Influence of Drying Methods on the Antibacterial, Antioxidant and Essential Oil Volatile Composition of Herbs: a Review

Drying is considered as the most common and fundamental technique for the postharvest preservation of herbs and is regarded as a good process to retain bioactive compounds. Past studies suggest that the choice of drying method and the parameters applied were able to influence the chemical and biological activities of herbs because pronounced differences in chemical content and composition were observed between the different drying methods. This has warranted numerous studies over the years to determine the influence of various drying methods on the content of bioactive compounds in functional food. However, reviews on the impact of drying on the bioactivity of dried herbs are rather scarce. Additionally, the influence of drying methods on the antibacterial activity of herbs has yet to be reviewed. Therefore, this paper attempts to provide a critical review on the influence of current drying process technology on the antibacterial and antioxidant properties, as well as the essential oil content, of various herbs. The use of innovative, new or existing drying technologies in preserving the active compounds was included in this paper. It was found that (i) no single drying method can be effectively used for the dehydration of all herbs; (ii) heat treatment can lead to biochemical changes (Maillard reaction), which increases the antibacterial activity; and (iii) innovative combined drying methods are promising in the production of herbs with high antioxidant activity and higher yields of total volatile concentrations.