Because of the coronavirus pandemic, many of us are limiting our trips to grocery stores by buying more canned and frozen goods since these have a longer shelf life than fresh produce. That has led to questions about the nutritional profile of canned and frozen goods. In this post, we’ll explore questions about frozen and canned goods, including how they’re produced, their nutritional profile, the liners used in canned goods, and their shelf life. We also examine the importance of frozen and canned food within the context of food security. As always, we write this with the awareness that having the ability to pick and choose is a privilege that not all of us have.
How are frozen fruits and vegetables produced?
The first step is blanching – steaming or boiling the produce for a short time – to preserve the fruit or vegetable by destroying pathogens that might shorten its shelf-life when frozen. Blanching also helps maintain color and flavor. The amount of time for blanching varies and depends on the item’s consistency and size. Blanching for too long will lead to nutrient loss. The blanching process ends with abrupt cooling, usually in a quick ice bath. Getting the produce from hot to cold quickly is important to prevent contamination by microorganisms and pathogens.
Second, the fruits/vegetables are drained, weighed, and packaged. According to the Food and Agriculture Organization of the United Nations, the packaging material should have “low permeability to both moisture and oxygen.” Finally, the fruits/vegetables are quickly frozen to prevent ice crystal formation, which can damage the produce. The produce makes its way through freezing chambers and freezing tunnels on trays or conveyor belts, which ensure an ideal distribution of cold air. Once frozen, the produce is further packed in thick secondary packing material such as cardboard to prevent ice crystal formation caused by temperature fluctuations. We usually don’t see this secondary packing material, because it gets removed at the grocery store.
How are canned fruits and vegetables produced?
We’ve been canning food since the 1800s, when brewer and confectioner Nicolas Appert answered a call from the French government for inventions to prolong the shelf-life of foods for the military. Canning consists of processing food and sealing it in an airtight jar or a can. It destroys any microorganisms that could spoil the food and, when done correctly, prevents recontamination.
Canning was first done in glass jars. Due to weight and broken glass hazards, tin cans were developed for commercial use. Today, most cans are tin-coated steel, and vacuum pouches (like those used for juice pouches) are increasingly used. Beverages can also be stored in aluminum cans, which are lighter and rust-resistant.
The canning process can be summarized as follows:
- Fruits and vegetables are washed, and sorted by ripeness and size.
- If needed, peeling and coring come next, with boiling, mechanical peelers, or lye.
- Cans or glass jars are washed with hot water and/or steam. The containers are then filled with the fruit/vegetable and topped up with liquid or syrup. These liquids have the sugars, spices, and salt needed to preserve the food and give it flavor.
- The container is then exhausted to remove any air. This helps preserve the food, extends shelf-life, and prevents bulging at high altitudes.
- A vacuum is created by heating or steaming the container or produced mechanically in a vacuum chamber.
- Cans are then double sealed and lids on jars are sealed.
- The containers are heat sterilized and then cooled. They are finally labeled and packaged.
Fruits and vegetables go through similar canning processes. However, fruits do not usually undergo the blanching or cooking step. Some fruits require additional processing such as depitting or peeling.
Why is it difficult to compare the nutritional profile of canned, frozen, and fresh fruits and veggies?
Before we delve into the actual data, it’s important to understand why there’s conflicting information on the relative nutritional value of fresh, canned, and frozen produce:
- Nutritional decay of fresh produce: Fresh fruits and vegetables lose nutrients as they sit on grocery shelves or in our fridge. Consequently, studies need to mimic the entire food supply-chain when they compare canned/frozen to fresh, rather than using fruits and vegetables that were just picked. As consumers, we should also consider this decay when we consider our purchasing habits. If we’re going to buy a head of broccoli but just leave it in the fridge for a week before cooking it, then we may want to consider purchasing frozen.
- Food preparation: Nutritional profiles of fresh produce can vary depending on how it’s prepared: baked, steamed, stewed, sauteed. So when studies compare the nutrient level of frozen/canned to food prepared at home, these are become further complicated by the many ways we prepare food at home. While the USDA has a dataset documenting nutrient retention for food prepared at home by different methods, there are gaps in this dataset that make it less useful than we would hope for making these comparisons. While it is difficult to find specific comparisons, we do know a few things in general:
- Boiling vegetables or fruits in lots of water leads to more nutrient loss. If you have to boil the produce, consider boiling in less water or find creative ways to use the cooking water.
- In many cases, steaming or microwaving leads to less nutrient loss, although this isn’t always the case.
It must be stressed, as noted by the USDA’s dataset, that all forms of fruits and vegetables are an important source of nutrients, even when prepared using nutrient-leaching methods.
