Disinfestation of stored corn using microwave energy

Transcription

1 AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: , ISSN Online: , Science Huβ, Disinfestation of stored corn using microwave energy 1 Vadivambal, R., 2 Deji, O.F, 1* Jayas, D.S., 3 White, N.D.G. 1* Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6, 2 Department of Agricultural Extension and Rural Development, Obafemi Awolowo University, Ile-Ife, Nigeria, 3 Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, MB, Canada, R3T 2M9, * *Phone: ; Fax: digvir_jayas@umanitoba.ca ABSTRACT Chemicals are widely used to kill insects in stored grain even though these affect the environment and the consumers. Use of microwaves to kill insects in stored grain could be an alternative for chemical methods. A pilot-scale industrial microwave system operating at 2450 MHz was used to kill larval and adult stages of three stored grain insects: Sitophilus zeamais, Tribolium castaneum and Plodia interpunctella in corn at 14, 16, and 18% moisture content. Mortality experiments were conducted at two exposure times of 14 and 28 s at power levels of 300, 400, 500 and 600 W. Complete mortality of S. zeamais and T. castaneum larvae and adults was obtained at 600 W for 14 s or at 500 W for 28 s, whereas for P. interpunctella larvae and adults, one hundred percent mortality was obtained at 500 W for 14 s or at 400 W for 28 s. The average surface temperature of corn was around o C when complete mortality of three species was obtained. Surface temperature distribution of corn after microwave heating was measured using an infrared thermal camera and temperature distribution was non uniform. Germination of microwave treated corn decreased as power or exposure time or both increased. Keywords: Microwave, disinfestation, corn, stored-grain insects, germination. INTRODUCTION Corn is a major source of food for both humans and animals and is grown in many parts of the world. The United States produced 42% of the world s corn, i.e., around million metric tonnes (Mt) in 2006, followed by China, Brazil, Mexico, and Argentina. Although, corn is mainly used as a livestock feed, it is used in a variety of food and industrial products including starches, sweeteners, corn oil, beverage, and fuel ethanol (US Grains Council, 2008). Insect pest infestation starts from the field and continues through storage and handling until the grain is processed for consumption. Various methods of insect control have been practiced to save the grain and the most common method is the use of chemical insecticides. The major problem associated with the chemical methods is that residues remain in the food grain potentially affecting the health of consumers. The chemical insecticides also affect the environment including air, water and the atmosphere. Hence, there is a search for an alternative method for killing insects in stored grain. Microwave disinfestation seems to have a great potential as an alternative method of killing insects in stored-grain. Use of microwave heating and drying has already captured a place in the agriculture and food industries. There has been a lot of research on microwave disinfestation of cereals especially wheat (Hamid et al., 1968; Hamid and Boulanger, 1969; Kirkpatrick and Roberts, 1970; Nelson and Stetson, 1974; Watters, 1976; Tilton and Brower, 1987; Shayesteh and Barthakur, 1996; Vadivambal et al., 2007) and of some food materials such as nuts (Wang and Tang, 2001) and cherries (Ikediala et al., 1999). However, not much research has been conducted on microwave disinfestation of corn, which is one of the major food crops in the developing countries. Maize weevil is considered a most important pest of stored maize in Africa (Longstaff, 1981). The major insects affecting corn are Sitophilus zeamais (Maize weevil) followed by Oryzaephilus surinamensis (L.) (Sawtoothed grain beetle), Tribolium castaneum (Herbst) (Red flour beetle), Cryptoletes ferrugineus (Stephens) (Rusty grain beetle), Rhyzopertha dominica (F.) (Lesser grain borer), Plodia interpunctella (Hubner) (Indian meal moth), Liposcelis bostrychophilus Badonnel, Latheticus oryzae (Waterhouse) (Longheaded flour beetle) and Cadra cautella (Walker) (Almond moth) (Peng, 1998). The objectives of this paper were: 1. To determine the mortality of three stored grain insects: Sitophilus zeamais, Tribolium castaneum,

