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When milk becomes acid, clumps are formed. By what means is this achieved?

The lowering of the pH of milk with the addition of a common acid, such as acetic acid (vinegar) affects the solubility of the milk protein casein. Casein can coagulate in acid conditions, forming clumps that are easily visible during this process. The food industry takes advantage of this effect in many of the processes used in the manufacture of dairy products, due to the realization of desirable properties of the finished products. For instance, during the production of yoghurt, casein coagulates and forms solid clumps in the course of the gel formation due to the activity of fermentative microorganisms.

The mechanism of this process can be explained as follows: Casein, as a curd protein, consists of many components bound up together into bundles termed “micelles.” A protein subunit stabilizes each of the micelles to keep them dispersed into separate micelles throughout the milk. Upon acidification, the protective protein subunit is disturbed and the micelles can clot together, which then makes the milk curdle/coagulate. Chemically, the casein micelle is destabilised and therefore, aggregated due to the decrease of its isoelectric charge to that of the isoelectric point. At that same time, the acidity of the medium increases the solubility of minerals so that organic calcium and phosphorus contained in the micelle gradually become soluble in the aqueous phase. Casein micelles disintegrate and casein precipitates, resulting in clumps.