Fermentation in Microbiology: Types, Process & Examples

Hi, I'm Kathy with Level Up RN. In this video, I’ll continue my microbiology review series by discussing fermentation. At the end of the video, I’ll include a short quiz to test your understanding of the key points—so be sure to stick around for that!


If you have our Level Up RN Microbiology Flashcards, grab your card on fermentation so you can follow along.

What is Fermentation?

Fermentation is an anaerobic process that releases energy from organic molecules like glucose. It begins with glycolysis but does not involve the citric acid cycle or the electron transport chain. As a result, fermentation only produces two ATP molecules through glycolysis.

The two main types of fermentation you should know are:

  1. Lactic Acid Fermentation
  2. Alcohol Fermentation

Let’s take a closer look at each.

Lactic Acid Fermentation

In lactic acid fermentation, pyruvate (produced in glycolysis) is reduced by NADH to form lactic acid. At the same time, NADH is oxidized back to NAD⁺, which is crucial because NAD⁺ is needed for glycolysis to continue producing ATP.

One major purpose of fermentation is to regenerate NAD⁺, ensuring ATP production can continue even in the absence of oxygen.

Examples of Lactic Acid Fermentation

  • Yogurt Production: Lactobacillus bacteria ferment lactose in milk, producing lactic acid. This acid lowers the pH, denaturing milk proteins, which causes the liquid milk to thicken into yogurt.
  • Muscle Cells During Exercise: When your respiratory and cardiovascular systems can’t supply enough oxygen, your muscles switch from aerobic respiration to lactic acid fermentation. This leads to the buildup of lactic acid, causing the burning sensation in your muscles during intense exercise (e.g., holding a wall squat for too long).

Alcohol Fermentation

In alcohol fermentation, pyruvate (produced in glycolysis) is first converted into acetaldehyde and carbon dioxide. Then, acetaldehyde is reduced by NADH to form ethanol, while NADH is oxidized back to NAD⁺. This process ensures that glycolysis can continue generating ATP.

Examples of Alcohol Fermentation

  • Alcoholic Beverages: Yeast ferments sugars into ethanol, which is the alcohol found in beer, wine, and spirits.
  • Bread Making: Yeast performs alcohol fermentation, producing carbon dioxide gas. This creates air bubbles in the dough, making it rise and forming the small holes you see in a slice of bread.
  • Biofuel Production: Ethanol, produced by fermentation, is commonly used as a renewable fuel source.

Fermentation vs. Anaerobic Respiration

Although both fermentation and anaerobic respiration occur without oxygen, they are not the same.

  • Fermentation only involves glycolysis.
  • Anaerobic respiration includes glycolysis, the citric acid cycle, and the electron transport chain—but uses a different final electron acceptor instead of oxygen.

Quiz Time!

Let’s test your knowledge with three quick questions:

  1. During lactic acid fermentation, what molecule is reduced by NADH to produce lactic acid?

    • Answer: Pyruvate

  2. During alcohol fermentation, pyruvate is converted into which two products?

    • Answer: Acetaldehyde and carbon dioxide

  3. How many ATP molecules are produced during fermentation?

    • Answer: Two

That’s it for this quiz! I hope you did great and found this video helpful.


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