In biochemistry, starch is a carbohydrate polymer (a biopolymer) in which the monomeric units are simple sugars.
Starch is often found in the fruit, seeds, or tubers of plants.
How to make starch from frosted potatoes
From the 1881 Household Cyclopedia
Potatoes much frosted will make very good starch, though it is a shade darker in color. All coarse clothes requiring to be stiffened, where whiteness is no object, may be done with starch made from potatoes greatly penetrated with frost. The best method of making potatoes into starch is to grate them down into water, then to take out all the refuse with the hand, and next to strain the whole of the water in which the potatoes have been grated through a thin cloth, rather coarse, or fine sieve, and afterwards frequently putting on and pouring off water until it comes clear from the starch, which is always allowed to settle or fall to the bottom of the vessel in which the operation is performed. An experiment was tried with a few potatoes that were put out to frost. They were grated down and made into starch powder. The produce of the fresh potato weighed 876 grains, while that of the frosted was only 412, being less than half the quantity.
The refuse of the potato, when taken from the sieve, possesses the property of cleansing woollen cloths without hurting their colors, and the water decanted from the starch powder is excellent for cleansing silks without the smallest injury to their color. In making hair-powder it has long been used, and is therefore well known.