Enzymes in vegetables trigger their growth and deterioration processes, sometimes leaving a very short window of opportunity to actually eat the vegetables fresh. Two methods of naturally preserving nutrients and flavor are canning and freezing fresh vegetables. The convenience factor of plucking a can off the shelf or a bag out of the freezer may present a more accessible method of preparing a quick, healthy meal. Besides the convenience factor, the question of whether to buy fresh vegetables every day or eat preserved produce may hinge on whether nutrients are lost in the process of canning or freezing vegetables.
Canned Compared to Fresh Vegetables
Research at the University of Illinois Department of Food Sciences and Human Nutrition concluded that canned and frozen vegetables retain most of their nutritional values. The authors of the study found that some vitamin C and possibly folate are lost during the heating process, but that the canning process did not affect other essential vitamins and minerals. Vitamin A, thiamin, potassium, dietary fiber, protein and calcium remain stable during the canning process.
Do-It-Yourself Canning
According to the Ohio State University Extension vegetable canning fact sheet, you should can vegetables within three hours of harvesting. If you anticipate a delay in canning, immediately refrigerate the vegetables to preserve freshness and nutritional values. Avoid soaking vegetables, which depletes them of flavor and nutrients. When vegetables are canned properly, they should retain many of their nutrients and flavors.
Spoilage of Canned Vegetables
The heating process in canning kills microorganisms and so limits the risk of contracting foodborne illnesses. If vegetables are not canned properly, however, or if the container is compromised, vegetables can spoil and pose a significant health risk. Before eating any home-canned or manufacturer-canned vegetable, check the jar or can carefully for leaks or bulging lids or walls, which indicate spoilage. If the can or jar spurts when opened, the vegetables are spoiled.
Canned Vegetables and Pregnancy
When considering which canned vegetables to buy, inspect not only the vegetables but the containers. Environmental Health News published an article that warns pregnant women about exposure to bisphenol A, or BPA. Women who ate canned vegetables everyday risked higher exposure to the BPA that lines most food cans. BPA seeps from cans into the vegetables. Women may consider replacing consumption of canned vegetables with fresh or frozen vegetables during their pregnancy.
Frozen Compared to Canned Vegetables
According to the Iowa State University Extension, freezing vegetables preserves more nutrients than canning. Freezing impedes growth of bacteria and molds and hinders enzymatic activity. The method preserves the nutritional value present at the time the vegetable is frozen. The U.S. Department of Agriculture recommends freezing vegetables at their peak and at 0 degrees or lower to preserve freshness and nutritional quality. Some freezing methods call for blanching prior to freezing. Blanching initially depletes some of the vegetables' nutritional value, but halts enzymatic activity and thus in the long-run actually preserves more of the vegetables' nutritional value.
Spoilage of Frozen Vegetables
The USDA advises that enzymatic activity and bacterial growth begin once you thaw vegetables. Once vegetables are thawed, they will spoil at the same rate as fresh vegetables. For this reason, handle thawed vegetables as you would fresh vegetables. Manufacturing methods of freezing destroy certain microorganisms through freezing at sub-zero temperatures. Home-freezing methods may not destroy all organisms, but cooking thawed vegetables will.
References
- University of Illinois Department of Food Sciences and Human Nutrition: Comparison of Selected Fresh, canned and Frozen Fruits, Vegetables, Legumes and Protein Foods
- Environmental Health News: Recipe for high BPA exposure: Canned vegetables, cigarettes and a cashier job
- Scientific America: Recipe for high BPA exposure: Canned vegetables, cigarettes and a cashier job
- Ohio State University Extension: Basics for Canning Vegetables
- United States Department of Agriculture: Freezing and Food Safety
- Iowa State University Extension: Freezing Fruits and Vegetables
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