Why does milk curdle when going sour?

Milk is a rather complex biological fluid consisting of mostly water, proteins, fat, vitamins, and minerals. However, proteins are responsible for curdling when milk goes sour. All proteins are made up of combinations of only 20 amino acids joined together to form a polypeptide. Of these 20 amino acids nine can carry a positive (basic amino acids) or negative (acidic amino acids) charge depending on the pH of the medium. Because the side chains of some of the respective amino acids can carry opposite charges, they attract each other like tiny magnets to keep the protein in its proper shape.

Unfolded proteins in the milk result in the formation of larger aggregates of protein matter

Although milk is pasteurized to reduce the amount of spoilage bacteria present, pasteurization is not sufficient to kill all the bacteria in milk. As milk ages in your fridge, the bacteria which survived slowly start to replicate and produce lactic acid which lowers the pH of the milk. This reduction in pH changes the charge of some of the amino acids thereby limiting their interaction with the oppositely charged amino acids. As a result, the protein cannot maintain its normal structure and starts to unfold. As the proteins unfold, the amino acids which are still charged will interacts with oppositely charged amino acids on other unfolded proteins in the milk resulting in the formation of larger aggregates of protein matter. As the pH is reduced further the size of these protein aggregated increase, resulting in the formation of curds which fall out of solution. The reduction in pH is also the reason why milk tastes sour once it goes off.?

Author: Dr. Stefan Hayward (PhD. Biochemistry)

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