Corn is generic term to describe the fruit (grain) of cereal plants in particular. Popping corn and sweet corn are just two of the many varieties of maize grown commercially. Each variety has different properties and is grown for different reasons.
The bulk of tissue within a grain of corn is called the endosperm. Endosperm is specialized storage tissue providing nutrients for the embryo when the seed germinates. It is also a source of carbohydrate for humans.
In popcorn the outer part of the endosperm is hard but the centre is soft. When the corn is heated the water in the central part turns to steam causing the seed to burst (the pop that we hear) and turn inside out. Sweet corn contains more sucrose in the endosperm than other varieties, which is what makes it sweet.
Maize, like all cereals, stores a carbohydrate food reserve in its seeds. Sugars enter the developing seed and are converted into starch. As the grain matures, excess water is removed leaving a hard, dry starch.
In the maize varieties used for most purposes the sugar is all converted to dry starch: this type of corn is called flint corn. In sweet corn the process is not completed by the time the plant is harvested, leaving the grain moist and sweet.
There are other forms of maize including flour corn where the starch remains soft- this was used by Native Americans because it is easy to grind and waxy corn which on milling produces flour with the texture of tapioca.
Corn is classified into five principal commercial types according to the structure of the kernel: dent maize, flint maize, flour maize, popcorn and sweet corn.
These crops were cultivated in the Americas before the arrival of Europeans, maize being the basic food plant of all pre-Colombian American civilizations. The grain of sweet corn contains a glossy, sweetish endosperm which is translucent when immature. A recessive gene on the fourth chromosome prevents the conversation of some of the sugar into starch. It is harvested when young and immature, when the kernels are plump but still soft and milky. As soon as the silk threads above the winter husk wither and turn brown, the cobs can be broken off, boiled and eaten as a vegetable. However, 20 minutes after picking, the sugar begins to converting to starch.
Therefore, corn on the cob should be absolutely fresh if one wants it to be sweet. For the same reason canned or frozen sweet corn is processed as soon as possible after harvesting. The grains of popcorn are small with a high proportion of very hard endosperm and a little soft starch in the centre. On heating, the moisture in the centre expands as steam, causing the grain to pop and explode, the endosperm becoming averted as a palatable and fluffy mass. Other types of corn will crack but will not explode.