Radio Frequency Heating for Dehydration and Pest Control of In-Shell Peanuts

Abstract

For many decades conventional wagon drying (curing) systems or batch/continuous flow drying systems have been used for peanut dehydration. Both are time and energy consuming. Radio Frequency (RF) dielectric heating has been applied to many types of food processing including tree nut. In this study, we would like to apply RF energy to dehydration and pest control of in-shell peanuts. RF is a fast and effective alternative drying method because heat is generated by the interaction between RF energy and moisture inside in-shell peanuts. In RF heating applications, the drying energy is evolved from inside the products. This will not only save time and energy, but will also eliminate or reduce thermal abuse to the products compared to the conventional processes. Despite the effectiveness of this method, it also presents some difficulties in its application. Primarily, RF heating or dehydration of peanuts process is non-uniform. Non-uniformity of RF heating of peanuts is due to many circumstances that have been discussed and taken into consideration in the RF treatment protocol. In this treatment protocol we overcome non-uniformity using multiple numbers of RF heating cycles with intermittent stirring and cooling. A set of 6 runs (each run consisting of a RF heating cycle followed by intermittent stirring and cooling cycle) were designed to dry peanuts at three different targeting temperatures of 40, 50 and 60 °C. The quality of peanuts after RF treatments at 40 or 50 °C are acceptable while the quality at 60 °C is not acceptable due to the percentage of broken and shelled peanuts. For the three targeting temperatures, the total RF heating time was in the range of 10 to 20 minutes. We found that the relation between peanut kernels’ temperature and the time of RF heating could be fitted in a straight line. In the second part of this work we studied the mortality of Red Flour Beetles (RFB), which are the most heat resistant insects that live in peanuts. We used a custom built heating block system manufactured by the Washington State University (WSU). We found different results from those in the literature on RFB. In our results RFB showed resistance to similar time-temperature combinations used before in others’ research. These results indicate that RFB in the southeastern area of the United States are more thermal resistant than those in the west coast. Further research with increased heating temperature, increased heating time or both are needed. In addition, it is necessary to review the nature of heat transfer differences between RF heating and conduction heating in the WSU heating block system.