Christine M. Bruhn, PhD, director of the Center for Consumer Research at the University of California–Davis and coauthor of a nutritional comparison of fresh, frozen, and canned fruits and vegetables, says the nutrition levels are about even, assuming that you’re buying your vegetables from a grocery store. “By the time vegetables are consumed, fresh, frozen, and canned have similar nutritional values,” she says. “Frozen is picked and frozen within hours, whereas fresh is placed in the shipping container and shipped across the country to the supermarket warehouse, then to the supermarket back room, the supermarket shelf, and finally to the consumer’s refrigerator.”
Bruhn says that while nutrients like vitamin A, minerals, and fiber are stable, others, like vitamin C, react with oxygen after the vegetable is picked and change chemically so that they no longer function the same way in our bodies. This is called oxidative degradation.
One study found that broccoli traveled an average of 2,095 miles before reaching the Chicago International Produce Market. That’s about four days, if a truck is driving 70 mph for eight hours a day.
Pam Becker, a spokesperson for General Mills, the parent company of the Cascadian Farm and Green Giant brands of frozen vegetables, says the majority of GM's vegetables are “harvested, cleaned, cut, and frozen within three hours.” Kim Emmer, a representative from Birds Eye Foods, also says its vegetables are “frozen within hours after picking.”
This cuts down on nutrient loss due to short-term storage. But because the vegetables are usually blanched before the flash-freezing process, certain heat-sensitive nutrients, such as thiamin, can decrease. Long-term storage of frozen vegetables (6 to 12 months) can further diminish vitamin C because the vegetables are still exposed to oxygen, even in the freezer. Overall, Bruhn’s comparison shows losses of vitamin C “due to the entire freezing process [ranging] from 10 to 80 percent, with averages around 50 percent.” For fresh vegetables, Bruhn cites a study that found a 10 to 75 percent loss of ascorbic acid (depending on the vegetable) after seven days of storage.
The best way to ensure you’re getting the most nutrition from your veggies? Buy them at your farmers’ market and eat them that week.