Quality Control Of Microwave Drying Green Turnip Based On Dielectric Properties

The drying of fruits and vegetables is growing in the drying industry.During the drying process,convenient quality inspection and control technology are necessary methods to improve the quality of dry products,reduce the energy consumption and improve the drying efficiency.Hot air drying of fruits and vegetables has problems such as long drying time,low quality and high energy consumption.Due to the fast drying of microwave drying technology,in this paper,the quality detection method and regulation mechanism of green turnip during microwave drying were studied.The dielectric properties were adopted to explore the quality detection method for fruits and vegetables.The dielectric constant and dielectric loss factor of green turnip were measured by using the coaxial probe method via a network analyzer in the frequency range of 16-3500MHz.The moisture content and temperature of green turnip were ranging from 15%to 93.20%and from 20 to 60℃,respectively.The relationships of frequency,moisture content,sugar content and material temperature with dielectric properties were investigated.The correlation models between moisture content and dielectric properties,sugar content and dielectric properties,sugar content and moisture content,dielectric properties and moisture content and temperature at 915MHz and 2450MHz were established.The feasibility of predicting moisture content and sugar content based on dielectric properties was examined.The quality detection of the moisture content and the sugar content of green turnip based on the dielectric properties was achieved.In addition,hot air,microwave continuous and microwave intermittent drying experiments for green turnip were carried out.By comparing the drying quality,it is proposed to regulate the microwave drying process based on dielectric properties.The change of energy demand during the drying process can be obtained via the establishment of the relationship between the dielectric properties of green turnip and the moisture content and the temperature.And then the microwave output power in the microwave drying process can be adjusted.The product quality can be improved and the energy consumption can be reduced when the drying temperature was controlled to 50℃.The results showed that the dielectric constant and the dielectric loss factor decreased with increasing the frequency from 16MHz to 3500MHz.The dielectric constant increased with increasing the moisture content.The dielectric loss factor increased with the increasing moisture content from 15%to 70%,and then decreased with further increasing the moisture content.The sugar content was negatively correlated with the moisture content.The variance analysis of the model indicated that the determination coefficient of each model was greater than 0.90 and P value was less than 0.05,which indicated that the model was accurate.The product quality and energy consumption of green turnip obtained by using different drying methods were analyzed.The results showed that the green turnip dry product based on the regulation of dielectric properties had the best rehydration,the lowest energy consumption and the highest drying efficiency.At the same time,the temperature of the green turnip could be well controlled during the drying process with the fluctuating around 50℃.This ensured that the energy absorbed by the material during the drying process was consistent with the energy provided by the microwave.The purpose of research is to improve the product quality,reduce the energy consumption and improve the drying efficiency.