Frozen and canned vegetables
Frozen.—Cook frozen vegetables according to package directions. For uniform cooking, thaw leafy vegetables just enough to separate the leaves before you put them in boiling water. Partly thaw corn on the cob before cooking it. If necessary, separate pieces of frozen vegetables with a fork during cooking.
Canned.—Commercially canned vegetables need reheating only. If liquid is not served with the vegetable, save it for other uses. (See Ways to Use Leftovers, p. [75].)
Unless you are absolutely sure home-canned vegetables have been processed correctly, bring them to a rolling boil. Then cover and boil for at least 10 minutes. Boil spinach and corn 20 minutes. If food looks spoiled, foams, or has an off-odor during heating, destroy it.