Effects Of Heat Treatment Of Bovine Milk And The Thermal-related Processes During Manufacture Of Milk Powder On Protein Aggregation And Coagulation Properties

The heat treatment of bovine milk is an important process in the dairy industry.For the reasons of sensory and nutritional requirements, the low intensity ofsterilization has become the universal desire of people. However, the high intensityof heat treatment was also used for the purpose of technological demands and safetyreasons. In China, the production of raw milk is imbalance in geography and season.Therefore, the use of milk powder has always received much attention. However, thethermal processes during the production of milk powder changed not only thenutritional characteristics of milk but also its technological applications. The aims ofthis study were to analyze the effects and mechanisms of the heat treatments and thethermal-related processes during the manufacturing of milk powder on thecoagulation properties, including the rennet-induced coagulation and acid-inducedcoagulation, to investigate the relationship of changes in the protein aggregation andphysiochemical characteristic with subsequent difference in the coagulationproperties of milk, and to solve the problems in rennet coagulation of reconstitutedmilk powder and its application in the production of cheese by chemicallycontrolling the aggregation of milk proteins during the production of milk powderand by use of certain lactic acid bacteria in the production of cheese, respectively.The research on the properties of rennet-induced gel and acid-induced gel fromdifferent heated milk showed that the high heat treatment had the increasinginhibitory effect on rennet coagulation and promoting effect on acid coagulation.Compared with raw milk in the same batch, heat treatment of70°C/15s decreasedthe hardness of texture by16.45%, while90°C/8min decreased by37.27%. Sizeexclusion chromatography (SEC) was used to analyze the protein distribution inmilk serum. The changes in serum protein aggregates (SPA) of heated milk beforecoagulation and in the residual SPA of whey after coagulation as affected bydifferent heat treatments were investigated for the first time. High correlations(|r|>0.75, P<0.001) were found between the changes of SPA and the variables ofcoagulation properties. The content of residual SPA depended on the heatingintensity of milk before coagulation.SPA was separated by SEC, and its characteristics and role in rennet-inducedcoagulation were studied. The results showed that the size of SPA increased with theincrease of heating temperature and time, ranging from~30nm at low intensity ofheat treatment to~100nm at high intensity of heat treatment. The rennet coagulationof raw milk with the addition of SPA was not inhibited, suggesting that the inhibition of heating on rennet coagulation did not derive from SPA. Subsequently,the heat-induced changes in casein micelle size, surface hydrophobicity and calciumion activity were investigated. In conclusion, the properties of casein micelles andcalcium ion activity were the main factors that caused the decrease of therennet-induced coagulation properties of milk.In this paper, the effects of thermal-related processes during the production ofmilk powder, including pre-heat treatment, concentration and spray drying on thecoagulation properties of milk were investigated. During the manufacture of milkpowder, the hardness of rennet gel gradually decreased, while that of acid gelshowed a gradual upward trend. The milk powder samples what experienced theprocesses of heat, concentration and drying decreased the hardness of their rennetgel by73.99-81.53%compared with raw milk in the same batch. In addition, thestudies on SPA, the sizes of casein micelles, the calcium ion activity and thecontents of protein secondary structures of milk powder showed that highdependence of these variables was found on the changes of the heat-relatedprocesses and their intensity.In order to investigate the heat-induced aggregation of the casein micelles withwhey proteins, especially with β-lactoglobulin and its role in milk coagulation, theelectron microscopy was used to study the microstructure of heated milk andreconstituted milk powder when combined with the technology of proteinimmunology. Using IPP software the differences of protein microstructures wereanalyzed. In results, the heat treatment did not cause dissociation and aggregation ofcasein micelles, but lead to the aggregation of β-lactoglobulin on the surface ofcasein micelles. It was also found that the casein micelles in reconstituted milkpowder displayed a large degree of dissociation and rearrangement as well as theaggregation of β-lactoglobulin with casein micelles. Therefore the qualitativechange in the microstructure of casein micelles was considered as the main reason ofdecrease of rennet coagulation properties in heated milk and reconstituted milkpowder.By addition of potassium iodate (PI) or isomaltooligosaccharide in raw milk，thecontrol and modification of SPA in subsequent heated milk and reconstituted milkpowder were studied. In results, PI significantly increased the concentration of wheyproteins and decreased the intensity of SEC protein aggregates peak B0and B1.Meanwhile, PI significantly improved the textural properties of rennet gel producedfrom heated milk or reconstituted milk powder. Furthermore, the microstructureresearch on the aggregation of casein micelle with in PI-processed milk alsocorroborated the modification of protein aggregation. Therefore, this finding provided a new idea for the improvement of rennet coagulation properties of heatedmilk and reconstituted milk powder.In addition, an improvements study of microbiology was performed on thecheese from reconstituted milk powder (CRMP). In results, the direct acidifiedCRMP resulted in a decrease of humidity: from65.0%in the first day to62.5%in21days and then to59.9%in45days. While the use of exopolysaccharide(EPS)-producing lactic acid bacteria kept the constant humidity of the cheese duringthe storage, improved the texture and increased slightly (4.6%) the yield of thecheese. Moreover, EPS-producing strain greatly changed the network of cheesemicrostructure, which was the main reason for the improvement of cheese textureand technical characteristics. In conclusion, the related theory and understanding ofthe study could provide the technical guidance for the development and productionof CRMP.