All-Season Production of Plant Indigo Based on Insights into Heat Stress for Strobilanthes cusia Leaves

Seasonal production is the major issue impeding industrial production of plant indigo. Indican is the major precursor in fresh Strobilanthes cusia leaves for indigo production. The change in indican content of leaves during storage was investigated to propose a novel process for all-season indigo production in this study. Little indican was detected in leaves after 144 h even at 4 °C. Hydrolysis of indican by β-glucosidase was the main cause for the indican decrease. Heat drying led to a significant decrease in the β-glucosidase activity. However, the increasing rate of temperature was not enough to rapidly inactivate the β-glucosidase activity, and the residual β-glucosidase activity led to a decrease in indican content from 85.86 mg/g DW to 9.43 mg/g DW at 105 °C within 5 min. Microwave drying could increase the internal temperature of leaves rapidly such that β-glucosidase was immediately inactivated. The components of microwave-dried leaves were close to those of fresh leaves, especially for indican, which was maintained stably for 32 weeks. The indigo production with microwaved leaves provided an indigo yield of 20.00 mg/g DW, which was 2.4 times higher than that with fresh leaves. The result indicated the feasibility of an all-season production of plant indigo with the new approach.