MATERIALS AND METHODS

These pests reduce the product quality by direct damage through feeding and indirectly by producing webbing and frass. It is estimated that annual losses of cereal grains due to insects and rodents are approximately 10% in North America and up to 50% in Africa and Asia (Hill, 1990). Post-harvest control of insects in wheat is essential under quarantine regulations in many countries. The traditional treatment is chemical fumigation due to low cost, fast speed in processing and ease of use. Insect culture and irradiation with microwaves specimens of Tribolium castaneum (Herbst) and Bruchidius incarnates for cultures were obtained from the rearing culture of Economic Entomology andAgricultural Zoology Dept. Fac. Agriculture Menoufia Univ. These insects were selected because of their economic importance as harmful and destructive pests to stored cereals and other food products worldwide. Tribolium castaneum beetles were cultured in 1.71 glass jars in wheat flour and brewer’s yeast (19:l = w/w) medium; while Bruchidius incarnates were reared on broad bean and brewer’s yeast (9:2 = w/w/w). Cultures and all test insects were maintained in a growth chamber at 27±1°C temperature and 75% relative humidity.

Fig. 1:	New device proposed for wheat protection at grain mill with microwaved chamber

For each trial, 10 insects were placed inside a 0.251 glass jar containing 50 g of whole wheat flour for T. castaneum and 50 g of broad bean for Bruchidius incarnates and positioned directly under the microwave applicator. A cheese cloth was used to cover the jar tops. Irradiation times were either continuous or intermittent and were set at 10, 20, 30, 40, 50, 60, 70 and 80 sec the power was varied from 10-100% of power level. A set of 30 insects was used as the control which was kept beyond the influence of microwaves at 23°C room temperature. The experiment for each growth stage of larvae and adults was replicated four times, mature larvae and l-3 days old adults were used in replications. An industrial microwave oven (Model Galanz, model P70B20AP-ST co Ltd., China.) with 2450 MHz frequency was used in the study. The microwave applicator was placed inside a housing (Fig. 1) constructed of a fibrous, microwave-absorbing material called Eccosorb CH 460. The distance between the applicator and the food surface was 5 cm. Any radiation that was transmitted, reflected, or missed the jars containing the insects was absorbed by the Eccosorb. The power outputs of the generator were set at 700 W.

Determination of mortality: The experiments were conducted with wheat samples at three infestation levels of 10, 20 and 30 adult insects per 50 g of sample. Fifty grams of sample were placed in the box and insects were added to the sample. The grain, along with the insects, was then placed on the conveyor belt and the sample was subjected to microwave energy. When the box came out of the conveyor it was gently lifted from the conveyor and the sample was spread on a sheet of study. The numbers of live and dead insects were considered dead if they failed to respond to gentle rubbing with a small brush. The sample was allowed to cool and the insects were checked for mortality again after 15 min. The experiment was repeated if insects went missing and the final count was not the same as the initial count. For the larval stage of T. castaneum and Bruchidius incarnates the experiments were conducted in the same way as for adult insects.

Quality analysis: Five samples of 50 g each were treated at 30, 60 sec at 20% power level, 60 sec at 40% power level and two control sample exposed and non exposed wheat were determined for various quality analyses like flour protein, flour ash, flour carbohydrate and moisture. All determined at Central Lab. At Faculty of Agriculture, Menoufia University.

Statistical analysis: All obtained data were analyzed by SPSS software computer program.

RESULTS AND DISCUSSION

Mortality of adult insects: The mortality percentages for all the insects at various power levels and exposure times are shown in Table 1. At a power level of 20% and an exposure time of 15 sec, the mortality of T. castaneum was 81%. As the power was increased to 40, 60, 80 and 100% the mortality also increased to 82, 82, 84 and 85%, respectively. As exposure time was increased, higher mortality was achieved. For example, at 75 sec with 80 and 100%, the mortality was 100%. When the exposure time was increased to 90 sec, 100% mortality was achieved at a power from 20-100%. LSD test shows that mortality of T. castaneum was significantly higher in 60,75 and 90 sec of exposed time at all power levels, whereas there was no significant difference in the mortality between 15, 30 and 45 sec at all power levels. The effect of power level and exposure time on mortality was the same for T. castaneum. While the mortality of Bruchidius incarnates was 81% with 15 sec exposure time with 20% power level. As the power was increased to 40, 60, 80 and 100% the mortality also increased from 81-84%, respectively. As exposure time was increased, mortality percentage was not achieved. The mortality was 100% when the exposure time was increased to 90 sec, with 60, 80 and 100% power levels. These results agreed with Mankin (2004), Lu et al. (2010), Birla et al. (2004), Wang et al. (2006), El-Naaggar and Mikhaiel (2011), Vadivambal et al. (2007) and Zhao et al. (2007).

