Effect Of Water Activity On The Interaction Between Lactose Crystallization And Protein Glycation In Simulated Milk Powder

The quality of milk powder is affected by environmental conditions.Improper storage will lead to chemical reactions between protein,fat and lactose,resulting in agglomerating,browning and even producing odor.Therefore,it is very important to explore the influence of environmental conditions on milk powder deterioration.In this study,the chemical changes of milk powder caused by water changes were studied.Simulated milk was constructed with casein,whey protein and lactose,and then stored at different water activity（Aw）for 15 d after spray-drying.Water adsorption behavior,lactose crystallization,glycation and the change of protein structure of simulated milk powder were investigated to analyze the interaction between lactose crystallization and glycation.This study could lay a foundation for revealing the quality deterioration mechanism of milk powder during storage.The main methods and results are as follows:（1）Water absorption isotherm,T₂ relaxation time,glass transition temperature（Tg）and crystallinity of casein-whey protein-lactose group（CN-WP-Lac）,casein-lactose group（CN-Lac）and whey protein-lactose group（WP-Lac）were determined by gravimetric method,low-field nuclear magnetic resonance technology,differential scanning calorimetry（DSC）and X-ray diffraction（XRD）to explore the changes of water content,water distribution and the effect of Aw on lactose crystallization in samples.The results showed that the content of free water in the three groups increased gradually and the molecules in the sample become more fluid with the increase of Aw.Water adsorption isotherm of CN-Lac group appeared a‘break’at Aw 0.54,while that of WP-Lac group appeared at Aw 0.43,indicating crystallization might occur at Aw0.54 and Aw 0.43,respectively.Tg in the three groups were lower than zero at Aw 0.54,0.54 and 0.43,respectively,and lactose crystallization appeared at the same conditions.This result indicated that Tg was not only correlated with Aw（p<0.05）,but also correlated with protein content（p<0.05）.CN-WP-Lac and CN-Lac groups only containedα-lactose monohydrate andα-/β-lactose 5:3 crystals,while WP-Lac group also containedα-/β-lactose 4:1 and anhydrousβ-lactose in addition to the above two crystals.Lactose crystallization in WP-Lac group was significantly higher than that in CN-WP-Lac and CN-Lac groups（p<0.05）at the same Aw condition.（2）The effect of Aw on different stages of protein glycation was investigated by the color change,available lysine,schiff bases,glyoxal（GO）,methylglyoxal（MGO）,N^ε-carboxymethyl lysine（CML）,N^ε-carboxyethyl lysine（CEL）,pyrraline（Pyrr）,pentosidine,melanoidins and fluorescent AGEs.The results showed that Aw was negatively correlated with available lysine content（p<0.05）and significantly positively correlated with b*value,Schiff base,GO,MGO,CML,CEL and fluorescent AGEs（p<0.05）,indicating Aw had a great influence on the early and late stages of glycation.（3）The changes of protein structure in simulated milk powder were characterized by scanning electron microscope（SEM）,polyacrylamide gel electrophoresis（SDS-PAGE）,Fourier transform infrared spectroscopy（FT-IR）,circular dichroism（CD）,fluorescence spectrometer and particle size analyzer.The results showed the protein bands migrated to high molecular weight with the increase of Aw.The whey protein bands in CN-WP-Lac group were gradually faint,while those in WP-Lac group were gradually disappeared.According to the correlation analysis,lactose crystallization intensity was negatively correlated with protein degradation degree（p<0.05）,and positively correlated with protein molecular weight（p<0.05）,indicating lactose crystallization promoted protein glycation.WP-Lac group showed the strongest lactose crystallization and the fastest reaction rate,while CN-Lac group was opposite.The rapid crystallization of whey protein was inhibited by the interaction between casein and whey protein in CN-WP-Lac group.