Effect of drying temperature together with light on drying characteristics and bioactive compounds in turmeric slice

Abstract Turmeric is an important source of curcuminoids. It is widely sold in the form of dried slices and powder for production of coloring agents, spices, traditional medicines, and cosmetics. In this study, turmeric slices were dried at five temperatures (40, 50, 60, 70, and 80 °C) under two conditions (without light exposure; noLE and with light exposure; LE) in a laboratory-made hot air dryer. For the LE condition, power and light intensity were 397 ± 29 W m−2 and 541 × 102 ± 42 × 102 lux, respectively. The Midilli and Kucuk model is the best for predicting drying characteristics of turmeric slices. Effective moisture diffusivities (Deff) and the drying rate constants (k) increased with the drying temperature and the light exposure. Light exposure and temperature did not significantly affect the color values of turmeric powder (p > 0.05). The three curcuminoids from fresh turmeric identified by HPLC were curcumin (17.88 ± 1.60 mg g−1 dry matter), demethoxycurcumin (12.34 ± 0.87 mg g−1 dry matter), and bisdemethoxycurcumin (19.84 ± 1.82 mg g−1 dry matter). The percentage changes of these curcuminoids after drying under noLE were higher at all temperatures compared to LE condition. Percentage changes of DPPH, ABTS, FRAP, and TPC after drying were not significantly influenced by the drying conditions (p > 0.05). The present study suggested that drying at 70 °C without light exposure was the best condition to preserve curcuminoids, color, total phenolic contents, and antioxidant capacity.