Step 1: Understanding the Concept:
Bleaching of spices like ginger is performed during post-harvest processing to improve the visual appearance (whiteness), prevent insect infestation during storage, and extend shelf life.
Step 2: Detailed Explanation:
The preparation of bleached or limed dry ginger ("Sonth") involves several key steps:
1. The fresh ginger rhizomes are peeled to remove the outer epidermal layer.
2. The peeled rhizomes are subsequently soaked in a slurry of 2% lime (calcium hydroxide, \(\text{Ca(OH)}_2\)) solution for approximately 12 to 24 hours.
3. This alkaline lime treatment neutralizes surface acids, bleaches the light-brown color of the rhizome to a desirable chalky-white finish, and deposits a protective coating of calcium carbonate on the dried surface.
This calcium coating acts as a strong physical and chemical barrier against storage insect pests (such as cigarette beetles) and prevents mold growth.
Using higher concentrations like 4% lime can lead to an excessively thick chalky residue and affect the chemical profile of the spice. Citric acid would not provide the white limed coating required for traditional bleached ginger.
Step 3: Final Answer:
Therefore, in dry ginger making, rhizomes are soaked in 2% lime during bleaching.