A. The color changes noted above do not mean the food is unsafe to eat. However, spoilage may also cause color changes. Any canned food that has an unusual color should be examined carefully before use ([p. 8]).
Q. Does ascorbic acid help keep fruits and vegetables from darkening?
A. Yes. The addition of ¼ teaspoon of crystalline ascorbic acid (vitamin C) to a quart of fruit or vegetable before it is processed retards oxidation, which is one cause of darkening of canned foods. One teaspoon of crystalline ascorbic acid weighs about 3 grams (or 3,000 milligrams).
Q. Is it all right to use preservatives in home canning?
A. No. Some canning powders or other chemical preservatives may be harmful.
Q. Why do the undersides of metal lids sometimes discolor?
A. Natural compounds in some foods corrode the metal and make a brown or black deposit on the underside of the lid. This deposit is harmless.
Q. When canned or frozen fruits are bought in large containers, is it possible to can them in smaller containers?
A. Any canned or frozen fruit may be heated through, packed, and processed the same length of time as recommended for hot packs. This canned food may be of lower quality than if fruit had been canned when fresh.
Q. Is it safe to leave food in tin cans after opening?