Mortality of insect larvae: The mortality percentages for all larval stage at various power levels and exposure times are shown in Table 2. At a power level of 20% and an exposure time of 15 sec, the mortality of T. castaneum was 75%.

Table 1:	Mortality percentage of T. castaneum larvae and adults exposed to combination of microwave heating in wheat flour

Table 2:	Mortality percentage of B. incarnates larvae and adults exposed to combination of microwave heating in broad bean

Table 3:	Quality aspects of wheat flour subjected to microwave irradiation

As the power was increased to 40, 60, 80 and 100% the mortality also increased to 77, 77, 78 and 78%, respectively. As exposure time was increased, higher mortality was achieved. at 90 sec with 100 and 99%. When the exposure time was increased the mortality was achieved at a power from 20-100%. LSD test shows that mortality of T. castaneum was significantly higher in 75 and 90 sec of exposed time at all power levels, whereas there was no significant difference in the mortality between 15, 30, 45 and 60 sec at all power levels. The effect of power level and exposure time on mortality was the same for T. castaneum. These results agreed with Mankin (2004), Lu et al. (2010), Birla et al. (2004), El-Naaggar and Mikhaiel (2011) and Vadivambal et al. (2007).

Quality analysis: The quality characteristics of wheat, i.e., flour protein, flour ash, flour carbohydrate, crude fiber and wheat moisture are shown in Table 3. Analysis of variance was performed between the control sample and treated samples. The results showed that there were significant differences in flour protein, total and hence there was no significant difference in flour ash, moisture and crude fiber (Macarthur and D’Appolonia, 1979). Campana et al. (1993) studied the physical, chemical and baking properties of wheat dried with microwave energy. They found that the protein content was not changed but the functionality of gluten was altered gradually with increasing time of exposure. Walde et al. (2002) studied the microwave drying characteristics of wheat and reported that there was no change in the total protein content of microwave-treated wheat samples but the structural and functional characteristics of wheat protein gluten were changed. The change in the functionality of gluten was observed by the absence of elasticity and stretchability of the dough. The researchers found that the use of microwave suitable for drying wheat while it is not suitable where the final products made out of flour required soft textural characteristics. Kaasova et al. (2002) investigated both chemical and biochemical changes during microwave treatment of wheat and found that advance in the baking quality was found at higher energy doses and higher end temperatures.

Based on experimental results, it can be concluded that there was no significant difference in the quality of flour ash, crude fiber and moisture of the wheat subjected to microwave energy.

Data in Table 3 showed that there is no difference in all contents of treated grains compared with un treated except in total carbohydrate which increased to 96.92% while it was 42.98% in control. it may be related to changing in the sugar content. this point want further studies.

CONCLUSION

The present study showed that the mortality percentages for all insects at various power levels and exposure times was increased, mortality percentage was not achieved. The mortality was 100% when the exposure time was increased to 90 sec. The mortality percentages for all larval stage at various power levels, was significantly higher in 75 and 90 sec of exposed time at all power levels, whereas, there was no significant difference in the mortality between 15, 30, 45 and 60 sec at all power levels. The quality characteristics of wheat, i.e., flour protein, flour ash, flour carbohydrate, crude fiber and wheat moisture are studied the microwave drying characteristics of wheat and reported that there was no change in the total protein content of microwave-treated wheat samples but the structural and functional characteristics of wheat protein gluten were changed. The change in the functionality of gluten was observed by the absence of elasticity and stretchability of the dough.