By: Zoe Blarowski April 18, 2016
It may seem like a natural assumption that fresh fruits and vegetables are more nutritious. But research shows this can be a false assumption. Frozen fruits and vegetables can be more nutritious than their unprocessed counterparts.
Brief Overview of Commercial Freezing Processes
Vegetables are typically blanched prior to freezing. This means they are submerged in boiling water for a few minutes. The purpose of blanching is to inactivate any naturally occurring enzymes that cause deterioration of the vegetables.
Fruit are usually not blanched prior to freezing. They are washed and frozen directly because of their delicate textures and higher acidity. This acidity will naturally slow down their deterioration process and help preserve them during freezing.
Both vegetables and fruit are frozen using modern technology that can freeze them very quickly. This prevents large ice crystals from forming inside the food and damaging cell walls. Fast freezing allows the food to retain its maximum flavor, texture and color after thawing.
What Research Tells Us
Various studies have been done on how processing and storage affects frozen fruits and vegetables compared to fresh. There are some consistent findings throughout much of the current research available.
Vitamin C – Freshly picked vegetables contain the highest amount of vitamin C. However, vitamin C begins to degrade immediately after harvest. For example, green peas have been shown to lose up to 51 percent of their vitamin C during the first 24 to 48 hours after harvesting.
Vitamin C is also sensitive to heat and water-soluble, so there is often some vitamin C lost during the blanching process for vegetables. On average, vegetables lose about 50 percent of their vitamin C during the freezing process.
This may sound like a lot, but this is often much better than a fresh vegetable. For instance, frozen spinach will lose 30 percent of its vitamin C after processing and one year of storage. Whereas, fresh spinach will lose 75 percent of its vitamin C after four days of storage at 4°C (39°F), a typical refrigerator temperature.
Because fruit is not heat processed prior to freezing, it retains vitamin C much better than frozen vegetables or fresh fruit. For instance, one study found that frozen blueberries and raspberries actually had significantly higher amounts of vitamin C than fresh berries stored for 3 days at 4°C (39°F).
B Vitamins – Like vitamin C, B vitamins, such as thiamin, riboflavin and folate, are also sensitive to heat and water-soluble. Not surprisingly, research results are similar to vitamin C. They showed a loss of B vitamins during the initial blanching process for vegetables, but the remaining vitamin B in the frozen, stored products remained stable and higher than fresh over time.
Vitamin A and Carotenoids – The primary source of vitamin A in fruits and vegetables is beta-carotene. Other carotenoids, such as lycopene, are also important for health.
Vitamin A and carotenoids are fat-soluble. This means they are not as sensitive to blanching and heat-processing methods. Especially in fruit, vitamin A and carotenoid levels appear to remain relatively similar after freezing compared to fresh.
In vegetables, the levels of vitamin A and carotenoids varies depending on the food. Frozen green peas are shown to have significantly less beta-carotene than fresh peas, whereas frozen broccoli and carrots have been found to have significantly higher amounts of beta-carotene than fresh.
Minerals – Naturally-occurring minerals, such as calcium or potassium, are stable when heated. Some mineral content of vegetables may be lost during blanching, but retention is generally high, ranging from 78 to 91 percent of the minerals staying in the frozen food. Fruit can be even higher as they are not blanched.
Antioxidants – These are also known as phenolic compounds. Antioxidants are a vital part of our diet that can counteract the harmful effects of aging. Fruits and vegetables contain hundreds of different types of phenolic compounds.
The blanching process is actually beneficial for phenolic compounds. They are naturally oxidized by the enzymes found in fresh vegetables, so inactivating them through a heat-treatment preserves the antioxidant levels in frozen vegetables.
In fruit, freezing is shown to have a minimal effect on phenolic compounds. Interestingly, frozen blueberries and some varieties of raspberries have actually been found to have a higher amount of antioxidants than fresh.
Other Important Considerations
The story doesn’t end simply at nutrient comparisons. Other factors also affect the final nutritional value of your fruits and vegetables.
Cooking – If you cook fresh or frozen fruits and vegetables while preparing them for your meals, this will continue to degrade the heat-sensitive vitamins.
Age of Fresh Produce – Unless you’re lucky enough to live in an area with lots of local farms that can grow fresh produce year-round, you’re likely buying your fruits and vegetables from a supermarket that’s shipped them in from hundreds or thousands of miles away. You don’t know when the fresh produce you’re buying was harvested or how long it’s been deteriorating in storage.
Picking Time – Fruits and vegetables have the highest nutritional value at their peak ripeness. Unfortunately, many types of produce are difficult to ship when they’re fully ripe because they’ll spoil easily. Many varieties are picked when they’re not ripe to simplify transport. These will have lower nutrient contents than frozen produce that is picked and processed when it’s at the perfect ripeness.
How to Maximize Your Frozen Fruits and Veggies
Commercially, frozen foods are kept at -18°C to -20°C (0.4°F to -4°F). This storage temperature has been shown to keep them at their best nutrient content for one year or longer. If they are stored at temperatures any warmer than this, frozen produce will start to slowly deteriorate.
The temperatures in a home freezer can vary depending on the freezer’s age, how often it’s opened, or its efficiency. Keep an eye on the temperature in your own freezer and try to use your frozen produce within one year or less to make sure it’s still at peak nutrition.
To use frozen fruits or vegetables, it’s best to put them directly into whatever dish you’re preparing. Leaving them out to thaw for any length of time at room temperature will cause them to oxidize and lose further nutrients.