CHARACTERISTICS OF MILK PROTEINS AS AFFECTED BY SOME TECHNOLOGICAL PROCESSING PARAMETERS

Abstract

Variation in heat treatments applied in dairy industry consequently, affects on milk characteristics especially milk proteins. Influence of various thermal processing on milk protein functionality is one of the most important factors. Cow milk was subjected for some researches to investigate the effect of different thermal processing on its characteristics. Unfortunately, the studies on buffalo milk in this topic are very little due its presence in a few countries. Therefore, this investigation was conduct to study the effect of some technological parameters such as heat process on the properties of the product and the study came into three parts. In the first part, fresh buffalo's milk was skimmed using laboratory milk separator (at 40 °C). The skim milk sample was divided into five portions. The first portion was left without any heat treatments to be as control. Other four portions were heat treated at 72 °C for15 sec, 90 °C for 10 min, 110°C for 10 min and 137 °C for 3 sec in circulator oil bath followed by cooling to 7 ºC. The processed skim milk was examined either when fresh or during storage for chemical composition, nitrogen distributions, ethanol stability, viscosity and flow behavior properties, protein break down and oxidation, amino acid composition, hydroxymethylfurfural and lactulose contents, colour parameters and surface tension. Treatments were also examined for composition of volatile compounds, acid soluble whey protein contents and microstructure. The results indicated that there was a significant increase (P˂0.05) in acidity, casein nitrogen, rennet coagulation time, dynamic viscosity, and surface tension values of heat treated samples than control. On contrary, non-casein nitrogen, whey protein nitrogen, and ethanol stability were significantly decreased. Storage of high heat treated samples at 110 ºC/ 10 min and 137 ºC/3 sec up to 90 days / 25°C led to significant increase in dynamic viscosity values. Flow behavior index value indicated that the higher heat treatment changed the flow behavior to be fully non-Newtonian type since the index (n-value) was lowered to the level of 0.716. Protein breakdown values as indicator by proteolysis were decreased with increasing the intensity of heat treatment, whereas the values increased during storage period up to 28 days at 7ºC and 25 ºC. A marked decrease in both of essential amino acids index and biological value of heat treated milk at 110º C/ 10 min and 137º C/ 3 sec. Consequently, heat treatments at 110ºC/10 min and 137ºC/ 3sec caused a decrease in both of protein efficiency ratio (PER) and chemical limiting score of lysine due to Millard reaction. Hydroxymethyl furfural contents was dramatically increased in milk samples treated at 110ºC/10 min or 137ºC/3 sec. Gas chromatographic profile of heated skim buffalo milk at 137ºC/ 3 sec showed formation dimethyl sulfide compound which defect off flavour. These off-flavours are due to the formation of sulfur compounds from the denatured whey proteins with different heat treatment and whey proteins were fully disappeared forming high molecular weight complexes in samples treated at 90ºC/ 10 min, 110 °C/10 min or 137 °C/3 Sec. Curd penetration value increased (i.e. firmness decreased) while the syneresis value decreased by exposing the milk samples to heat treatments which indicate a strength reduction in rennet gel due to higher water binding capacity of formed gel. Heat treatments of milk samples caused a slight increase in surface tension values, probably due the denaturation of whey protein and formation of aggregates.