## 3 Results and Discussion  
### Evolution of VOC Profiles During Successive Kefir Grain Passages  
As shown in Tables S1-S3, a total of 82 volatile organic compounds (VOCs) were identified across all samples, including both unfermented juice and fermented beverages. These VOCs were classified into seven chemical families: organic acids (2 compounds), alcohols (24), aldehydes (10), ketones (4), esters (24), furans (2), and a miscellaneous group labeled as \"other compounds\" (16). In the unfermented kiwi juice, 14 VOCs were identified, including three alcohols, one ketone, three esters, and seven compounds classified as \"other compounds\", with a total VOC concentration of 9.14 \(\pm\) 0.43 mg/L (Table S1). In the fermented samples from the first kefir grain passage, the number of VOCs was either equal to (as in Bev5-24 h) or greater than that in the unfermented juice, with the exception of Bev2-24 h, which was the only sample to exhibit a lower VOC concentration (6.12 \(\pm\) 0.20 mg/L) compared to the fresh juice. Among all samples in this passage, Bev8-24 h contained the highest number of VOCs (29), while Bev6-24 h showed the highest overall concentration (20.42 \(\pm\) 0.55 mg/L). During the second kefir grain passage, only Bev5-48 h exhibited a similar number of VOCs to the unfermented juice, while all other fermented samples showed higher values. However, Bev2-48 h and Bev8-48 h had lower VOC concentrations, measuring 6.23 \(\pm\) 0.21 and 8.78 \(\pm\) 0.26 mg/L, respectively. The highest number of VOCs (32) was detected in both Bev3-48 h and Bev9-48 h, whereas Bev6-48 h had the highest overall concentration (21.37 \(\pm\) 0.55 mg/L). In contrast, the fermented drinks from the third batch of cultures contained both a greater number and higher concentrations of VOCs compared to the unfermented juice. Bev9-72 h exhibited the highest number of VOCs (36), and Bev6-72 h showed the highest concentration (26.94 \(\pm\) 0.96 mg/L). These results suggest that VOC production depended on the shaking rate, the inoculum size, and the number of kefir grain passages. Shaking rates ranging from 38 to 86 rpm were effective for producing fermented beverages with a high number of VOCs, as observed in Bev8-24 h (86 rpm), Bev3-48 h (38 rpm), and both Bev9-48 h and Bev9-72 h (86 rpm). The influence of the inoculum size on VOC production appeared to depend on both the shaking rate and the number of fermentation passages. For instance, the proportions of milk kefir grains for the samples with the highest VOC counts were as follows: Bev8-24 h (PMKG = 1.32% \(w/v\)), Bev3-48 h (PMKG = 4.33% \(w/v\)), and both Bev9-48 h and Bev9-72 h (PMKG = 3.00% \(w/v\)). Moreover, the highest VOC concentrations observed in Bev6-24 h, Bev6-48 h, and Bev6-72 h suggest that a shaking rate of 25 rpm, combined with a 3.00% (\(w/v\)) inoculum, is optimal for producing kefir-like beverages rich in volatile compounds. On the other hand, by the third passage, the kefir grains appeared to be better adapted to the acidic conditions of the unfermented kiwi juice (pH = 3.27 \(\pm\) 0.17), which likely contributed to the enhanced production of volatile compounds (Tables S1-S3). The low production of organic acids in the beverages from all three kefir grain passages, compared to other VOCs (Tables S1-S3), may be attributed to the low pH of kiwi juice, particularly in comparison to UHT whole milk (pH = 6.70; [25]). This acidic environment likely restricted the formation of organic acids. Furthermore, these compounds may have been further metabolized by yeasts present in the inoculum into other volatile compounds, such as alcohols, esters, or ketones [26]. In a previous study, kefirs were produced through 24 h batch fermentations of UHT whole milk using the same type of inoculum as employed in the present study, across four kefir grain passages [25].