Tomatoes should also be stored on the counter, and actually do best when processed because the lycopene is released when the tomato cells break down. Heating tomato products makes the lycopene two to three times easier for our bodies to absorb.
Antioxidants in carrots, called carotenoids, are also more abundant and useable when carrots are cooked.
Along with plant chemicals, the vitamins found in these foods are utilized when combining them with either water or fat. The vitamins A, D, E and K are soluable with fat, some is needed in your food. That's generally not a problem, so remind yourself to put some fat in celery dressing or topping vegetables to enhance intake.
Vitamins B and C are soluble with water, but here's where it gets tricky. Cooking with too much water for too long leaches the vitamins from the food, along with leaving a mushy consistency. Steaming or microwaving in a small amount of water until they're tender but crisp is your best bet.
Believe it or not, how you slice food also affects the nutritional benefits. Garlic and onions offer more of their antioxidant compounds when crushed prior to cooking. In fact, if garlic is allowed to sit crushed 10 minutes prior to cooking, it allows those plant chemicals to form and keep those arteries clear.
If garlic is cooked whole, it releases no antioxidant affects at all.