Optimization of Two Step Drying Process of Porphyra by Hot Air and Microwave and Quality Evaluation

In this paper, a two-step hot air and microwave drying method was used to dry Porphyra, and the drying process was optimized. Taking the total drying time of Porphyra, the contents of protein and total sugar in dried Porphyra as indexes, the optimal drying process was obtained by using central composite design, model fitting by either quadratic polynomial or neural network, and genetic algorithm optimization. The results showed that the total drying time for sequential hot air drying and microwave drying was shorter than that for sequential microwave drying and hot air drying. In the case of sequential hot air drying and microwave drying, the total drying time required to reduce the moisture content to 0.1 g/g DM was the shortest (62.5±1.4 min), if the hot air drying temperature and time were 65 oC and 1 h, and the subsequent microwave power was 400 W. Comparing the contents of protein and total sugar in dried Porphyra obtained under the optimal process conditions of the two drying methods, the quality of dried Porphyra obtained by sequential hot air drying and microwave drying was better than sequential microwave drying and hot air drying. A highest protein content of 68.82±1.46 mg/g DM was obtained, when the hot air drying temperature and time were 48 oC and 3 h, and the subsequent microwave power was 240 W. Moreover, if the samples were first air dried at 65 oC for 2 h and then microwave processed at 240 W to the end, the total sugar content was the highest (124.72±7.17 mg/g DM). Generally, the sequential hot air drying and microwave drying is suitable for the dehydration of Porphyra in terms of drying time and product quality.