Effect Of Microwave On Aspergillus Parasiticus

Rice is one of the main food crops in China, with an average annual production of 1.8-2.0 million tons. However, millions of rice was damaged by pest or mildew during storage. Although chemical fumigation and conventional methods such as low temperature and controlled atmosphere could achieve certain effect, they could result in some environmental problems, insect resistance and so on. In order to resolve this problem, an efficient, safety, environmental protection method was required. Microwave treatment shows considerable potential for killing insect and preventing mildew in food. The current study showed that the microwave with high-energy density could kill mould and pest effectively. However the effects of low energy density microwave on mould was limited. In this study, Aspergillus parasiticus were isolated from the rice. Subsequently the effects of low-energy density microwave on the growth characteristics, cell permeability, DNA of the fungus were investigated. The effects of microwave on the activity and structure of biological macromolecules such as catalase and lactate dehydrogenase were studied. The aim was to provide optimal processing parameters for the industrial application of microwaves. The results were as follows:1. Isolation and identification of rice mold. Three moulds, from rice processing and storage, such as Aspergillus flavus, Penicillium chrysogenum, Aspergillus parasitic were preliminary evaluated by shape and microscopic observing, in which the highest detection rate was Aspergillus parasitic.2. The effect of Microwave on growth characteristics of moulds was studied. With low-energy density microwave treatment and water-bath heating, residual moulds decreased and the growth rates slowed down. At the same time, the semi-lethal temperature (50-55℃) and lethal temperature (>75℃) of mould with microwave treatment were lower than those of the water-bath heating (semi-lethal temperature 60-65℃; lethal temperature >90℃). The inhibitory effects with microwave treatment was better than water-bath heating.3. The effects of microwave on mould. With the low-energy density microwave treatment and water-bath heating, the nucleic acid permeability, protein permeability, conductivity, as well as the Ca²⁺ permeability increased, and the surfaces of mould became rough. Meanwhile swelling appeared. Dehydrogenase activity increased followed by decrease. And the brightness of DNA bands decreased. The effects to the mould cell with microwave were more obvious than water-bath heating. The cell damages increased with extending microwave processing time.4. The effect of microwave on hydrogen peroxide and lactate dehydrogenase structure and activity. With the low-energy density microwave treatment and water-bath heating, theα-helix andβ-sheet of catalase and lactate dehydrogenase turned intoβ-turn and random coil. Additionally -SH ratio and hydrophobic index of catalase and lactate dehydrogenase increased, and the activity gradually declined. The effects of microwave on protein structures were greater than the water-bath heating and there were no significant effects on theβ-turn and random coil.5. Mechanism of microwave treatment on mold. Microwave treatments caused DNA degradation and damaged the structures of DNA and enzymes, and increased the permeability of cell membranes. The normal metabolic functions of cells were disrupted, leading to fungal death. Meanwhile, the growth rate of residual bacteria reduced.The growths of residual bacteria are closely related to the conductivity increased, while the conductivity, Ca²⁺permeabiliry, protein penetration and penetration rate of DNA were closely related to the death rate.