When it comes to Food Preservation, we've been there, done that, now serving 119 tips in 11 categories ranging from Answer User Questions to Vinegars.
Citrus juice - lemon, lime or even orange juice works best to keep apples, pears, bananas, peaches and other fruits from turning brown. The acid in these juices stops the oxidation.
Frozen vegetables and fruits can actually be more nutritious than fresh, because they're packaged immediately after harvesting and the nutrients stay at their peak. Look for plain, 100% fruits and vegetables. Avoid anything mixed with cheese, topped with sauce, or anything with added sugar.
Separate the leaves and rinse well, then blot dry and bunch them together. Wrap the head in a double layer of paper towel and store in a freezer bag in the crisper. The lettuce leaves will be more crisp than when you brought it home, and it will last a long time.
If you don't plan to use bananas right away, buy less-ripe ones or a few of each. Heat and air causes bananas to ripen. They can be stored at room temperature, but the warmer it is the faster they will ripen.
To freeze bananas, peel them, then wrap them tightly in freeze-proof cellophane or put them in a plastic container. They can be frozen for up to a month, but please note that they will turn brown.
Use straight from the freezer in smoothies or ice creams.
The easiest way to keep your sliced apples from turning brown is to keep them in a bowl of water. If you are placing your apples on a serving dish, you can brush them with water that has a small amount of lemon juice added to it.