Study Of The Effect Of System Factors On Heat Induced Aggregation In Bovine Milk

Heat treatment during dairy product processing is one of the most critical aspects,which will have a significant impact on the quality and organoleptic properties of dairy products.In recent years,the changes of protein during heat treatment and the interaction between proteins have been extensively studied.In order to understand the system factors in milk,such as pH,calcium ion concentration and the effect of phosphate on milk protein,especially the influence of heated milk protein aggregation,this experiment separated whey and casein by ultra-high speed centrifugation.The soluble heat-induced protein aggregation were separated by an ultrafiltration centrifuge tube.Polyacrylamide gel electrophoresis and reversed-phase high performance liquid chromatography were used to analyze the changes in protein composition in soluble heat-induced protein aggregation.At the same time,the particle size,BS light intensity and TSI of the sample were detected by laser particle size analyzer and stability analyzer.The effects of system factors on the protein aggregation formed after heat treatment and the stability of the the sample were discussed.The protein composition of the aggregation was analyzed by polyacrylamide gel electrophoresis.It was found that the components of the aggregation are mainly?-lactoglobulin and ?-casein,with a small amount of a-lactalbumin,as-casein and ?-casein.The changes in protein composition of the samples at different pH were determined by reversed-phase high performance liquid chromatography.It was found that when milk was heated at pH 6.4-6.8,the denatured whey protein was adhere to the micellar phase of casein micelles forming a whey protein and casein aggregation.The proportion increases as the pH decreases.At pH 7-7.4,the proportion of soluble aggregation in the sample gradually increased.Similar results were also observed by particle size analysis.Observed by the stability analyzer,it was found that when the pH was lower than 6.6 and higher than 6.8,the stability of the sample was in a poor state,and the stability became worse as the pH decreased or increased.The changes in protein composition were determined by reversed-phase high performance liquid chromatography when different amounts of calcium chloride were added.It was found that the amount of soluble aggregation in the sample was reduced when no calcium chloride was added.The amount of soluble aggregation in the sample decreases as the calcium ion concentration increases.This means that whey protein tends to form a micellar phase aggregation with casein micelles rather than dissociate from micelles to form soluble aggregation when increasing calcium ion concentration.The average particle size of the sample also increases as the calcium ion concentration increases.A similar conclusion was also obtained from the stability analysis that the sample stability became worse as the calcium ion concentration increased.The effect of sodium hexametaphosphate on the protein aggregation was observed by RP-HPLC,and it was found that casein protein dissociated from the micelle was also found in the unheated sample with the addition of sodium hexametaphosphate.The amount of dissociation is proportional to the amount of sodium hexametaphosphate added.After heating,the dissociation of ?-casein was increased,while the dissociation of as-casein and?-casein was slowed down.The amount of soluble aggregation after heating increased as the amount of sodium hexametaphosphate added,but it was found by observing the particle size and stability that the addition of sodium hexametaphosphate reduced the average particle size of the sample and improved the stability of the sample.