Last canning season, a frustrated reader told us he couldn’t find answers to the following questions: Does cooking vegetables remove nutrients? And are canned vegetables healthy for us? Here’s detailed information on how the canning process affects the nutritional quality of the foods we eat.
People preserve their home-grown produce for a multitude of reasons, including to save money, but largely because they believe the answer to the question “Are canned vegetables healthy?” is a resounding “Yes!” Health-conscious people grow gardens to optimize the nutritional quality of the produce they eat by harvesting at peak ripeness and then canning it for their home pantries. But canning, especially pressure canning of low-acid produce, involves high heat. So, does cooking vegetables remove nutrients, and are home-canned vegetables healthy?
Nutritional Benefits of Fresh Food
Let’s begin by considering the nutritional quality of fresh, unprocessed food, which possesses both macronutrients and micronutrients. Macronutrients are proteins, carbohydrates, and lipids or fats. These are chemically and structurally determined and incredibly difficult to change. The primary ways they can be altered is through the breaking down of starches or denaturing of proteins in high heat.
Here, I’ll primarily focus on micronutrients — minerals, vitamins, and phytochemicals. Minerals are inorganic and typically pure elements, such as calcium, iron, and magnesium. Plants get minerals from the soil in which they’re grown, absorbing them as they take up water. Healthy soil produces nutritious plants.
Vitamins, on the other hand, are organic substances we need but cannot make ourselves. Plants are able to synthesize many vitamins that our bodies can, in turn, use after we eat the plants.
Phytochemicals are another micronutrient, although not as well-researched as vitamins and minerals because of their complexity. Phytochemicals are plant chemicals with potential health benefits to humans. Research is continually being done to identify phytochemicals and their roles. Phytochemicals may serve as antioxidants, enhance immunity, intensify cellular communication, detoxify carcinogens, cause cell death in tumor cells, and repair damaged DNA. Made by the plants themselves, they typically protect the plant from sun damage, insects, drought, and microorganisms. Phytochemicals are also responsible for much of the flavor and color of the produce we eat.
Home Canning Nutriution: Are Canned Vegetables Healthy?
Fully ripe — but not overripe — produce holds the highest nutritional content. Vitamin E, which is a collection of eight fat-soluble compounds that work the same in the human body, is especially high at peak ripeness. In peppers, researchers found that vitamin E content increased as chlorophyll degraded, allowing the color of the peppers to show.
Luckily, it’s not all bad news when we consider the answer to the following question: Does cooking vegetables remove nutrients? Some nutrients are made more bioavailable — that is, able to be used by the human body — when the food is cooked or processed. This is especially true of phytochemicals, because the heat from processing breaks up their cellular matrix, making it easier for our bodies to digest and absorb the nutrients. Some foods that become more bioavailable after cooking include spinach, carrots, and sweet potatoes. Besides some phytochemicals, iron, calcium, and potassium are made more easily digestible when the vegetable containing them is cooked.
Does Cooking Vegetables Remove Nutrients?
On the other hand, water-soluble vitamins are quite sensitive and can degrade easily. High heat through cooking or processing tends to damage these vitamins, lowering the amount present in the food. These include vitamin C and the B vitamins, such as thiamin, riboflavin, B6, and folate. (Vitamin B12 is synthesized by certain bacteria and therefore doesn’t occur in most plant products.) High heat can lower a food’s vitamin C content by 50% or more. Some studies suggest that canning can reduce vitamin C by up to 90% in some vegetables. Studies also suggest heat can destroy at least 10% to 30%, and up to 70%, of the B vitamins in produce. But more is typically leached into the aqueous solution in which the food is stored (see “Does Canning Liquid Affect Vegetable Nutrients” below). Regarding root vegetables, canning heat causes the loss of approximately 10% to 20% of available phosphorous, thiamin, beta-carotene, beta-cryptoxanthin, lycopene, and lucein and zeaxanthin. Folate and vitamin C were reduced 20% to 25%. The losses of vitamin C, thiamin, and folate in other vegetables fell in the range of 10% to 15%.
Tomatoes experience much lower nutrient loss due to heat, partly because they’re usually processed in a hot water bath (which their high acidity allows) rather than pressure canned. They lose mostly folate (70%) and choline (10%). Raw tomatoes are considered a good source of vitamin C, potassium, folate, and vitamin K.
Are canned vegetables healthy for us? Luckily, some nutrients that are more stable in heat are niacin, pantothenic acid, and biotin. Minerals such as calcium, magnesium, iron, sodium, zinc, and copper aren’t affected by heat, so their content in food doesn’t change with heat processing. Any mineral loss in canned food is likely due to minor leaching by the water in which the food is processed.
