Reaction kinetics of lipid oxidation and proteolysis in stored UHT milk

UHT milk samples processed at 140 degreesC were stored at 9, 15, 25, 35, 45 and 55 degreesC and analysed for changes brought about by lipid oxidation and proteolysis. The TBA value increased from initial 0.012 to the levels o f 0.021, 0.023, 0.032 and 0.065 after 16 weeks of storage at 15, 25, 35 and 45 degreesC whereas it was 0.22 following storage for 12 weeks at 55 degre esC. The level of free amino groups measured 18.14 mu mol eq of glycine/ml in fresh sample and increased to 21.92, 23.08, 28.40, 33.56 and 36.06 mu mo l eq of glycine/ml of milk after 16 weeks of storage at 9, 15, 25, 35 and 4 5 degreesC. At 55 degreesC temperature, the increase was up to 32.90 mu mol eq of glycine/ml of milk following 12 weeks of storage. The 2 reactions we re thus distinctly storage temperature dependent and could be best describe d by zero order reaction kinetics. Storage temperature of 35 degreesC was c ritical for proteolytic changes. The Arrhenius plot for lipid oxidation dat a yielded 3 straight lines for the temperature ranges of 15-25, 25-45 and 4 5-55 degreesC whereas only 2 straight lines were observed for proteolysis i n the temperature ranges of 9-35 and 35-55 degreesC. Activation energy, ent halpy, activation entropy and free energy of activation were also obtained for these reactions.