Difference Between Lactic Acid and Alcoholic Fermentation

Main Difference – Lactic Acid vs Alcoholic Fermentation

Lactic acid fermentation and alcoholic fermentation are two types of anaerobic respiration methods. Therefore, both lactic acid and alcoholic fermentation do not require oxygen. The main difference between lactic acid and alcoholic fermentation is that lactic acid fermentation produces lactic acid molecules from pyruvate whereas alcoholic fermentation produces ethyl alcohol and carbon dioxide. Alcoholic fermentation of yeast is used in the food industry to produce wine and beer. Lactic acid fermentation occurs in the muscle cells when they are run out of oxygen.

Key Areas Covered

1. What is Lactic Acid Fermentation
– Definition, Mechanism, Importance
2. What is Alcoholic Fermentation
– Definition, Mechanism, Importance
3. What are the Similarities Between Lactic Acid and Alcoholic Fermentation
– Outline of Common Features
4. What is the Difference Between Lactic Acid and Alcoholic Fermentation
– Comparison of Key Differences

Key Terms: Alcoholic Fermentation, Anaerobic Respiration, Carbon Dioxide, Cytosol, Glucose, Heterolactic Fermentation, Homolactic Fermentation, Lactic Acid Fermentation, Pyruvate

What is Lactic Acid Fermentation

Lactic acid fermentation refers to a metabolic process by which glucose is converted into the metabolite: lactate and cellular energy. Generally, lactic acid fermentation is carried out by bacteria such as Lactobacillus and yeast. It occurs in two major steps: glycolysis and fermentation. Both glycolysis and lactic acid fermentation occur in the cytosol. Glycolysis is the first step of both lactic acid and alcoholic fermentation. During glycolysis, hexose sugars are broken down into two pyruvate molecules. Pyruvate is composed of three carbon molecules. Two NADH molecules and four ATP molecules are produced by this process. Since two ATP molecules are consumed by the process itself, the net yield of ATP is two in the fermentation. The process of lactic acid fermentation is shown in figure 1.

Difference Between Lactic Acid and Alcoholic Fermentation

Figure 1: Lactic Acid Fermentation

Two types of lactic acid fermentations can occur in the cytosol. They are homolactic fermentation and heterolactic fermentation.

Homolactic Fermentation

In homolactic fermentation, two lactic acid molecules are generated by the action of the enzyme lactate dehydrogenase. The chemical reaction for homolactic fermentation is shown below.

C₆H₁₂O₆ → 2 CH₃CHOHCOOH

Heterolactic Fermentation

In heterolactic fermentation, lactic acid, ethanol, and carbon dioxide are produced with the aid of both lactate dehydrogenase and pyruvate decarboxylase enzymes. The chemical reaction of heterolactic fermentation is shown below.

C₆H₁₂O₆ → CH₃CHOHCOOH + C₂H₅OH + CO₂

Muscle cells typically undergo aerobic respiration in the presence of oxygen. But when the oxygen supply is poor, they undergo lactic acid fermentation. This may occur due to extreme exercising or severe straining. Muscle cells undergo lactic acid fermentation to fulfill quicker energy requirements. The production of lactic acid in muscles may cause cramps or stiffness in muscles. The structure of a lactic acid molecule is shown in figure 2.

Difference Between Lactic Acid and Alcoholic Fermentation_Figure 2

Figure 2: Lactic Acid

Lactic acid fermentation has a wide range of applications in the food industry. Lactobacillus spps are used in the production of yogurt, cheese, sauerkraut, kefir, and imparting a sour taste to food.

What is Alcoholic Fermentation

Alcoholic fermentation refers to a metabolic process by which glucose is converted into ethanol and carbon dioxide. It is mainly carried out by yeast and some other bacteria. Alcoholic fermentation also occurs in the cytosol in the absence of oxygen. The two pyruvate molecules produced in the glycolysis are converted into ethanol and carbon dioxide. The fate of the pyruvate in the alcoholic fermentation is shown in figure 3.

