Question 8 — A student observes that curd - Questions at the end of the chapter - The Invisible Living World: Beyond Our Naked Eye

Last updated at July 15, 2026 by

Q 8
Question

A student observes that when curd is left out for a day, it becomes more sour. What can be two possible explanations for this observation?

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Answer
Why does curd left out get more sour?
Curd is left out for a day
It stays at a warm room temperature
↓
Lactobacillus keeps multiplying
These bacteria grow well in warm conditions
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More lactose is fermented
The bacteria keep feeding on the milk sugar
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More lactic acid is produced
Lactic acid is what makes curd sour
↓
So the curd tastes more sour than before
The bacteria keep multiplying and keep making more lactic acid, so the curd turns more sour.

Two explanations

  • 1. Lactobacillus bacteria in the curd keep multiplying when left in warmth.
  • 2. As they grow, they keep making more lactic acid.
  • More lactic acid makes the curd taste more and more sour.
More bacteria and more lactic acid make the curd grow sour.
← Back to: 2.4.2 Microorganisms and food
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