Brazil is the largest producer of different concentrated juices. The reduction of water activity (Aw) level in these products, caused by high concentration of sugar, increases its shelf life and determines the taste. The Aw also affects the stability of these juices and controls the growth rate of microorganisms. All these features make the Aw the parameter, which should be carefully monitored during production processes in the food industry. Commercially available equipment measures only the value of the Aw after equilibrium between the sample and ambient is reached. In contrast to this approach we present the Surface Acoustic Wave (SAW) sensor adjusted for real time Aw and water vapor saturation dynamics (SD) measurements in juices. The sensitive element of the sensor is the graphene oxide film of nanometer-scale thickness deposited by the SAW atomization. Water activity of solutions of sucrose with different concentrations (0.861<Aw<0.982), which are used as models of concentrated juices, was measured with the developed sensor. Adsorption curves for water molecules were experimentally obtained and the sensitivity of the sensor was measured to be 2375 Hz/RH%, which gave 0.6% accuracy of measurements. Measurements of SD showed that the saturation time (ST) necessary for the gas phase to reach equilibrium with the product is 35-45 sec. Thermal treatment of the sucrose samples (121oC/15 min) made the ST increase to 57.4 sec. The results show that the SD measurements can not only determine the Aw of juices but also analyze the treatment to which the sample was subjected.