Effect of Co-fermentation with the Apple Endophyte Torulaspora delbrueckii on the Synthesis of the Flavor Substances of Apple Vinegar

Torulaspora delbrueckii S1, an endophyte isolated from apples, was used in the fermentation of apple vinegar to explore its influence on the flavor of apple vinegar. The volatile flavor substances and organic acids of apple vinegar co-fermented with T. delbrueckii S1 and Saccharomyces cerevisiae (experimental) and that fermented with S. cerevisiae (control) were detected by headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) and high performance liquid chromatography (HPLC), respectively, and their differences in flavor substances were compared through biometric analysis. Heatmap cluster analysis showed that T. delbrueckii S1 had no significant effect on the synthesis of volatile substances in apple vinegar, and the apple vinegars could be well distinguished by orthogonal partial least squaresdiscriminant analysis (OPLS-DA). The results of differential metabolites showed that phenethyl acetate, contributing to the typical apple aroma; and phenyl ethanol, benzaldehyde and benzyl alcohol, contributing to floral and fruity aromas, were more prominent in the experimental group, while ethyl caproate, ethyl decanoate and ethyl laurate, responsible for fruit and laurel oil-like aromas, were more prominent in the control group. The results of co-occurrence network analysis showed that T. delbrueckii S1 could enhance the transmissibility of volatile flavor compounds, and consequently increase the complexity of metabolites. In addition, the box plot of organic acids showed that T. delbrueckii S1 could increase the contents of L-lactic acid, L-malic acid and acetic acid. In this study, the effects of apple endophytic fungus T. delbrueckii S1 on the synthesis of flavor compounds in apple vinegar were analyzed, and it provided a basis for further utilization of this strain.