Fruits begin to lose some of their nutritional value as soon as they're harvested, but proper cooling and preserving can minimize that loss over a long period, according to a National Institute of Food and Agriculture publication on canning. Boiling fruits and vegetables changes the physical property of the produce through heat and by immersing them in water. Exposure to light, air and naturally occurring enzymes can also reduce the nutrients in fruit.

Boiling fruit, for canning as an example, accelerates the the loss of many vitamins. As much as one-half to one-third of vitamins A and C, thiamine and riboflavin are lost in cooking. Most other vitamins, however, see far less reduction in the boiling or cooking phase. However, with every year that passes, a boiled and canned fruit may lose 5 to 20 percent annually of vitamins A and C. And if you use a copper pot, you will lose even more vitamins, as compared to stainless steel.

The advantages of boiling and canning fruit, or cooking with fruit to avoid buying processed items like pre-packaged pies, is that you can save money and have the peace of mind knowing you're eating only what you want to consume and not a lot of chemicals and preservatives. If you can steam your fruits and vegetables, you will retain more vitamins than if you boil them.

One of the most obvious exceptions to the rule of lost nutrients due to cooking can be found in tomatoes. Research has shown that cooking tomatoes actually releases more of the antioxidant lycopene than would be found in raw tomatoes. Scientists suggest that cooking breaks down the cell walls to allow more lycopene to be released. Scientists have also helped bio-engineer tomatoes to include more lycopene, a substance that is associated with lowering the risk of certain cancers, particularly prostate cancer.

To help maximize the amount of vitamins you do get in your fruit, pick fruit that was harvested when it was fully ripe. Most fruits ripened on the plant have more nutritional value than those picked when they're still a little green. Experts suggest adding fresh fruits and vegetables to water that is already boiling, rather than letting the produce soak as the water heats up. Similarly, keep fruit in the refrigerator until you are ready to eat it or cook it. And remember that the longer a food is stored, canned, frozen, refrigerated or otherwise separated from the plant, the greater the loss of vitamins and nutrients.

While it may seem that fresh fruit is always the more nutritious option, frozen fruit often may have more vitamins and other nutrients. Fruits are usually frozen soon after they are harvested, while fresh fruit can spend days being transported to stores and days in your home before being eaten. As a result, you may want to think about using frozen fruit in recipes unless you know you can get to the store the day you're cooking to buy the freshest fruit available.