A high carbohydrate food, sweetcorn has been an important nutritional resource for thousands of years, although the corn of our ancestors was a starchier, less tender version of today's corn. Corn that is cultivated today falls into two main categories: sweet corn and field corn. Sweet corn, which was not widely cultivated until the mid 1800s, is harvested at an immature stage, so that its kernels are tender and juicy; at their peak of flavour, they contain 5% to 6% sugar by weight. Field corn, on the other hand, is harvested at a mature, predominantly starchy stage, dried to a more hardened state, and used in a multitude of ways, as livestock feed and, after refining, in a wide array of processed food, drinks and cooking oils as well as in many non food products, such as fuel, paper, and plastics. Popcorn is a field type corn with thick walled kernels; when heated, steam is trapped inside the dried kernels, causing them to explode.

Generally speaking, canned and frozen corn are about equal in nutritional value to fresh corn. However canned corn usually has both salt and sugar added, making it somewhat higher in calories and sometimes substantially higher in sodium than fresh cooked kernels. Despite its name, canned "creamed" corn has no milk or cream added; it is prepared with sugar and cornstarch, which further raise its calorie (but not its fat) content.

For sweetcorn, freshness means staying cool, since warmth converts the sugar in the kernels into starch. In the supermarket sweetcorn should be displayed in a refrigerated display ( however it rarely is in the UK). ideally, it should have been picked the morning you buy it. The corn should not be piled high in the display, or it will generate its own heat, hastening the conversion of sugar to starch. If you're making a trip to the country to get fresh picked corn, take a cooler in which to pack it.

Check that the husks are fresh looking, tight, and green (not yellowed or dry); strip back part of the husk to see whether tightly packed rows of plump kernels fill the ear. The kernels at the tip should be smaller (large kernels at the tip are a sign of overmaturity), but still plump rather than shrunken. Pop a kernel with your fingernail: Milky juice should spurt out. If the liquid is watery, the corn is immature; if the skin of the kernel is tough and its contents doughy, the corn is overripe. The stalk of a freshly picked ear of corn will be green and moist; if it is opaque and white, or dry and brown, the corn is several days old and will not be very sweet. The silk should be moist, soft, and light golden, not brown and brittle.

To best enjoy fresh sweetcorn's flavour, "the sooner the better" is a rule of thumb, as gardeners will tell you "walk slowly down the garden, pick your sweetcorn, and run back to the kitchen". Try not to store corn for more than a few hours; cook it as soon as possible after it is picked, and be sure to refrigerate it the moment you get home if you are not cooking it immediately. (At room temperature, sweet corn loses its sugar six times faster than at 32 degrees (F), up to half its total sugar in one day.)