November 2015 Issue

Treasures of Frozen Produce
By Judith C. Thalheimer, RD, LDN
Today's Dietitian
Vol. 17 No. 11 P. 30

Nutritious, convenient, and cost-effective, frozen produce can help add more fruits and vegetables to clients' plates.

Low in calories, packed with underconsumed nutrients, and known to decrease risk of chronic diseases, fruits and vegetables are an essential part of any healthful dietary pattern, but Americans aren't getting enough.1

The USDA recommends adults aim for 21/2 to 3 cups of vegetables and 11/2 to 2 cups of fruits every day.2,3 A July 2015 analysis of national survey data indicates that, from 2007 to 2010, one-half of the US population consumed less than 1 cup of fruit and 11/2 cups of vegetables daily.4 A full 76% didn't meet fruit intake recommendations, and 87% didn't meet vegetable intake recommendations.4 And those numbers are getting worse. According to the 2015 State of the Plate, the Produce for Better Health Foundation's report on America's consumption of fruits and vegetables, per capita intake of fruits and vegetables in the United States has decreased 7% in the last five years.5 Fresh, frozen, canned, and dried fruits and vegetables, and juice all count toward intake.5 Of the fruits and vegetables eaten at home, only 1.6% of fruit and 14% of vegetables are frozen.5 Increasing the public's knowledge of the benefits and uses of frozen produce, however, could help decrease the gap between recommended and actual intake of fruits and vegetables.

The Freezing Process
To understand the pros and cons of frozen produce, it helps to have a basic knowledge of the freezing process. Commercial freezing of small fruits and berries began around 1905 in the eastern part of the United States. Freezing vegetables proved more difficult. Enzymes in the vegetables led to deterioration and off-flavors over time.6 "The enzymes that cause browning and deterioration are still active under commercial and household freezer temperatures. You'll need to get to -80º centigrade to inhibit activity," says Gene Lester, PhD, national program leader for food quality with the USDA Agricultural Research Service division of Nutrition, Food Safety, and Quality. It wasn't until 1929 that food scientists discovered that blanching vegetables before freezing disabled the enzymes.6 "Many people don't realize that vegetables have to go through a quick heating process before flash freezing to knock out these enzymes," Lester says. "This par-boiling or blanching also kills any microbes, so it's good for food safety." Most fruits aren't blanched before freezing.6

In 1928, a technologist named Clarence Birdseye launched the modern freezing industry by inventing a way to quick-freeze packaged foods. Birdseye's innovation led to the commercial large-scale freezing of fruits and vegetables.6 "The food industry continues to seek better technology to provide foods that are nutritious, safe, and high quality," says Roger Clemens, DrPH, CFS, CNS, FACN, FIFT, FIAFST, an adjunct professor of pharmacology and pharmaceutical sciences at the University of Southern California School of Pharmacy's International Center for Regulatory Science. One innovation is individual quick freezing (IQF), in which fluidized bed freezers use highly pressurized air to suspend small food particles, like blueberries, in the air while they freeze to prevent clumping.6 "Processes like IQF allow food to be harvested, washed, cut, blanched, frozen, and packaged within an hour of harvest," Clemens says.

"If you ever picked produce from your own garden and tried to freeze it, it can be a mess," says Barbara Ruhs, MS, RDN, owner of Neighborhood Nutrition. "Mine come out as one big block. The technology and the study of how to make frozen fruits and vegetables come out of the bag individually is pretty amazing."

"People may not realize that no chemicals are needed to freeze produce," Clemens says. "Vegetables are generally washed a few times in clean water and then frozen with no additives and no processing other than cutting and blanching."

"We usually think of packaged foods as chemically processed, but this is technology without chemicals," Ruhs says. "The preservative is the freezing process."

Frozen in Time
The key to the nutritional value of any produce, fresh or frozen, appears to be time. Fruits and vegetables are most nutritious when they're fully ripe, but the fresh produce destined for supermarkets is picked early so it isn't overripe when it arrives at the store. "Because fruits and vegetables in stores were picked before they were fully ripe, they had less time to develop a full spectrum of vitamins and minerals," says Mary Ann Moylan, RD, LDN, CDE, an in-store nutritionist for Ahold USA's Giant Superstore supermarket in Willow Grove, Pennsylvania. Foods that are going to be flash frozen are allowed to ripen fully. "The benefit of in-field processing is you can wait and pick things at their peak of ripeness," Lester says.

After produce is picked, the level of some nutrients begins to decline. A study of antioxidants in fresh vs frozen produce conducted for the British Frozen Food Federation found that levels were similar in freshly picked and frozen produce, but antioxidants in fresh produce declined over time, reaching a low point in as little as three days.7 The quick turnaround time from field to freezer not only allows foods to ripen fully but also eliminates the delay that can lead to loss of nutrients. "The nutrients in frozen produce are frozen in time," Ruhs says.

