Crops are grown with one ultimate purpose in mind: the harvest. Harvest can begin shortly after planting if you consider that when you thin root crops such as carrots, you can eat the small baby carrots. Leafy greens such as lettuce or spinach can be harvested leaf by leaf as needed for salads. Chives and other herbs are also harvested as needed throughout the growing season.
Year-round harvest can be achieved with careful planning. Crops are planted continually, every three weeks or so, to prevent all the plants from maturing at the same time. Cold-season crops that are given some protection with a cold frame can be left in the ground well into the winter in cooler climates and harvested when needed.
Fruits are harvested as they ripen. Commercially grown fruits may be stored in controlled environments with a higher ratio of carbon dioxide than found in the air to inhibit ripening. These must be specialized rooms that are entered wearing protective equipment because the air is not healthy to breathe. Ethylene is a gas used to induce ripening in apples, tomatoes, and bananas. It is also used to harvest commercially grown pineapples, blackberries, blueberries, and cherries. Rotten apples produce high amounts of ethylene and can cause other fruits that are stored with them to overripen. Therefore, it is best to store apples separately from other fruits.
Crops may be harvested by hand (for example, tomatoes, which are prone to damage) or by specialized machines (such as bean pickers) that have been designed specifically for that purpose. Crops fresh from the field may still be warmed from the sun and have what is called field heat. The heat present in the plant causes rapid decay after harvest and can be reduced by harvesting early in the day and immediately storing the produce in a cooler.
Postharvest handling of the fruit or vegetable differs for each crop. Some may need to be dried prior to storage. Crops that have soil on them should be washed thoroughly prior to drying. Perishable crops, like leafy greens, last longer if they are stored in a refrigerator with the stems in a container of water and a plastic bag placed over the top. Harvested produce that is not sold fresh or dried may be frozen, canned, fermented, or pickled. Frozen foods have become more popular in recent years than canned goods, although canning is still the preferred method for preserving tomatoes.
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