Response Surface Optimization of Goat Milk Kefir Fermentation for Improved Physicochemical and Microbiological Quality

Introduction: Goat milk kefir is gaining increasing attention as a functional fermented dairy product because of its digestibility, nutritional value, and potential probiotic properties. However, its quality is strongly influenced by fermentation conditions, including grain-to-milk ratio, temperature, and fermentation time. This study aimed to optimize goat milk kefir production using a Box–Behnken design combined with response surface methodology to establish a reproducible protocol with targeted physicochemical and microbiological characteristics.

Methods: A three-factor, three-level Box–Behnken design comprising 14 experimental runs, including center points, was applied. The independent variables were grain-to-milk ratio, fermentation temperature, and fermentation time. The evaluated responses included pH, titratable acidity, syneresis, total solids, proteolytic activity, lactic acid bacteria, yeasts and molds, and total aerobic mesophilic count. Data were analyzed by ANOVA and fitted to second-order polynomial models to determine the effects of fermentation parameters and identify optimal processing conditions.

Results: All three factors significantly affected kefir quality. The experimental ranges were pH 4.0–4.9, titratable acidity 60–115 °D, syneresis 22–40%, total solids 11.3–13.4%, proteolytic activity 0.38–0.95 mg Gly/mL, and lactic acid bacteria 6.9–9.1 log CFU/mL. Increasing grain-to-milk ratio and temperature accelerated acidification and microbial growth, whereas longer fermentation enhanced proteolysis. The optimized region achieved LAB counts above 9.0 log CFU/mL, titratable acidity above 110 °D, and syneresis below 30%.

Conclusions: The optimized conditions, namely 1.5% kefir grains, 30 °C, and 22 h, provide a reproducible protocol for producing goat milk kefir with strong probiotic potential and controlled technological quality. These conditions may support future shelf-life and pathogen-control studies.