Role of microorganisms in the fermentation

One tiny microscopic microbial cell will not produce industrially significant volume of fermentation products. However, when we talk about billions and billions of microbial cells in the fermentation vat, the volume and concentration of fermentation products is industrially and economically significant! The diversity of fermentation products produced by the microorganisms is attributed to the rich diversity of microorganisms which have diverse metabolisms that can yield various types of fermentation products.

FERMENTATION

• Fermentation -- oxidation of an organic compound in the absence of external electron
acceptor (no oxygen required). Uses SLP (substrate-level phosphorylation).
• Respiration -- oxidation of an organic compound where oxygen is the final electron
acceptor. Uses ETS (electron transport system) as well as SLP.
• Anaerobic respiration (unique to bacteria) -- oxidation of organic compounds where
an external substrate other than oxygen serves as final electron acceptor. Exs: nitrate, sulfate, carbon dioxide.

Lactic acid fermentation
• pyruvate + NADH lactic acid + NAD+
• found in many bacteria: lactic acid bacteria, Bacillus, also in some protozoa, water
molds, even human skeletal muscle
• Responsible for souring of milk products yogurt, cheese, buttermilk, sour cream, etc. Excellent keeping properties.
• Some bacteria produce only lactic acid = Homolactic fermenters
• Other bacteria produce other products as well; ethanol, CO2, lactate, etc. =
Heterolactic fermenters
Alcoholic fermentation
• pyruvate acetaldehyde + CO2
• acetaldehyde + NADH ethanol + NAD+
• Found in many fungi, yeasts, some bacteria.
• Very important in human applications. Bread, alcoholic spirits


Formic acid and mixed acid fermentations
• pyruvate (3-C) + CoA Acetyl-CoA (2-C) + formic acid (1-C) • HCOOH CO2 + H2
• found in many bacteria, very common in enterics (Gram-negative facultative anaerobic rods, include E. coli and other common intestinal tract denizens)
Useful in identification: 2 common variants
Mixed acid fermentation: Some bacteria use several pathways, produce ethanol, formic acid, acetic acid, lactic acid, succinic acid, CO2, and H2. Note lots of acid, lower pH than
many other fermentations. 
Note: ATP yield via mixed acid is ~2.5 ATP/glucose, a bit higher than straight lactic acid fermentation.


Butanediol fermentation:
Butanediol produced, also much more CO2, and H2

Roles of fermentation in nature

• Fermentations play major role.
• large part of cellulose ingested by herbivores is excreted in undigested form.
• Wherever organic matter accumulates, bacteria can grow and remove oxygen (by respiration), leading to anaerobic conditions that favor fermentation.
• Even in lab cultures (test tubes of media), bacteria eat up all available oxygen, rely largely on fermentation unless vigorous aeration is maintained! Bacteria are pigs, gorge
themselves at every opportunity!
• Beside bacteria, fermentations also carried out be protozoa, fungi, even animal muscle
tissues (only works as temporary energy supplement).


What substances can be fermented?
• must have intermediate oxidation state (o.s.)
• if totally oxidized (-CO)n cannot be fermented
• if totally reduced (-CH2)n, cannot be fermented
• must be convertible to a substrate for substrate level phosphorylation (usually into some glycolytic step)
• Many sugars can be fermented. Also amino acids (e.g. by Clostridia, oxidizing one
amino acid and using a different amino acid as electron acceptor.)


Respiration
• Use an external electron acceptor. Oxygen as prototype.
• The "problem" with fermentation is that, by using an organic molecule as a terminal
electron acceptor to be discarded as waste, cell is losing out on potential to further oxidize organic molecule, get more energy.
• Alternative solution is to use some non-organic molecule that has a low redox potential, can accept electrons and become some reduced molecule. Oxygen is perfect
for this, has extremely low redox potential, and becomes reduced to water, the "perfect" waste product for an aqueous environment.
• To transfer electrons (and protons, H+) to oxygen, need special oxidase enzyme. In
mitochondria, this is a cytochrome, cyt a. In bacteria, different cytochromes; in E. coli, cyt o or d.





There are numerous examples for fermented food
 Fish sauce
In production of fish sauce, eviscerated fish is mixed with salt and placed in fermented tanks to allow liquefaction for about six months. The collected liquid is further ripened for few more months. Halophillic microbes are involved in this fermentation process. Streptococcus, Micrococcus and Bacillus species predominate. This product is dark colored with a distinct aroma.

Sauerkraut 
This refers to fermented cabbage. Normal microflora in cabbage is involved in the fermentation process under anaerobic conditions. Leuconostoc mesenteroides and Lactobacillus plantarum is involved. Temperature is a crucial factor in the control of fermentation. If the temperature is below 21 degrees Celcius, Lactobacilli outgrow. L. mesenteroides require a lower temperature below 21 0C. Acidity created by Lactobacilli prevents the growth of L. mesenteroides.

Pickels
Pickels consist of vegetables like cucumber, onions, chilies etc. Lactic acid bacteria such as Leuconostoc mesenteroides, P. cerevisiae, L. brevis, and L. plantarum are involved in the fermentation process these bacteria also take part in fermentation of olives.

Soy sauce 
In production of soy sauce, a mixture of soybean and wheat flour is inoculated with Aspergillus oryzae and Aspergillus soyae. These fungi digest complex starch and produce sugars which facilitate the growth of bacteria. Anaerobic bacteria carry out fermentation to produce soy sauce.

Beer and Ale Malted beverages are produced by brewing. Mainly the yeasts are involved in the process. Yeasts convert fermentable sugars to ethanol and carbon dioxide. As yeasts do not produce enough amylases to hydrolyze starch in barley grains, they are germinated prior to brewing. Hops which are added for bitterness have an inhibitory effect on gram positive bacteria. Saccharomyces carlsbergensis is the principle organism used. This species is subjected to various genetic modifications to increase the efficiency of the fermentation. In addition to ethanol and carbon dioxide, yeasts produce a small amount of glycerol, acetic acid and aromatic esters. Ale is a top fermented beverage with Saccharomyces cerevisiae.

Wine
Wine is made from grape juice in large scale. Yests; Saccharomyces cerevisiae var. ellipsoideus is the culture used in wine fermentation. High temperature is not suitable for this fermentation as yeasts die while low temperature allows the growth of lactic acid bacteria.