Influence of various drying methods on physicochemical characteristics, antioxidant activity, and bioactive compounds in Centella asiatica L. leaves: a comparative study

This comparative study aimed to evaluate the effects of different drying methods such as solar drying, shade drying (SHD), freeze drying (FD), oven drying, and microwave drying on the physicochemical properties, bioactive components, and antioxidant activity of Centella asiatica. The results showed that out of all the treated samples, FD-treated samples showed the lowest moisture content (2.4%), the lowest water activity (0.24%), and the highest rehydration ratio (5.51%). For samples treated using different drying methods, significant differences in Commission on Illumination – LAB (L*, a*, and b*) values and total color difference (E) were observed. FD-treated samples showed the minimum color change (E) and highest lightness (L*). Additionally, upon Fourier-transform infrared spectral analysis, no major changes in the functional groups were observed between C. asiatica leaves processed using different drying methods. FD-treated samples showed the highest antioxidant activity followed by SHD-treated samples, as measured by 2,2-diphenyl-1-picrylhydrazyl and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid radical scavenging assays. The phenolic (chlorogenic acid, rutin, kaempferol, and quercetin) and triterpene saponin (madecassoside, asiaticoside, madecassic acid, and asiatic acid) contents of the dried samples of C. asiatica were measured using high-performance liquid chromatography, which showed that the FD method allowed for the highest retention of phenolic and triterpene saponins among the tested drying techniques. The physicochemical characteristics, antioxidant potential, and bioactive retention of the samples that underwent FD treatment were superior to those of other methods, and therefore, FD can be employed as the first-line drying technique for processing C. asiatica leaves.