Effects of different drying methods on nutritional composition, physicochemical and functional properties of sweet potato leaves

In this study, effects of microwave-vacuum (MVD), hot-air (HAD), and vacuum-freeze (VFD) drying methods on the nutritional composition, physicochemical, and functional properties of sweet potato leaves (SPL) were investigated. VFD-SPL showed higher total dietary fiber, vitamin C, E, B-1, and B-2, and mineral Mg, P, and Zn content than MVD- and HAD-SPL. MVD-SPL showed highest water absorb index, oil absorption capacity, swelling power and solubility. HAD-SPL had smallest particle size (D[4,3] = 56.85 mu m), which was further verified by scanning electron microscopy findings. Total polyphenol content of VFD-SPL was highest (6.30 g CAE/100 g DW), followed by MVD- and HAD-SPL (6.06 and 4.72 g CAE/100 g DW, respectively), and the same order was found for antioxidant activity. Although nutritional and functional properties of MVD-SPL were intermediate, the higher physicochemical properties, drying rate, and lower cost suggest MVD has greater potential as a new drying method of SPL. Practical applications Microwave drying can be used to enhance the transportation of heat and moisture inside the drying products, and further increase drying rate and lower the drying temperature due to the fact that vacuum enables moisture to evaporate at a lower temperature, thus maintaining good product quality, particularly for materials with active ingredient and heat-sensitive components. In this study, the higher physicochemical properties, drying rate, and lower cost than VFD- and HAD-SPL suggest that MVD has greater development potential as a new drying technology in SPL powder production.