Canned corn does not taste the same as boiled corn on the cob. That's because canned vegetables are processed in order to stay ready for consumption for long periods of time. Processing affects flavors, odors, appearance and vitamin content.
Canned and Fresh Vitamins
Canned vegetables do have the same vitamins fresh vegetables have. Canned vegetables slowly lose vitamin content over time and during preparation, while fresh vegetables quickly lose vitamin content and then rot.
Fate of Vitamins in Fresh Vegetables
Even if vegetables are kept cool, they may lose half of their vitamins within two weeks of harvesting. This is because vegetables have such high water content that interacting with oxygen and microorganisms makes them perishable. Active food enzymes and moisture loss also contribute to spoilage. Vitamin C is highly water soluble and is greatly affected by heat. Without proper cold storage, a vegetable such as spinach can lose its entire vitamin C value in ambient temperatures within four days of harvest. Cooking your produce further destroys nutrients.
Fate of Vitamins in Canned Vegetables
Canning vegetables requires heating them for long periods of time at high temperatures. Vitamins such as vitamin C, thiamine, riboflavin and B6 are reduced during canning, but the amount of nutrient loss varies by vegetable. For example, canned spinach may only have about 34 percent of its original thiamine content, but asparagus could still have about 75 percent of its thiamine. However, the slow, hot cooking processes that deplete vitamins also prevent food poisoning. Heat-resistant botulism spores grow in the absence of air (i.e. in sealed cans) and can be deadly if not first destroyed by high boiling water temperatures for long periods.
Perhaps the best way to acquire the benefits of vegetable vitamins is to grow your own produce. You can eat them fresh or can them yourself. To maintain optimal quality of your vegetables, begin processing them within six to 12 hours of harvest. For canning in boiling-water baths, your canner should be deep enough to leave at least 1 inch of water over jar lids during heating. Select only the best quality vegetables and jars. After canning, store your vegetables in temperatures between 50 and 70 degrees Fahrenheit.
Remember that commercial canned foods that may bear the Five-A-Day logo have met the FDA's definition of "healthy" foods, which only regulates fat, saturated fat, cholesterol and sodium content, not vitamins. Usually, canned vegetables meet the requirement of sodium levels below 480 mg per serving, enabling them to earn the logo.