Headspace volatiles and physical characteristics of vacuum-microwave, air, and freeze-dried oregano (Lippia berlandieri Schauer)

Mexican oregano (Lippia berlandieri Schauer) was dried using a freeze drier, conventional hot air drier, and a recently developed vacuum-microwave drier. The effect of the drying method on the relative content of major flavor volatiles, rehydration rate, color, and structural integrity of the herb was evaluated. Dynamic headspace analysis of volatiles present in fresh or dried oregano revealed that beta -myrcene, alpha -terpinene, gamma -terpinene, p-cymene, and thymol were the major volatile compounds of the plant. The level of thymol in vacuum-microwave-dried oregano was found to be comparable to that of fresh or freeze-dried samples (1.3 times the thymol concentration of air-dried). Air-dried samples of oregano were darker, were less green, and exhibited lower rehydration rates than those prepared by vacuum-microwave or freeze-drying. Structures of vacuum-microwave and freeze-dried oregano appeared to be quite similar as observed on electron micrographs.