How does a spoonful of old curd turn a bowl of warm milk into fresh curd?
The starter curd carries Lactobacillus bacteria.
The starter curd carries Lactobacillus bacteria. Added to warm milk, they ferment the milk sugar (lactose) into lactic acid. The rising acidity lowers the pH, which makes the milk protein (casein) coagulate, setting the milk into the semi-solid curd. The bacteria also multiply and add to the flavour and to the curd's vitamin B12 — which is why a little old curd 'seeds' a whole new batch.
Key point
Lactobacillus ferments lactose to lactic acid; the acid coagulates milk protein into curd.
Common mistake
thinking the milk sets simply because it's warm.
Memory tip
curd's bacteria make acid → milk protein clots → curd sets.
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