Development Of Drum Dryer And Mathematical Modeling Of Drum Drying Processing Of Pumpkin Powder

Drum dryer is widely used in food industry because of its easy operation, good adaptability, high thermal efficiency, short drying time and low equipment cost. Compared with the other drying methods, drum drying has the advantage of pregelatinization which can improve the grade of gelatinization of the product and make the product to be instant edible. Moreover, product dried by drum drying has a higher quality of rehydration because of increasing porosity and complete inner structure. Pumpkin powder with high economic value is one of the most popular pumpkin products. Hot wind drying and spray drying are widely used in producing of pumpkin powder at present. However, both of them have some disadvantages such as high temperature, long drying time, low quality, and highly lost of nutrients of hot wind drying method and its production. For spray drying although the production maintains a relatively large amount of the nutrients and is characterized by good solution and satisfactory taste, it still cannot be widely used in the industry owing to its high investment of spray dryer and high energy consumption and operation cost. In order to design and manufacture a test bed of drum drying which can be used to dry many different kinds of materials, the drying theory and the structure of the drum dryers were deeply studied in this paper. By using the drum drying method, a new type of pumpkin powder was developed and the processing was optimized. According to the heat and mass transfer mechanism, the drying kinetic was analyzed and a numerical model was developed in the study. The main research contents and results were summarized as follows:1. A test bed of single drum drying was developed. The configuration and operating principle of the equipment was deeply analyzed. The test bed of drum drying can be used to dry different kinds of materials. The test indicators and functions of the test bed not only are consistent with the actual production but also meet the experimental requirements.2. The product processing of a new kind of pumpkin powder with drum drying was developed. The determined process steps include selection, cutting, cooking, mashing, drum drying and smashing. Through numerous experiments, the best cooking process was determined:small pumpkin pieces should be blanched in 80℃water for 2min, then be put into running water to cool down with its core temperature getting to room temperature, and finally be steamed for 4min. The drying processing was optimized by using the Orthogonal Rotatable Central Composite Design. Considering the higher product quality, the optimum data were developed: flow mass is 40 kg/h, rotary speed is 1.00 r/min, and vapor pressure is 0.39 MPa.3. Drum drying characteristics of pumpkin pulp were investigated. The drum drying process was divided into two phases:pulp phase and flake phase. It was indicated from the results that most of the moisture evaporated during the pulp phase; the higher the steam pressure was the higher the drying rate became. It was also shown that during the flake phase only falling rate period took place. As the steam pressure increased, on the one hand, the initial moisture content of flake phase became less and the drying rate decreased, on the other hand, the decreasing of the drying rate slowed down and the drying time was much shorter. The experimental moisture loss data were fitted with 10 thin-layer drying models. Consequently, of all the drying models, a semi-theoretical Midilli - Kucuk model was fitted best.4. During the flake phase, variation of the temperatures of the product and the outer surface of the drum dryer was analyzed. Results showed that both of the temperatures increased with displacement under different vapor pressures. It can also be indicated that variation of the temperatures were related to variation of the drying rate, the faster the drying rate was the slower the temperature of the product increased, the faster the drying rate decrease the faster the temperature of the outer surface of the drum dryer increased.5. The heat and mass transfer model of the product during flake phase was established and the equations were solved under steady-state condition. The experimental data of the pumpkin pulp dried by drum drying were used to validate the simulation results. The experimental and simulation results had a high correlation value which shows that the model could be used to predict the variation of the product temperature with displacement.6. Comparison analysis on sensory quality, dissolvability, nutritional quality, yield and energy consumption of the pumpkin powder with drum drying, spray drying and hot wind drying was done. Results showed that compared with the spray drying pumpkin powder with drum drying had a similar quality on sensory and dissolvability, higher yield, but lower nutritional preservation rate. The product quality of the hot wind drying was the worst. The energy consumption of drum drying decreased 35.89% per kilogram water compared with spray drying.