Study On Microwave Freeze Drying Of Banana Chips

Freeze drying is the best drying method in quality of dried sample. It also shows thedrawbacks include high energy consumption, long drying period and high cost. On thecontrary, dielectric drying shows higher drying rate but worse quality of production.In this study the applied range of dielectric drying was developed, the aim of thisstudy was developing the drying method by combining freeze and dielectric drying,which can process production with high drying rate as well as prefect productionquality. Banana was presented as the material.Firstly, to improve the energy consumption of freeze drying (FD), the FD anddielectric drying was combined as a new drying technique and it was called“microwave freeze drying (MFD)”. Microwave power and starch content of bananawas set as the index to evaluate the drying performance. The results indicated theincreasing microwave power and decreasing starch content would accelerate thedrying rate. At the same time higher microwave would bring the phenomenon such ashigher crispnessand better rehydration ability but also accomplished with higherswollen rate and worse drying uniformity. The best parameter of MFD drying bananachips was middle starch content processed by2W/g microwave power. The restructedbanana/potato chip was also studied to evaluate the influence of banana starch as theraw materials.Secondly, the energy consumption was studied. For the same weight of samples,MFD can save33.8%energy and40%drying time compared with FD. Usingdifferent drying parameter in desorption drying stage can further reduce the dryingtime and energy consumption. The energy consumption model also establised in thispart.Thirdly, because of high starch content of banana, The structure and morphology ofbanana starch dried by radical field included27,915and2450MHz dielectric dryingand MFD was investigated. The results indicated that dielectric drying can acceleratethe hydrolysis, which made the molecular mass be decreased. The XRD diagramsshowed the starch from banana was A-structure that after drying the crystallinechanged to B-structure and the crystallinity increased significantly. Higher dielectricfrequency boosted the drying rate but poor uniformity of temperature distribution washard to avoid. The high temperature area could exceed the gelatinization temperaturethus changed the structure of starch. This phenomenon caused tangled starch granuleas the result increased the particle size. Because the ice existed during MFD, the hydrolysis and de-structure was restricted during drying. For this reason thecrystallinity and Mw showed less different with the samples without drying.Finally the modified MFD dryer called pulse spouted microwave freeze drying(PSMFD) was developed. The captured infrared photos showed longer spouting timeand less spouting interval made the better uniformity of temperature distribution.Mathematical analysis data indicated higher microwave power made the bananatemperature rise fast but showed poor uniformity. The evaluation method of dryinguniformity was also established in the part.