Study On Uniformity Of Berry Puree Under Microwave Assisted Foam-mat Drying

Berry fruit is rich in a variety of active compounds and beneficial for human's health with high medical and health value,and has a great potential to be developed.However,fresh berry fruit is easy to decay and deteriorate owing to soft texture and high moisture.Drying is an effective way to prolong shelf life and increase types of agricultural products.Berry powder transformed by dehydration may greatly meet the demands of consumers and markets.Microwave assisted foam-mat drying(MFD)is suitable for the dehydration of materials with thermal sensitivity compounds due to the advantages of high drying efficiency,good quality and high energy utilization.Nevertheless,a major disadvantage for the dehydrated berry is non-uniform drying quality due to the uneven distribution of electromagnetic fields,the thermophysics properties and dielectric properties of the material.In this study,the uneven distribution of dehydrated berry under MFD was studied.The effects of drying parameters in terms of microwave intensity,puree thickness,air velocity on the uniformity of temperature and moisture content of dehydrated puree were determined under MFD.Moreover,the model of heat and mass transfer for the process of MFD was established to analyze reasons of the unevenness and the effects of drying conditions on homogeneity.Introduction of the tempering stage in the process of MFD,the effects of MFD parameters in terms of microwave intensity,intermittent ratio,microwave drying time per cycle in process of intermittent MFD(IMFD)on the uniformity of dehydrated puree were investigated by central composite design method.The optimal technology parameters of IMFD were obtained,which provide the theoretical basis for the industrial applications of MFD technology.The specific conclusions were shown as follows:(1)The effects of MFD parameters in terms of microwave intensity,puree thickness and air velocity on the uniformity of temperature and moisture content of dehydrated puree was studied.The results showed that the moisture and temperature of dehydrated puree were regular distribution,and the moisture content increased and the temperature decreased from the edge to the center of puree.The uniformity of temperature and moisture content of dehydrated puree and overall average temperature increased significantly with microwave intensity.In addition,the uniformity of temperature and moisture content of dehydrated puree increased and overall average temperature decreased significantly with the decrease of puree thickness and the increase of air velocity.The suitable process parameters were developed as microwave intensity of 10-12 W/g,puree thickness of 4-6 mm,and air velocity of 3-4 m/s,to produce the high uniformity berry puree under MFD based on research results and the production situation.(2)The regularities of heat and mass transfer in MFD were analyzed.The results showed that the microwave absorption in edge of puree was higher,and microwave gradually attenuated along its propagation direction(from the edge to the center of berry puree),resulting in the non-uniformity of drying rate of puree.Microwave absorption increased and the difference of drying rate reduced within puree with the increase of microwave intensity to improve drying uniformity of berry puree.Furthermore,the attenuation of microwave within puree was aggravated with the increase of puree thickness.Therefore,the uniformity of moisture and temperature decreased.Besides,the temperature gradient within berry puree was significantly decreased with the increase of air velocity to improve the uneven phenomenon of moisture and temperature.(3)A tempering stage was introduced in the process of MFD,the influence of processing parameters on drying uniformity was investigated by central composite experiment design method,and the optimization of parameters of IMFD was obtained.The results showed that the greater microwave intensity resulted in the higher uniformity of moisture content and temperature within berry puree.The uniformity of moisture content and temperature can be improved effectively by shortening microwave drying time per cycle and increasing the intermittent ratio.And the highest temperature of the puree increased and then decreased with the increase of microwave intensity.The intermittent radio had a negative correlation with the highest temperature.However,the highest temperature increased slowly very much when the intermittent radio was less than 2.The microwave drying time per cycle had a significantly positive correlation with the highest temperature.Moreover,the optimum process parameters for IMFD were obtained microwave intensity of 12 W/g,intermittent ratio of 2.80,microwave drying time per cycle of 130 s,respectively.These findings may provide valuable theoretical basis to ensure the uniformity of MFD berry puree in industrial applications.