The relationship between electric impedance spectroscopy (EIS) parameters and the conventional parameters of pH, acidity and hardness was discussed to evaluate the technique’s applicability as an on-line monitoring system of food processing automation, specifically yogurt processing. To determine pH, acidity and hardness, impedance (Z) obtained with an EIS measurement system was used during the yogurt processing at different temperatures. Time course curves for both pH and acidity had only gentle slopes showing the progress of lactic fermentation; however the time course for hardness had bending points indicating the start of coagulation. Also, these bending points were found at the time course for the Z ratio obtained as 100 kHz Z divided by 0.1 kHz Z. The relationships between Z ratio and both acidity and hardness have respective linear functions and the calculated acidity and hardness using the Z ratios showed agreement with those measured in the yogurt processed from the same materials. The EIS technique is applicable for monitoring yogurt processing and is more sanitary and non-destructive method than the conventional method.