Curdling milk is the first step to separating the solids (fat and some protein), from the liquid (whey protein and water). There are two ways to curdle milk, either with acids, like lemon juice or vinegar or with an enzyme such as rennet.
The whey is removed, salt is added, and the curds are cut into smaller pieces and heated to release more whey. The additional whey is drained off, which leaves clumps of casein. Those clumps are pressed into molds and left to age (dry) for various periods of time.
During manufacturing of ricotta cheese, queso fresco, or Indian paneer, acid is used to curdle milk with the present of heat. Rennet contains enzymes that catalyze the degradation of casein proteins in milk. About 80 % of protein in milk is casein, the rest is whey proteins.
Curds that are formed with rennet have a gel-like consistency, allowing them to be stretched and molded unlike curds formed with acid. Cheeses with good melting qualities, such as mozzarella, are produced with rennet.
The lactic acid formed from the fermentation of the milk sugar caused the clotting of the major milk protein, casein. Casein micelles are the components from which cheese is made. The casein micelles determine the stability of milk products during heating, concentration and holding.
Caseins and whey proteins are major components of milk proteins which affect the cheese making process. Cheese yield quality, curd firmness, syneresis rate and moisture retention are affected by the casein portion of milk. Milk with a low casein content (2.27%), gave lower cheese yield per 100 kg milk (7.38 kg) than milk which was richer in casein (2.48%, yield 7.94 kg).
Cheese processing: A milk curdling activity
