Thermal indicators in milk, which had been subjected to one of the six industrial processes of thermization, pasteurization, direct and indirect UHT-sterilization, pre-sterilization and in-bottle sterilization, were studied. The following three indices of heat damage were analyzed by high performance liquid chromatography (HPLC), hydroxymethylfurfural (HMF), lactulose and acid-soluble beta-lactoglobulin(beta-LG). Average amounts found were 1710 mg/L of beta-LG and 2.49 mu mol/L of HMF in pasteurized milk. In UHT milk, the amounts for direct and indirect processes were 389 and 322 mg/L of beta-LG, 12.0 and 250 mg/L of lactulose and 5.6 and 8.7 mu mol/L of HMF. In sterilized milk the amounts were 1120 mg/L of lactulose and 22 mu mol/L of HMF, without any detectable presence of undenatured whey proteins. On the basis of the time/temperature profiles, a sterilization factor, expressed as seconds, was defined for each thermal treatment. By applying discriminant analysis each industrial process could be classified independently at the 95% confidence level (pasteurization, UHT-treatment and in-bottle sterilization), but direct-UHT treated milk could not be discriminated from indirect-UHT milk, nor thermized milk from raw bulk milk. The simultaneous application of several heat-induced parameters improves the classification of industrial processed milks, and is therefore a useful tool for optimization of the processing conditions.
