DESIGN AND TESTING OF A PROTOTYPE AUTOMATED OHMIC THAWING UNIT
Ohmic heating, or electroconductivity heating, is a process of applying electricity to a substance, and the resistance of that substance generates heat. The substance used most in this study was frozen blocks of shrimp. The traditional method used to thaw frozen shrimp is to immerse the shrimp in warm water. One major concern of using water as a medium to transfer heat is that there is a limited supply of potable water. Thawing shrimp by electrical currents is intended to save the water supply. The major concern of ohmic thawing, on the other hand, is to prevent "runaway heating," which occurs when all of the current flows through a less resistant path with the block resulting in cooked shrimp. Two ways to prevent this localized heating is control current levels generated through the block and monitoring the surface temperature of the block, in which case both methods were used cooperatively in this study.
A mobile ohmic unit was built with the capabilities of simultaneously thawing two blocks of shrimp, as well as to be transported from the lab to a shrimp processing plant. The temperature at which the computer shut off the current was slightly above the freezing point of the shrimp. The voltage was continually adjusted automatically to maintain constant current in the block. When currents were detected too high, the unit either reduced the voltage level or shut power off to that particular region of the block. At no time after any of the experiments were warm shrimp detected.
Quality attribute comparison test were conducted to determine whether the ohmic system had any affect on the shrimp compared to the shrimp thawed from the immersion tanks. The shrimp were evaluated based on microbiology, moisture content, and sensory evaluation. The results showed no significant difference between the shrimp thawed by the ohmic system and the shrimp thawed by the immersion method.