Elucidating the microbiota-metabolite interplay in hurood and chula: An integrated multi-omics investigation

Hurood and chula, traditional fermented dairy products from Xilingol, show flavor and quality differences due to variations in production processes. Therefore, how heating and kneading impact their microbial, non-volatile metabolite, and volatile organic compound (VOC) composition warrants exploration. In this study, shotgun metagenomic sequencing revealed the differential microorganisms between hurood and chula. Ultra-performance liquid chromatography-tandem mass spectrometry identified 47 differential metabolites. Among these, organic acids and their derivatives, benzene and substituted derivatives, free fatty acids (FFAs), and lysophosphatidylcholines showed the highest content in chula, whereas lactose, melibiose, and histamine were significantly enriched in hurood, suggesting that the heating and kneading process affected galactose and histidine metabolic pathways. In contrast, headspace solid-phase microextraction gas chromatography-mass spectrometry identified four differential VOCs with elevated levels in hurood. These compounds functioned as key aroma contributors, imparting richer and more complex aroma characteristics that included creamy, oily, fatty, caramel, coconut, woody, spice, and maple notes. Correlation analysis indicated that the differential microorganisms were involved in metabolite dynamics and VOC accumulation, and further revealed that they drove the directed accumulation of VOC through regulating FFA release and transformation and by regulating the Maillard reaction of small peptides, thereby underpinning their distinct flavor profiles. This study provides important insights into the heating- and kneading-induced formation of flavor and quality in traditional fermented dairy products from Xilingol, thereby facilitating the precise control of traditional processes and subsequent product quality improvement.