But while fresh vegetables lose some nutrients over time, frozen vegetables may lose some in the blanching process. Blanching denatures enzymes that have deleterious effects on vegetables, but it also impacts water-soluble nutrients. According to Lester, any heating is detrimental to vitamin C content. "Ascorbic acid, also known as vitamin C, is one of the most heat-sensitive compounds," Lester says. "For C, it's not the boiling water, it's the exposure to heat. Even steaming causes losses of this powerful antioxidant. Other water-soluble nutrients, like B vitamins, are less affected. As long as you're not soaking, even cut vegetables, very little loss of B vitamins and carotenoids will occur during quick blanching or steaming."

While blanching reduces vitamin C content, C levels still may be higher in frozen produce than in fresh that has been sitting in a market. "Vitamin C is very prone to degradation by enzymes naturally found in foods," Clemens says. "C content is actually greater in many flash-frozen foods than that found in foods in a store's perimeter or farmers' market because the blanching denatures the enzymes that would typically break it down over time."

Denaturing enzymes protects produce from browning and degradation and stops the breakdown of vitamin C, but it also can produce less desirable effects. According to research by Dosz and Jeffery, the enzyme responsible for the cancer-fighting power of broccoli is destroyed during blanching. The enzyme, myrosinase, is responsible for the formation of sulforaphane, a cancer-preventive agent. Interestingly, their research discovered that the addition of small amounts of daikon radish to frozen broccoli provided the myrosinase necessary to kick-start sulforaphane formation once the broccoli thawed. Until commercial processors take their advice, home cooks can sprinkle frozen broccoli with raw cruciferous vegetables like radish, cabbage, or even spicy mustard before cooking.8,9

Fresh vs Frozen
The Frozen Food Foundation conducted studies with the University of California, Davis, and the University of Georgia to compare the nutrient content of fresh and frozen produce. "Both studies reveal that frozen fruits and vegetables are just as rich in nutrients as fresh, and often more so," says Adrienne Seiling, executive director for the Frozen Food Foundation.10,11 Similarly, a 2015 study published in the Journal of Agriculture and Food Chemistry compared levels of vitamin C, riboflavin, alpha-tocopherol, and beta-carotene over time in fresh vs frozen corn, carrots, broccoli, spinach, peas, green beans, strawberries, and blueberries. Overall, the vitamin content of the frozen produce was comparable to or higher than that of the fresh, except for β-Carotene, which was lower in frozen peas, carrots, and spinach, and riboflavin, which was lower in frozen peas and broccoli.12

The message that frozen produce can be just as nutritious as fresh produce on store shelves seems to be getting through to consumers. "According to an opinion poll we conducted in April 2015, 69% of Americans 18 and older feel that frozen foods are just as nutritious as fresh ones," Seiling says.

Cost and convenience are other issues of concern to consumers. Moylan sees customers who are reluctant to buy fruits and vegetables because they think they're more expensive than other foods, or they think they're time-consuming to prepare, "but that isn't really accurate," Moylan says. "Frozen fruits and vegetables can be very economical, and they can really save time in food preparation." Ruhs has had similar experiences with her clients. "For those who may live alone or who don't really like to cook, frozen produce can be a big help," Ruhs says. "Frozen vegetables are already prepped and cook in a few minutes. Plus, there are always sales and coupons in the frozen food aisle."

In 2011, the USDA published a cost comparison of fresh, frozen, and canned produce. The study (based on 2008 numbers) concluded that neither fresh nor processed fruits and vegetables are consistently cheaper. Frozen raspberries, for example, were less expensive per edible one-cup equivalent than fresh, but frozen strawberries were more expensive. Leafy greens like spinach and kale were more economical in the frozen section. Taking all available forms of produce together, the study concluded that it was possible to eat 2 cups of fruit and 21/2 cups of vegetables per day for between $2 and $2.50.13

Waste is another factor affecting the food budget. Studies show that 31% to 40% of the postharvest food supply in the United States is wasted. In a consumer survey published in PLoS One in 2015, respondents indicated they throw out more fruits and vegetables than other foods.14 "Since frozen produce won't spoil," Ruhs says, "you're not wasting food or money."

Cooking With Frozen Produce
"Frozen produce allows consumers to prepare a wider variety of foods with little effort, so they can get more fiber and phytochemicals in their diet," Ruhs says. "There's so much variety and so much innovation, from time-savers like prediced onion to medleys of different colored carrots to blends with whole grains and beans. And veggies in sauces can appeal to almost any palate, offering an opportunity for people to try new tastes without the investment in rarely used ingredients and cooking time."

The USDA's MyPlate dietary guidance recommends covering one-half the plate at every meal with fruits and vegetables.15 Moylan offers her clients suggestions for how frozen produce can help them reach this goal. "Frozen fruit is great year-round for tossing into parfaits; topping cereal, yogurt, or cottage cheese; adding to mixes to make more nutritious muffins or pancakes; or, of course, blending into smoothies," Moylan says. "Frozen veggies work great in smoothies too, and frozen stir-fry combinations are a wonderful addition to egg dishes at breakfast." For lunch and dinner, Moylan recommends tossing frozen vegetables into stir-fries, homemade soups, casseroles, stews, sauces, lasagnas, and even stuffing. She recommends adding frozen vegetables toward the end of cooking, since they don't take long to cook.

