Q: Can avocadoes be frozen and still be edible? Do I need to add anything to them — salt, sugar?
A: There must be a bumper crop of avocados, because this question has come in several times recently! Whole or pieces of avocado do not freeze well, but puree does. Depending on what you might use the puree for later, you may sweeten it or not. But you should add lemon juice or powdered ascorbic acid (vitamin C) to protect the color.
Mash the pulp and mix in either 1 tablespoon of lemon juice or 750 milligrams of powdered vitamin C for each quart of pulp, or for each two avocados. Pack into a rigid container leaving a half-inch of headspace. Or pack into zipper-seal, freezer-weight plastic bags. Leave headspace but squeeze as much air out of the bag as possible.
Adding salt is probably not a good idea, because salt will make many fats go rancid faster in the freezer. Since avocado has so much fat, you might get off flavors after several months of storage if you add salt now.
Avocado puree can be used for dips, as a spread on bread and for making creamy salad dressings. There are recipes for avocado breads, cakes and pies as well as avocado-based soups.
Q: If some fresh fruits and vegetables don't need to be refrigerated when they're fresh, why do they have to be refrigerated after they're cooked?
A: Fresh produce has a lot of natural defenses it uses to protect itself while alive. Most fresh produce on our counters still is alive. It's using oxygen and putting out carbon dioxide. Many fruits produce ethylene, the "ripening gas," to speed up their own and other fruits' ripening.
Produce may have a natural wax on its skin to make it harder for bacteria to get in or moisture to get out. While it's alive, it can keep its pores closed and moisture inside. Fuzzy leaves or skins also help keep bacteria at a distance. Produce can keep all its natural juices, sugars and proteins safely stored inside its cells. And some produce has natural enzymes that will inactivate bacteria that enter.
But once we slice, dice, peel, chop or cook, we've killed the produce. Now, the natural defenses are gone. Enzymes and proteins are no longer active — they're just food for bacteria (or us). Waxes are removed. All the juices and sugars are now out in the open, exposed for bacteria to use. So cooked foods and even just peeled or sliced fruits and vegetables are more likely to allow bacteria to grow.
Cooking does make them safer initially, because it kills off any bacteria that are present. But unless you keep the food packaged in an intact, sterile container, it soon will be re-contaminated. Recontamination can happen from serving spoons or dishes, other utensils or containers, or from whatever happens to be floating past in the air.
There have been several outbreaks of food poisoning traced to sliced fruits such as tomatoes and cantaloupes. And cooked vegetables have long been recognized as potentially hazardous foods that must be refrigerated to be kept safe. So protect yourself and use the cold.