Analysis of A1‐type and A2‐type β‐casein in Maiwa Yak and Pien‐niu milk by HPLC‐high‐resolution MS and tandem MS

Abstract In this study, a peptide‐based method employing ultra high performance liquid chromatography electrostatic field orbitrap high‐resolution mass spectrometry and triple quadrupole mass spectrometry was developed for quantification of A1‐type and A2‐type β‐casein in milk from Yak, cows, and their offspring of crosses, Pien‐niu. The specific peptides of A1‐type and A2‐type β‐casein were screened and confirmed by protein software after analysis of high‐resolution mass spectrometry. The multiple reaction monitoring method was established based on the qualitative results, and isotope‐label peptides were used as internal standards. The linear correlation coefficients of this method were >0.99. The relative standard deviations of repeatability test were 0.2‐3.6%. The recovery rate ranged from 93.3 to 114.4% with relative standard deviations <6% at three different spiking levels. The method was applied to analyze 45 milk samples from different species. The results showed that β‐casein in Yak and Pien‐niu milk was about 30% higher than that in cow milk. Furthermore, the β‐casein in the Yak milk only contains A2‐type β‐casein. A1‐type and A2‐type β‐casein coexist in most samples of Pien‐niu and cow milk, a few samples contain only one type of β‐casein. These results provide further understanding in nutritional value of milk from Yak and Pien‐niu.