Comparison of Fermented Milk Characteristics of Tibetan Kefir Grains, Lactobacillus helveticus LZ-R-5 and Common Lactic Acid Bacteria and On-line Monitoring Application of Low-Field NMR in Storage Period

Three types of fermented milk were obtained from Tibetan kefir grains,Lactobacillus helveticus LZ-R-5(separated from Tibetan kefir grains)and Chuanxiu licorice yoghurt baking powder fermented pure milk. The growth characteristics of three kinds of fermented milk during fermentation and storage were studied. Using the low-field nuclear magnetic resonance(LF-NMR)technique studied the distribution and migration patterns of water in three kinds of fermented milk during storage. Combined with the water holding capacity and rheological properties,it explored whether the moisture migration and rheological characteristics of fermented milk existed correlation. The results showed that during the fermentation,the pH of the three fermented milks gradually decreased,and the number of viable lactic acid bacteria increased first and then decreased. When the number increased first and then decreased,the number of viable cells of LZ-R-5 fermented lactobacilli was gradually decreasing. During the storage of the three types of fermented milk,the total moisture content and the free water content gradually decreased,and Horizontal relaxation time T₂₂ decreased first and then increased. In addition,the changes in water holding capacity,apparent viscosity,G' and G″ of the three types of fermented milk were consistent. Therefore,LF-NMR technology could be applied to the online monitoring of moisture during storage of fermented milk,which would provide a theoretical basis for the rapid determination of quality of fermented milk.