Kefir is an artisanal product that is gaining scientific attention due to its increase in consumption attributed to its potential health benefits. The effect on the quality attributes of kefir grains of different origin (household and commercial) and preserved with different methods (drying and freezing) was evaluated to standardize a domestic and semi-industrial process. Chemical (protein, lactose. lactic acid, ethanol, and acetic acid) as well as microbial properties (total plate count and total yeast count) were monitored during the experiment. Results show a statistical difference between the kefir grains in terms of acetic acid and ethanol percentages, which leads to the conclusion that there is a difference in the microbial populations that produce these products. Lactic acid, protein, and lactose parameters are statistically the same as well as the growth in biomass and the total population of yeast. Our results suggest that both types of kefir grains maintain a similar performance and that their metabolic capabilities are stable throughout time (taking into account that the household grains have been productive for years), meaning that domestic or semi-industrial processes could be easily standardized.

Kefir, a fermented milk product, differs from yogurt due to its unique microbial composition, offering a broad spectrum of health benefits. Given its global popularity and high cost, there is a significant trend towards domestic kefir production. This study explores the optimization of kefir fermentation using Costa Rican domestic kefir grains, assessing the effects of temperature, agitation, and initial starter culture concentration. A central composite rotatable design and response surface statistical approach were employed to evaluate these parameters. Microbial growth data were fitted into a quadratic model, revealing significant interactions, particularly with temperature affecting both lactic acid bacteria (LAB) and yeast populations. Optimized fermentation conditions were established at 25°C, 0 rpm, and 5 g/L initial biomass, under which final microbial populations reached 9.45±0.13 log(cfu)/mL for yeast and 9.23±0.06 log(cfu)/mL for LAB. The specific growth velocity for kefir biomass was 0.029 1/h, and the total acid production rate was 0.060 g/(L h). Notably, the acetic acid production was significantly less than lactic acid, indicating a dominance of LAB over acetic acid bacteria, which is crucial for the desired flavor and health benefits of kefir. Additionally, microbial enumeration on glucose-yeast extract, calcium carbonate agar, and Rogosa agar showed distinct colony formations, highlighting the complex microbial interactions within kefir. This comprehensive dataset suggests that the performance of non-commercial starter cultures can be significantly improved under controlled conditions, providing a basis for developing guidelines for domestic kefir production. This study not only optimizes kefir production but also ensures that home-prepared kefir can meet quality standards, potentially enhancing its nutritional and therapeutic benefits.