2 and Plodia interpunctella in 14, 16, and 18% moisture content corn at various microwave power levels and exposure times, 2. To study the surface temperature distribution in corn using thermal imaging, and 3. To determine the germination potential of microwave treated corn. METHODS AND MATERIALS Samples: Corn (Zea mays) variety Pioneer 39B93 (grown in Manitoba and harvested in September 2008) was selected for the study. Initial moisture content was determined by drying 10 g of unground grain, in triplicate, at 130 ± 2 o C for 16 h (ASABE, 2006) in a hot air oven (Model No. 130D, Thelco Laboratory Oven, Winchester, VA) and was expressed as percentage on a wet mass basis. The corn was then conditioned to 14, 16, and 18% moisture content by adding a calculated quantity of distilled water. The conditioned grain was kept in a refrigerator for 72 h for uniform moisture distribution and the moisture content of the grain was verified using the hot air oven. One moisture content representing each category (straight, tough, and damp) was selected for the study (Muir, 2001). The conditioned grain was then stored in sealed bags in the refrigerator until used for the experiments. Stored-grain insects: Three major stored-grain insects S. zeamais, T. castaneum, and P. interpunctella were selected for the study. The S. zeamais culture was started with individuals obtained from a farm in Oak Bank, MB in 1995 and maintained on whole wheat at 30 o C and 70% RH. Tribolium castaneum were obtained from Landmark, MB in 1998, and grown in the laboratory on whole wheat flour and brewer s yeast at 30 o C and 70% RH. To obtain different life stages, adults were placed on wheat flour and brewer s yeast and kept for different periods in environmental chambers (CONVIRON, Controlled Environments Limited, Winnipeg, MB) at 30 o C and 70% RH. Plodia Interpunctella was obtained from a pet shop in Winnipeg, MB (2008). The mortality of larval and adult stages of all three insect species was determined. Pilot-scale industrial microwave system: The mortality experiments were conducted in a continuous, pilot-scale, industrial microwave system (Fig. 1) operated at 230 VAC, 60 Hz, 23 A (Model No: P24YKA03, Industrial Microwave Systems, Morrisville, NC). The microwave system consists of a belt assembly, an applicator, a fan and heater assembly and a control panel. The maximum speed of the conveyor was 6 m/min. The sample holder was a polystyrene rectangular box of dimension 30 cm x 3 19 cm x 1 cm. The experiments were conducted with corn samples at 14, 16, and 18% moisture content and with 10 insects per 50 g of sample. The experiments were carried out at two different exposure times. At the maximum speed (6 m/min) of the conveyor, it took 14 s for the sample to pass the applicator and at a speed of 3 m/min the sample was exposed to microwave energy for 28 s. Preliminary studies were conducted to determine the power necessary for complete kill of insects. Based on the preliminary results, the power levels selected for the study were 300, 400, 500, and 600 W. Determination of mortality: Fifty grams of sample were placed in the sample holder along with the adults or larvae of the selected species. The grain, along with the adult insects, was then kept on the conveyor belt and the sample was subjected to microwave energy. When the box came out of the conveyor it was gently taken out and the sample was spread on a sheet of paper. The numbers of live and dead insects were counted. The adult insects were considered dead if they failed to respond to gentle rubbing with a small brush. The sample was held at room temperature for 15 min and the insects were checked for mortality again. When the number of insects recovered was not 100%, the same experiment was repeated until 100% recovery was achieved. A minimum of three replicates were done for all the mortality experiments. Control mortality was determined by allowing the grain and the insect to pass on the conveyor at zero power. The mortality of larval stages of T. castaneum and P. interpunctella was determined in the same way as the adult insects. Since S. zeamais deposits an egg inside the kernel, to obtain the kernels infested with S. zeamais larvae, 10 adult insects were kept in a vial with 10 grains and left in an environmental chamber for 48 h. The kernels were dipped in acid-fuchsin to stain the egg plugs in order to determine the infested kernels, following the method described by Frankenfield (1950 cited by Gudrups 2001). Acid fuchsin stains the egg plug a pink color, which was then identified with the use of a microscope. The kernels with eggs were then kept in the environment chamber for 22 days, for the eggs to develop into a late larval stage. The kernels were then treated with microwave energy and kept in the chamber for 15 days for adults to emerge. If no adults emerged, then larvae were considered killed. An uncooled focal planar array type infrared thermal camera with 320 x 240 pixels was used to take thermal images of the microwave-treated grain (Model: Therma-CAM TM SC500 of FLIR systems, Burlington, ON, Canada; Spectral range: µm).

