Some foods just don’t freeze as well as others. Why not? Crispy, crunchy fruits and veggies like celery, lettuce, cucumbers, and apples that we usually eat raw are more likely to disappoint when frozen because they will be much softer and limp when thawed. These foods have high water content combined with fragile cell walls in their tissue, and when they are frozen the water expands, breaking cell walls and turning the texture to mush. Similarly, citrus fruits, watermelon, grapes, and tender-leafed herbs like basil also become unpleasantly mushy after thawing. (Frozen grapes, however, are worth trying as a sweet frozen treat, yum!)
Cooking fruits and vegetables also softens cell walls, so foods that we are used to eating cooked are good candidates for freezing, such as tomatoes (so long as you aren’t planning to slice them onto a sandwich after freezing).
Undesirable flavor changes also occur in some foods when they are frozen. Green peppers and garlic become overly strong and even bitter, whereas onion tends to lose flavor. Spices and seasonings may also be affected; for example, cloves, pepper, and celery seasoning get stronger and bitter, curry develops and off-flavor, and salt loses flavor. Adding salt also runs the risk of increasing rancidity if added to foods containing fat. To avoid these flavor changes, season prepared foods when reheating for serving instead of prior to freezing, or just season them lightly before freezing.
For much more information about freezing foods, visit the NCHFP website, General Freezing Information.