Kratom (Mitragyna speciosa) is an ethnobotanical plant with high medicinal value that has been historically utilized in Southeast Asia as a traditional remedy for fatigue mitigation and productivity enhancement. Recently, it has attracted widespread attention, particularly in North America and Europe, for its potential for pain management and alleviating opioid withdrawal symptoms and has become an important source for future drug development. Postharvest processing represents a key step in the kratom farm-to-pharm chain where products undergo chemical modifications before reaching consumers. In this study, we investigated how different withering durations, drying temperatures, and lighting conditions could affect kratom postharvest product qualities. Leaves were harvested from cultivar H and mixed well before randomly assigned to one of four withering durations (0, 12, 24, and 72 h) at 25 ℃ in the dark and subsequently one of five drying temperatures (-40, 25, 40, 60, and 80 ℃). Additionally, leaves were dried at 25 ℃ under either light or dark. Overall, withering at 25 ℃ significantly increased mitragynine concentration. Compared to 0 h withering, a 12 h withering followed by drying at -40, 25, 40, or 60 ℃ increased mitragynine concentrations by 117%, 17-123%, 16-61%, and 43-103%, respectively. The 12 h withering increased the concentration of speciogynine and paynantheine by 27-28% and 35-67%, respectively, when leaves were dried below 40 ℃. In contrast, speciociliatine levels initially decreased during 12 or 24 h withering but subsequently elevated after the withering duration increased to 72 h. Drying temperatures and light exposure generally had little effect on the biosynthesis of most of the alkaloids. However, an alternation in kratom powder color was noticed for those with a short withering duration and subsequently a high drying temperature. Other minor alkaloids including corynoxine A, corynoxine B, speciofoline, isospeciofoline, mitraphylline, and ajmalicine were below the lower limit of quantifications. Taken together, our study shows that withering and subsequent drying temperatures have significant effects on the color and content of bioactive compounds of kratom, and further research on optimizing kratom postharvest processing is needed.
