Question

Which of the following pH values is responsible for the curd formation by Lactobacillus?
[1 mark]

• A. Around 6.7
• B. Around 8.7
• C. Around 7
• D. Around 4

Answer 1

The correct option is D Around 4

Curd is a food product obtained by the fermentative activities of the lactic acid bacteria (LAB) such as Lactobacillus on milk. The pH of the milk generally lies within a narrow range of 6.5-6.7 (slightly acidic).

To prepare curd, a small amount of curd is added to the milk as inoculum; the lactic acid bacteria (LAB) present in the inoculum (curd) start multiplying by feeding on the lactose (sugar) present in the milk. These bacteria anaerobically breakdown lactose to form lactic acid. As the bacteria multiply, the lactic acid concentration increases, thereby reducing the pH of milk to 4 or less within a few hours.

At around pH 4 (highly acidic pH), the milk protein casein starts to coagulate/aggregate to precipitate as curd. Thus, option d is the correct answer.

The pH 7 is considered to be neutral and 8.7 is considered to be alkaline pH. Curd is not formed at neutral or alkaline pH values by Lactobacillus.