Yeast and certain bacteria perform ethanol fermentation where pyruvate (from glucose metabolism) is broken into ethanol and carbon dioxide. The net chemical equation for the production of ethanol from glucose is:
C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)
Ethanol fermentation has used the production of beer, wine, and bread. It’s worth noting that fermentation in the presence of high levels of pectin results in the production of small amounts of methanol, which is toxic when consumed.
Lactic Acid Fermentation
The pyruvate molecules from glucose metabolism (glycolysis) may be fermented into lactic acid. Lactic acid fermentation is used to convert lactose into lactic acid in yogurt production. It also occurs in animal muscles when the tissue requires energy at a faster rate than oxygen can be supplied. The next equation for lactic acid production from glucose is:
C6H12O6 (glucose) → 2 CH3CHOHCOOH (lactic acid)
The production of lactic acid from lactose and water may be summarized as:
C12H22O11 (lactose) + H2O (water) → 4 CH3CHOHCOOH (lactic acid)
Hydrogen gas is produced in many types of fermentation (mixed acid fermentation, butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate fermentation) as a way to regenerate NAD+ from NADH. Electrons are transferred to ferredoxin, which in turn is oxidized by hydrogenase, producing H2. Hydrogen gas is a substrate for methanogens and sulfate reducers, which keep the concentration of hydrogen low and favor the production of such an energy-rich compound,but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus.
As an example of mixed acid fermentation, bacteria such as Clostridium pasteurianum ferment glucose producing butyrate, acetate, carbon dioxide, and hydrogen gas: The reaction leading to acetate is:
- C6H12O6 + 4 H2O → 2 CH3COO− + 2 HCO3− + 4 H+ + 4 H2
Glucose could theoretically be converted into just CO2 and H2, but the global reaction releases little energy.
Modes of operation
Most Industrial fermentation uses batch or fed-batch procedures, although continuous fermentation can be more economical if various challenges, particularly the difficulty of maintaining sterility, can be met.
In a batch process, all the ingredients are combined and the reactions proceed without any further input. Batch fermentation has been used for millennia to make bread and alcoholic beverages, and it is still a common method, especially when the process is not well understood. However, it can be expensive because the fermentor must be sterilized using high pressure steam between batches.Strictly speaking, there is often addition of small quantities of chemicals to control the pH or suppress foaming.
Batch fermentation goes through a series of phases. There is a lag phase in which cells adjust to their environment; then a phase in which exponential growth occurs. Once many of the nutrients have been consumed, the growth slows and becomes non-exponential, but production of secondary metabolites (including commercially important antibiotics and enzymes) accelerates. This continues through a stationary phase after most of the nutrients have been consumed, and then the cells die.
Fed-batch fermentation is a variation of batch fermentation where some of the ingredients are added during the fermentation. This allows greater control (process control) over the stages of the process. In particular, production of secondary metabolites can be increased by adding a limited quantity of nutrients during the non-exponential growth phase. Fed-batch operations are often sandwiched between batch operations.
The high cost of sterilizing the fermentor between batches can be avoided using various open fermentation approaches that are able to resist contamination. One is to use a naturally evolved mixed culture. This is particularly favored in wastewater treatment, since mixed populations can adapt to a wide variety of wastes. Thermophilic bacteria can produce lactic acid at temperatures of around 50 degrees Celsius, sufficient to discourage microbial contamination; and ethanol has been produced at a temperature of 70 °C. This is just below its boiling point (78 °C), making it easy to extract. Halophilic bacteria can produce bioplastics in hypersaline conditions. Solid-state fermentation adds a small amount of water to a solid substrate; it is widely used in the food industry to produce flavors, enzymes and organic acids.
In continuous fermentation, substrates are added and final products removed continuously. There are three varieties: chemostats, which hold nutrient levels constant; turbidostats, which keep cell mass constant; and plug flow reactors in which the culture medium flows steadily through a tube while the cells are recycled from the outlet to the inlet. If the process works well, there is a steady flow of feed and effluent and the costs of repeatedly setting up a batch are avoided. Also, it can prolong the exponential growth phase and avoid byproducts that inhibit the reactions by continuously removing them. However, it is difficult to maintain a steady state and avoid contamination, and the design tends to be complex.Typically the fermentor must run for over 500 hours to be more economical than batch processors.
Selecting Yeast in Beer Brewing and Wine Making
Humankind has benefited from fermentation products, but from the yeast’s point of view, alcohol and carbon dioxide are just waste products. As yeast continues to grow and metabolize sugar, the accumulation of alcohol becomes toxic and eventually kills the cells (Gray 1941). Most yeast strains can tolerate an alcohol concentration of 10–15% before being killed. This is why the percentage of alcohol in wines and beers is typically in this concentration range. However, like humans, different strains of yeast can tolerate different amounts of alcohol. Therefore, brewers and wine makers can select different strains of yeast to produce different alcohol contents in their fermented beverages, which range from 5 percent to 21 percent of alcohol by volume. For beverages with higher concentrations of alcohol (like liquors), the fermented products must be distilled.