Rheological characteristics of goat and sheep milk

Rheology represents the properties of both solid and liquid foods, where texture is the rheology related to solid foods, and viscosity is the rheology of fluid foods. Three categories of tests measure textural characteristics of solids foods, empirical (ball compressor, penetrometer, curd tension meter), imitative (texturometer, texture profile analysis [TPA]), and fundamental tests (small amplitude oscillatory shear analysis [SAOSA], torsion analysis [TA]). Regardless of animal species, milk casein gels are mainly responsible for the rheological properties of cheese and other dairy products. In normal fluid milk, the viscosity is affected by the state and concentrations of fat, protein, temperature, pH, and age of the milk. Average milk viscosity has been determined for goats at 2.12 cPas, sheep 2.48 cPas, camels 2.8 cPas, buffaloes 2.2 cPas, and cows 1.7 cPas. Heating decreases the dynamic viscosity, but it increases at the point of coagulation. Sheep and goat milk have the same proteins as cow milk, but their proportions and genetic polymorphs differ widely, which explains different rennetabilities and considerable theological variations in cheesemaking. Casein micelle structure is similar in goat, sheep, and cow milk, but differs in composition, size and hydration. Sheep milk has caseins richer in calcium than cow caseins, it is also very sensitive to rennet, because of higher beta/alpha(s)-casein ratio, and coagulation proceeds faster than in cow milk. Rheological studies with Monterey Jack cheese from goat milk found knitting with progressing aging time, less hardness, less shear stress values, and more rubberiness. High correlations were noted between SAOSA scores and proteolysis in cheeses. Terrincho sheep cheese showed increased hardness, fracturability, gumminess, chewiness, yelloweness, decreased adhesiveness, resilience, and cohesiveness during ripening. Frozen storage of soft goat cheeses had minimal effects on textural qualities, which has valuable market implications. Feta cheeses showed increased compactness and porosity, when goat milk had been added to sheep milk. Yogurt studies, including Labneh from the Middle East, found highest viscosity for sheep followed by goats, cows, and camels, and viscosity increased with solids contents. Three different transient viscosity stages were described mathematically, and camel milk varied least in viscosity during yogurt gelation. Viscosity decreased with increasing angular velocity of the inner cylinder, suggesting that yogurt behaved as a shear-thinning non-Newtonian fluid. Rheological properties are important monitors of quality control in dairy processing and in scientific research. (c) 2006 Elsevier B.V. All rights reserved.