Saccharomyces boulardii CNCM I-745 as a Probiotic Adjunct in Kefir: A Proof-of-Concept Study Comparing Traditional Grain and Industrial Starter Cultures

Probiotic yeasts are increasingly proposed as adjuncts in fermented dairy products, but their behavior in kefir is still poorly described. This proof-of-concept study examined the effect of Saccharomyces boulardii CNCM I-745 supplementation on kefir produced with two traditional grain cultures and two industrial direct-vat-inoculation cultures during 21 days of cold storage at 4 °C. Microbiological, physicochemical, and sensory parameters were monitored on days 1, 7, 14, and 21. The starter culture type was the main source of variation, with traditional grain kefirs showing higher microbial counts and better sensory scores than industrial cultures. S. boulardii did not change pH, titratable acidity, Lactococcus spp., or total mesophilic aerobic bacteria, indicating that it can be added without disturbing the established kefir microbiota or its acidification pattern. The probiotic yeast increased the total yeast count, slightly modulated Lactobacillus spp., and gave a small improvement in taste–aroma scores. In the yeast-free industrial culture, S. boulardii maintained viable counts above 6 log CFU/g throughout storage, showing that it can act as the sole yeast source in kefir matrices that lack indigenous yeast. Traditional grain kefirs kept a more stable overall sensory quality across 21 days than industrial cultures. The multivariate analysis confirmed two largely independent quality dimensions, one related to lactic acid bacteria and acidity and another to sensory perception. The study supports the use of S. boulardii as a probiotic adjunct in kefir and provides preliminary effect-size information for future, adequately replicated trials.