3 Control panel 2. Microwave applicator 3. Conveyor 4. Thermal camera,5. Data acquisition system Fig. 1. Industrial microwave system with a thermal camera The thermal sensitivity of the camera was 0.07 o C at 30 o C. While taking thermal images, the emissivity of the grain was set as 0.98 for all the experiments. From the thermal image obtained for each sample, the maximum, minimum and average temperatures were extracted using ThermaCAM Researcher 2001 software (FLIR systems, Burlington, ON, Canada) and the differences between the maximum and minimum surface temperatures ( t) of the grain were obtained. Determination of germination: Germination of the corn seeds subjected to different levels of microwave power was assessed by plating 25 seeds on Whatman no. 3 filter paper in a 9-cm diameter Petri dish saturated with 5.5 ml of distilled water (Wallace and Sinha, 1962). The plates were placed in a plastic bag to prevent desiccation of the filter paper and kept at 25 o C for 7 d. On the seventh day the germinated seeds were counted and the germination percentage was calculated. Statistical analysis: A factorial experiment of three factors (moisture content, power, exposure time) with 3x5x2 (three moisture content: 14, 16, 18%; five 20 power levels: 0, 300, 400, 500, 600 W; two exposure times: 14, 28 s) was performed. Three replications were done for determination of mortality and germination with control samples for all the experiments. Analysis of variance (P < 0.05) with mean separation by Scheffe s test was used to check the significance between the mortality of insects at different grain moisture contents, power levels and exposure times (SAS, 2002). RESULTS AND DISCUSSION Mortality of Sitophilus zeamais: The mortality of larval and adult stages of S. zeamais, T. castaneum, and P. interpunctella are given in Table 1. The control mortality for the larval and adult stages of the three insects was zero. The mortality of adult S. zeamais for 14% m.c. corn at 300, 400, 500, and 600 W at 14 s exposure time was 40, 73, 93, and 100%, respectively. The mortality at 300, 400, 500 and 600 W for 28 s exposure time was 77, 87, 100 and 100%, respectively. Mortalities in the similar range were obtained for 16 and 18% moisture content corn. There was no significant difference in the mortality of adults at 14, 16, and 18% moisture content (F=1.98; df=2, 60; P=0.1465) corn while the mortality

4 increased with either an increase in power (F=437.32; df=4, 60; P<0.0001) or exposure time (F=18.95; df=1, 60; P<0.0001). The mortality of S. zeamais larvae at 300, 400, 500, and 600 W for 14 s exposure time was 40, 63, 90, and 100%, respectively. Similar mortalities were obtained for 16, and 18% moisture content corn. There was no significant difference in the mortality of S. zeamais larvae at various moisture content (F=0.07; df=2, 60; P=0.9356) whereas the mortality was significantly higher at higher power (F=588.59; df=4, 60; P<0.0001) and exposure time (F=51.20; df=1, 60; P<0.0001). There was no significant difference in the mortality between S. zeamais larvae and adults. Mortality of Tribolium castaneum: The mortality of T. castaneum adults at 14% m.c. corn at 300, 400, 500 and 600 W for 14 s exposure time was 27, 50, 83 and 100%, respectively. The mortality of T. castaneum adults at 300, 400, 500, and 600 W at 28 s exposure time was 67, 97, 100 and 100%, respectively. One hundred percent mortality was achieved at 500 W for 28 s exposure time or at 600 W for 14 s exposure time corresponding to mean surface grain temperature of 63 and o C, respectively. Mortality of T. castaneum adults were not significantly different at various moisture contents (F=1.95; df=2; 60; P=0.1516) whereas mortality increased significantly with power (F=263.39; df=4, 60; P<0.0001) and exposure time (F=78.68; df =1, 60; P<0.0001) The mortality of T. castaneum larvae at 300, 400, 500 and 600 W at 14 s exposure time was 37, 60, 90 and 100%, respectively. The mortality increased significantly as the power (F=429.94; df=4, 60; P<0.0001) or exposure time (F=50.58; df=1, 60; P<0.0001) increased while there was no significant difference in the mortality with respect to moisture content (F= 0.79; df=2, 60; P=0.4584). There was no significant difference in the mortality of T. castaneum larval stages at 14, 16 or 18% m.c. corn. The mortality of T. castaneum larvae was significantly higher than the adults at all moisture content. Mortality of Plodia interpunctella: The mortality of P. interpunctella adults at 300, 400, 500, and 600 W for 14 s exposure time was 60, 77, 100, and 100%, respectively and the mortality of P. interpunctella larvae were 67, 87, 93, and 100% respectively. There was no significant difference in the mortality of P. interpunctella adults (F=0.36; df=2, 60; P=0.6984) and larvae (F=0.49; df=2, 60; P=0.6168) at 14, 16 and 18% m.c. corn. Comparing the mortality of larvae of the three stored grain insects by mean separation method, the larvae of P. interpunctella was the most susceptible followed by T. castaneum and S. zeamais with no significant difference between the latter two. When the adult mortalities were compared, P. interpunctella adults were the most susceptible followed by S. zeamais. The least susceptible of the three were the T. castaneum adults. The mortality could be related to the size of the insects. The mortality of P. interpunctella and T. confusum during microwave treatment was studied by Shayesteh and Barthakur (1996). Higher mortality was observed in larger sized P. interpunctella compared to smaller sized T. confusum. The size of P. interpunctella, S. zeamais and T. castaneum adults are 13-18, , and mm long, respectively (Agriculture Canada, 1981; Sinha and Watters, 1985). The greater the size, higher will be the microwave absorption resulting in increased mortality of the insect. Also, the P. interpunctella adults were very fragile. The wet masses of P. interpunctella, T. castaneum, S. zeamais adults are: 8.4, 2.0, and 1.4 mg, respectively (White and Sinha, 1987). Halverson et al. (2003) studied the disinfestation of S. oryzae, T. castaneum and R. dominica in wheat and reported that young larvae and eggs were less susceptible than the pupae due to decreased radiation coupling caused by the much smaller size of young larvae and eggs. Differences in larval and adult susceptibility seem to vary among species (Nelson, 1996). Lethal temperatures for mortality of insects vary not only with the species but also with the developmental stage of insects (Fields, 1992). Tilton and Brower (1983) stated that the embryonic stage of an insect is a time of extreme radio sensitivity and adult insects are more radio resistant than the other stages (Hasan and Khan, 1998). Imamura et al. (2004) studied the effect of softelectron treatment on the life stages of T. castaneum, P. interpunctella, and Callosobruchus chinensis (L.). Their results indicated that soft electrons at 170 kv effectively killed the eggs, larvae and pupae of T. castaneum and P. interpunctella but the adults survived after long periods of exposure (5-10 min) but were killed by the treatment for 10 and 15 min, respectively. These studies indicate that adult stage is the most tolerant to radiation treatment compared with the early life stages because of limited cell division. 21

5 Tateya and Takano (1977) observed no significant differences in insect mortality when adult S. oryzae was treated in 12.3 to 16% m.c. wheat. Shayesteh and Barthakur (1996) observed that mortality was significantly reduced at 12% moisture content compared with 6 and 9% moisture content at each input power level. We also observed that there was no significant difference in the mortality of all the three adult and larval insect species in 14, 16, and 18% moisture content corn. The complete kill of larvae and adults of all three insect species were obtained for a combination of 600 W for 14 s exposure time corresponding to a temperature of o C or 500 W for 28 s exposure time corresponding to a temperature of 63 o C, respectively. The end use quality of grain is not affected until they are held at over 60 o C (Banks and Fields, 1995). To ensure processing quality, it was recommended that cereal grain should not be heated above 55 o C or to a maximum of 60 o C to maintain milling and baking quality (Brooker et al., 1974). For corn, safe temperature recommended for commercial use was 54.4 o C (130 o F) (Hall, 1980). The temperature reached during microwave heating for complete kill of insects was o C, which lies within the safe limits suggested in the literature. Hence, the microwave treatment could kill the insects without affecting the end use quality of the grain. Surface temperature distribution in corn: The average temperature, minimum, maximum and difference between the maximum and minimum ( t) temperatures of 14, 16 and 18% moisture content corn are summarized in Table 2. The average surface temperature of 14% m.c. corn at 0, 400, 500 and 600 W for 14 s exposure time was 28, 45, 49 and 55 o C, respectively while the average surface temperature of 300, 400, and 500 W at 28 s exposure time were 46, 55, and 63 o C, respectively. The average temperatures increased with an increase in the power or exposure time or both. Similar range of temperatures was obtained for 16 and 18% m.c. corn. Non-uniform temperature distribution was observed during microwave heating. The surface temperature distribution during microwave heating is shown in Fig. 2 for 14% m.c. corn at various power levels. The minimum temperature of 14% corn at 300 W for 28 s was 38 o C, whereas the maximum temperature