Effects of post-harvest drying on the essential oil and glandular trichomes of Lippia scaberrima Sond.

Lippia scaberrima (Verbenaceae) is an aromatic, medicinal shrub which is indigenous to southern Africa. Quantitative and qualitative analyses of the essential oil were conducted using gas chromatographic techniques. The effects of post-harvest drying on trichome structure, as well as on essential oil yield and composition, were investigated. Limonene, carvone and 1,8-cineole were found to be the major components of the oil. Loss of mass, due to both water and volatile compounds, was determined on a daily basis during drying of the plant material. Using the results derived from these determinations, oil yields were adjusted to a fresh weight basis. This was done to compensate for increased yield resulting from increased loading of dried material. Oil yields were observed to increase by approximately 42% (m/m) during the first four to six days after harvest, where after a significant decrease was observed. A short period (four to six days) of post-harvest drying is therefore beneficial to improving oil yields of L. scaberrima. Concentrations of 13 individual components were investigated and these results indicated that drying does affect the oil composition. This factor should be taken into account if consistent economic production of the oil is envisaged. It was found that extended periods of drying, once the plant has lost most of its moisture, leads to a rapid reduction in oil yield. Scanning electron microscopy (SEM) corroborated the changes observed in the oil yield, since the appearance of the trichomes remained unchanged during the first five post-harvest days, but signs of glandular evacuation became evident after 10 days of drying.