Balancing fermentation control and aroma complexity in semi-spontaneous kiwifruit fermentation through Saccharomyces...

Recent research indicates the great potential of spontaneous fermentation microbiota for enhancing the aromatic complexity of fruit wines. To evaluate the impact of Saccharomyces cerevisiae inoculum size on fermentation performance and flavor formation in a semi-spontaneous fermentation system, this study systematically compared inoculated fermentation (IF), spontaneous fermentation (SF), and semi-spontaneous fermentation with graded Saccharomyces cerevisiae inoculation levels (SF + 20%, SF + 50%, and SF + 100% of the standard inoculum). Physicochemical properties, organic acids, volatile compounds, microbial communities, and sensory characteristics were comprehensively analyzed. The results demonstrated that increasing the S. cerevisiae inoculum level significantly influenced the fermentation performance and flavor formation. Compared with SF, the high-inoculum regimen (SF + 100%) effectively reduced acetic acid content from 3.93 g/L to 0.28 g/L, a level comparable to that of the IF group, thereby indicating effective control of acetic acid bacteria during fermentation. More importantly, this regimen retained a detectable fraction of indigenous microbiota, which may have contributed to the accumulation of phenolic compounds and key aroma-active esters (e.g., ethyl hexanoate, isoamyl acetate) and terpenoids (e.g., geraniol, damascenone). Relative odor activity values (ROAV) analysis revealed that this treatment exhibited the greatest abundance and contribution of aroma-active compounds contributing to fruity and floral characteristics. Electronic nose data clearly discriminated among the five fermentation protocols, and sensory evaluation confirmed that the SF + 100% group achieved the highest scores for fruity aroma, ester aroma, and overall acceptability. In summary, semi-spontaneous fermentation with a high S. cerevisiae inoculum level (1.0 × 10⁷ CFU/mL) achieved a favorable balance between fermentation control and aroma complexity. These findings provide a practical strategy for optimizing controlled spontaneous fermentation and improving the quality of specialty fruit wines.