Who doesn't apprecaite the convenience of frozen produce every once in a while? It's ready to cook, requires zero prep, and there's no risk of losing a finger while choppng away. Since it's National Frozen Food Month, we want to dig a bit deeper: With so many options lining the grocery store aisles, choosing how to buy veggies (and then prepare them once at home) can be mind boggling. But when nutrition is the deciding factor, what’s the best way to get the biggest bang per nutritional buck?
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Uncooked, fresh produce is typically thought to be the most nutritious — but it does vary from food to food. One study showed higher levels of lutein, a carotenoid that can prevent macular degeneration and eye problems, in uncooked spinach when compared to cooked . Foods with high amounts of vitamins B and C are best fresh because these vitamins are water-soluble, meaning they can be leeched from the food during processing. But the nutritional quality in fresh versus frozen varieties also depends on the type of veggie. Brassica veggies — like cabbage, kale, Brussels sprouts, and cauliflower — are typically better fresh than frozen because they retain more phytochemicals and antioxidants, which studies suggest may actually help prevent certain cancers .
But this doesn't mean that fresh produce is immune to nutrient loss — and the farm-to-grocery store process may be to blame. The freshness of a ripe tomato or strawberry isn't measured from when it hits the grocery store shelf — it begins right after harvesting. Once a fruit or veggie is harvested, it begins to release heat and lose water (a process called respiration), impacting its nutritional quality. Between pest-controlling sprays, transportation, handling, and plain ol' time, fresh produce at the store might have lost roughly half it's original amount of nutrients.
Fortunately for those who rely on convenient bags from the freezer aisle, studies suggest frozen veggies have just as many nutrients (if not more) as their fresh counterparts  . Fresh fruits and veggies produce enzymes (trypsin and chymotrypsin) that cause loss of color, flavor, and nutrients just after harvest. But the reaction can be stopped by deactivating the enzyme — which freezing can do — leaving the frozen veggies with more nutrients  . (When done right, that is — the storage process can also cause some nutrients to be lost because of oxidation.) Foods that are best frozen are those with high amounts of fat-soluble nutrients, like vitamin A, cartenoids, and vitamin E, because they’re more stable during food processing and storage (like blanching and freezing). However, be careful with the prep: Studies suggest thawing frozen veggies before cooking can actually speed up vitamin C losses in frozen peas, spinach, okra, and green beans .
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The bag is mixed on whether cooked veggies are any "worse" nutritionally. Some research suggests cooking bright veggies (like tomatoes, carrots, and sweet potatoes) can also obliterate essential nutrients (specifically, carotenoids) . But other studies show the overall nutritional quality of a cooked veggie depends on the type of cooking. Despite the common notion that cooking kills nutrients, boiling or steaming some veggies (like broccoli and carrots) may actually boost levels of free radical-trapping antioxidants and phytochemicals like carotenoids and polyphenols . However, methods like stir-frying have been found to deplete key nutrients like chlorophyll, protein, and vitamin C in broccoli .
The best bet for cooking veggies to get the most nutrients for your buck? Go easy on the temperature and cooking time, and limit the amount of water you use for cooking veggies loaded with vitamins B and C (remember, water soluble vitamins will quickly vanish in the presence of H2O). And here's some good news for the microwave chef: Studies suggest microwaves have little effect on the nutritional quality of fruits and veggies, much like conventional ovens .
- Foods with high amounts of vitamins B and C are best fresh because the vitamins are water-soluble, meaning they’re usually dissolved in food processing. In the fresh produce section, go for bell peppers, citrus fruits, cabbage, and berries.
- Studies suggest frozen veggies have just as many nutrients as (if not more than) their fresh counterparts.
- Foods that are best frozen are those with high amounts of fat-soluble nutrients, like vitamin A, carotenoids, and vitamin E, because they’re more stable during food processing and storage (like blanching and freezing). So when hitting the frozen aisle, go for carrots, leafy greens, and broccoli.
- Boiling or steaming some veggies (like broccoli and carrots) may actually boost levels of free radical-trapping antioxidants and phytochemicals like carotenoids and polyphenols.
- The best bet for getting the most nutritional value out of cooked veggies is to go easy on the temperature and cooking time, and cook with little to no water.
Photo by Caitlin Covington
Originally posted February 2012. Updated March 2013.
What's your take: fresh or frozen produce? Share your opinions in the comments below or tweet the editor @ksmorin!