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.
4. More mineral matter than most other foods.
Lying mostly just beneath the skin.
5. Other substances in such small quantities as to be of little food value.
To find out how much water a potato contains, pare and weigh it. Lay it in a warm, dry place, weighing it every day until it ceases to lose weight by evaporation of its moisture. Compare the final weight with what it weighed at first. The difference between these shows how much water the potato contained.
To show mineral matter in potatoes, heat a bit of potato in a crucible or evaporating dish over a Bunsen burner till only gray ashes are left.
Let a potato lie in a dark, warm place until it sprouts. Bring it to the light from time to time and observe the growth of the sprouts, also any change in the size of the potato. What do you think the sprouts feed on? Would a sprouted potato be as nutritious as an un-sprouted one?
The tuber is a storehouse of starch for the nourishment of the young shoot. In potatoes dug too early the starch is immature or unripe. In those kept too long after digging the starch has been partly changed to gum, a substance more serviceable than starch to the growing plant, but not so nutritious for man.
Potatoes are best (fullest of starch) in the fall and winter. Select those of regular shape, of medium size, and with smooth skin. A bushel of very large potatoes gives the purchaser less than a bushel of smaller ones, which pack more closely. Green bitter spots are caused by the potatoes' being grown too near the surface of the ground. Keep them in a cool, dry place. If sprouts appear pick them off.
Is the skin of a potato thick in proportion to the eatable part? Do we need to take off a thick paring?
By experiment it has been found that potatoes pared before being boiled lose much of their food value during cooking; for nearly one-fifth of the mineral matter, with some other soluble substances, and a little starch, passes into the cooking water. The longer the potatoes lie in water before they are cooked, the greater is this loss. New potatoes are best cooked in their "jackets." Any but imperfect or very old potatoes may be cooked this way.
They will not be as white as if they had been pared before cooking; but if one wishes to be economical, food-value should not be sacrificed to appearance. Old potatoes may have to be soaked to restore water lost by drying. Always pare potatoes thinly, and take out eyes with the point of the knife. To make the loss from pared potatoes as small as possible, put them at once into boiling water and as quickly as possible bring it to the boiling-point again.