Do not freeze these foods because their structure will deteriorate in the freezer. Some commercially prepared foods, such as already frozen potatoes, can be substituted in recipes that you want to freeze, since they are flash frozen and treated to maintain quality. If you want to freeze a food and you're not sure if it will keep well, wrap and freeze a small portion, then thaw and reheat. If the quality of the food is acceptable to you, go ahead and freeze the entire recipe.
Lettuce, tomatoes, radishes, and cucumbers do not freeze well because their cell structure breaks down. You can freeze fresh tomatoes, but then only use them in cooked dishes like tomato or spaghetti sauce.
Watermelon, grapes, apples, and citrus fruits do not freeze well. Generally, any fruit with a high water content loses quality when frozen.
COOKED EGG WHITES
Cooked egg whites become rubbery when frozen. You can freeze them in a salad or sandwich filling if finely chopped and mixed with other ingredients.
Mayonnaise separates when frozen. In a casserole, mayonnaise can freeze well. Use cooked salad dressings instead of mayonnaise.
Cooked and raw potatoes become mushy when frozen. Use purchased frozen potato wedges, chunks, or hash browns when making a casserole to be frozen.
FLOUR THICKENED SAUCES
These sauces and gravies separate when thawed. Freeze without the thickening agent, then add flour when reheating.
CORNSTARCH THICKENED SAUCES
These sauces and gravies thickened with cornstarch separate when thawed. Freeze without the thickening agent, then add cornstarch when reheating.
If you want to freeze cooked pasta, mix it with a sauce or gravy before freezing. Plain cooked pasta becomes too mushy if frozen alone.
The cell structure of salad greens is destroyed when frozen; do not freeze.
Cream cheese becomes crumbly and grainy when frozen by itself. It will freeze well when mixed with other ingredients, like milk or cream, in a sauce.
SPICES AND HERBS
Freezing can change the quality and strength of spices and herbs. If you want to freeze a highly spiced recipe, especially one containing cloves, paprika, peppers, and curry, add the seasonings during reheating. Season lightly when preparing the food for the freezer.
From Linda Larsen, Your Guide to Busy Cooks.