A. Most people do not realize the crucial importance of freshness when it comes to produce. Whenever we slice into a vegetable or fruit we expose the cut surfaces to heat, light and oxygen -- the nutrient destroyers. This exposure results in the oxidation of phytonutrients and loss of some nutritional value of the fruit or vegetable. An example of oxidation can be seen by cutting an apple, pear or banana and letting it sit in the open a few minutes. The “browning” you observe is evidence of the oxidation that robs it of its nutrients. Antioxidant protection can be seen by taking the type of same fruit or vegetable and cutting it up but this time apply some “fruit fresh” crystals and notice how the crystals prevent oxidation, which is the antioxidant protecting the fruit. Some fresh orange juice will work too. In the natural state, fruits and vegetables are provided protection from the air because of their skins. Even though they still “go bad in time,” the protection of their skins is lost when they are cut up, diced or sliced.
According to Dr. Ray D. Strand, M. D. — Specialist in Nutritional Medicine "Fresh salads and cut vegetables and fruit lose more than 40 to 50 percent of their value if they sit for more than three hours". All I know is that I incorporate tons of fruits and vegetables into my eating plan to receive their benefits. If I were not concerned about nutrients, I would just plow down Big Macs and the like and call it a day. But since that is not the case, and I am going to eat premium foods, WHY would I allow nutrient depletion just so I could have the "convenience" of pre-sliced produce? A good rule of thumb is "If I cut it now, I eat it now."
On a side note: It is best to tear lettuce into bite size pieces rather than cut it with a knife. Tearing it will break the bonds between the cells, making for less water loss. Cutting lettuce slices through cells, allowing the water to escape. I have also heard that cutting lettuce (especially with a metal knife) will cause it to turn brown more rapidly, possibly due to a chemical reaction between the metal and the lettuce.