- Fruit and vegetable varieties: The nutritional content of different varieties of fruits and vegetables vary. For example, green apples and red apples have different vitamin and sugar content. With dozens of apple varieties available, this comparison is very complicated. Even within a single variety, the location where the crop was grown can impact nutrient content. Consequently, getting identical fruits or vegetables, where the only variable is how the fruit/vegetable was stored, is challenging.
- Food processing: We know that boiling or blanching fruits or vegetables can lead to the loss of certain nutrients, particularly water-soluble ones (see below). As previously outlined, the canning and freezing process usually involves a boiling step. But for some produce, we also include a boiling step at home. For example, we usually boil or steam corn. Therefore, to compare the nutritional content of canned and fresh corn, we should compare canned corn to boiled fresh corn, instead of raw corn. However, many studies compare canned and frozen produce with their raw counterparts. When it comes to corn, this may seem obvious, but when it comes to canned tomatoes or frozen spinach, it’s a step that studies may not include.
Are canned and frozen fruits and veggies as nutritious as fresh produce?
The answer is: “it depends”, but overall, when fruits and vegetables are cooked the same way they are in our homes, then they are roughly equivalent.
For the sake of simplicity, I divided nutrients into several groups: water-soluble (using Vitamin C as an example), lipid-soluble vitamins, minerals, and fiber.
Water Soluble Vitamins – Vitamin C
Here are some important points about Vitamin C in canned and frozen produce:
- This in depth review highlights that the loss of vitamin C in canned and frozen food can range anywhere between 10 and 90%. Since this is a water-soluble nutrient, most of the loss happens when fruits and vegetables are boiled, whether it’s part of the blanching process to freeze/can foods or boiling fresh produce at home. So it’s important to avoid over-boiling produce or to try using the water that produce was boiled in.
- For products that are not cooked during the freezing process, such as spinach or peas, frozen products tend to have more vitamin C than canned, since the latter are blanched.
- Tomatoes experience a 30% loss in vitamin C when they’re boiled. But canned and fresh-boiled tomatoes actually have fairly equivalent vitamin C levels if, for example, you make tomato sauce where the water the tomatoes are boiled in is used.
- Frozen goods that are exposed to oxygen will lose vitamin C more quickly than frozen goods that are properly stored. In contrast, once a canned good has been processed, it keeps its vitamin C content over time. So make sure you repackage your frozen produce tightly once open: not only will they keep their nutrients better over time but they’ll also suffer from less cosmetic damage like freezer burn.
Lipid Soluble Vitamins
Lipid soluble vitamins include vitamin A, vitamin E, and carotenoids. Here are some important points about lipid soluble vitamins in frozen and canned goods:
- Since these vitamins are not water soluble, they are not lost in the cooking process like vitamin C, whether this cooking occurs in the prep of canning or freezing or at home.
- Vitamin A does oxidize over time when exposed to air. So when you open a bag of frozen produce, make sure you pack it tightly.
- In many studies, an increase in vitamin A/carotenoids was observed in canned produce. This may be because the boiling and blanching process break down plant cell components, making the vitamin more accessible and measurable. The caveat is that excessive heat may actually degrade vitamin A and carotenoids. These observations were also noted for vitamin E.
Similar to water-soluble vitamins, some minerals including calcium, potassium, and sodium can be lost in the cooking or boiling process. Since minerals can be lost in the boiling process, home cooking of fresh produce can actually lead to more mineral loss than what is seen in frozen/canned goods, since canned and frozen goods seldom over-boil the produce (if you’re like me, you might be boiling carrots and potatoes to mush, so that they can puree more easily. I probably have to revise this practice). As previously noted, consider boiling in little water and using the water if you cook at home.
Once a canned good has been processed and packaged, its mineral content can actually increase due to absorption of minerals from the brine. However, this is not always a good thing. For sodium, its absorption in canned goods may increase levels and may be a concern for people who need to limit their salt intake. However, there is some evidence suggesting that rinsing canned goods can eliminate excess sodium. If increased sodium in your diet is a concern, consider purchasing canned goods that are labeled as “low-sodium” or “no salt added”.
Medical organizations are quite concerned that we’re not getting enough fiber in our diet. The good news is that canned and frozen foods have roughly the same amount of fiber than their fresh counterparts. Roughly the same amount of roughage 🙂
Canned goods are an easy and quick way to increase fiber in one’s diet. This is particularly true when it comes to beans, since dried beans are time consuming to prepare. A bean salad or chili can be quickly made using canned beans.
How long is frozen food safe?
