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How Curd Sets From Milk GK Facts, Overview & Study Guide
The transformation of liquid milk into semi-solid curd is a biochemical and microbiological fermentation process driven by the action of Lactic Acid Bacteria on milk proteins and sugars. In its raw, natural state, milk is an oil-in-water emulsion containing dispersed fat globules, dissolved lactose sugar, minerals, and suspended proteins. The primary protein in milk is casein, accounting for roughly eighty percent of total protein content, with the remaining twenty percent composed of soluble whey proteins. Under normal neutral conditions, casein exists in stable colloidal suspension as spherical colloidal aggregates called casein micelles. These micelles remain uniformly dispersed in liquid milk because their outer protective coating, formed by kappa-casein, carries a net negative electrical charge that generates repulsive electrostatic forces, preventing individual micelles from clumping together.
The setting of curd begins when a small quantity of pre-existing curd, known scientifically as an inoculum or starter culture, is introduced into lukewarm milk. This starter culture contains active strains of beneficial lactic acid bacteria, predominantly Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus, and Streptococcus thermophilus. These bacteria produce the enzyme lactase, which metabolizes milk's primary disaccharide sugar, lactose, converting it into lactic acid through anaerobic fermentation. As bacterial colonies multiply, the continuous accumulation of lactic acid increases the concentration of hydrogen ions, causing the pH of the milk to drop progressively from its fresh, near-neutral level of six point six down toward an acidic threshold of four point six.
For food scientists, biotechnologists, and competitive examination candidates, the setting of curd demonstrates the physical chemistry of protein isoelectric precipitation. At a pH of roughly four point six—which corresponds precisely to the isoelectric point of casein—the negative surface charges on the kappa-casein outer layer are neutralized by surplus hydrogen ions. With electrostatic repulsion eliminated, hydrophobic forces take over, causing the destabilized casein micelles to aggregate, denature, and cross-link into an interlocking three-dimensional microscopic protein mesh. This protein matrix traps liquid whey, dissolved minerals, and suspended fat globules, converting fluid milk into a gelatinous curd. Beyond altering texture, bacterial fermentation enriches curd with Vitamin B12 and probiotic cultures, while making it easily digestible for lactose-intolerant individuals.
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