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Dielectric Properties of Yogurt for Online Monitoring of Fermentation Process

Guo, Chaofan, Xin, Le, Dong, Yuehan, Zhang, Xueying, Wang, Xuejiao, Fu, Hongfei, Wang, Yunyang
Food and bioprocess technology 2018 v.11 no.5 pp. 1096-1100
Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, correlation, dielectric properties, fermentation, impedance, monitoring, pH, titratable acidity, yogurt
Yogurt is one of the most popular dairy products fermented by Lactobacillus bulgaricus and Streptococcus thermophilus. The present study introduced dielectric properties as a new technique for yogurt fermentation monitoring. Dielectric properties of cow milk and yogurt at different time of fermentation were measured from 10 to 3000 MHz at 42 °C using open-ended coaxial-line probe technology by an impedance analyzer. Meanwhile, pH and titratable acidity of yogurt at different fermentation time were measured. Results showed that the dielectric loss factor of yogurt was positively correlated with fermentation time and had an irregular change at the endpoint of fermentation (7 h, pH ≈ 4.6). The polynomial determination coefficients of dielectric loss factor with pH and titratable acidity decreased with increasing frequency and were found highest at 10 MHz (0.927 and 0.963, respectively). The linear coefficients of determination between measured values and calculated values were 0.990 for titratable acidity and 0.965 for pH at 10 MHz, respectively. Accordingly, dielectric loss factor can be a promising indicator for online monitoring of industrial yogurt fermentation process.