was 52 o C. The average surface temperature was 46 o C. This shows the range of temperature observed in the same sample. The surface temperature distribution during microwave heating of cereals and oilseeds were studied by other researchers (Manickavasagan et al., 2006; Vadivambal, 2009). The average surface temperatures of wheat, barley and canola were between , and o C, respectively during microwave heating at 500 W power (Manickavasagan et al., 2006). The dielectric properties of a material are responsible for the ability of the material to store and dissipate energy (Mudgett, 1982). Dielectric constant (ability of material to store charge when used as a capacitor dielectric) for rye, oats, and sunflower seeds is 6.0, 4.9, and 2.0, respectively (dielectric constant of water is 80.3). The average temperature of 14% m.c. rye at 0, 200, 300, 400, and 500 W for 28 s were 27.6, 49.2, 59.0, 72.1, and 82.5 o C, respectively, and the average temperature of oats were 27.2, 33.1, 35.5, 46.6, and 51.4 o C, respectively. The average temperature of sunflower seeds at 8, 10, and 12% m.c. at 0, 200, 300, 400, and 500 W for 28 s were 23.3, 30.6, 35.1, 39.2, and 43.6 o C, respectively. The higher dielectric constant for rye shows that rye heats faster than oats and sunflower seeds (Vadivambal 2009). The dielectric constant for corn is 3.6 (Khrone, 2007). The statistical analysis showed that surface temperature of 16% m.c. corn was higher than the 14 and 18% m.c. corn. Germination: The results of germination of corn exposed to microwave energy for 14 s and 28 s are presented in Fig. 3 and Fig. 4, respectively. The control germination for 14, 16, and 18% m.c. were: 89, 91 and 92%, respectively. The germination of 14% m.c. corn at 300, 400, 500, and 600 W for 14 s exposure time were 85, 80, 52, and 35%, respectively. As the exposure time was increased to 28 s, the germination at 300, 400, 500 and 600 W were 76, 39, 19 and 5%, respectively. Similar results were obtained for 16 and 18% m.c. corn. The germination of corn was significantly decreased as the power or exposure time increased confirming that power (F=721.57; df=4, 60; P<0.0001) and exposure time (F=480.63; df= 1, 60; P<0.0001) have significant effect on germination whereas moisture content had no significant effect (F= 0.28; df= 2, 60; P=0.7575) on germination. Blanco et al. (1977) studied the effects of low level microwave radiation on germination and growth rate in corn seeds. In their study, the growth of corn seedlings was completely inhibited at low power level of 10 mw/cm 2. The reason for complete inhibition was stated to be due to a loss of turgor resulting from water loss, since full turgor pressure is necessary for the growth of plant cells. 22

6 Table 1: Mortality percentage (average of three replication ± standard deviation) of larval and adult stages of three stored grain insect species. Insect life stage Power, Moisture content, % W Exposure time, s Sitophilus zeamais larvae Sitophilus zeamais adult Tribolium castaneum larvae Tribolium castaneum adults Plodia interpunctella larvae Plodia interpunctella adults ±10 63± ±5.8 67± ± ± ±5.8 87±5.8 63±5.8 87± ± ± ± ± ± ±10 77±5.8 53±5.8 67± ±5.8 53± ± ±5.8 80±10 77± ±10 77± ± ± ± ±5.8 80± ± ±5.8 43±5.8 60± ±20 87± ±0 90±10 77±5.8 90± ± ± ± ± ± ±10 60±0 30±10 57± ±20 97±5.8 43± ± ± ± ± ± ± ± ± ±10 80±10 63± ± ± ± ± ± ± ±10 97±5.8 63± ±5.8 67±5.8 77± ± ± ±5.8 87± ± Table 2: Surface temperature (average ± standard deviation in o C) distribution in corn Temperature m.c., % Average Minimum Maximum Max-Min. ( t) Power, W Exposure time, s Control ±0.1 46±1.4 45±0.5 55±0.6 49±0.2 63±0.7 * 55±0.6 * 16 27±0.2 45±0.5 43±1.5 56±0.1 48±0.8 62±0.7 * 52±1.2 * 18 28±0.1 47±0.3 44±0.8 56±0.9 49±1.0 63±0.8 * 54±1.2 * 14 27±0.1 38±0.4 37±0.2 43±0.2 39±0.6 49±1.1 43± ±0.2 36±0.8 36±1.2 44±.07 39±0.5 48±1.2 41± ±0.1 39±1.0 38±0.8 45±1.2 39±1.5 49±1.6 43± ±0.1 52±3.7 52±0.6 67±1.0 60±1.8 74±2.0 67± ±0.1 52±2.1 50±2.6 67±1.5 57±2.8 74±1.1 62± ±0 54.4± ± ± ± ± ± ± ± ±06 24± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±1.8 * Temperature at which one hundred percent mortality was obtained. Shivhare et al. (1991) studied the effects of different factors on microwave treatment of corn, the drying characteristics and the quality of corn seed. They suggested that high seed germination could be maintained by using microwave power levels of 0.25 W/g of wet seeds. Though some of the studies have shown that exposure to microwave enhances germination, Campana et al. (1993) studied the 23 physical, chemical and baking properties of wheat dried with microwave energy and concluded that germination capacity was decreased by exposure to microwave energy. The decrease in germination capacity was related to the final temperature and the initial moisture content of the grains.

