关键词: 牦牛乳, 酸奶, 奶渣, 牦牛乳制品, 稳定碳同位素, 稳定氮同位素

Abstract:

【Background】Stable isotope fingerprinting technology has been widely adopted in the origin traceability study of dairy products. However, most of them are focused on comparing the differences between the stable isotopes of raw milk and milk products. Nevertheless, the fractionation effect of stable isotopes on dairy products processing and the application of stable carbon and nitrogen isotopes for origin tracing of yak dairy products are still unclear.【Objective】In this study, yak yogurt and yak milk dregs were used as the study subjects to determine the changes in stable carbon and nitrogen isotope and the fractionation coefficients and correlations of yak dairy products at key points during processing, to investigate the stable carbon and nitrogen isotope characteristics of yak dairy products from different origins, so as to provide the theoretical and technical supports for origin traceability of yak dairy products.【Method】A total of 196 samples were collected from the Nerong and Jiali counties of Nagqu City, Tibet Autonomous Region, obtain five key sampling points for yogurt processing (yak milk, yak milk boiled for 5 min, sample after strain addition, fermentation at 40 ℃ for 6 h, and yogurt) and four key sampling points for milk dregs processing (yak milk, skimmed yak milk, skimmed yak milk boiled for 10 h, and milk dregs). The stable carbon and nitrogen isotope ratios were determined using an elemental analysis isotope ratio mass spectrometer (EA-IRMS). The differences and correlations between the stable carbon and nitrogen isotopes at the key sampling points for yogurt and milk dregs processing were determined using one-way ANOVA comparative analysis and Pearson correlation analysis, respectively. Furthermore, the differences in stable carbon and nitrogen isotopes between yogurt and yak milk and milk dregs and yak milk with different origins were determined using a two-factor ANOVA.【Result】The fractionation of stable carbon and nitrogen isotope during yogurt processing was as follows: δ¹³C_{yak milk} >δ¹³C_{40 ℃ fermentation for 6 h, yak yogurt} >δ¹³C_{samples after adding strain}, fractionation coefficient between 0.9996 and 1.0009, Δ_{Yak milk-yak yogurt} was 0.48‰; δ¹⁵N_{boiling 5 min yak milk, 40 ℃ fermentation for 6 h, yak yogurt}>δ¹⁵N_{yak milk}, fractionation coefficient was between 0.9993 and 1, and Δ_{Yak milk-yak yogurt} was -0.61‰. The correlations between the stable carbon and nitrogen isotopes at some key sampling points were significant. During milk dregs processing, δ¹³C_{yak milk, boiled 10 h skimmed sample, yak milk dregs}>δ¹³C_{Skimmed yak milk}, fractionation coefficient was between 0.9995 and 1.0005, Δ_{Yak milk-yak dregs} was 0. A significantly negative correlation was observed in δ¹³C at some key sampling points, while no significant difference was observed in δ¹⁵N for each key point sample and the fractionation values were 0. The stable carbon and nitrogen isotopes of dairy products from different origins significantly differed, with δ¹³C and δ¹⁵N being enriched in yak dairy products from Nerong County compared to Jiali County.【Conclusion】The fractionation of δ¹³C and δ¹⁵N was observed during yak dairy products processing. The addition of strains, fermentation, and centrifugal defatting processes resulted in different δ¹³C ratios, while heating induced changes in the sample δ¹³C and δ¹⁵N. Although stable isotope fractionation occurred during yak dairy products processing, its influence was less than the origin. Therefore, the stable carbon and nitrogen isotopes could be applied to trace the origin of yak dairy products.

Key words: yak milk, yogurt, milk dregs, yak dairy products, stable carbon isotope, stable nitrogen isotope