Effect of high relative humidity on dried Plantago lanceolata L. leaves during long-term storage: effects on chemical composition, colour and microbiological quality.

INTRODUCTION Modern phytotherapy and quality assurance requires stability data on bioactive metabolites to identify and minimise decomposing factors during processing and storage. A compound's stability in a complex matrix can be different from the stability of the purified compound. OBJECTIVE To test the stability of iridoids and acteoside and quantify changes in colour and microbiological quality in a common herbal tea, dried P. lanceolata leaves during exposure to high-humidity air. To test the contribution of fungi to metabolite decomposition. METHODOLOGY Dried P. lanceolata leaves were exposed to atmospheres of different relative humidity (75, 45 and 0%) for 24 weeks. Changes in aucubin and catalpol concentration were determined by CE-MEKC, and those in acteoside on TLC. Colour and chlorophyll-like pigments were measured by different spectrophotometric methods. The number of fungi was monitored; 10 strains were isolated from the plant drug, and their ability to decompose the analytes of interest was tested. RESULTS During incubation at 75% relative humidity (RH), aucubin, catalpol and acteoside concentrations decreased by 95.7, 97.0 and 70.5%, respectively. Strong shifts were detected in CIELAB parameters a* and b* (browning) as a result of conversion of chlorophyll to pheophytin. Intensive microbial proliferation was also observed. Changes at 45 or 0% RH were typically insignificant. Seven of the 10 isolated fungal strains could decompose both iridoids, and five could decompose acteoside in vitro. CONCLUSION It was shown that exposure to water results in loss of bioactive molecules of P. lanceolata dried leaves, and that colonising fungi are the key contributors to this loss.