Curd is made at home with nothing more than milk and a spoonful of starter. Simple as it looks, many people find their curd does not set properly, water collects on top, and it turns sour within hours even in the fridge. A report in the Kolkata daily Ei Samay explains why, and what to do: setting curd is a fine balance of bacteria, temperature and time.
The water on top is whey separating from the curd, and there are three main causes. The first is milk too low in fat. Full-fat milk carries more milk solids; with thin milk the protein network is weak and the whey separates.
The second is moving the bowl while the curd sets. Shifting it again and again breaks the protein bonds, and liquid seeps out of the set curd and rises to the top. The third is the wrong temperature: if the milk is too hot or too cold, the bacteria cannot do their work.
Whether curd turns sour depends on fermentation. The bacteria turn lactose, the natural sugar in milk, into lactic acid, which sets the curd and gives it a mild tang. If fermentation runs too long, or there are too many bacteria, the acid builds up and the curd becomes very sour. Curd left out at room temperature in hot weather sours quickly too.
The report gives a few simple rules. Boil the milk well and set the curd while it is still lukewarm. Stir in a very small amount of starter gently. Cover the bowl and leave it where it will not be moved. Put it in the fridge as soon as it has fully set, because the cold almost stops fermentation and keeps the curd from souring further. And remember that curd sets fast in summer, so it does not need long outside the fridge.
Source: Ei Samay
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