Effects Of Heat Treatment Of Bovine Milk And The Thermal-related Processes During Manufacture Of Milk Powder On Oxidized Flavor

Attention has been paid to the production and application of milk powder dueto geographical, seasonal and other factors that limit raw milk supply. There aresome flaws both in the direct consumption of milk powder and in the alternative ofraw milk used in food after recovery of milk powder. The formation of oxidizedflavor during the production and storage of milk powder was a bottleneck thatrestricted the consumption and application of milk powder. The purpose of thepresent study was to investigate the effect of the heat treatment of bovine milk andthe thermal-related processes, including pre-heat, concentration and spray drying,during the manufacture of milk powder on the oxidized flavor compounds (OFC); toevaluate the environment of lipid oxidation and the exposure of the OFC precursorin process of bovine milk; to analyze the effect of the processes on the morphologyand surface composition of milk powder particle; and to research their effects andpotential mechanism on the formation of OFC both in the process sectors and instorage link of milk powder.Firstly, the solid phase microextraction-gas chromatography-mass spectrometrywas used to analyze the volatiles in different heated milk, the content of OFCincreased with increasing the intensity of heat treatment. Compared with the rawmilk, the heat treatment at90℃for more than2min significantly increased thecontent of hexanal (P<0.05), and the heat treatment of90℃for20min increased thecontent of hexanal by2.70times. The regression relationship between OFC andheating parameters were built: the correlation coefficient was greater than0.86. Theheat treatment changed the oxidative environment of bovine milk: the treatment of90℃for20min reduced DPPH· scavenging activity by23.15%and increasedperoxide value (POV) by37.23%compared with the raw milk. The research on thethermal destruction of OFC precursor (milk fat) found that the low intensity of heatdid not cause the significant changes in the distribution of fat globules. Meanwhile,in milk fat globule membrane, the contents of XDH/XO and BTN proteins whichhave protective effect on the milk fat gradually destructed with the increasingintensity of heat treatment.The research on effect of the thermal-related processes during the production ofmilk powder on the formation of OFC showed that the contents of OFC in theconcentrated milk or in milk powder were higher than those in raw milk or in heatedmilk and that the concentrated milk had higher contents of OFC compared with the milk powder. Besides, a higher content of OFC in distillate even than that in itscorresponding concentrated milk was found, suggesting that the severe lipidoxidation happened during concentration of milk with the loss of flavor compounds.According to the study on oxidative environment and exposure status of flavorprecursors, bovine milk decreased its DPPH· scavenging activity and increased itsPOV after the pre-heat, concentration or spray drying; compared to unconcentratedmilk, the d3,2of concentrated milk significantly increased (P<0.05) due to theaggregation of milk fat globules; the composition of XDH/XO and BTN proteinschanged by the processes of pre-heat and concentration and thus affected theirprotective effects on milk fat. In conclusion, the changes in the oxidativeenvironment and exposure of flavor precursors caused by the processing of bovinewere the reason and starting point for the formation of OFC.The particle characteristics of milk powder have a close effect on the formationof oxidized flavor during the storage. Based on the analysis of composition, only theprocess of drying significantly affected the surface free-fat content in milk powderparticles (P<0.05); the contents of oleic acid, linoleic acid and the total unsaturatedfatty acid in surface free-fat did not change significantly during the storage of milkpowder; the surface composition of particles that was affected by the thermal-relatedprocesses showed a difference from the composition of total milk powder. Inaddition, the sizes of milk powder particles ranged from33.24to53.60μm by thequantitative statistics of images. The porosity of milk powder ranged from1.73to17.09mm3/g by static nitrogen adsorption method. The effects of the thermal-relatedprocesses on these morphological characteristics of milk powder particles weredetermined, that is, the characteristics were under control by these processes.Levels of OFC in reconstituted milk had the increased tendency. It was alsofound that the formation of OFC during storage of milk powder could be influencedby the thermal-related processes. Total anti-oxidative activity of reconstituted milkpowder after storage almost depleted and POV substantially increased. Based on thecharacteristics of milk powder particles and concentrations of OFC during storage,the morphology of milk powder particles rather than the surface composition wasthe main influencing factor in the formation of OFC during storage of milk powder.A free radical was detected in stored milk powder. The radical was in the formof a single peak with a g value of2.0054. The intensity of free radical increased withincreasing the storage time. In addition, the study on the control of OFC displayedthat the addition of ascorbic acid resulted in milk powder with low contents of OFCwhether in light-exposed or light-protected storage, and with the formation of newfree radical. Finally, the reconstituted milk powder was used in the production of yogurt. Inresults, aldehydes, methyl ketones, olefinic aldehydes, ketenes, furan andbenzothiazole were detected in the yogurt produced from reconstituted milk powder.However, the yogurt that was produced with the newly produced milk powder hadthe low contents of these compounds. It was also found that the flavor metabolismof cultures during the fermentation of yogurt could be influenced by heat intensityof bovine milk. These results could expand existing apprehension on the applicationof milk powder in the manufacture of yogurt.