Because low-acid foods must reach temperatures of 240 to 250 degrees Fahrenheit during pressure canning to kill Clostridium botulinum spores, vitamin loss is unavoidable because of high heat.
Does Canning Liquid Affect Vegetable Nutrients?
Water-soluble vitamins tend to dissolve into the canning medium; that is, the liquid in which the produce is canned and stored.
Root vegetables are typically high in potassium, folate, manganese, vitamin C, beta-carotene, and many B vitamins. In addition to the nutrients lost through the heat of processing, root vegetables lose an additional 10% to 15% of vitamin C, thiamine, and folate, as well as a few minerals, into the canning medium. Other vegetables lose an additional 10% of vitamin C, thiamin, and niacin. Folate is decreased by an additional 20%. Beta-carotene, lycopene, lutein and zeaxanthin, and riboflavin are only marginally reduced by the canning medium.
Legumes are considered a good source of B vitamins, iron, copper, magnesium, manganese, zinc, and phosphorous. Legumes are unique because their nutrition is different from other vegetables. They leach 10% to 20% of their inherent calcium, zinc, phosphorous, sodium, beta-carotene, beta-cryptoxanthin, lycopene, and lutein and zeaxanthin into the canning medium. They lose 25% to 35% of iron, magnesium, potassium, riboflavin, and vitamin C, and significant amounts of copper (45%), B6 (50%), thiamin (60%), and folate (70%).
Fruits are quite varied, but they generally lose 10% to 20% of potassium, copper, thiamin, riboflavin, niacin, and vitamin B6. They lose 25% of beta-carotene, beta-cryptoxanthin, lycopene, and lutein and zeaxanthin. Folate and vitamin C decrease by about 50%.
Do Canned Foods Lose Nutrition Over Time?
Vitamin C degrades drastically over time, as do many phytochemicals. Anthocyanin, an antioxidant that often gives berries and other produce their deep blue, purple, or red coloring, was found to be particularly susceptible to time as well as heat. Phytochemicals change over time. Some degrade after produce is harvested, while some increase for several months during storage of the processed food. Hundreds of phytochemicals have been identified, with more being discovered and researched each year. The study of phytochemicals is still a relatively new subject in food nutrition and not well-understood. Beta-carotene, beta-cryptoxanthin, lycopene, lutein and zeaxanthin, and other carotenoids tend to be more bioavailable after cooking or processing. Isoflavones are fairly heat-resistant, as are the stilbenes found in some berries, such as blueberry. Many fruits, vegetables, and grains have some isoflavones, and soy and legumes are particularly rich in these compounds. Another heat-resistant phytochemical is the lignans found in nuts and legumes, and some flavonoids in apples, berries, and cherries actually increase with heat.
Subcategories of phytochemicals can contain both heat-resistant and heat-sensitive species. These include the proanthocyanidins found in fruits and some vegetables and the isothiocyanates in cruciferous vegetables. The sterols found in some plant oils aren’t heat-resistant and tend to oxidize in high temperatures.
The temperature at which you store your home-canned food matters. In one study, certain nutrients in black currant jam were monitored after a storage time of 13 months. The jam stored at 46 degrees retained vitamin C and anthocyanins better than jam stored at room temperature. Jams stored at 99 degrees didn’t retain any vitamin C at all.
Summation: Does Cooking Vegetables Remove Nutrients?
Light, heat, oxygen, and moisture content are the factors that most affect how nutrients hold up over time in storage. Little oxygen remains in jars following the canning process, so this is less of a factor in nutrient oxidation. Moisture content is also set and unchangeable. Focus on the factors you can control: harvest and storage. Harvest produce at peak ripeness and process it immediately. Store home-canned foods out of direct light and in consistently cool temperatures.
But are canned vegetables healthy, in general? Unless all the food you consume has just been harvested, you’ll be losing its nutrients. Allowing it to sit on the counter for several days without processing will significantly reduce produce’s inherent nutrients. While this may feel discouraging, you’re probably still getting plenty of nutrients by eating any fruits and vegetables, whether processed or fresh. Definitely don’t skimp on safe canning practices in an attempt to preserve vitamins. Losing a little vitamin C is much better than suffering botulism poisoning!
Rebecca Sanderson grew up in a small town with a backyard full of animals. Her background in health and nutrition gives her a unique perspective on animal husbandry and her own gardening and food preservation.
Originally published as “Preserving Nutrition” in the August/September 2023 issue of MOTHER EARTH NEWS magazine and regularly vetted for accuracy.
