Fruits And Vegetables Microwave Drying Characteristics Of Pulsed Electric Field Pretreatment

Pusled electric field (PEF) is an alternating electric field which is evoked by high electric power through surge circuit. PEF technology is mainly used in the process of bio-chemical sterilization, enzyme passivation and the pre-treatment of juice extra, osmotic dehydration and drying of fruits and vegetables. The effects of PEF pre-treatment on osmotic dehydration and/or hot air drying of apples, red bell pepper, potato, carrot, etal, were studied by abroad scholars. The literal results showed that PEF pre-treatment induced higher osmotic dehydration rate and drying rate of fruits and vegetables, less energy consumption and a little undesired influence on the qualities of products. However, most the researches are about the impact of PEF pre-treatment or combines with osmotic dehydration on hot air drying characteristics of fruits and vegetables. There is little information obtained which is about the effects of PEF pre-treatment on other drying characteristics such as microwave drying characteristics of fruits and vegetables. So the problem that effects of PEF pre-treatment on microwave drying characteristics of fruits and vegetables was proposed in this study.Here, materials included carrots and potatos, and the average drying rate of samples under microwave drying, the rehydration ratio of carrot slices and the shrinkage ratio of potato slices were experiment indexes. Both orthogonal design and quadratic regression orthogonal design were used in this study. Statistical analysis and indexes optimization were carried out using SPSS and Matlab solftware, respectively.Firstly, four factors (pulse frequency, electric field intensity, microwave power density and slices thickness), quadratic regression orthogonal experiments were conducted to study the effects of PEF pre-treatment on microwave drying characteristics of carrot slices. Secondly, three factors (pulse frequency, electric field intensity and microwave power density), quadratic regression orthogonal experiments were carried out to learn the impact of PEF pre-treatment on microwave drying characteristics of potato slices. After experiments, curves of microwave drying rate of samples with PEF pre-treatment were obtained; equations of drying kinetics and indexes were established, and equations of drying kinetics were predictively analyzed as well. Optimal combinations of parameters were acquired after indexes synthetically optimized. The results from this study showed that PEF pre-treatment could improve microwave drying rate of carrot slices and potato slices; the Page's model produced a good fit for the drying curves of PEF pre-treated samples in section; the effects of electric field intensity as well as the interaction of pulse frequency and microwave density, thickness on the average drying rate of carrot slices were significant; and the influence of pulse frequency on the rehydration ratio of carrot slices was significant as well; the impact of pulse frequency on the average drying rate of potato slices was fairly remarkable; the shrinkage ratio of potato slices was markedly influenced by the interaction of pulse frequence and electric field intensity, and was comparatively impacted by the quadratic effect of pulse frequency. The optimal combinations of parameters that affected the average drying rate and the rehydration ratio of carrot slices were pulse frequency 30Hz, electric field intensity 2.0kV/cm, microwave power density 1.0W/g, slices thickness 4.0mm; and the optimal combinations of parameters that influenced the average drying rate and the shrinkage ratio of potato slices were pulse frequency 30Hz, electric field intensity 1.0kV/cm and microwave power density 1.0W/g. The results that received under optimal conditions revealed that PEF pre-treatment increased experiment indexes values inducing less drying time and better qualities of products.To investigate the influence of PEF pre-treatment on the rehydration ratio of carrot slices and the shrinkage ratio of potato slices under hot air drying, two fators (pulse frequency and electric field intensity), three levels, orthogonal experiments were done in the end. The results indicated that the hot air drying time of carrot slices with PEF pre-treatment didn't reduce and the rehydration ratio was lower than that of the control samples; PEF pre-treatment could increase the hot air drying rate of potato slices inducing less drying time, but the diameter size of potato slices markedly changed and the products were obvious curly.In a word, PEF pre-treatment can enhance the microwave drying rate of fruits and vegetables, for instance carrot and potato, and improve the qualities of products.