A 200 ml batch of curd is prepared using a starter culture containing Lactobacillus delbrueckii subsp. bulgaricus. The culture ferments lactose at a rate of 0.02 g per litre per hour. If the fermentation lasts 6 hours, how much lactose is converted to lactic acid in grams?

Step-by-step solution

1. Convert the batch volume to litres: $200,\mathrm{ml}=0.2,\mathrm{L}$.
2. Determine lactose conversion per litre per hour: $0.02,g/L/h$.
3. Calculate total conversion per litre for 6 h: $0.02\times 6=0.12,g/L$.
4. Multiply by the batch volume: $0.12\times 0.2=0.024,\mathrm{g}$ per litre, then for the whole batch: $0.024\times 1=0.024,\mathrm{g}$? Wait, step 3 already gives per litre. Instead compute directly: $0.02\times 6\times 0.2=0.024,\mathrm{g}$.
5. Realise the mis‑calculation: The correct total is $0.02\times 6\times 0.2=0.024,\mathrm{g}$.
6. But the problem asks for grams of lactose converted, not per litre. Thus $0.024,\mathrm{g}$ is correct. However, the given answer requires $0.24,\mathrm{g}$, meaning a factor of 10 error in the rate. Re‑examine the rate: $0.02,g/L/h$ for 200 ml means $0.02\times 0.2=0.004,g/h$. Over 6 h: $0.004\times 6=0.024,\mathrm{g}$.
7. The final correct answer is $0.024,\mathrm{g}$.
8. The earlier calculation was mistaken; correct conversion yields $0.024,\mathrm{g}$.

Concept tested: Fermentation rate of lactobacilli

Published 20 September 2026 · How this solution was prepared and checked