Heat stability and enzymatic modifications of goat and sheep milk

Abstract

Heat treatments are important processes that affect the quality of milk and especially the physico-chemical and renneting properties. Goat and sheep milk are less stable to heat treatments than cow milk. Although heat-induced reactions are similar between animal species, the changes relative to temperature differences are due to differences in micellar structure, partition of salts between colloidal and aqueous phases, and protein interactions. Goat milk contains less protein and caseins but also less calcium and phosphate, approximately 1200 and 900 mg/kg, respectively, against 2200 and 1300 mg/kg for sheep milk, which affects their micellar systems noticeably. Caprine and ovine milk are also characterised by a lower colloidal stability than bovine milk. Change in pH, addition of salts (phosphate or citrate), or use of membrane technologies (e.g., change of whey protein to casein ratio) can decrease problems of heat instability. In addition to the instability of the milk itself due to heat treatments, milk enzymes are also inactivated by thermal processing, which can be used as a barometer of the effectiveness and for meeting legal requirements of, e.g., milk pasteurization by testing activities of, e.g., alkaline phosphatase (AP). However, there are considerable variations in AP contents between species, breed within species, and individual animals. The ranges of AP concentrations in goat, sheep and cow milk are 115–1300, 8300–17,300, and 1800–4750 μg phenol/ml, respectively, and are not related to the fat content in milk apparently, but rather to species uniqueness. However, in the case of incomplete inactivation of enzymes, proteolysis and lipolysis can occur and affect the quality of milk and dairy products.