The heating process during canning destroys from one-third to one-half of vitamins A and C, riboflavin, and thiamin. For every year the food is stored, canned food loses an additional 5 to 20% of these vitamins. However, the amounts of other vitamins are only slightly lower in canned food than in fresh food.
Most produce will begin to lose some of its nutrients when harvested. When produce is handled properly and canned quickly after harvest, it can be more nutritious than fresh produce sold in stores.
When refrigerated, fresh produce will lose half or more of some of its vitamins within 1 to 2 weeks. If it's not kept chilled or preserved, nearly half of the vitamins may be lost within a few days of harvesting. For optimum nutrition, it is generally recommended that a person eat a variety of foods.