7 More et al. (1992) studied the effect of microwave heating on the germination of sorghum (Sorghum bicolor (L.) Moench) grain at 12, 14, and 16% m.c. and the microwave oven had nine power levels with output power of 650 W at 2450 MHz. One hundred gram samples were treated at three power levels PL3, PL6 and PL9 for 30 or 60 s. The results of the experiment showed that at 14 and 16% m.c. for PL3 and PL6, the germination was higher than the control and for 12% m.c., the germination was same as the control. Whereas there was a significant reduction in the germination of sorghum treated for 60 s at PL6 and PL9. Their result showed that moisture content was not a significant factor during microwave treatment in many grain quality characteristics. CONCLUSIONS The results from our study show that microwaves have a potential to kill insects in corn. One hundred percent mortality of T. castaneum and S. zeamais adults and larvae were obtained at 600 W for 14 s exposure time or at 500 W for 28 s exposure time corresponding to a mean surface temperature of o C and 63 o C, respectively. There was no significant difference between the mortality of adult and larval stages of three insect species at 14, 16, and 18% m.c. corn. Among the three insect species studied P. interpunctella was the most susceptible to microwave energy followed by S. zeamais and T. castaneum with no significant difference between the latter two. The germination of corn decreased with increase in power or exposure time or both. Fig.3. Germination percentage of 14, 16, and 18% m.c. corn exposed to 14 s exposure time and various power levels Fig.4. Germination percentage of 14, 16, and 18% m.c. corn exposed to 28 s exposure time and various power levels ACKNOWLEDGEMENTS We thank the Canada Research Chairs program and the Natural Sciences and Engineering Research Council of Canada for providing financial support for this study. REFERENCES Agriculture Canada (1981). Red flour beetle. Insect Identification Sheet No.75. Ottawa, Ontario: Agriculture and Agri-Food Canada ASABE (2006). Dielectric properties of grain and seed. ASAE D In ASABE Standards 2006, St. Joseph, Michigan: ASABE Banks, J. and Fields, P (1995). Physical methods for insect control in stored-grain ecosystems. In: Jayas, D.S., White, N.D.G. and Muir, W.E. (eds) Stored-Grain Ecosystem, New York, NY: Marcel Dekker Inc., pp Blanco, J.B., Bristo, J.M. and Romero-Sierra, C (1977). Effects of low-level microwave radiation on germination and growth of corn seeds. Proceedings IEEE. 65: Brooker, D.B., Bakker-Arkema, F. and Hall, C. W (1974). Drying Cereal Grains. Westport, Connecticut: AVI Publishing Company Campana, L.E., Sempe, M.E. and Filgueria, R.R (1993). Physical, chemical, and baking properties of wheat dried with microwave energy. Cereal Chem. 70(6): Fields, P.G (1992). The control of stored-product insects and mites with extreme temperatures. J. Stored Prod. Res. 28(2):

8 Control 400 W, 14 s 500 W, 14 s 600 W, 14 s Fig. 2. Thermal images of microwave heated corn at 14% m.c. 25

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