Tapioca Starch (Best Option)
1:1 (1 tablespoon tapioca starch per 1 tablespoon arrowroot powder)Tapioca starch is a fine, neutral-flavored starch that thickens liquids similarly to arrowroot powder by gelatinizing starch molecules, creating a smooth and glossy texture. It is highly effective at thickening at lower temperatures, which suits oatmeal's cooking process.
To use tapioca starch successfully, dissolve it in a small amount of cold water before adding to hot oatmeal to prevent clumping. Avoid prolonged cooking after adding to preserve its thickening power.
Compared to arrowroot, tapioca starch produces a slightly chewier mouthfeel but retains the creamy consistency and clarity in oatmeal, making it an excellent substitute.