Main Difference - Lactic Acid vs Alcoholic Fermentation

Figure 3: Alcoholic Fermentation

The conversion of pyruvate into ethanol and carbon dioxide occurs in two steps. The first reaction is catalyzed by the enzyme pyruvate decarboxylase while the second reaction is catalyzed by the alcohol dehydrogenase. The net ATP production is same in both lactic acid and alcoholic fermentation as the second reaction of both processes do not yield ATP. The two NADH molecules produced by glycolysis are used in the second step in both lactic acid and alcoholic fermentation. Alcoholic fermentation is used the bread baking. The carbon dioxide produced by alcoholic fermentation cause the bread to rise. Alcoholic fermentation is also used in the production of alcoholic beverages such as wine, beer, whiskey, vodka, and rum.

Similarities Between Lactic Acid and Alcoholic Fermentation

Lactic acid and alcoholic fermentation are two types of anaerobic respiration mechanisms.
Both lactic acid and alcoholic fermentation are catabolic processes.
Both lactic acid and alcoholic fermentation do not require oxygen.
Both lactic acid and alcoholic fermentation occur in the cytosol.
Both lactic acid and alcoholic fermentation break down glucose molecules into two pyruvate molecules.
Both lactic acid and alcoholic fermentation occur in two steps: the first step of both lactic acid and alcoholic fermentation is glycolysis.
Both lactic acid and alcoholic fermentation produce heat.
Both lactic acid and alcoholic fermentation produce four molecules of ATP.
The net ATP gain is two in both lactic acid and alcoholic fermentation.
The two NADH molecules produced in the glycolysis are used in the second step of both lactic acid and alcoholic fermentation.
Both lactic acid and alcoholic fermentation are less efficient in the production of ATP when compared to aerobic respiration.

Definition

Lactic Acid Fermentation: Lactic acid fermentation refers to a metabolic process by which glucose is converted into the metabolite: lactate and cellular energy.

Alcoholic Fermentation: Alcoholic fermentation refers to a metabolic process by which glucose is converted into ethanol and carbon dioxide.

Occurrence

Lactic Acid Fermentation: Lactic acid fermentation occurs in Lactobacillus spps, yeast, and muscle cells.

Alcoholic Fermentation: Alcoholic fermentation occurs in yeast and other microorganisms.

Products

Lactic Acid Fermentation: Lactic acid fermentation produces lactic acid molecules from the pyruvate.

Alcoholic Fermentation: Alcoholic fermentation produces ethanol and carbon dioxide from the pyruvate molecule.

Enzymes

Lactic Acid Fermentation: Lactate dehydrogenase and pyruvate decarboxylase are the two enzymes involved in the lactic acid fermentation.

Alcoholic Fermentation: Pyruvate decarboxylase and alcohol dehydrogenase are the two enzymes involved in the alcoholic fermentation.

In the Food Industry

Lactic Acid Fermentation: Lactic acid fermentation is used in the production of yogurt and cheese.

Alcoholic Fermentation: Alcoholic fermentation is used in the production of bread, beer, wine, and vinegar.

Conclusion

Lactic acid and alcoholic fermentation are two mechanisms involved in the respiration in the absence of oxygen. Both types of fermentation occur in the cytosol. Glycolysis is the first step of both lactic acid and alcoholic fermentation, which produced pyruvate. Lactic acid fermentation produces lactic acid molecules from pyruvate while alcoholic fermentation produces ethanol and carbon dioxide from pyruvate. The main difference between lactic acid and alcohol fermentation is the products of each fermentation.

Reference:

1. “Lactic Acid Fermentation.” BiologyWise, Available here.
2. “Glycolysis and Alcoholic Fermentation.” The Institute for Creation Research, Available here.

Image Courtesy:

1. “Figure 07 05 02” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Lactic-acid-skeletal” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
3. “Pyruvate decarb 1” By Cwernert at en.wikipedia – Transferred from en.wikipedia to Commons by Ronhjones. (CC BY-SA 3.0) via Commons Wikimedia