Nutrition can be tricky. With a laundry list of vitamins and nutrients needed daily, fitting all of them into a couple of rushed meals can feel like an impossible chore. Especially when you consider those nutritional labels might not all be as accurate as they appear. Nutritional labels are based on ideal levels of nutrient contents, but a multitude of factors affects how healthy a certain food product is -- sometimes for the better, but usually for the worse.
When it comes to produce, for example, it's a long list. Growing location and season, plant cultivar, maturity when harvested, processing procedures, shipping and storage methods, shipping duration, washing and cooking are among the many factors that can alter the levels of nutrients in a certain product; the effect those variables have on different commodities varies greatly from product to product and nutrient to nutrient.
In addition, studies on nutrient values aren't always terribly helpful. While pulling cans off the shelf and produce from the bins can determine the nutrients that a certain regional population is receiving, it doesn't speak much to the overall nutritional differences between freezing, canning and selling products fresh.
And that's the big question here. Many consumers believe if they eat fresh food -- which is typically more expensive -- they're getting a healthier meal. In some instances, however, this isn't the case.
Studies at the other end of the spectrum have focused entirely on processing at a specific growing site in order to measure the effect of processing, but ultimately, they don't follow the products to their destination to determine the impact of transport and storage [source: Rickman, et al. and Rickman, et al.]. Others puzzlingly study nutrients that aren't prone to diminishing by studying ones that are, while some don't bother to take into account what happens to the food once it hits the kitchen counter [source: Rickman, et al. and Rickman, et al.]. Even changes in moisture content can muddy comparisons.
So as you can see, the issue gets pretty complex. On the next page, let's look at some examples of how these many elements -- especially the big three: processing, storage and shipping -- can cause a fluctuation in those essential nutrients we all need.
Canned, Frozen or Fresh: A Balancing Act
Basically, fruit and vegetable processing works like this. When canning, you expose foods to a lot of heat. When freezing, you expose them to a short burst of heat and then you cool them. When it comes to fresh, you don't heat them at all (although you can chill them). Then the food -- whatever its end form -- gets transported to where a customer can buy it.
When you heat food (and this includes cooking it), you destroy or at least slow down the bacteria and enzymes that contribute to spoiling and decay. Freezing food does the same thing. These actions decrease the levels of some nutrients, but in rare instances, also simultaneously increase the levels of others. Take tomatoes, for example. When you heat tomatoes, you amp up levels of lycopene (a great antioxidant) but in the process you sacrifice some vitamin C.
Processing also stops moisture loss and interaction with oxygen. And while there are a few other elements that could be at work (for instance, canning can involve adding additional ingredients like sodium, sugar and water that may affect nutrient levels), for our purposes, let's just focus on the main differences.
Another factor in the overall equation is when the product is harvested. Fresh produce is typically picked before it's ripe, so unlike frozen or canned products, it often doesn't come with a full load of nutrients from the get-go. What nutrients the produce does have will start to diminish over time. Even if it looks fresh, a premium appearance can be deceiving. Generally, freezing and canning slow nutrient loss.
What you have then, is a balancing act. Canned and frozen food might be somewhat depleted during processing, but once that initial loss is over, they can start to make gains on fresh food. This means that a 6-month-old can sitting on the shelf could be healthier for you than its fresh compadre over in produce, languishing for weeks waiting to catch a customer's eye.
If you eat all your fruits and vegetables directly upon picking them at peak ripeness, then you're in great shape; but obviously, most people don't have that kind of year-round access to a wide range of produce.
So does all this send you on a slippery slope straight to the bottom of the food pyramid? It doesn't have to -- cover all your bases and try to get a good mix of canned, frozen and fresh fruits and veggies the next time you hit the grocery store. If you eat the recommended five to nine, sooner or later you'll probably stumble across everything you need.
Related HowStuffWorks Articles
- "Frequently Asked Questions." California Tomato Growers Association. (9/21/2009) http://www.ctga.org/index.php?option=com_content&view=article&id=26&Itemid=25
- "General Canning Information." National Center for Home Food Preservation. (9/25/2009) http://www.uga.edu/nchfp/how/general/how_canning_preserves_foods.html
- Hough, Andrew. "Frozen food 'healthier than fresh produce', scientists claim." Telegraph. Sept. 11, 2009. (9/25/2009) http://www.telegraph.co.uk/health/healthnews/6170232/Frozen-food-healthier-than-fresh-produce-scientists-claim.html
- Kern, Rebecca. "Myth Buster: Fresh Vegetables Are Better Than Frozen." AARP. June 11, 2009. (9/25/2009) http://bulletin.aarp.org/yourhealth/healthyliving/articles /myth_buster_are_fresh_vegetables_better_than_frozen_.html
- "Lettuce, Leafy Greens and E. Coli." Science Daily. Sep. 7, 2007. (9/25/2009) http://www.sciencedaily.com/releases/2007/09/070902193834.htm
- Rickman, Joy, Barrett, Diane and Bruhn, Christine. "Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds." Journal of the Science of Food and Agriculture. 2007. (9/25/2009) http://postharvest.ucdavis.edu/datastorefiles/234-779.pdf
- Rickman, Joy, Barrett, Diane and Bruhn, Christine. "Nutritional comparison of fresh, frozen, and canned fruits and vegetables II. Vitamin A and carotenoids, vitamin E, minerals and fiber." Journal of the Science of Food and Agriculture. 2007. (9/25/2009) http://www.mealtime.org/uploadedFiles/Mealtime/Content/jsfaarticle_partiiucdavis_may07.pdf
- Roberts, Tammy. "Cooked or raw, tomatoes are a healthy choice." University of Missouri Extension. May 5, 2009. (9/23/2009) http://missourifamilies.org/features/nutritionarticles/nut306.htm
- "Storage Time and Temperature Effects Nutrients in Spinach." Science Daily. Feb. 28, 2005. (9/23/2009) http://www.sciencedaily.com/releases/2005/03/050323124809.htm
- "The Basics of Blanching." Free Culinary School. June 12, 2008. (9/25/2009) http://freeculinaryschool.com/the-basics-of-blanching/
- "Tomatoes: cooked better than raw?" FoodNavigator.com. April 23, 2002. (9/25/2009) http://www.foodnavigator.com/Science-Nutrition/Tomatoes-cooked-better-than-raw