Study of two-stage microwave extraction of essential oil and pectin from pomelo peels

Pomelo peels were first processed by a solvent-free microwave extraction (SFME) for essential oils, then by a hot-solvent microwave extraction (HSME) for pectin. SFME was superior to the conventional hydrodistillation (HD) method for essential oil extraction and HSME was better than acidic solution method for pectin extraction in terms of extraction efficiency and yield of targeted component. Chemical composition analysis by GC–MS showed that SFME did not affect the quality of essential oils. By using the response surface methodology, the optimal conditions of HSME for pectin was found at microwave power of 520 W, solvent pH value of 1.5 and extraction time of 5.6 min. Surface view by optical microscope (OM) and cross sectional view by scanning electron microscope (SEM) of the peels suggested that microwave can enhance the extraction process by two distinct mechanisms: one attributes to the diffusion across the intact oil gland while the other involves the convection through the broken oil gland.