Flow and Structural Characteristics of Concentrated Frozen Buffalo’s Milk

Abstract

The effects of freezing and heat temperature on the rheological behavior and structural of buffalo's milk samples were studied. Rheological properties of raw, pasteurized, whole and skim milk during freezing period of 12 weeks at (-22 ºC ± 2) were measured in a rotational viscometer at temperature (30˚C). Differences between shear stress values of ascending and descending curves were negligible so, no remarkable hysteresis was observed. Obtained shear stress values were found to be dependent on type, treatment and fat content of the milk. The maximum obtained of shear stress values for pasteurized whole milk and raw whole milk were 30.9 and 30.7 dynes/cm 2 at shear rate 1312 s-1 for pasteurized whole milk and raw whole milk, respectively. The obtained flow curves for the raw skim milk showed almost linear relationship between shear stress and shear rate values, which in turn express a Newtonian behaviour. Increasing the fat in milk did not greatly influence the obtained shear stress values as well as the linear characteristic of the obtained flow curves. Pasteurized milk either whole milk or skim milk being frozen at (-22 ºC ± 2) gave the highest viscosity values, compared with that of the other tested raw milk samples. Freezing storage for 12 weeks caused an increase in the values of consistency coefficient of (k-values) of frozen milk concentrates, while the flow behaviour index (n-value) tended to be slightly lower than the unity indicating a shifting towards the non-Newtonian behaviour of the milk concentrates. Transmission electron micrographs (TEM), of fresh and frozen whole concentrated milk obtained from ultrafiltration of whole milk caused changes in size, distribution, and average diameter of casein micelles. The fresh whole concentrated milk from UF showed a roughly spherical shape, in various sizes. The appearance of frozen whole concentrated milk from UF milk concentrated three times (3X) also exhibited nearly spherical shapes with a wide range of sizes.