From a public safety point of view, foods with low acidity (a pH more than 4.6) need sterilization under high temperature (116-130 °C). To achieve temperatures above the boiling point requires the use of a pressure canner. Foods that must be pressure canned include most vegetables, meat, seafood, poultry, and dairy products. The only foods that may be safely canned in an ordinary boiling water bath are highly acidic ones with a pH below 4.6, such as fruits, pickled vegetables, or other foods to which acidic additives have been added.
Canning is a way of processing food to extend its shelf life. The idea is to make food available and edible long after the processing time. A 1997 study found that canned fruits and vegetables provide as much dietary fiber and vitamins as the same corresponding fresh or frozen foods, and in some cases, even more. The heating process during canning appears to make dietary fiber more soluble, and therefore more readily fermented in the colon into gases and physiologically active byproducts. Canned tomatoes have a higher available lycopene content.
I am not snobbish about canned vegetables. We eat them every week in my household. Some folks won't eat them because they consider canned vegetables too low-class for their lifestyle. Other folks claim canned vegetables are void of nutrients and therefore not worth consuming . I never felt that way about them. Especially during the winter, when most fresh vegetables are out of season, I find canned vegetables to be a real money saver. The USDA's Food Pyramid recommends everyone eat at least 3 servings of vegetables per day. When trying to reduce one's caloric intake, it makes sense to eat at least 4 to 6 serving of vegetables a day. Canned vegetables, when purchased 2 for $1, cost only about 15f¢ per 1/2-cup serving. This is an almost unbelievable bargain in terms of healthy eating.
Canned Veggie Savings: To save the most on canned vegetables, there are a few details to remember. Generally store brands are the least expensive. All of us should be watching our sodium intake. Luckily many stores now have their own brands of no-salt-added canned vegetables. Low sodium canned vegetables taste fresher than their salted counter parts. At my local super-store they cost less than 50D¢ per 15-ounce can.