science Q

When bacteria grows and multiplies exponentially, it convert lactose in milk to lactic acid, and this lactic acid is sour, more lactic acid, simply means bacteria kept multiplying and in fact this yogurt/curd is more healthy.

As you are using curd to indicate yogurt I believe you are from India, in India, yogurt is/was called KHATTA, means sour, and people put salt and pepper and guzzled in gallons, and worked in the fields like tigers, in the burning sun on the earth, we never ever considered it bad, that was long long time ago, but timing have changed and tastes may changed, and new generation is very picky and following west, that yogurt/curd should be sweet.