One of the obstacles for prserving food by freezing is arresting the ripening process. The produce contains enzymes which cause ripening to take place. Freezing greatly slows the action of these enzymes, but does not halt it altogher. If you've ever noticed that after a period of several months unblanched frozen corn on the cob doesn't taste as fresh as it did earlier in the storage period then you are observing this in action.
Blanching (heating, but not cooking) the produce will destroy these enzymes (which are extremely sensitive to heat), thus halting the ripening process while leaving the food undercooked enough that thawing and cooking the stored food won't end up with the food OVER cooked. The nutritional value is preserved much better than food which is canned, as the high temperature and long processing time to can non-acid foods destroys much of the vitamin content.
The goal of blanching is to use a process to heat the food as quickly as possible to a sufficient temperature throughout, and then to cool it again as quickly as possible, usually in a cold water bath. This results in the freshest possible food when thawed later.
Most fruits don't require blanching. We freeze peaches, strawberries, blueberries, and blackberries raw.
Most good cookbooks (I like the Joy of Cooking) have processing times and procedures for boiling water or steam blanching.
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