Streptococcus, Enterococcus, Lactococcus, Pediococcus, and various Lactobacillus species. These bacteria ferment the sugar to lactic acid as the only fermentation product.
They have the enzyme aldolase, the key enzyme in glycolysis, which breaks down fructose 1, 6-bisphosphate to glyceraldehyde 3-phosphate (a triose phosphate), and hence use glycolytic pathway (glycolysis) for fermentation.
Pyruvate, the end product of glycolysis, is directly reduced to lactic acid with the enzyme lactate dehydrogenase. Lactic acid is quite important in dairy industry where it is used for souring milk and also for production of various types of cheese, yoghurt, and other dairy products.
Certain species of Streptococcus play a major role in ‘dental caries’ formation due to production of lactic acid on tooth surface; lactobacilli occur in human digestive tract and help in digestion of milk; Lactobacillus acidophilus is added to milk diet of those unable to digest milk carbohydrates.
ii. Heterolactic fermentation:
In contrast to homolactic fermentation, some members (Leuconostoc, some Lactobacillus species) of lactic acid bacteria group carry out heterolactic fermentation (Fig. 26.9) in which they produce mainly ethanol and CO2 along with lactic acid.
Heterofermentative bacteria do not follow the glycolytic pathway (glycolysis) because they lack enzyme aldolase and therefore cannot break down fructose 1, 6-bisphosphate to glyceraldehyde 3-phosphate.
Instead they use phosphoketolase pathway to oxidise glucose 6-phosphate to 6-phosphogluconate and then to decarboxylate this to ribulose 5-phosphate, which is broken down to glyceraldehyde 3-phosphate and acetyl phosphate by means of the enzyme phosphoketolase.
Glyceraldehyde 3-phosphate is converted ultimately to lactate (lactic acid) with the production of one molecule of ATP, while the acetyl phosphate accepts electrons from the NADH generated during the production of ribulose 5-phosphate and is thereby converted to ethanol without yielding ATP.
Because of this, the heterofermenters produce only one molecule of ATP from glucose instead of the two molecules produced by homofermenters. Because the heterofermenters decarboxylate 6-phosphogluconate, they produce CO2 as a fermentation product, whereas the homofermenters produce little or no CO2.
Heterolactic fermentation causing bacteria can be isolated from plants, silage, and milk. Leuconostoc genus is used in wine production, in fermentation of vegetables such as cabbage (sauerkraut) and cucumbers (prickles), and in the manufacture of buttermilk, butter, and cheese.
3. Butyric Acid (and Butanol) Fermentation:
Butyric acid fermentation is carried out by species of Clostridium. A number of Clostridium species ferment sugars and produce butyric acid as a major end product. Some species also produce butanol. However, clostridia lack a cytochrome system and a mechanism for electron transport chain/oxidative phosphorylation hence obtain ATP only by substrate-level phosphorylation.
The biochemical steps in the butyric acid and butanol fermentation from sugars are well understood and given in Figure 26.10. Glucose is converted to pyruvate via glycolysis, and pyruvate is split to acetyl-CoA and carbon dioxide (CO2). Two acetyl-CoA molecules interact forming acetoacetyl-CoA, which then is reduced to butyryl-CoA through certain intermediate steps.
Butyric acid fermentative clostindia ferment butyryl-CoA into butyric acid (butyrate), whereas butanol fermentative clostindia convert butyryl-CoA into butanol. During the early stages of fermentation pathway, butyric acid is the predominant product, but as the pH of the medium drops, synthesis of butanol initiates.
4. Formic Acid Fermentation:
Many bacteria, especially members of the family Enterobacteriaceae (e.g., Escherichia, Enterobacter, Salmonella, Proteus) ferment pyruvate to formic acid (formate) and other products. This conversion is called formic acid fermentation. Formic acid may be converted to H2 and CO2 by enzyme formate dehydrogenase (Fig. 26.11).
5. Mixed Acid Fermentation:
Mixed acid fermentation is relatively common and is named so because the end-products are a complex mixture of acids (particularly acetic, lactic, succinic and formic acids) along with ethanol (Fig. 26.12). If formate dehydrogenlyase is present, the formic acid is degraded to H2 and CO2 (as discussed in formic acid fermentation).
Mixed acid fermentation is carried out by the same bacteria of family Enterobacteriaceae that cause formic acid fermentation. The proportions of the products vary depending on the bacterial species. This fermentation can be detected by methyl red (MR) test.
6. Butanediol Fermentation:
This fermentation is the characteristic of Enterobacter, Serratia, Erwinia, and some species of Bacillus (Fig. 26.13). Pyruvate is converted to acetoin, which is then reduced to 2-3-butanediol with NADH. Some bacteria, as species of Klebsiella, carry out both the butanediol and mixed acid fermentations. Butanedoile fermentation can be detected using the Voges-Proskauer (VP) test.
7. Propionic Acid Fermentation:
Propionic acid bacteria (genus Propionibacterium) carry out propionic acid fermentation characteristically. Either lactic acid, produced by the fermentative activities of other bacteria, or glucose can be fermented by these bacteria. The enzymatic reactions leading from glucose and lactic acid to propionic acid are shown in Fig. 26.14.
Propionic acid fermentation from lactic acid is important in Swiss cheese manufacture where the fermentative production of CO2 causes characteristic holes.
8. Mixed Amino Acid Fermentation:
Microorganisms also ferment substances other than carbohydrate (glucose). For convenience, members of genus Clostridium (e.g., C. sporogenes, C. botulinum) often can ferment mixtures of amino acids by oxidizing one amino acid and using another as an electron acceptor.
This process is called Stickland reaction. This reaction generates NH3, H2S, fatty acids, and CO2. This fermentation causes unpleasant odour of some wines and cheeses and is also responsible partly for the horrible smell of a gangrenous wound. The fermentation is quite useful for microorganisms growing in anaerobic protein-rich environments. The stick land reaction of a mixture of alanine and glycine is given in Fig. 26.15.
