Nutritional, sensory and physico-chemical characterization of protein-standardized UHT milk

The protein content of 1.7% fat milk was standardized 'downwards' from similar to 3.4% to values in the range similar to 3.2-2.6%, by mixing with ultrafiltration (UF) permeate obtained from skim milk or quarg acid whey. Normal or protein-standardized milk products were subjected to direct (150 degrees C for 2.7 a) or indirect (138 degrees C for 1 s) ultra-high temperature (UHT) heat treatments and stored at 4 or 25 degrees C for up to 12 weeks. Standardization with skim milk permeate caused no change in the milk pH (6.7) and all milk products had excellent stability during direct or indirect UHT heating. Upon storage at 4 or 25 degrees C for up to 12 weeks no age gelation occurred, and increases in apparent viscosity and sedimentation were minimal. Nutritional quality was slightly reduced by the addition of skim milk permeate, due mainly to minor decreases in the content of protein, calcium, phosphorus and potassium. Sensory quality was similar to normal UHT milk, as shown by periodic triangle sensory tests. The use of acid whey permeate for downward protein standardization led to a better profile of micronutrients, compared to milk with skim milk permeate. In other respects its use was unsatisfactory; although the pH shift upon addition of acid whey permeate to milk was minor (from pH 6.7 to pH 6.50-6.65), neutralization to pH 6.8 was required to avoid protein coagulation during direct or indirect UHT heating. In both direct or indirect UHT milk, high apparent viscosity and excessive sedimentation developed upon storage and off-flavours were unacceptably strong.