Natural starches contain a mix of two basic starches -- a long, straight-chain starch, amylosa, and a short, brached-chain starch, amylopectin. A starch's characteristics change according to its differing proportions of amylose and amylopectin.
Grain starches like wheat, corn or oats contain 22 to 27 percent amylose, a relatively high amount. They:
- Are clear when hot but cloudy when cold. (Sauces with flour are opaque hot or cold because flour contains things other than starch.)
- Set up thick enough to slice with a knife.
- Become spongy and leak watery fluid when frozen and thawed.
- Thicken just below the boiling point of water and can be held hot without damage.
- Reheat without thinning.
- Thin if stirred once cool and firmly set.
Root starches like tapioca and arrowroot and waxy starches contain up to 99 percent amylopectin. They:
- Are crystal clear hot or cold.
- Are thickest when hot at their gel temperature. Thin a little when cooled. Set up in a thick, clear, glossy coating -- not firm enought to cut.
- Freeze and thaw nicely without change.
- Thicken at lower tmeperatures than grain starches.
- Thin when vigorously stirred, hot or cool.
In many cases, it is easy to pick the right starch. For a coconut cream pie, you need a starch that thickens enough to slice -- go with flour or cornstarch. For a stir-fry, you need a clear coating -- go with cornstarch because it is clear when hot. But a cherry pie presents problems because you need clear hot or cold. This would mean a root starch like tapioca or arrowroot. While this will give you a clear, thick coating, it will thin a little when reheated. You can mix starches -- use mostly tapioca for clarity and just a little cornstarch to make it thicken and reheat well.
Asian grocery stores are a great place to buy starches. They have arrowroot, potato starch, rice starch, tapioca starch (a powder), wheat starch, etc. at a fraction of their cost in regular stores.