Optimizing drying time for Centella asiatica (L.) Urban: metabolomic insights into dehydration effects on primary and secondary metabolites

Drying has been considered a preservation technique for reducing water activity, preventing microbial growth, and preserving the quality of aromatic and medicinal plants. Therefore, an understanding of the changes in plant metabolisms due to dehydration during drying and the resulting changes in the active components of medicinal crops is required. Centella asiatica (L.) Urban is one of the important medicinal plant for consumption or medicinal purposes with its four most abundant triterpenoids, including two sapogenins (asiatic acid, AA; madecassic acid, MS) and saponins (asiaticoside, AS; madecassoside, MS). This study investigated the effects of the rate of dehydration on C. asiatica using a metabolic approach and identified the proper drying time to obtain the highest active components. In fresh samples (0-h drying condition), the highest AA content and TCA-related components (citrate, glutamate, and aspartate) levels were observed. As drying progressed, even minimal drying (6 h) induced metabolic changes by suppressing photosynthesis. With extended drying time, a significant time-dependent increase in amino acid production was observed. While amino acid accumulation progressed, an increase in MA content was observed at 12 h of drying along with an increase in CabAS gene expression levels. Subsequently, representative stress-related amino acids (GABA and proline) levels rose over time, peaking at 24 and 48 h of drying, respectively. At 48 h of drying, when the moisture in the C. asiatica had disappeared, an increased level of CaAS expression (involved in biosynthesis of α-amyrin, the precursor of AA and MA) was observed. At extreme dehydration (96 h of drying), increased levels of CaGT expression (involved in the glycosylation of AA and MA to produce AS and MS) were recorded. Consequently, these elevated biosynthesis gene expression levels resulted in increased saponins, including AS and MS content. However, beyond 96 h of drying, all the metabolites underwent degradation. This study highlights that metabolic responses during drying significantly alter centellosides by stimulating diverse metabolic pathways. Optimizing the drying period would maximize active components (MS and AS) in C. asiatica, thereby enhancing its pharmaceutical value.