Comparative Volatilome Profiling of Milk Kefir and Cereal-Based Kefir Analogues Fermented with Milk and Water Kefir Grains

Milk and water kefir are traditional fermented beverages attracting considerable scientific and commercial interest, due to their microbial diversity, health-enhancing properties and consumer preferences. They are linked to tradition, meet the tastes of global consumers, and are model microbiomes/fermented products of considerable relevance. In this study, a Direct Injection Mass Spectrometry (DIMS) technique, i.e., Proton Transfer Reaction Time of Flight Mass Spectrometry (PTR-ToF-MS), was exploited for the rapid and non-invasive profiling of volatile organic compounds (VOCs) during the fermentation of i) milk, ii) oat, iii) corn, and iv) barley, using both milk kefir grains and water kefir grains as fermentation microbiomes. The impact of the supplementary inoculation of a strain of Lactiplantibacillus plantarum M5MA1-B2 of biotechnological relevance, in association with kefir microbiomes, has also been evaluated. The intensity 62 ions, corresponding to volatile compounds belonging to the chemical classes of acids, aldehydes, ketones, esters, alcohols, lactones and sulfur compounds, was monitored at 13 different times, representative of the different phases of the overall fermentation time of 48 h, offering a time-scale omics approach in microbiomes study (i.e. kefir microbiomes in milk and in cereal flours). The addition of L. plantarum M5MA1-B2 to kefir grains serves as a suitable model for demonstrating the perturbation of a single-strain starter culture when inoculated in matrices fermented by complex microbiomes. A complex modulation was detected, with an overall intensity-reducing effect for many VOCs, including acetoin, diacetyl, acetaldehyde, and ethanol. Heptanal/2-heptanone (in cereal-based matrices) 2-nonanone (in all the matrices) are good markers of strain addition with respect to milk kefir grains. This study also contributed to extending the volatilome of milk kefir, describing for the first time 19 VOCs associated with this matrix (e.g. propenoic acid, methanethiol, hexenal, 2,4-heptadienal).