Yogurt fermentation through ohmic heating and electrical conductivity based process monitoring

This study investigated the potential of electrical conductivity-based ohmic fermentation monitoring for set yogurt production. A milk-starter culture mixture was prepared by inoculating Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus into ultra-high-temperature sterilized milk. The temperature of this milk-starter culture mixture ohmically increased to a target temperature of 43 °C within 3.20 ± 0.12 min under 10 V/cm. It maintained the ohmic fermentation for 10 h under 5 V/cm. During ohmic fermentation, the conductivity increased from 0.69 ± 0.01 S/m at 0 h to 0.99 ± 0.03 S/m at 10 h. Electrical conductivity was converted to electrical conductivity fermentation index (CF_i), which increased from 0 up to 1 by the end point of fermentation. CF_i values were empirically modeled based on the physicochemical qualities of yogurt (pH, titratable acidity, viscosity, and total soluble solids) and proliferation of lactic acid bacteria (LAB). The CF_i estimation model showed good correlations for physicochemical indices and LAB growth. It successfully estimated the pH drop in the milk-starter culture mixture with a first order coefficient (β₁) of −2.099, second order coefficient (β₂) of −0.335, and coefficient of determination (R²) of 0.998. Furthermore, CF_i exhibited a good correlation with LAB growth (R² = 0.963). Thus, the application of electrical conductivity data and its CFi model for real-time monitoring of physicochemical changes and microbial growth during ohmic fermentation reinforces its effectiveness and potential in yogurt production.