Abstract— Fermentation is the process where bacteria convert lactose to lactic acid. When milk is fermented to make yogurt, its elasticity increases accordingly. pH meters have traditionally been used for fermentation process monitoring based on acidity. Ultrasonic systems can provide a rapid, accurate, inexpensive, simple and non- destructive method to on-line monitor the properties of food during process operations. This paper evaluates the use of ultrasonic measurements to characterize yogurt fermentation process by correlating acoustic properties and fermentation process characteristics. This research shows the correlation between fermentation time and acoustic attenuation as well as acoustic velocity. It also shows the effect of temperature on the received signal attenuation and velocity for yogurt and milk. This paper also proposes a fuzzy logic inference system to model the yogurt fermentation process. The relative peak to peak amplitude and the relative time of flight are used as an input to the fuzzy inference system.