Diversity and dynamics of microbial communities during spontaneous fermentation of different cultivars of apples from China and their relationship with the volatile components in the cider.

Microorganisms involved in spontaneous fermentation are critical in cider productivity, quality, and aroma. This study examined the composition and dynamic changes of indigenous microbial communities during spontaneous fermentation of three apple cultivars (Qin Guan, Fuji, and Qin Yue) from Shaanxi, China, and their influence on cider's volatile components. Samples were collected at three time points and analyzed using high-throughput sequencing and HS-SPME-GC-MS. A total of 2961 bacterial and 618 fungal operational taxonomic units (OTUs) were identified, distributed among 20 bacterial and 5 fungal phyla. Alpha diversity analysis and Permutation multivariate analysis of variance (PERMANOVA) demonstrated that the cultivar significantly influenced fungal diversity and microbial composition at the start of the fermentation (p < 0.05). Moreover, the particular microbial communities inherent to the cultivar were maintained at the end of the fermentation. The characteristic fungal genera were Hanseniaspora in Qin Guan, Rhodosporidiobolus and Pichia in Fuji, and Debaryomyces in Qin Yue. A total of 43 volatile compounds were identified, among which 40 exhibited significant differences across different cultivars (p < 0.05). The characteristic volatile flavor components of Qin Guan included Isobutyl acetate, Ethyl 3-methylbutanoate, Hexyl acetate, Ethyl 2-hydroxypropanoate, 1-Hexanol, 1-Heptanol, and 2-Methylpropanoic acid. Fuji's characteristic component was Diethyl butanedioate, while Qin Yue's was (E)-3-Hexen-1-ol. Using partial least squares regression (PLSR) and Spearman analyze the correlation between microbiota and volatile compound production, the abundances of Hanseniaspora uvarum, Pichia kudriavzevii, Vishniacozyma victoriae, and Meyerozyma guilliermondii were positively correlated with the production of esters and higher alcohols. This research demonstrated that different apple cultivars influence the microbial community composition and, in turn, impact the cider aromas during spontaneous fermentation.