S. Jiao

Determining Radio Frequency Heating Uniformity of Mixed Beans for Disinfestation Treatments

Chickpeas and lentils are two important legumes grown in the U.S. that need phytosanitary treatments before export to several countries, but it is difficult to artificially infest them with live cowpea weevil for radio frequency (RF) treatment validation. To evaluate the more readily infested black-eyed peas and mung beans as surrogates for chickpeas and lentils, the thermal and dielectric properties of black-eyed peas and mung beans at selected moisture contents were measured and compared with those of chickpeas and lentils. Temperature differences between black-eyed pea and chickpea or between mung bean and lentil were determined in a pilot-scale 27 MHz RF unit. The results showed that the dielectric constant and loss factor of black-eyed pea and mung bean increased with increasing moisture content and temperature, which was in good agreement with the trends observed in chickpea and lentil. After 6 min of RF heating, temperatures in black-eyed pea (moisture content of 8.8% w.b.) were 6°C higher than those in chickpea (moisture content of 7.0% w.b.), while after 10 min of RF heating mung bean temperatures (moisture content of 10.2% w.b.) were 4°C higher than lentil temperatures (moisture content of 7.1% w.b.) under the same treatment conditions. By reducing the moisture contents in black-eyed pea and mung bean to 2.6% and 3.7% w.b., respectively, their final temperatures were about 3.5°C and 3.7°C lower than those of chickpea and lentil. This would result in conservative insect mortality results when using black-eyed pea and mung bean as surrogate hosts for validation of pest control treatments in chickpea and lentil.

Developing Low Pressure Treatments for Disinfesting Fresh Fruits

To develop low pressure/low temperature (LPLT) disinfestation treatments for fresh apples, the LPLT system stability and performance were evaluated first. Different life stages of coding moth were treated in hypobaric chambers maintained at 10°C and 1.33 kPa with nearly saturated humidity to evaluate the tolerance of insects. Weight loss, color, firmness, titratable acidity (TA), and soluble solid content (SSC) were selected as quality parameters to evaluate the quality changes of “Red Delicious” apples after LPLT treatment. Results showed that the regulating system kept pressure to within ±1% of the set point, with a chamber leakage rate of 0.009 kPa/h and LP system leakage rate of 0.480 kPa/h. The most tolerant life stage for codling moth under LPLT treatment conditions was 5th instar larvae. Insect mortality increased with increasing LPLT treatment time to more than 98% after 12 days at 10°C temperature and 1.33 kPa pressure. The quality of apples was maintained well after 15 days of LPLT treatment, suggesting that LPLT technology has potential as an alternative non-chemical disinfestation treatment method for apples.

Developing Radio Frequency Treatment Protocol for Disinfesting Coffee Beans

The objective of this study was to develop novel non-chemical treatments for postharvest disinfesting coffee beans using radio frequency (RF) energy. A pilot-scale 27 MHz, 6 kW RF unit was used to study heating uniformity, develop a treatment protocol, and evaluate quality attributes in treated coffee beans. After comparing 3 selected electrode gaps, an appropriate gap of 14 cm was obtained to raise the central temperature of 2.75 kg samples to 48°C using RF energy by 4.6 min, compared to more than 237 min using forced hot air at 48°C. RF heating uniformity in beans was improved by adding forced hot air, and back and forth movements. The final temperatures reached 50.4°C and 50.6°C, resulting in uniformity index values of 0.060 and 0.073 for interior and surface temperature distributions, respectively. No significant differences in weight loss, moisture content, and color were observed between RF treatments and unheated controls.

Quality and Mold Control of Enriched White Bread by Combined Radio Frequency and Hot Air Treatment

This study explored the application of radio frequency (RF) energy in conjunction with conventional hot air treatment to develop feasible heat treatments of bread loaf in a 6 kW, 27.12 MHz pilot-scale RF system. Treatment parameters were selected based on minimum time-temperature conditions required for 4-log reduction of P. citrinum spores and on acceptable bread quality. Much lower mean product temperature and shorter holding time were used for control of P. citrinum spores with combined RF and hot air treatment compared to conventional heating alone. Heating bread to 58°C or higher resulted in 4-log reduction of P. citrinum spores isolated from moldy bread. The storage life at room temperature (23°C) was extended by 28±2 days for the treated white bread.

Developing Postharvest Disinfestation Treatments for Legumes Using Radio Frequency Energy

There is an urgent need to develop technically effective and environmentally sound phytosanitary and quarantine treatments for the legume industry to replace chemical fumigation. The goal of this study was to develop practical non-chemical treatments for postharvest disinfestations of legumes using radio frequency (RF) energy. A pilot-scale 27 MHz, 6 kW RF unit was used to investigate RF heating and consequent quality attributes in treated chickpea, green pea, and lentil samples. Only 5-7 min was needed to raise the central temperature of 3 kg legume samples to 60°C using RF energy, compared to more than 275 min when using forced hot air at 60°C. RF heating uniformity in product samples was improved by adding forced hot air, and back and forth movements on the conveyor at 0.56 m min-1. The final temperatures exceeded 55.8°C in the interior of the sample container and 57.3°C on the surface for all three legumes, resulting in low uniformity index values of 0.014-0.016 (ratio of standard deviation to the average temperature rise) for the interior temperature distributions and 0.061-0.078 for surface temperature distributions. RF treatments combined with forced hot air at 60°C to maintain the target treatment temperature for 10 min followed by forced room air cooling through a 1 cm product layer provided good product quality. No significant differences in weight loss, moisture content, colour or germination were observed between RF treatments and unheated controls.