Fermentation is a metabolic process that allows organisms to extract energy from carbohydrates in the absence of oxygen. This anaerobic pathway is one of the oldest forms of energy generation on Earth, predating the oxygen-rich atmosphere. Humans have utilized this process for millennia for the production and preservation of foods and beverages. Fermentation transforms raw ingredients into products with unique textures, flavors, and extended shelf lives, as seen in the making of bread, cheese, and alcoholic drinks.
The Fundamental Mechanism of Fermentation
This process is an alternative way for a cell to produce adenosine triphosphate (ATP), the cell’s energy currency, when oxygen is unavailable. The initial step for nearly all forms of fermentation is glycolysis, where a glucose molecule is broken down into two molecules of pyruvate, generating a net gain of two ATP molecules. This breakdown reaction also reduces the coenzyme Nicotinamide Adenine Dinucleotide (NAD+) to NADH.
Glycolysis requires a continuous supply of NAD+, but in anaerobic conditions, the cell cannot use oxygen to recycle NADH back to NAD+. Fermentation serves the specific purpose of regenerating NAD+ by transferring the electrons from NADH onto an organic molecule, usually pyruvate or a derivative. This recycling step ensures that glycolysis can continue running, allowing the cell to maintain a steady supply of ATP.
Lactic Acid Fermentation
Lactic acid fermentation is a single-step process where pyruvate generated by glycolysis is directly converted into lactate. The enzyme lactate dehydrogenase catalyzes this reaction, simultaneously oxidizing NADH back to NAD+. This pathway is common in bacteria and is also utilized by animal cells.
This anaerobic process occurs in human muscle tissue during periods of intense exertion when oxygen delivery cannot keep pace with the energy demand. The temporary accumulation of lactate allows the muscle cells to continue producing ATP until oxygen delivery is restored. Lactic acid bacteria, such as those in the Lactobacillus genus, drive the use of this process in the food industry.
Bacteria ferment milk sugars, such as lactose, into lactic acid, which rapidly lowers the acidity. This drop in pH causes milk proteins to thicken, coagulating the product into the texture characteristic of yogurt. The resulting acidic environment acts as a natural preservative by inhibiting the growth of spoilage microorganisms, a mechanism also used to preserve foods like sauerkraut, kimchi, and pickles.
Alcoholic (Ethanol) Fermentation
Yeasts, particularly Saccharomyces cerevisiae, and some bacteria perform alcoholic fermentation. This pathway converts pyruvate into two products: ethanol and carbon dioxide (CO2), requiring two enzymatic steps. The first step involves converting pyruvate into acetaldehyde and releasing CO2 as a gaseous byproduct.
In the second step, acetaldehyde accepts electrons from NADH, converting it into the final product, ethanol. Crucially, this transfer oxidizes the NADH back into NAD+, which is immediately available to keep the glycolysis pathway operating. This regeneration mechanism is the main biological reason for the process.
The industrial applications of this pathway center on these two end products. In the brewing and winemaking industries, ethanol is the desired chemical component of the beverages. The released CO2 is beneficial in bread making, where the gas forms bubbles that cause the dough to rise, creating a light and airy texture before the ethanol evaporates during baking.
Other Biologically Significant Fermentation Pathways
Beyond lactic acid and alcohol, various other fermentation pathways exist that yield different organic end products. Butyric acid fermentation, carried out by spore-forming bacteria in the genus Clostridium, produces butyric acid, hydrogen gas, and CO2. Butyric acid is a short-chain fatty acid associated with the aroma of rancid butter, but it is also a metabolite produced by beneficial microbes in the mammalian gut.
Another process is propionic acid fermentation, performed by bacteria such as Propionibacterium species. This pathway converts pyruvate through a complex series of reactions to yield propionic acid. Propionic acid is responsible for the sharp, nutty flavor profile and the formation of the characteristic holes, or “eyes,” found in Swiss-type cheeses.