Estimation of hardness of cooked rice during retort sterilization by electric impedance measurement

Since retort pouch food is sterilized under high temperature and high pressure, the physical properties are supposed to change with the progression of sterilization. However, an adequate technique to measure the state of food during sterilization has not yet been established. In this study, we measured the electrical impedance of rice samples (rice-water system) in a tray-type retort container continuously throughout the sterilization process. In addition, we measured the hardness of rice grains in retort pouches that were sterilized for various sterilizing times using a material tester. We found that the impedance vector locus for the samples was plotted linearly, and the slope of the regression line of the vector locus was evaluated as an indicator of the changes in the sample. During the heating process (13 minutes at room temperature to 120℃ of retort chamber temperature) and sterilization process (30 minutes at 120℃ of retort chamber temperature), the slope value decreased with processing time. After the sterilization process started (after 13 minutes from the start of the heating process), the central temperature of the sample was almost constant, but the slope value continued to decrease. We thought that the change in the slope value represented some kind of physical and/or chemical change in the sample. Samples with different sterilizing times showed decreasing hardness of rice grains as well as decreasing slope values as the sterilizing time increased. It was determined that there is a high correlation (R**2 = 0.693) between the slope value and the hardness of rice grains. The results showed that the above is one possible method to non-destructively and continuously measure how the hardness of retort-packed rice changes during sterilization and to evaluate changes with slope values used as an indicator of hardness.