Frozen produce can play an important role in increasing fruit and vegetable intake throughout the day. "People really need to rethink the frozen food aisle," Ruhs says. "Frozen produce is a great choice for many reasons: it's nutritious, it's an affordable option, it's convenient and quick, and it limits food waste because it won't spoil."

Clemens adds, "Frozen and fresh produce can coexist in a healthful dietary pattern."
Lester agrees: "Fresh is always good, but with the convenience of frozen, there's no reason why you can't have all your recommended servings of fruits and vegetables every day." By explaining the benefits and nutritional value of frozen produce and offering simple tips for working frozen fruits and veggies into everyday cooking, nutrition professionals can help clients and patients increase their fruit and vegetable intake and improve their overall health.

— Judith C. Thalheimer, RD, LDN, is a freelance nutrition writer, a community educator, and the principal of JTRD Nutrition Education Services.

1. US Department of Agriculture, US Department of Health and Human Services. Dietary Guidelines for Americans, 2010. 7th ed. Washington, DC: US Government Printing Office; 2010.

2. All about the vegetable group. website. Updated August 19, 2015.

3. All about the fruit group. website. Updated July 27, 2015.

4. Moore LV, Thompson FE. Adults meeting fruit and vegetable intake recommendations – United States, 2013. MMWR Morb Mortal Wkly Rep. 2015;64(26):709-713.

5. State of the plate: 2015 study on America's consumption of fruit and vegetables. Produce for Better Health Foundation website.

6. Barbosa-Cánovas GV, Altunakar B, Mejía-Lorío DJ. Freezing of fruits and vegetables: an agribusiness alternative for rural and semi-rural areas. Food and Agriculture Organization of the United Nations website.

7. Antioxidants in fresh and frozen fruit and vegetables: impact study of varying storage conditions. University of Chester website.

8. Quick D. Researchers preserve cancer-fighting properties of frozen broccoli. Gizmag website. August 7, 2013.

9. Dosz EB, Jeffery EH. Modifying the processing and handling of frozen broccoli for increased sulforaphane formation. J Food Sci. 2013;78(9):H1459-H1463.

10. Rickman JC, Barrett DM, Bruhn CM. Nutritional composition of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds. J Sci Food Agric. 2007;87:930-944.

11. New study encourages consumers to think frozen when buying fruits & vegetables. Frozen Food Foundation website. Updated November 14, 2013.

12. Bouzari A, Holstege D, Barrett DM. Vitamin retention in eight fruits and vegetables: a comparison of refrigerated and frozen storage. J Agric Food Chem. 2015;63(3):957-962.

13 Stewart H, Hyman J, Buzby JC, Frazáo E, Carlson A. How much do fruits and vegetables cost? EIB-71, US Department of Agriculture, Economic Research Service. Published February 2011.

14. Neff RA, Spiker ML, Truant PL. Wasted food: U.S. consumers' reported awareness, attitudes, and behaviors. PLoS One. 2015;10(6):e0127881.

15. MyPlate. website. Updated July 28, 2015. 


Apple Chicken Stir-Fry

Serves 4

1 lb cubed boneless, skinless chicken breast
1 T vegetable oil
1/2 cup onion, vertically sliced
13/4 cup frozen carrots, thinly sliced
1 tsp dried basil, crushed
1 cup frozen Chinese pea pods
1 T water
1 medium baking apple, cored, and thinly sliced
2 cups cooked brown rice

1. Stir-fry cubed chicken breast in vegetable oil in nonstick skillet until lightly browned and cooked. Remove from skillet.

2. Stir-fry onion, carrots, and basil in oil in same skillet until carrots are tender. Stir in pea pods and water; stir-fry two minutes.

3. Remove from heat; stir in apple. Add to chicken. Serve hot over cooked rice.

Nutrient Analysis per serving
Calories: 330; Total fat: 7 g; Sat fat: <1 g; Trans fat: 0 g; Cholesterol: 66 mg; Sodium: 116 mg; Total carbohydrate: 31 g; Dietary fiber: 5.5 g; Sugars: 6 g; Protein: 30 g


10-Minute Veggie Soup

Serves 6

2 29-oz cans low-sodium chicken broth
1 14.5-oz can diced tomatoes, no salt added
1 tsp dried basil
1/8 tsp salt
1/8 tsp pepper
1/2 tsp onion powder
3/4 cup macaroni, dry
3 cups frozen mixed vegetables (broccoli, cauliflower, and carrot mix works well)

Combine chicken broth, diced tomatoes, basil, salt, pepper, and onion powder. Bring to a simmer and add pasta and frozen vegetable mix. Cook for six minutes and remove from heat.

(Note: Pasta won't be cooked all the way through. Let soup sit for five minutes, and pasta will become soft. Soup will then be ready to serve.)

Nutrient Analysis per serving
Calories: 122; Total fat: 2 g; Sat fat: <1 g; Trans fat: 0 g; Cholesterol: 0 mg; Sodium: 151 mg; Total carbohydrate: 19 g; Dietary fiber: 2 g; Sugars: 5 g; Protein: 9 g