Development of a predicative model for identification of loss of stability and proteolysis occurrence in UHT milk

ABSTRACT The aim of this work was to develop a predictive model for identifying loss of stability and sedimentation in UHT milk by determining the enzymatic activity of aminopeptidasis in milk by spectrophotometry. Samples of raw milk, pasteurized and UHT after filling for 6 months in Southern Brazil were analyzed. Acidity, freezing point, fat, total solids, nonfat solids and density were analyzed in raw and pasteurized milk. Raw milk samples were also subjected to psychrotrophic count analysis and aminopeptidasis activity and UAT samples of stored milk were analyzed for degree of proteolysis through sensory analysis and aminopeptidasis activity. Sensory changes were observed in smaller storage time for samples of raw milk originated with psychrotrophic count above 107 CFU ml-1. There was no correlation between aminopeptidasis activity and proteolysis and there was also no significant correlation between physicochemical parameters and the occurrence of proteolysis in stored milk. The model was unable to predict loss of stability and occurrence of proteolysis in UHT milk.

摘要 本研究旨在通过分光光度法（spectrophotometry）测定牛乳中的氨肽酶（aminopeptidasis）活性，构建用于识别超高温灭菌乳（UHT milk）稳定性丧失与沉淀问题的预测模型。研究分析了巴西南部地区经灌装储存6个月后的生鲜乳、巴氏杀菌乳及超高温灭菌乳样品。针对生鲜乳与巴氏杀菌乳，分别检测了酸度、冰点、脂肪含量、总固形物含量、非脂固形物含量与密度；同时对生鲜乳样品开展嗜冷菌计数（psychrotrophic count）与氨肽酶活性测定，并通过感官分析（sensory analysis）与氨肽酶活性测定，评估储存后UAT乳样品的蛋白水解（proteolysis）程度。结果表明，嗜冷菌计数高于10^7 CFU·mL^-1的生鲜乳来源样品，其感官品质劣变出现的储存时长更短。本研究未发现氨肽酶活性与蛋白水解程度间存在相关性，同时储存乳的理化参数（physicochemical parameters）与蛋白水解现象的发生也无显著关联。本研究构建的模型无法有效预测超高温灭菌乳的稳定性丧失与蛋白水解问题。