Dielectric properties of cowpea weevil, black eyed peas and mung beans with respect to the development of radio frequency heat treatments

Authors: JIAO, S - Washington State University; Johnson, Judy; TANG, J - Washington State University; TIWARI, G - Washington State University; WANG, S - Washington State University

Submitted to: Biosystems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2010
Publication Date: 5/30/2011
Citation: Jiao, S., Johnson, J.A., Tang, J., Tiwari, G., Wang, S. 2011. Dielectric properties of cowpea weevil, black eyed peas and mung beans with respect to the development of radio frequency heat treatments. Biosystems Engineering. 108(3):280-291.

Interpretive Summary: Cowpea weevil is a serious pest of legumes, and may interfere in export marketing of these products. Chemical fumigants are commonly used to disinfest product of this pest, but various regulatory and environmental concerns have generated interest in non-chemical alternatives such as radio frequency heating. To validate the efficacy of such treatments, large amounts of product must be infested with cowpea weevil and treated. To accomplish this, black-eyed peas and mung beans are being considered for use as surrogate host legumes, because they are better hosts for cowpea weevil when compared with the target legumes (chickpeas and lentils). By way of evaluating the suitability of black-eyed peas and mung beans, their dielectric properties were measured and compared to those of cowpea weevil immature stages and adults. For both insect and legume samples, the dielectric constant and loss factor decreased with increasing frequency but increased with increasing temperature and moisture content. Comparison of the dielectric loss factor of cowpea weevil immature stages and adults with legumes at commonly used industrial frequencies of 27 (RF) and 915 (MW) MHz showed that cowpea weevils should differentially heat faster than the legumes, with the differential heating reduced in MW heating when compared to RF heating. Dielectric properties of black-eyed peas and mung beans were similar to the target legumes, suggesting that their use as surrogate hosts should not bias efficacy studies. Penetration depths calculated for black-eyed peas and mung beans suggests that industrial RF heating systems could be used to disinfest these products as well, and RF treatment has much larger penetration depth than MW treatment.

Technical Abstract: In developing radio frequency (RF) and microwave (MW) disinfestation treatments for chickpeas and lentils, large amounts of product infested with cowpea weevil must be treated to validate treatment efficacy. To accomplish this, black-eyed peas and mung beans are being considered for use as surrogate host legumes, because they are better hosts for cowpea weevil when compared with the target legumes. Dielectric properties are very important parameters for developing RF and MW treatments and may be used to estimate heating uniformity and penetration depth. Dielectric properties of black-eyed pea and mung bean flours at four moisture content levels as well as cowpea weevil immature stages and adults were measured with an open-ended coaxial probe and impedance analyzer at frequencies of 10–1800 MHz and temperatures of 20–60°C. For both insect and legume samples, the dielectric constant and loss factor decreased with increasing frequency but increased with increasing temperature and moisture content. Comparison of the dielectric loss factor of cowpea weevil immature stages and adults with legumes at commonly used industrial frequencies of 27 (RF) and 915 (MW) MHz showed that cowpea weevils should differential heat faster than the legumes, with the differential heating reduced in MW heating when compared to RF heating. Penetration depths calculated for black-eyed peas and mung beans suggests that industrial RF heating systems could be used to disinfest these products as well, and RF treatment has much larger penetration depth than MW treatment.