The Canning Process
Canning is one of the best studied forms of food preservation (7,8). Using heat to cook or can foods of all types destroys microorganisms that can cause food spoilage or foodborne illnesses. The sterilization process occurs inside the can as the food uniformly is heated; no preservatives are used or needed. Selecting the conditions for heat treatment is done to maintain the food's taste, texture, nutritional value and microbiological safety. The commercial canning process, developed by Nicolas Appert more than a century ago, is essentially unchanged. Home and commercial canning are similar, though commercially the temperatures and containers used are more carefully controlled. Steel cans can withstand the high heat, pressure and movement that occurs inside the big pressure cookers (called "retorts") used in processing plants. Steel cans also conduct the heat to the food rapidly and allow penetration of heat to the center of the product. This means canned foods are the safest form of food, because the conditions used are designed to preserve microbiological safety as well as nutritional and sensory quality.

Canned Foods Preserved Naturally
No additives are needed to preserve canned foods. When salt (sodium chloride) is added, it is only for flavor, and it can be eliminated without compromising food safety. Calcium chloride sometimes is added to tomatoes or other vegetables because it helps to maintain their shape. The added benefit is that calcium is an essential nutrient.

Packed With Nutrients
Vegetables and fruits are canned when they are at their peak of quality to ensure they will look and taste good. Generally, the time between harvest and processing is minimized by having processing plants located close to the growing areas. Fresh produce, on the other hand, may spend seven to 14 days on the road and in the supermarket before it reaches the consumer’s home.

Many consumers do not realize canned fruits and vegetables are good sources of nutrients, in spite of nutrition labels. Nutritional labels are used more often by consumers to determine how much fat or calories are provided. Using nutritional labels for comparison among fruits and vegetables – as well as between forms (fresh vs. canned vs. frozen) – is not important to most consumers and may not be considered trustworthy sources of information.

For most consumers, nutritional labeling is the major point-of-contact information source. However, consumers seem to be suspicious of information provided by manufacturers. A recent survey of label accuracy by the Food and Drug Administration (FDA) showed more than 90 percent of the labels are accurate. When there were discrepancies between laboratory analysis and label values, these could be attributed to analytical difficulties in determining a nutrient. This is why many manufacturers underestimate the amount of a nutrient in a canned food product.

Canned is the Key Ingredient
A perusal of cookbooks and cooking magazines reveals canned foods are important ingredients, particularly in today’s fast food preparation society. Consumers who use canned foods cut preparation and cooking time, while obtaining a wide variety of fruits, vegetables, legumes and meats all year long.

What's on the Label?
The nutrient values that appear on the food label are based on two sets of dietary standards: Daily Reference Values (DRVs) and the Reference Daily Intakes (RDIs). The values obtained for this study were based on both the DRVs and RDIs established by the FDA. Not all of the nutrients listed in Tables 1 and 2 are on the labels, however. The FDA requires only a limited number of the nutrients. Any additional information is at the discretion of the manufacturer.

DRVs are presented on the label as Percent Daily Values. The Percent Daily Values are calculated from the DRVs and usually are based on a 2,000 calorie diet for adults and children over the age of four. Table 1 is a list of the current DRVs based on a 2,000 calorie diet.

In the past, the U.S. Recommended Dietary Allowances (RDAs) were used as standards for labeling purposes. RDIs now have replaced the U.S. RDAs on the nutrition labels. In developing the new nutrition labels, the FDA wanted to avoid the possibility of confusion between the U.S. RDAs and the values on which they were based, the RDAs. Table 2 is a list of the RDIs.

Why Are the Listed Nutrients Important to Us?

Although there are RDIs for many nutrients, only a selected group appear in our tables and on the labels. Those listed below with a brief explanation of their functions in the human body are shown in our tables (11).

Vitamin A
Vitamin A in the diet comes in one of two forms: retinoids, found in animal products, and carotenoids (over 500 forms found in plants). This vitamin is important in both night and color vision. In addition, vitamin A is critical for the activity of mucus-forming cells in the human body. Carotenoids are classified as antioxidants, which provide protection for the cells of the body. Vitamin A values in the tables are expressed as IUs or International Units; carotenoid content is converted to these values. Fruits and green and yellow vegetables, such as apricots, carrots, peaches, pumpkin, spinach and sweet potatoes are all high in carotenes that can be converted to vitamin A in the body. Tomatoes contain an important carotenoid called lycopene that has been suggested as an anticarcinogen, particularly for prostate cancer.

Vitamin C

Vitamin C has a variety of functions in the human body. Also known as ascorbic acid, it is critical for collagen synthesis, acts as an antioxidant, enhances iron absorption and plays a role in the synthesis of vital compounds within the body’s cells. Apricots, berries, oranges, pineapple, potatoes, spinach and tomatoes are good sources of vitamin C.

Calcium
Calcium is essential for the human body. This mineral plays a role in cell regulation, nerve transmission and the maintenance of strong teeth and bones. In general, fruit and vegetable products provide only small amounts of calcium. However, taken in the context of a varied diet, canned vegetables and legumes can add to calcium intake. Canned salmon is a very good source of calcium because it contains some of the small bones that are softened by the heating process.

Iron
Iron is a part of both hemoglobin and myoglobin. This nutrient is essential for the uptake and release of oxygen in the body. Cooked peas, spinach, navy beans and red kidney beans are all good sources of iron.

Thiamin
Thiamin, a B vitamin, is important for enzymatic activities in the body’s cells when in its coenzyme form. It participates in carbohydrate and energy metabolism. Green beans, peas and dried beans are all very good sources of thiamin.

Folate
Folate plays a key role in the formation of the bases used in the genetic material RNA and DNA. Recent evidence has shown it is a particularly important nutrient for the fetus during gestation, and pregnant women should consume increased amounts of this vitamin (12). Additional research shows a relationship between elevated homocysteine levels in the blood and cardiovascular disease and stroke in the elderly, and folate plays an important role in the metabolic homocysteine pathway (13,14). Cooked beets, asparagus, pinto beans and spinach, as well as raw spinach, are all important sources of folate.