Preparation, Quality and Purine-lowering Activity of Functional Yak Yoghurt during Storage Period

In order to develop a purine-reducing functional yak yoghurt, Lactobacillus paracasei 259 and commercial starter was used as the fermentation strain in this experiment. The inoculation amount of yak yoghurt strain was optimized by single factor experiment, and the quality changes of yak yoghurt during post-ripening period and storage period were observed. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the changes of flavor substances during post-ripening period and storage period, and the purine-lowering activity of yak yoghurt was determined by high performance liquid chromatography (HPLC). The results showed that the post-ripening time of yak yoghurt was 24 h, the sensory score was the best when the inoculation amount of Lactobacillus paracasei 259 was 5%, the number of viable bacteria was 3.1×10⁸ CFU/mL, the acidity was 96°T, and the inhibition rate of xanthine oxidase (XOD) was 51.51%. Yak yogurt with 5% inoculum of Lactobacillus paracasei 259 had a viable bacterial count higher than 10⁷ CFU/mL during 28 d of storage, and the acidity was lower than 105.4°T. With the increase of storage time, the pH and water holding capacity decreased gradually, and the sensory quality, hardness, cohesion, viscosity, consistency and XOD inhibitory activity increased first and then decreased. The main flavor substances in yak yogurt after ripening were alcohols, esters, ketones, aromatic heterocycles and sulfur compounds. The acids increased at storage period compared to the post-ripening period. The relative content and quantity of alcohols were the highest, which gave the yogurt a fruity, floral and sweet flavor. The degradation of inosine and guanosine in yak yogurt during the storage period firstly increased and then decreased, up to 84.6% and 86.5%, respectively. It can be seen that yak yogurt fermented by Lactobacillus paracasei 259 is of better quality, inhibits xanthine oxidase activity, effectively degrades inosine guanosine, and has good potential for application in the prevention and adjunctive treatment of hyperuricemia.