Optimization of microwave vacuum drying of okra and the study of the product quality

Microwave vacuum drying (MVD) was reported to be an efficient method to dehydrate vegetables. In this study, fresh okra was dehydrated by this technique. Orthogonal design was used to optimize the drying parameters (microwave power, vacuum level, and loading density) to achieve high drying rate and product quality. Microwave power (500, 700, and 900 W), vacuum level (45, 60, and 75 kPa) and loading density (3, 4, and 5 kg/m(2)) were set up as the treatment factors. Drying rate, energy consumption, rehydration rate, color change, total phenol, and flavonoid contents were evaluated as the responding quality parameters. The optimal drying parameters of okra were identified as 900 W of microwave power, 60 kPa of vacuum level and 5 kg/m(2) of loading density. Fresh okra was then dehydrated under MVD, hot air drying, and vacuum freezing drying. Results showed that microwave vacuum drying method was most efficient, with the drying rate as high as 2.36 +/- 0.05 g/min. The contents of total phenol and flavonoid in the products of microwave vacuum drying had no significant difference from that of vacuum freeze drying and were significantly higher than that of hot air drying. Practical applications MVD is an efficient and energy-saving technology, compared with the conventional hot air drying or vacuum freeze drying. Meanwhile, it could retain the color, flavor, nutrients, and bioactive compounds in the product, thus it is a promising method to dehydrate vegetables and fruits. Okra is rich in nutrients and is of increasing demand worldwide. Whereas it tends to spoil even at refrigerated temperature, so that a practical drying art is essential to be developed. This study proved the feasibility and efficiency of dying okra by MVD, and worked out an optimized drying condition under MVD, which could be a guide in okra processing industry.