Question

Which test can be used to check the purity of lactic starter cultures?

• A. Oxidase test
• B. Catalase test
• C. Indole test
• D. Creatine test

Hint:

All true Lactic Acid Bacteria (such as lactococci, lactobacilli, and streptococci) are catalase-negative.
Any positive reaction (bubbling with \( \text{H}_2\text{O}_2 \)) in a starter culture is an immediate indication of contamination.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Lactic Acid Bacteria (LAB), which comprise the vast majority of dairy starter cultures, are physiologically characterized as Gram-positive, non-sporeforming, and catalase-negative.
They lack the heme-containing enzyme catalase, which breaks down hydrogen peroxide into water and oxygen.

Step 2: Detailed Explanation:
Let us analyze how this biochemical property is used to verify culture purity:
Because pure lactic starter cultures are catalase-negative, adding a few drops of hydrogen peroxide (\( \text{H}_2\text{O}_2 \)) to a pure culture will not result in any gas bubble formation.
If the starter culture is contaminated with common non-starter organisms (such as coliforms, aerobic sporeformers, micrococci, or molds), these contaminants will produce the catalase enzyme.
When hydrogen peroxide is added to a contaminated culture, the catalase enzyme from the contaminants rapidly decomposes it, producing visible oxygen gas bubbles:
\[ 2\text{H}_2\text{O}_2 \xrightarrow{\text{Catalase}} 2\text{H}_2\text{O} + \text{O}_2 \uparrow \]
This quick, reliable biochemical reaction makes the catalase test an ideal diagnostic tool to check for contamination and ensure the purity of lactic starter cultures.

Step 3: Final Answer:
The Catalase test is used to check the purity of lactic starter cultures.