Dr. Diane Barrett, fruit and vegetable products specialist at the University of California Davis, recently addressed this issue in a concise report called Maximizing the Nutritional Value of Fruits and Vegetables.
Our Foodfacts.com research indicates that she reviewed literature on how the nutritional value compares among fresh, frozen and canned fruits and vegetables.
She notes these items are often most health promoting when at their peak of maturity, but routinely eating them that way can be a challenge.
Many people don’t have home gardens where they can pick and eat something when it’s ripe and ready. For those who do, of course, even on balmy Vancouver Island, you can’t grow a wide variety of produce in the winter, and most local farmers’ markets aren’t open selling stuff picked that morning.
So, to ensure we get our daily recommended intake of fruits and vegetables, many shop at supermarkets. That produce, much of which is imported, can spend several days or even months getting from field to table, with stops at warehouses, trips in trucks and time spent at the supermarket before it’s finally sold.
Fruits and vegetables contain a high percentage of water, and once harvested their source of nutrients is gone, but even when detached from the root or vine, they are still alive and respirating.
Barrett says that, during storage, temperature and humidity must be carefully controlled to maintain low rates of respiration, prevent moisture loss and maintain eating quality. Most growers have developed systems to ensure their produce is handled with care, but loss of nutrients can occur until it is finally eaten.
Vitamin C is water soluble and sensitive to oxygen, heat and light, which make it prone to loss during both home cooking of fresh fruits and vegetables and thermal processing, such as canning. Because of that, vitamin C loss is often used to gauge nutrient degradation. For example, Barrett says vitamin C losses in vegetables stored at 4 C for seven days can range from 15 per cent for green peas to 77 per cent for green beans. Keeping some produce even more chilled than that slows the vitamin C loss. When broccoli is stored at 0 C for seven days, no vitamin C is lost, but when stored at 20 C, 56 per cent is lost.
Canning fruits and vegetables exposes them to high temperatures, which in vegetables can degrade vitamin C by 10 to 90 per cent, depending on what’s canned. Barrett says vitamin C losses during freezing were slightly lower, but could rise if what was being frozen was blanched and/or improperly stored, such as being subject to freezing temperatures that fluctuate as can occur in a frost-free home freezer.
In her report, Barrett describes how other nutrients are affected, both positively and negatively, in stored fresh, and frozen and canned, fruits and vegetables. For example, the beta-carotene content in carrots refrigerated 14 to 16 days increased by 10 per cent. Processed canned tomato products have a higher lycopene content than fresh tomatoes.
In conclusion, Barrett says that fruits and vegetables should be consumed soon after harvest, or post-harvest handling must be controlled so that degradation does not occur, and that includes at home. If the latter two things are not done, she says fresh produce, by the time it’s consumed, might be nutritionally similar to frozen or canned versions.
Barrett says a good diet should include a variety of fruits and vegetables, whether canned, frozen or fresh. Reading the nutrition facts on the label of frozen or canned goods can also help you determine what to eat.