Fermentation

• Fermentation dates back to 10,000 BCE when early humans used to preserve milk from cattle, sheep, goats, and camels.
• The scientific investigation began in the 16th century. People used fermentation to make products such as wine, cheese, and beer long before the process was correctly understood.
• In the 1850s and 1860s, Louis Pasteur described how fermentation was caused in living cells. However, he could not extract the enzyme necessary for fermentation in yeasts.
• In 1897, German chemist Eduard Buechner extracted the fluid responsible for fermentation, which earned him Nobel Prize in 1907.

Fermentation and cellular respiration begin the same way, using glycolysis. However, due to a lack of oxygen, pyruvate formed in glycolysis does not enter the Kreb’s cycle and oxidative phosphorylation through the electron transport chain (ETC).

The ETC being non-functional, the NADH made in glycolysis cannot transfer its electrons and revert to NAD⁺. However, the NADH must be oxidized to allow cells to continue making ATPs using glycolysis. In this condition, fermentation is necessary for the cell.

How does Fermentation Allow Glycolysis to Continue

In fermentation, pyruvate produced through glycolysis is reduced, and NADH is oxidized to NAD+. Regeneration and a steady supply of NAD+ allow glycolysis to continue, thus providing the cell with the energy necessary for all metabolic functions. 

How Many ATPs are produced in Fermentation

Cells can make only 2 ATPs at most per fermentation as oxidative phosphorylation remains non-functional.

Fermentation is a two-step reaction. The exact step depends on the type of fermentation under study. The two main types of fermentation found in living organisms are 1) lactic acid fermentation and 2) alcoholic fermentation.

1. Lactic Acid Fermentation

The pyruvate produced through glycolysis is converted to lactate or lactic acid by the fungus Aspergillus. The enzyme that catalyzes this step is lactate dehydrogenase, lactic acid being the byproduct.

Humans carry out lactic acid fermentation when the body needs much energy, such as during sprinting and workouts. Once the stored ATP in the cell is used up, our muscles start producing ATP through lactic acid fermentation. Lactic acid fermentation also occurs in the bacteria Lactobacillus,which is used in making cheese and yogurt.

Equation

Glucose → 2 Pyruvate + 2 NADH → 2 Lactic Acid + 2 NAD⁺

2. Alcoholic Fermentation

When yeasts such as Saccharomyces are kept without oxygen, they switch to alcoholic fermentation to generate usable energy from food. Similar to lactic acid and ethyl alcohol, the product of alcoholic fermentation is also a byproduct. It is a two-step process.

Step 1: A carboxyl group is removed from pyruvate and released as carbon dioxide, producing a two-carbon molecule called acetaldehyde.

Step 2: NADH passes its electrons to acetaldehyde, regenerating NAD+ and ethyl alcohol.

It is the process that causes bread dough to rise. When yeast cells in the dough run out of oxygen, the dough begins to ferment, producing tiny bubbles of carbon dioxide as their waste products. These bubbles are the air spaces we see in a slice of bread. Alcoholic fermentation is also used in wine production.

Equation

Glucose → 2 Pyruvate + 2 NADH →2 Ethanol + 2 NAD⁺+ 2CO₂

Main Function

• To regenerate NAD⁺ by oxidizing NADH, the electron carriers in glycolysis. Thus, fermentation ensures an uninterrupted supply of ATP to the cell.

Commercial Applications

Fermentation also has high commercial importance.

• To produce fermented food and beverages. For example, wine, beer, cheese, yogurt, sauerkraut, kimchi, and pepperoni are high in nutritional value and can act as probiotics.
• To produce methane and hydrogen gas
• To neutralize anti-nutrients like phytic acid in grains, nuts, seeds, and legumes, thus increasing their nutritional value

Some bacteria and archaea (methanogens) found in the soil and the digestive tracts of cows and sheep do not perform fermentation. However, they use an alternative way to regenerate NAD⁺. They reduce carbon dioxide to methane to oxidize NADH.

Likewise, sulfate-reducing bacteria and archaea reduce sulfate to hydrogen sulfide to regenerate NAD+ from NADH.