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 degrees C within 3.20 f 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 f 0.01 S/m at 0 h to 0.99 f 0.03 S/m at 10 h. Electrical conductivity was converted to electrical conductivity fermentation index (CFi), which increased from 0 up to 1 by the end point of fermentation. CFi 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 CFi 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 (beta 1) of -2.099, second order coefficient (beta 2) of -0.335, and coefficient of determination (R2) of 0.998. Furthermore, CFi 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.