Nutrition Study Findings
COMPARISON OF SELECTED FRESH, CANNED AND FROZEN FRUITS,
VEGETABLES, LEGUMES AND PROTEIN FOODS
What Makes this Study Different from the 1995 Analysis?
In our 1995 comparative nutritional analysis of canned, frozen and fresh fruits and vegetables, we confirmed canned fruits and vegetables are, in general, nutritionally equivalent to their fresh and frozen counterparts. The information presented came from the existing USDA nutrient data bank and other sources, as accessed by a popular software program used by nutritionists. In addition, we gathered data from labels provided by manufacturers. The values presented in that study still are valid. However, the RDIs used for the calculation of the percent RDI were based on a proposed change in the RDIs which did not take place. The present comparison uses the values currently in effect. We also used the USDA nutrient data base exclusively, since that is now available electronically on the Internet, making it a resource available to many more individuals.
We have included a number of nutrients not shown in the 1995 study; for example, potassium and folate – two nutrients that are dietarily important and provided by fruits and vegetables. Information on calcium and iron is presented consistently.
The number of foods in the study was expanded to include 12 fruits, 14 vegetables, seven legumes and three protein foods (chicken, tuna and salmon). Fresh, fresh-cooked and canned products are in the tables where possible. Examples of commercial brands are included for all foods, being representative of the products available to the consumer.
Another addition to this study is the nutritional analysis of five popular recipes that can be made with fresh, frozen or canned ingredients. Since canned foods provide convenience in preparation, as well as comparable quality in finished products, it is important to know how well they stack up nutritionally.
How Do the Canned Foods Compare?
From a nutritional standpoint, fruits and vegetables are low in calories and fat, and are important dietary sources of vitamins (particularly vitamins A and C and folic acid), minerals (potassium, in particular) and fiber. They contain no cholesterol and can contribute substantially to fiber intake, a food component almost always low in American diets. Conventional wisdom has said fresh produce always is better than processed. Our findings in this (as well as our 1995) study show canned fruits and vegetables generally stack up very well against fresh. Although there is some loss of vitamin C content during heat processing, canning usually results in stable levels of most essential nutrients (8). The amount of a vitamin or mineral or fiber in canned food remains the same, even after one to two years of storage. Detailed nutrient information about selected fruits and vegetables are given in the tables and the brief summaries that follow.
Some canned fruits and vegetables high in vitamin A are apricots, carrots, pumpkin, spinach and sweet potatoes. Vitamin A is present as carotenes, specifically B-carotene, which have both vitamin and antioxidant activity. Carotenes are very stable during the canning process and little is lost. In fact, some analyses indicate carotenes are more available for measurement and use by the body following heat treatment. Lycopene, a carotene that occurs in tomatoes, seems to be more effective in preventing prostate cancer when it is consumed after heating or canning (15,16).
Good to excellent sources of vitamin C among the fruits and vegetables are apricots, asparagus, grapefruit, oranges, pineapple, spinach, strawberries and tomatoes. Although some vitamin C is lost during the heat treatment, much of it dissolves in the cooking liquid and can be recovered by using the liquid in soups and sauces. The vitamin C that is retained in the product remains stable during the shelf life (usually two years) of canned food.
Most vegetables and dried, cooked or canned beans also are very good sources of folate or folic acid. Much less information is available about the stability of this important nutrient during processing, and no label information is required for comparison. However, looking at the information available from the USDA nutrient composition database, we see canned vegetables and beans can provide 20 to 40 percent or more of the RDI for folate. Folic acid is similar to vitamin C in stability, so we can probably assume it is still there when the can is opened. Some vegetables that are not great sources of vitamins A and C can be very good sources of folate – beets and peas, as well as dried beans, are good examples.
Thiamin, one of the B-complex vitamins, is obtained by eating meats or legumes. Although this is a B-vitamin that is not particularly stable to heating, it survives the canning process well. This makes canned meats and beans comparable to freshly cooked food. All dried beans must be cooked for hours to soften and make them palatable. For this reason, canned beans compare favorably with home-cooked.
Another essential nutrient that is not often referred to in nutrition articles and not always on the label is potassium. Together with sodium, potassium helps to regulate fluid retention in the body and influences blood pressure and kidney function. Fruits, vegetables and legumes often are excellent sources of potassium. This mineral is retained during canning, making canned foods as good of a source as fresh or frozen.
Dietary fiber in fruits, vegetables and beans is essential in boosting fiber intake to recommended levels. Apples, blackberries, blueberries, cherries, carrots, pears, beans and peas provide this non-caloric food component in the form of cellulose and pectins. Canning does not affect the content of dietary fiber and may even make it more soluble, and therefore more useful.
Protein and Calcium
Canned poultry and fish are comparable to fresh-cooked poultry and fish in their nutritional value. These foods are considered to be protein sources. Canning does not affect their protein content in any way. A benefit of canning fish is that there is calcium in the small bones that are cooked sufficiently enough to soften them so they are consumed. Therefore, canned fish has more calcium than the fresh-cooked product.
Are There any Surprises in the Study?
From a scientific standpoint, the results obtained in the second study confirm our findings from the 1995 analysis. Although the databases differed somewhat, the values for concentrations of nutrients were very similar. The percentages are somewhat altered because some of the RDIs and DRVs used in the current analysis were slightly different. The expanded lists provide a different array of foods – legumes and poultry/fish and some additional nutrients. Regardless of which food and what nutrients we examine, the bottom line is the same. Canned foods are an excellent alternative to fresh and frozen, because they provide the nutrients we expect from any specific food group.
What Can We Tell Consumers?
For the layperson, reading nutritional labels can be a challenge. Although many consumers say they read labels, they usually are checking for calories or fat content. Only a limited number of vitamins or minerals can be listed. Consumers may believe this means other nutrients are not present. Sometimes the importance of a particular food lies in the missing nutrient. Consumers should be confident that if a food is suggested as being "high in nutrient X," then the form (canned, frozen or fresh) will not alter that.
Canning is one of the safest ways to preserve foods. The high heat process, used for many decades, kills microorganisms that cause foodborne illnesses. Rarely is an outbreak of food-related sickness caused by commercially canned products. The rapid heating methods, high temperatures, the integrity of the can and its conductivity all contribute to the success of the process. Shelf life of canned foods is at least two years.
No preservatives are used in canning. Fruits may have sugar or syrup added to enhance flavor and maintain texture, so caloric value is increased. Alternative packing liquids, such as juice, give consumers a choice. Salt (sodium chloride) is added to some vegetables, beans, meats and mixed foods (such as soup), in part because consumer testing has shown the taste of salt is important to most people, so it routinely is added. If reducing sodium intake is a health concern, many manufacturers have low-sodium alternatives. Calcium chloride, often found in canned tomato products, is added to maintain texture in whole or diced pieces. The calcium then becomes available as a nutrient. Mixed foods will contain flavorings and spices that enhance flavor.
Using canned vegetables and beans in soups and stews provides the same nutritional value as the fresh ingredients likely would provide. Because canned foods already are cooked, they require only minimal further cooking time. Research studies in scientific journals show that once processed, little additional loss of nutrients occurs in subsequent cooking steps. Therefore, using canned foods in casseroles, soups and stews saves preparation and cooking time, as well as energy, while providing the same nutritional value as fresh foods.
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