Opened canned food can be frozen, but never in its original metal can; transfer the contents to a glass or plastic container. Once the can is opened, the tin layer is exposed to air and a metallic taste can develop; the contents also expand as they freeze and can deform the can. Freezing is still not always the right choice for vegetables whose texture will deteriorate.
The inside of a sealed metal can has a protective coating, and the contents do not contact air until the can is opened. As soon as the lid is opened, that balance changes: air enters, metal is exposed at the cut edges of the coating, and the acid in the food starts to contact the metal. The metallic taste that develops over a few days comes from this. For that reason, an opened can—whether its contents are going into the refrigerator or freezer—should not be stored in the can; transfer them to another container. This rule applies regardless of the type of canned food.
Freezing adds a second reason. A metal can does not flex, and the liquid inside expands as it freezes; the can bulges, its seam is strained, and it may sometimes split open. A bulging can can also be mistaken for a sign of spoilage, and afterward it is hard to tell why it bulged. A glass or plastic container solves both problems: transfer the contents, leave two to three centimeters of headspace, and tighten the lid after the food has frozen. Write the food name and freezing date on the container.
How well canned foods freeze depends on their contents. Legumes and whole-kernel foods such as chickpeas, beans, corn, and peas give an acceptable result when frozen; they soften slightly, but the difference is not noticeable in soup, salad, or other dishes. Tomato products—tomato paste, diced tomatoes, and tomato sauce—freeze very well and become a convenient ready-to-use supply when frozen in portions. Tuna can also be frozen after draining, but its flakes break apart after thawing, so it is better used in recipes that are cooked.
Some foods should not be frozen. High-moisture canned vegetables expected to stay crisp—pickled vegetables, gherkins, and some diced cucumber products—lose their texture when frozen. Creamy or milk-based canned soups separate and look grainy after thawing. For these foods, the right solution is not freezing but using them within a few days while refrigerated in a sealed container. If the amount left is already small enough, freezing is not worthwhile anyway; buying a can in the right size is the solution next time.
Everything discussed here applies to factory-canned food after opening; the safety rules for home canning in jars are a separate topic outside the scope of this article. If you will not freeze the opened canned food, keep it refrigerated in a sealed container and write the opening date on it. Thaw frozen canned food in the refrigerator and use it promptly after thawing; do not refreeze thawed canned food, because a second freeze-thaw cycle significantly worsens both its texture and flavor.