According to the USDA Food Safety Information, frozen food can be stored indefinitely *if stored appropriately*. The problem is, most foods aren’t stored appropriately: they remain exposed to air and temperature fluctuations every time we open our freezers. Consequently, most frozen food gets freezer burned and discolored over time. But food that is freezer burned is still safe. We just don’t like it because of the change in texture and appearance. There are many simple ways to use freezer burned fruits and vegetables, like in soups and smoothies.
How long is canned food safe?
According to the USDA, many canned foods are also good indefinitely, although there are exceptions. Canned meat, for example, cannot be stored indefinitely.
The USDA has specific recommendations regarding damaged cans. They recommend not using cans that are swollen and immediately discarding these cans. Swelling indicates that the contents are contaminated with the bacteria that causes botulism.
Dents can cause small cracks leading to bacterial contamination. The USDA does not recommend using deeply dented cans because it is difficult to identify swelling in these items. However, cans with small dents are still safe to use. The USDA distinguishes a deep dent as “one that you can lay your finger into. Deep dents often have sharp points.” The USDA also highlights that a dent along the seam of a can is particularly dangerous since it can crack the seam.
Cans with mild rusting are safe, but severe rusting can expose the canned good to bacteria and make it unsafe. Mild rusting is on the surface of a can, and can be removed by rubbing it with a paper towel.
Is the BPA lining in canned goods safe?
BPA (bisphenol A) is a chemical used to line the interior of canned goods to prevent rusting and corrosion. There are concerns about the safety of BPA, particularly that it can interfere with our body’s hormones. The FDA has funded multiple long term studies in rodents and primates to determine the safety of this product. The organization states that the amount of BPA that is found in canned food is minimal and safe at the levels found. This guidance also aligns with findings from Health Canada. Out of an abundance of caution, the FDA has banned the use of BPA in baby bottles and in formula packaging.
However, finding a safer alternative to BPA is not straightforward. When the BPA controversy first came to light, food producers quickly removed BPA, only to replace it with alternative and less studied chemicals (including other bisphenols). Manufacturers cannot just get rid of can liners because they are needed to prevent corrosion of the metal by the food or beverage inside, which would pose a food safety risk. The risks of some alternative liners are still unknown. But, researchers and food production companies are now cautiously optimistic that they have found replacements that can be soon used throughout the industry. (Article continues after infographic)
Is access to canned goods a solution to food insecurity
In the United States, canned and frozen food are often brought up as solutions to the issue of “food deserts”. The USDA defines a food desert as “areas where people have limited access to a variety of healthy and affordable food”. As such, food with long shelf life may seem like an easy solution that can help individuals in food deserts: canned and frozen fruits and vegetables can be a rich source of nutrients for individuals without regular access to fresh alternatives.
But access to canned and frozen foods will not necessarily improve food security on its own. The USDA highlights that “access” to fresh produce is not just the absence of grocery stores, but also the availability of economic resources to afford a healthful diet. In addition, many food deserts are found in urban areas where racial and economic structures have led to food insecurity. Consequently, building more grocery stores in rural areas or merely stocking more canned goods is not a solution to food accessibility, since it does not address the economic causes of food insecurity. The economic factors may still get in the way even if “access “is improved.
Frozen and canned goods of any variety, including off-brand, conventional, or organic, can be just as good and more affordable than fresh produce. But consumers beware: canned and frozen are not always cheaper, particularly when a crop is in season.
Among all the articles that I read, my biggest realization was how quickly nutrients in fresh produce decrease in our fridge. It made me shift my perception and realize that eating fresh produce soon after purchase will decrease food waste and also be more healthful. But this necessarily means more trips to the grocery store, which is not a good option during a pandemic. Even outside the pandemic, trips to the store require transportation and time, which are limiting for many individuals.
In general, canned and frozen foods carry a stigma that I hope this article helps alleviate. Canned goods are not “less than”. Frozen produce is not “worse”. They are nutritious alternatives, often picked at the peak of their ripeness. They are pre-processed, which can save energy, time, and can be greatly beneficial to individuals who are physically impaired, or don’t have the time or inclination to cook. If you struggle with the taste or texture of frozen and canned foods, there are many resources online that can guide you as you prepare them.
All forms of fruits and vegetables are an important source of nutrients, even when prepared using nutrient-leaching methods. Fruits and vegetables cannot be replaced by a pill or supplement. Most of us do not eat enough fruits and vegetables so finding ways to incorporate more produce – fresh, frozen, or canned – into our diets is extremely beneficial. Ultimately, if you opt for frozen or canned goods, you can rest assured that the produce you are offering your family is healthful and safe.