Influence of drying conditions on degradation of curcuminoids and bioactive compounds in turmeric(Curcuma longa L.)

Abstract

Turmeric is being used as spice, traditional medicine, food coloring, and food supplement. Solar drying is a simple and cost effective method for producing dried herbs and spices in many parts of the world. However, the effect of temperature and light during solar drying on several properties of turmeric is still ambiguous. This study investigated the influences of drying temperature and light exposure on the drying behavior, color, curcuminoids contents (curcumin, demethoxycurcumin, and bisdemethoxycurcumin), total phenolic contents, and antioxidant capacity of turmeric. Fresh turmeric rhizomes at the maturity of 9 - 10 months were used in all experiments. The turmeric slices dried in a parabola-type greenhouse solar dryer were bright orange-yellow while the sun dried products were dark brown-orange. To investigate the influence of light on total curcuminoids and color of turmeric powder, the powder was directly exposed to mimic natural sunlight for 0, 10, 20, 30, and 40 h. The results showed that the light significantly affected b*, C*, and h° and reduced total curcuminoids content. On the drying study, Page and Midilli Kucuk models could well describe the drying characteristics of turmeric slices in the temperature range 40 – 80 °C. However, Page model was more suitable because of its simplicity. Effective moisture diffusivities and the drying rate constants increased with the drying temperature and the light exposure. The results showed that drying at high temperature led to shorter drying time and resulted in preservation of color quality in the dried product. This might be due to the retardation of enzymatic browning reaction at high temperature. The percentage changes of these curcuminoids after drying under the without-light condition were higher at every temperature compared to the with-light condition. The combination of light exposure and drying temperature was further investigated under polycarbonate (UV impermeable) and poly(methyl metacrylate) (UV permeable) covers at 40 – 70 °C. It was found that light exposure was a major factor for deterioration of color and curcuminoids contents. Percentage changes of DPPH, ABTS, FRAP, and TPC after drying were not significantly influenced by temperature and light exposure. The results suggested that drying at 70 °C without UV light was the best condition to preserve curcuminoids, color, total phenolic contents, and antioxidant capacity. Drying under these conditions resulted in shorter drying time without negative impact on curcuminoids contents. This fundamental knowledge can be further applied for the optimization of the drying process for turmeric slices in a solar dryer in order to improve quality and reduce cost and energy consumption.

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