Studies On Technology And Mechanism Of High-efficiency Microwave Freeze Drying For Typical Vegetable Products

It is well known that freeze drying produces the highest quality dried foods. However, a major problem with conventional freeze drying (FD) is the long drying time needed, which in turn leads to high energy consumption and high capital costs. This was partly due to the poor heat transfer rate associated with the conventional electric heating method which transfers heat for drying by conduction. To increase the freeze drying rate, utilization of microwave heating instead of the traditional conduction heating has been proven to give better heat and mass transfer rate, this technique is called microwave freeze drying (MFD). But MFD is not so efficiency when applied in dehydration of high moisture content materials due to very small values of dielectric constant and loss factor of frozen water, which also results in difficulty to control product quqlity. In this research, vegetable soup and potato were selected as test materials, a high-efficiency MFD method based on improving dielectric properties of the materials was put forwared according to theoretical and experimental study. The new drying method will solve the problem of long drying time, high energy consumption and high costs existing in conventional freeze drying vegetables on premise of good product quality.Firstly, the effects of microwave power, material thickness, material load and material temperature on the drying characteristics and quality of vegetable soup was investigated. Taking total drying time, specific energy consumption and sensory quality as response variables, response surface methodology was employed to optimize the optimal microwave freeze drying conditions. The results showed the optimal drying conditions were microwave power 1.0-1.4 W/g, material thickness 20 mm, and material temperature 52-58℃.Secondly, the dielectric properties of food materials are affected by the chemical composition, consequently, the microwave absorption ability and drying rate are affected. Hence, the effect of the three different food seasoning ingredients (i.e., NaCl content, sucrose content and sodium glutamate content) on dielectric properties and MFD process of instant vegetable soup was investigated. The results showed that the dielectric properties of both frozen and unfrozen vegetable soups were significantly influenced by food seasoning ingredients. The dielectric loss factor of frozen vegetable soup increased with increasing NaCl, sucrose and sodium glutamate content, and NaCl content showed the most significant effect. The MFD time of instant vegetable soup could be reduced by adding NaCl, sucrose and sodium glutamate. The NaCl and sucrose addition had significant effect on drying rate and sensory scores. The addition levels of the three food seasoning ingredients when used in combination were optimized using RSM. An optimal zone was reached at NaCl content of 3.2-5.3%, sucrose content of 2-6.8%, and sodium glutamate content of below 4.5%, respectively. The mechanism of effect of NaCl, sucrose and sodium glutamate on material dielectric properties was also discussed, and it was concluded that the dielectric constant and loss factor of frozen vegetables soup increased due to seasoning ingredients addition making more water unfrozen.Thirdly, a comparison of FD and MFD for instant vegetable soup with different dielectric properties was carried out. It was found that the MFD rate was significantly faster than the FD rate. Furthermore, the dielectric constant and loss factor of frozen vegetable soup significantly increased by adding seasoning ingredients, which improved the microwave absorption ability of the material, resulting in a further increase of MFD rate; while the FD rate could not be improved by addition of the ingredients. The energy consumption and product quality comparison showed that suitable addition of food seasoning ingredients could improve MFD rate of vegetable soup and reduce drying time, which was an ideal way to achieve high efficiency MFD process on the premise of good product quality.Fourthly, the effects of osmotic pre-treatment on MFD characteristics of potato slices and the key quality attributes viz. color, texture and rehydration ratio, of the dried products was investigated. It showed that partial osmotic treatment is effective in improving the MFD rate. However, excessive osmotic dehydration has an adverse effect on product quality. Osmotic pre-treatment with salt concentration below 5% and/or sucrose concentration below 30% prior to MFD resulted in dried products of good quality with shorter total drying times relative to MFD of untreated samples.Finally, the microstructural changes of potato slices with and without blanching in MFD and FD process were investigated, and the final dried product was also tested for vitamin C content, color, rehydration, starch content, texture and sugar content. Results for both unblanched and blanched potato tissues, crystal growth during sublimation drying stage was observed to cause structural damage to the cell walls. Blanched tissue suffered more damage during the freezing process. Interestingly, microwave freeze drying yielded product similar in quality to that obtained in vacuum freeze drying with conductive heating.