Optimize Detection Method Of Lactulose In Milk And Applied In Evaluation Of Dairy Quality

Based on optimized the ISO enzymatic method which detect lactulose content in milk samples, experiments were conducted to investigate the effects of lactulose content in liquid milk under different processing conditions.Experiment 1A new assay for the determination of lactulose in milk samples was developed by optimizing the previous ISO enzymatic method. Lactulose was hydrolyzed to fructose and galactose at 50℃for 1 h byβ-galactosidase of 300 Unit in this proposed method. The recovery rate of the advanced method was 98.6%, with RSD＜5%. There was no significant difference for lactulose in milk samples between this new method and ISO method (P=0.33). However, with the new method, the testing time was about 9h shorten, and the probability of microbial contamination was significantly decreased, compared with that of the normal one.Experiment 2Experient were conducted the effects of sterilization temperature, sterilization time and the supplementation of reconstituted milk on the concentration of lactulose in the Pasteurized processing conditions or the ultra heat treated processing conditions. The results showed that when Pasteurized, the content of lactulose in milk was linearly increasing with the ascending of sterilization temperature (85℃, 90℃, 95℃, 100℃) and sterilization time (15s, 30s, 45s, 75s, 90s) (R~2＞0.94). And the impact of sterilization temperature is more crucial than that of sterilization time. The content of lactulose in milk was positively correlated with content of reconstitued recovery milk (R~2=0.99). While processed via ultra heat, the content of lactulose in milk was curve increasing with the ascending of sterilization temperature (137℃～142℃) and sterilization time (2s, 5s, 15s). Therefore, the distribution of heating time could be deduced from the concentration of lactulose in UHT milk. However the content of lactulose in milk was negatively correlated with the added ratio of recovery milk (R~2=0.97 ).Experiment3Four brands of commercial UHT milk were used to study effect on the variation of lactulose content of commercial UHT milks during storage at room temperature (19±2℃). The results indicated that the lactulose content in the UHT milk was relatively stable before 42 days and increased gradually with prolonged shelf-life which associated with producing technology.