Effect of homogenization pressure and milk composition on some physical properties of yogurt

Abstract

Milk which contained 0, 2, or 4% milk fat and 9, 1 1, or 13% milk solids-not-fat was pasteurized at 90'C for 10 min and homogenized at 2500, 5500, 8500, 11500, or 14500 psi prior to the manufacture of yogurt. The effect of homogenization pressure on the syneresis, firmness, and viscosity of yogurt was investigated. Syneresis of yogurt was tested by centrifuging the yogurt followed by measurement of the free whey. Firmness of yogurt was measured using a Texture Analyzer to determine the force needed to penetrate the yogurt to a depth of 10 mm. The viscosity of the yogurt was determined with a Brookfield DV-111 rheometer and the data was transformed into initial viscosity and decreasing rate of viscosity. Homogenization pressure significantly (P <.01) affected syneresis, firmness, and the viscosity of yogurt. Increasing the homogenization pressure increased the viscosity of yogurt, increased the firmness of yogurt, and decreased syneresis in the yogurt. However, this effect was influenced by the composition of the yogurt. The greatest effect of homogenization pressure on firmness and viscosity was observed in yogurt which contained 4% milk fat with the highest firmness and viscosity observed at a homogenization pressure of II 5 00 psi. There is no advantage in using homogenization pressure greater than II 500 psi. The effect of homogenization pressure decreased as the concentration of milk fat decreased. Homogenization pressure up to 14500 psi had a slight effect on the Theological properties of non-fat yogurt. Homogenization pressure in excess of 14500 psi might be required to induce observable changes in non-fat yogurt. Yogurt which contains milk fat can be made with a lower concentration of milk solidsnot-fat without affecting the Theological properties of the yogurt by homogenizing the milk at II 500 psi rather than just 2500 psi.