Q1.
Observe the changes in the glass bowls. Write your predictions and observations in Table 2.4.
Answer
Bowl A (lukewarm milk, kept warm) sets into curd; bowl B (cold milk, kept in the refrigerator) stays liquid. Here is Table 2.4 filled in.
| Change in the appearance of milk | Change in the colour of milk | Possible reason | |||
|---|---|---|---|---|---|
| Bowl A | Bowl B | Bowl A | Bowl B | ||
| Prediction | Will thicken and set | Will stay liquid | No real change | No change | Curd sets faster in a warm place |
| Observation | Milk has turned into curd — thick and set, tastes a little sour; some clear liquid may separate at the edge | Milk has not curdled; it is still liquid, but it might be a little sour | Stays white, looks slightly more opaque | No change — still white | Lactobacillus from the added curd multiplies quickly in warm milk and ferments it; in the cold the same bacteria grow far too slowly |
Tip: the spoonful of curd you add is the starter — it is not an ingredient of the milk but a supply of living bacteria. That is why a fresh, live spoon of curd sets a bowl of milk while a sour, old one often does not.