Retention of the harpagoside content in dried Harpagophytum procumbens (Devil's Claw) root through controlled drying and the application of near infrared spectroscopy (NIRS) as rapid method of determination

[摘要] ENGLISH ABSTRACT:The effect of drying conditions on the retention of harpagoside, a glucoside iridoid found in theindigenous Southern African plant Harpagophytum procumbens (Devil's Claw), was investigated.Additionally, the feasibility of using near infrared spectroscopy (NIRS) as a rapid and nondestructiveanalytical tool for the prediction of the iridoids harpagoside (HS), 8-p-coumaroylharpagide (8pCHG) and moisture contents, as well as the HS:8pCHG ratio in Devil's Claw wasalso studied.Due to the purported medicinal activity ofthe tuberous secondary roots, Devil's Claw is ofcommercial interest with a well established export market in Germany. The stability of HS duringprocessing has, however, never been investigated and the current study evaluated the effect ofthree drying methods (sun, tunnel and freeze-drying), as well as three tunnel-drying temperatures(40°C, 50°C and 60°C, dry bulb temperature) on HS content.Methanolic extracts (70% methanol-water) were analysed by high-performance liquidchromatography (HPLC) and a multiple extraction test (n = 12) indicated a 91.70% recovery forHS. The comparative drying results showed significant (P < 0.05) differences between tunnel andfreeze drying on the one hand and sun-drying on the other, with the latter being the mostdetrimental technique. In comparing tunnel-drying temperatures, at a constant relative humidity(RH) of30%, drying at 40°C showed the lowest retention ofHS, differing significantly (P < 0.05)from that at 50°C. Drying at 60°C did not differ significantly (P > 0.05) from either of the othertwo conditions, but also resulted in lower retentions ofHS content than drying at 50°C.Overall, freeze-drying showed excellent results for HS retention (1.565% ± 0.394%; mean± standard deviation), but economic constraints limit the commercial application of this dryingmethod. Although further study is required to optimise tunnel-drying conditions, this methodindicated acceptable and practical results (1.526% ± 0.396%), especially with conditions set to50°C and 30% RH (1.750% ± 0.438%). Future research should concentrate on elucidating thedegradation mechanisms of HS during processing, transport and storage of dried Devil's Clawroot.The analysis of dried, ground Devil's Claw root by NIRS was referenced to HPLCanalyses of the same methanolic extracts used in the drying studies. The spectra were generatedon two different NIRS instruments by measuring diffuse reflectance of the powder whilstcalibrations were performed by partial least squares regression. Both independent data setvalidation and full cross-validation were used to evaluate the performance and predictive abilitiesof the various models. Excellent moisture content (MC) calibrations were developed with a standard error of prediction (SEP) ofO.24% (range: 2.44% to 10.43%) and correlation coefficient(r) of 0.99. For the given MC range, the model performed very well against the standard error oflaboratory (SEL) ofO.14%.Considering the sensitivity of the HPLC reference method (SEL = 0.035%), the HScontent calibration (range: 0.693% to 2.244%) performed admirably with an SEP ofO.134%. Theavailable range was, however, relatively small and future NIRS studies should incorporate a largerrange if possible. The 8pCHG model (range: 0.069% to 0.290%) performed similarly, with astandard error of cross-validation of 0.028% and SEL of 0.007%.By inclusion of a second species, H. zeyheri, both the 8pCHG content and especiallyHS:8pCHG ratio (range: 1.84 to 34.48) calibrations were evaluated as possible methods ofdistinguishing between species. With 8pCHG values of ca. 0.9%, good separation was achievedfor the first model but, contrary to previous studies, the ratio was found to be less successful.Principle component analysis of the spectra, however, showed great potential as a qualitative toolfor this purpose.It is believed that NIRS can become an invaluable tool for the division of Devil's Clawinto export classes, depending on HS content. This content is, however, dependant on the dryingconditions used and drying parameters should be optimised to ensure the best possible quality andretention ofHS.

[发布机构] Stellenbosch University