What’s healthiest? Fresh, frozen or canned foods?
In general, organic, local, fresh is best, followed by frozen, then canned. This is not an absolute rule; frozen produce can be a better choice if it was frozen shortly after picking compared to fresh produce that traveled 3,000 miles to your grocery store – and then sat in your fridge for several days before you ate it.
Canned fruits and veggies typically have added sugar or salt, and sometimes preservatives and colors, too. Draining or rinsing off this salty water does decrease the amount of salt you consume, but you also lose the water-soluble vitamins and minerals (vitamin C, B vitamins, potassium), which have leeched into the salty water. Using this water as a soup base is one way to retain some of those nutrients in your diet.
Vitamin stability is affected by heat light, air, and pH. What happens to the food before you get it (agricultural practices, maturity, storage, handling, fermentation, drying, freezing, canning, and blanching), and after you get it (home food processing, storage, purchasing practices, cooking practices, food preparation, and reheating), all affect the amount of nutrients the food delivers to you by up to 20-fold!
In general, vitamins E, C, B1, folate, and retinol are the most delicate and likely to be lost through cooking, processing or simply the passage of time. Minerals and macronutrients (protein, carbohydrate, fats) are typically the most stable and most likely to survive.
If you are serious about retaining the most nutrients, here are the details:
The Produce Department
Fruits that stay ripe, like grapes, have pretty stable vitamin content. Fruits that spoil quickly, like raspberries, lose about half their vitamin C in a week. Many vegetables lose delicate nutrients (vitamins E, C, B1, folate, and retinol) rapidly as soon as they are picked.
The Frozen Food Aisle
Commercial food is usually blanched before it is frozen. In this process, plant foods are cooked with hot water or steam for a short period of time. This decreases the water-soluble nutrients by 10-50%. Thawing further destroys vitamin C levels, so it’s best to cook frozen vegetables while they are still frozen. Leafy green vegetables lose more nutrients than other vegetables because of their large surface area. As the months pass, the produce in your freezer will continue to lose its nutrients, so you should finish it off within 6 months. Although frozen foods have lost some of their flavor, freezing is generally a good way to preserve the nutrients and texture of many foods.
The Canned Food Aisle
Foods that are going to be canned are cooked at high temperatures and usually for a longer period of time when compared to commercial freezing. The temperature and duration of cooking are crucial factors; fewer nutrients are lost when cooking at high heat for a short period of time than at low heat for a long period of time. Vitamin B2 is more sensitive to light than to heat, but this can be tightly controlled in industrial processing. Vitamins A, E and beta-carotene are susceptible to destruction by air, but this can also be controlled during industrial processing.
During canning, green peas lose about 80% of their thiamin, 73% of their vitamin C, and 35% of their folate. The amount of time canned foods spend on the shelf also affects their nutritional value. For example, the vitamin C content can decrease another 20% over the course of a year.
Unlike vitamins, minerals are not destroyed by light, heat, pH, or oxidizing agents. Minerals are removed during physical processing or may leach into water. Cooked vegetables that have been frozen for 6 months do not have iron or zinc losses.
Salmon, which is typically cooked before being put in the can and then cooked again during canning, loses some of its carotenoids during pre-canning cooking, and another 15-35% during the actual canning. Some companies place the raw, fresh fish in the can, then seal and cook only once to minimize nutrient loss. These companies tend to offer wild-caught, environmentally friendly fish but may not be stocked in your grocery store. Fortunately, you can find them on the internet.
Interestingly, the calcium content of some fish increases as a result of the canning process. Fish that are canned with their bones, like sardines, have about 10-20 times more calcium than their fresh form. During canning, the calcium-rich bones soften, so they can be easily eaten. Fish that are not canned with their bones, like tuna, lose about half their calcium due to canning. However, these fish, whose bones are not normally eaten, are not a good source of calcium anyway.
Although there are large variations among foods and among individual nutrients within foods, it is safe to say that the more a food is processed, the more nutrients are destroyed. Your best nutritional bet is organic, local, fresh produce, followed by frozen, then canned.
Raven Bonnar-Pizzorno, MS (Nutrition)
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- USDA National Nutrient Database for Standard Reference, Release 19 (2006)