Yeast Viability and Molecular Detectability in Direct-Vat-Set Starter Cultures Used for Kefir Production

Kefir beverages require direct-vat-set starter cultures, and their sensory properties depend directly on the yeast. Yeasts provide mixed fermentation and develop the corresponding sensory characteristics. However, regulatory requirements for direct-vat-set starters remain insufficiently detailed, and control methods are mostly limited to culture-based approaches. Therefore, assessing yeast viability and molecular detectability is a relevant research task. This study compared five commercial lyophilized starter cultures. Yeast counts were determined by plating on Sabouraud agar supplemented with chloramphenicol, followed by counting the number of colony-forming units. Molecular detection involved a DNA analysis with the ribosomal DNA ITS region amplified using ITS1F/ITS2R primers. The amplification products were analyzed by agarose gel electrophoresis. Viable yeasts appeared in three of the five samples (2.8 × 10⁵ – 6.2 × 10⁸ CFU/g). The remaining two samples showed no growth, yet one of them was found to contain yeast DNA detected by PCR. Probably, these non-viable cells or cells in a VBNC (viable but non-culturable) state could not be detected by culture-based methods. The results confirmed the variability in the yeast component of commercial direct-vat-set starter cultures but demonstrated the insufficiency of culturebased methods. The combined use of microbiological and molecular approaches provided a more comprehensive assessment of the starter microbiota.