Drying of Gymnema sylvestre Using Far-Infrared Radiation: Antioxidant Activity and Optimization of Drying Conditions

Abstract

The leaf extracts of Gymnema sylvestre consist of secondary metabolites which are well known for antioxidant activity. This study aimed to measure the drying characteristics of G. sylvestre leaves under far-infrared radiation (FIR) and to optimize the specific energy consumption for drying and antioxidant activity of ethanol-water extract of dried leaves. Fresh leaves were harvested and exposed to combinations of four different temperatures (125, 150, 175 and 200 °C) and exposure times (5, 10, 15 and 20 min). Drying kinetics, energy consumption, color changes, total phenolic content (TPC) and antioxidant activities were quantified. Both temperature and drying time have significant (p < 0.05) effects on drying characteristics and antioxidant activity. The equilibrium moisture content was achieved at 200 °C and 18 min. The specific energy decreased and total color changes increased with temperature. Under lower temperatures (125 and 150 °C), TPC and antioxidant activity increased with exposure time, whereas higher exposure time (20 min) with high temperatures (175 and 200 °C) significantly decreased TPC and antioxidant activity. The highest TPC of 30.5 mg TAE/g leaf-fresh weight was achieved at 200 °C and 15 min. The optimal drying conditions achieved from the dissimilarity function method were 200 °C and 8.4 min.

Optimization of Black Tea Drying Temperature in an Endless Chain Pressure (ECP) Dryer

“…The specific energy consumption (SEC) showed a decreasing trend with increasing drying temperature (Table 1). Similar findings have been reported in previous studies on drying tea [11] and herbal leaves [10]. The SEC of treatment T1 was 48.24 MJ/kg of moisture, which was significantly higher (p < 0.05) compared to T3 and T5.…”

“…The TPC values of tea from treatments T2 and T3 were found to be comparable. The TPC values showed an increasing trend with the drying temperature, which is widely reported in the literature [10,21]. This observation could be attributed to the breakdown of extractable TPC from complex phenolic compounds [10].…”

“…The TPC values showed an increasing trend with the drying temperature, which is widely reported in the literature [10,21]. This observation could be attributed to the breakdown of extractable TPC from complex phenolic compounds [10]. However, increasing the temperature beyond a threshold level could degrade the TPC [10].…”

“…The energy wastage of tea leaves drying was calculated using Equation ( 8). The methodology outlined in the study by Abhiram et al [10] was employed to measure the total TF and TR in tea infusions from triplicate samples. The ratio between TF and TR was then calculated.…”

“…The total phenolic content (TPC) of the dried tea leaf samples was measured using the Folin-Ciocalteu method, as described in [10]. Briefly, a 10% solution of Folin-Ciocalteu reagent was prepared, then 5 mL of this solution was mixed with 0.05 mL of tea infusion.…”