Distinct metabolomes and quality characteristics of vacuum-assisted osmosonic-pretreated Curcuma longa L. rhizomes subjected to different drying methods

This study sought to investigate the impact of four drying methods, thus, sun (S) drying, freeze (F) drying, relative humidity convective (RHC) drying and infrared (IR) drying, on the metabolomes and quality characteristics of vacuum-assisted osmosonic-pretreated turmeric (Curcuma longa L.) slices. The quality characteristics examined included contents of marker compounds, antioxidant activities, total phenolic content (TPC), total flavonoid content (TFC), chromatic properties, sensory characteristics, rehydration ratios (RR), and moisture diffusivities. Using LC-Q/TOF-MS and chemometric analyses, the metabolomes of variously dried turmeric samples were examined. In this regard, we found distinct metabolome differences between the samples. These differences translated into differential levels of three marker compounds, thus, curcumin, demethoxycurcumin and bisdemethoxycurcumin as captured by HPLC-DAD analysis. Overall, F drying gave the best results for all parameters assessed except for effective moisture diffusivity. Specifically, the F dried samples possessed the highest contents of the marker compounds, curcumin (3.99 ± 0.01 μg/mg), BDMC (0.275 ± 0.0006 μg/mg), DMC (0.170 ± 0.0007 μg/mg). The F dried samples also possessed the highest antioxidant activities, TPC, and TFC, chromatic and sensory characteristics. There were also marked differences in the rehydration ratios (RR), and the moisture diffusivities of the various dried samples. The RHC dried samples ranked second to the F dried samples for all quality parameters examined. In summary, for all the quality characteristics examined, the following order in terms of suitability and potential industrial large-scale applicability of the various drying methods was deduced: F drying > RHC drying > IR drying > S drying.