This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
What is a potato? a root? Let us see if by examining one we can find out. On its surface are little scars, called "eyes." If a potato be buried in the ground in mild weather or kept in a warm, dark place, what happens? It sprouts; that is, the eyes send out green shoots that in time have leaves. These eyes, then, must be buds, and the potato a stem, not a root; for, ordinarily, roots do not bud. A thickened underground stem, like this of the potato, is a tuber.1 Potato roots are slender and fibrous.
1 The sweet potato is a true root, but from its resemblance to a tuber is called a tuberous root.
Fig. 6. - Potato-plant.
Potatoes are grown from cuttings, not from seed, each piece planted being cut so that it has two or more eyes. Why not leave one eye only? Potatoes are planted in April and May, and harvested mainly in early autumn.
A. Pare and grate a piece of raw potato. Squeeze it in a piece of cheese-cloth held over a bowl. Rinse what remains in the cloth with cold water, and squeeze it as dry as you can. What does it look and feel like?
B. Let the liquid in the bowl stand until a white sediment settles; then pour it off carefully. Add a little water to the sediment and boil it. Does it act like anything you have seen before?
C. Mix one teaspoonful of cornstarch with one tablespoonful of cold water, add one-fourth cupful of boiling water, and stir until clear. Do the same with laundry starch. Dissolve about one teaspoonful of salt in one-fourth of a cupful of water; do the same with one teaspoonful of sugar. Add a few drops of tincture of iodine to a test-tube of water. Pour a little of this iodine solution into each of the starch pastes. What happens? Try a few drops in the salt solution; in the sugar solution. Has it the same effect on these as on the starch? (Plate V.)
Starch is turned blue by iodine. Since no other substance is affected in this way, iodine serves as a test for starch.
D. Add a drop or two of iodine solution to the white substance obtained from the potato. What do you think it is? Test a slice of potato for starch.
1 Analysis (plural, analyses) means separation into parts. Chemists have made complete analyses of all kinds of foods. We can make only rough analyses of a few foods, and test them for the substances contained in them in considerable quantities.
1. Plant fibre (partly cellulose).
Similar to fibre of wood. Forms walls of cells, or little divisions, in the potato.
Too tough to be digested; therefore of little food value.
About 75 % of the weight of the potato. Fills cells.
White insoluble powder floating in water in cells; with boiling water forms a jellylike paste.
The chief foodstuff in potatoes.