It’s clear that fruits and vegetables are key components of a nutritious diet. What’s not so clear is what impact various forms of preservation have on the nutrients in these foods.
Are there significant differences in the nutritional content of fresh, frozen, and canned fruits and vegetables? If the answer to this question is yes, then how should the differences impact our purchasing habits?
What happens to nutrients when fruits and vegetables are harvested?
Most fruits and vegetables are comprised of 70–90% water. Once they are removed from their sources of nutrients (vines, trees, or plants), they experience moisture loss, nutrient degradation, and potential microbial spoilage. The longer a fruit or vegetable has been off its nutrient source the worse its quality.
Fresh fruits and vegetables
According to researchers, freshly picked fruits and vegetables are attractive, flavorful, and packed with nutrients. When consumed within hours or a couple days of being picked, they are typically superior in nutritional content. Unfortunately, most Americans are not able to supply all of their fruit and vegetable needs through home gardens, so they must rely on grocery stores and farmers’ markets.
“Fresh” fruits and vegetables purchased at these venues may be costly and many are not rich in nutrients. The fruits and vegetables that are available in stores were likely harvested weeks or months prior to being made available to consumers. When stored under refrigeration and in high relative humidity, these produce items appear to be in great shape. However, they may be low in many nutrients, especially water-soluble vitamins. These vitamins are sensitive to heat, light, and oxygen, so they degrade rapidly after harvest. Vitamin C is a noteworthy example. As much as 77% of the vitamin C found in freshly picked green beans is lost after 7 days of refrigerated storage.
Processed fruits and vegetables
Methods used to process fruits and vegetables (canning, freezing, dehydrating, etc.) allow fruits and vegetables that only grow in specific parts of the world and/or only grow at certain times during the year to be consumed worldwide throughout the whole year. Canned and frozen vegetables are often less pricy than their fresh counterparts because they are easier to transport and have longer shelf lives.
Most of the nutrient loss that occurs in these produce items happens when they are heated. Frozen fruits and vegetables are often blanched (a short-time heating step) to deactivate enzymes before they are frozen. This treatment results in some loss of nutrients. Canned fruits and vegetables are exposed to high temperatures during canning, resulting in more significant nutrient loss. In fact, canned fruits and vegetables lose 10-90% of their vitamin C during processing. Losses during freezing preparation are slightly lower. Interestingly, ascorbic acid is added to some fruits and vegetables to prevent browning, so these may actually have higher concentrations of vitamin C than their fresh counterparts.
Fat-soluble nutrients (e.g., vitamins A and E, lycopene) may be released from the cellular matrices of fruits and vegetables by thermal, freezing, high-pressure, or other preservation methods. This may increase the bioavailability of these nutrients (i.e., the body may be able to access and use them more easily).
It is important to note that the nutritional content of frozen and canned fruits and vegetables remains fairly stable during storage (i.e., they do not lose many nutrients as they sit in a can on a shelf or in a bag in the freezer).
Two additional factors should be considered when evaluating canned fruits and vegetables. First, substances contained in the cans or their linings, namely BPA, can leach into the produce. BPA is an endocrine disruptor that is associated with a number of health concerns (you can read more about BPA here). Second, many vegetables are canned with high sodium liquids and fruits are often canned with sugary syrups. These fruits and vegetables can always be rinsed, but research shows that some vitamins and minerals may be lost during rinsing.
Protein, fiber, and fatty acids do not appear to be significantly influenced by canning or freezing. Keep in mind, though, that much of the fiber in fruits and vegetables is found in their peels or skins. If these are removed during processing then the fruits and vegetables will have less fiber than a fresh version that hasn’t been peeled.
Which should we consume?
Though it’s a simple answer, it’s true: We should eat fruits and vegetables!
Freshly picked produce items stored for a very short time do provide maximal nutrients. However, if these are not available or if “fresh” fruits and vegetables have been stored for a lengthy period of time, frozen and canned varieties are just as beneficial, especially if they were frozen or canned soon after being harvested. In fact, some vegetables, such as tomatoes, have greater concentrations of certain nutrients after they have been processed.
For more information, you can check out the research conducted by researchers at the University of California—Davis.
This research makes me feel more comfortable with the fact that we eat a lot of frozen vegetables. I think I’ll also be more mindful of how long I store produce before eating it!
What about you? Do you have access to freshly picked fruits and vegetables or do you purchase a lot of frozen and canned varieties? How do you make sure your family eats lots of fruits and vegetables?