volatile organic compounds (VOCs) Bacteria Biohydrogen Dark fermentation Fermentation Fermentative bacteria Flow cytometry Frequency dependent polarizability Frequency-dependent polarizability anisotropy Hydrogen production Microscopic observations Optical technique Physiological state Physiology Polarization Process parameters Spore-forming bacteria dairy fermentation gas chromatography–ion mobility spectroscopy (GC-IMS) high-temperature ion mobility spectrometry (HTIMS) kefir microbial communities next-generation sequencing (NGS) non-negative matrix factorization (NNMF) non-targeted screening (NTS) using machine learning qPCR quantitative polymerase chain reaction (qPCR) traditional and commercial kefir