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.