Chemistry of “Stretch” of Mozzarella Cheese

The development of curd stretch is governed by the amount of calcium that is available for casein crosslinking. The key to producing pasta-filata cheese is to attain simultaneously the appropriate moisture content in the curd at the time of stretching and the level of casein-associated calcium that is necessary for the curd to stretch in hot water or brine. Curd that contains too much casein-associated calcium will tear and fracture during stretching and fail to attain a plastic, stretchable consistency. On the other hand, too little casein- associated calcium results in complete loss of structure and stretch when the curd is worked in hot water.Both the total calcium content of curd and the distribution of total calcium between the casein associated and soluble states determine the amount of casein-associated calcium at the time of stretching. Total calcium content depends on the amount of calcium that is lost to the whey when acid is produced by the starter during cheese making. Calcium distribution, on the other hand, is governed by the curd pH at the time of stretching: a low pH favours calcium in the soluble state while a high pH favours calcium in the casein-associated state. Therefore, controlled acidification, demineralization and dehydration, coupled with attaining a critical curd pH at the time of stretching, are the key technological parameters for all pasta-filata cheeses.

In the calcium rich environment of milk, the casein precipitates out of milk as dicalcium paracaseinate, entrapping fat, insoluble minerals and some sugar. At a pH between 5.2-5.4, resulting from the development of acidity, some of the calcium of the dicalcium paracaseinate gets dissolved, leading to the formation of monocalcium paracaseinate.This when heated to 54 °C or higher becomes smooth, pliable and stringy and retain fat. If acidification is excessive, generally below pH 5.2, monocalcium paracaseinate will continue to lose calcium and form paracaseinate, which may stretch, but has difficulty in retaining fat. The curd generally does not stretch above pH 5.6.