Fermentation

Fermentation is a special type of anaerobic respiration that does not use the electron transport chain.

Respiration, whether aerobic or anaerobic, is a catabolic process.

For anaerobic respiration to occur, a molecule must be able to substitute for oxygen, such as sulfate SO₄^-2. When no such molecule can functionally replace oxygen, the traditional metabolic pathways cannot be used. Lucky for cells (and for us!), there is another pathway to generating energy: fermentation.

Definition of Fermentation in Biology

Now that we know that aerobic and anaerobic respiration are, let's look at the definition of fermentation.

• Remember: fermentation is a type of anaerobic respiration!

Fermentation occurs in two phases: glycolysis and NADH oxidation. The first phase, glycolysis, breaks down glucose with the help of NAD⁺, which generates ATP. The second phase, NADH oxidation, regenerates NAD⁺.

Well, that’s a lot of new terminology. So, like catabolism, let’s break all these terms down!

Essential Molecules

Glucose

Let's start with glucose. Glucose (Fig. 2) is a sugar whose chemical formula is C₆H₁₂O₆. It is a chemical compound in the carbohydrate family because it contains carbon (C), hydrogen (H) and oxygen (O), with a 2:1 ratio of two hydrogens for every oxygen. Glucose is considered the main source of energy for most living organisms.

Next, we have ATP. ATP (Fig. 3) is short for Adenosine triphosphate.

The structure of ATP consists of one adenosine bonded to three phosphates. When a phosphate bond is broken, energy is released. The resulting molecule, when missing a phosphate, is called adenosine diphosphate, or ADP. ADP can even lose a second phosphate, thus becoming adenosine monophosphate, or AMP.

NADH

The final significant molecule is nicotinamide adenine dinucleotide, or NADH (Fig. 4). In its reduced form, it will lack a hydrogen (H⁺) and be identified as NAD⁺.

Fermentation Process in Biology

In biology, fermentation is the metabolic process cells use to generate energy, or ATP, without the primary assistance of oxygen. It occurs in a simpler fashion than aerobic respiration, in that it takes only two steps: glycolysis and NADH oxidation.

Glycolysis

The first step in anaerobic respiration, glycolysis, occurs in the cytoplasm.

Glycolysis occurs in a long sequence of steps, summarized below:

1. Phosphorylation, wherein two ATP molecules, one after the other, lose a phosphate group to a glucose molecule.

2. The modified glucose molecule splits into triose phosphate.

3. NAD⁺ take a hydrogen (H⁺) from the triose phosphate.

4. ADP takes a phosphate from the modified triose phosphate. The modified molecule is now a phosphoglycerate. This step generates ATP.

5. Phosphoglycerate is modified into phosphoenolpyruvate, losing H₂O in the process.

6. ADP takes a phosphate from phosphoenolpyruvate, resulting into our end product, pyruvate, or pyruvic acid. This step generates ATP.

The previous list is an abridged summary. The full ten steps of glycolysis can be seen in this diagram (Fig. 5).

Fig. 5. The full ten steps of glycolysis. licensed by CC BY-SA 4.0

Glycolysis can be summarized and simplified into a single equation:

Glucose + 2 NAD⁺ + 2 ATP ⇾ 2 Pyruvate + 2 NADH + 4 ATP + 2 H⁺

Pyruvate, or pyruvic acid, is the resulting molecule from glycolysis. Its chemical formula is C₃H₃O₃^-. Pyruvate will undergo more catabolism in the next phase. Look! Four ATP molecules have been produced and can be used for energy. We have ATP to power our cells!

So, why can’t we just stop here? Unfortunately, the NAD⁺ required for glycolysis is in short supply. To produce more ATP, the cell needs a supply of both sugar and NAD⁺. The next phase, fermentation, will meet this demand.

NADH Oxidation

At this point, after glycolysis, we have an overabundance of pyruvic acid (pyruvate) and NADH. And, we also need to generate more NAD⁺ to continue the synthesis of ATP via glycolysis.

But, how can we regenerate the NADH to NAD⁺? Here, oxidation will solve our issue.

Oxygen is the most common oxidizing agent, but unfortunately under anaerobic conditions oxygen is in very low supply. Yet, by definition, oxidation can still occur! Oxygen is the most common oxidizing agent because of its high electronegativity, but in its absence, some cells have found a substitute.

During fermentation, NADH becomes oxidized to NAD⁺, which replenishes the demand for NAD⁺ during glycolysis. The oxidization is made possible by some catalyst, such as an enzyme, yeast, or bacteria. There are two major types of fermentation, named after their by-product:

• Alcohol fermentation: Fermentation of pyruvate by yeast, which yields ethyl alcohol as a by-product. It is used to make wines, beers, and spirits.

• Lactic acid fermentation: Fermentation of pyruvate by bacteria, which yields lactic acid as a by-product. It is used to make cheeses and yogurts.

To summarize, the combined metabolic phases of glycolysis and NADH Oxidation are displayed below (Fig. 6).

Difference between Cellular Respiration and Fermentation

You may be asking yourself, why isn't all respiration anaerobic? Anaerobic respiration has distinctly less metabolic pathways than aerobic respiration. To answer that question, we observe how many ATP can be produced from one single molecule of glucose.

Aerobic respiration can yield up to 36 ATP per molecule, whereas anaerobic respiration can only yield a paltry two ATP per molecule of glucose.

There lies the true power of oxygen! Look on the bright side, with fermentation, we get to enjoy the flavorful fermented food by-products.

Products of fermentation

Chemically, fermentation produces by-products such as lactic acid, ethanol, hydrogen gas, carbon dioxide, ATP, acetone and some proteins or enzymes. Yet, most people are familiar with the fermentation process because it gives rises to a unique food category: fermented foods.

Fermentation has yielded many fermented foods that have enriched our lives. Wine, beer, and liquors are fermented food beverages that humankind has enjoyed since the dawn of civilization.

Another early fermentation process is the further fermentation of ethanol beverages into vinegar, using lactic acid fermentation. Vinegar has provided humans with its preservative properties during the pickling process, in times when mechanical refrigeration was unavailable.

Cheese is another well-loved fermented food, where milk sugars are converted into lactic acid. Similarly, yogurt and sour cream are fermented foods derived from milk sugars. Lactic acid gives these fermented foods their sour taste.

Fermented sausages, such as salami or pepperoni, are a type of meat preservation process used globally. In Asian countries, fermented beans, such as miso, are popular fermented foods derived by lactic acid fermentation.

Lactic acid fermentation of cabbage yields sauerkraut and kimchi. Soy sauce and Worcestershire sauce can be added to the list of fermented foods derived from lactic acid fermentation.