Starch

Starch is a mystery, something looks like a powder.

Potato starch
We can extract starch out from potatoes. Just squeeze some peeled potatoes to extract whity liquid out of them. Then hydrate the liquid for quite some time and it will become starches.

On below link, someone has extracted 16g of starches from 200g of potatoes.
 * 片栗粉の作り方 - http://ws-plan.com/tsukuru/katakuriko.html

Usages
In pastry, e.g. sponge cake, it is used to keep the cake moist and give a soft texture.