Do not store your potatoes in the refrigerator. There are a couple of reasons for this. Firstly, the colder temperature will convert the starch of the vegetable into sugars. This breaks down the consistency of the flesh and produces a mushy potato, likely to be discolored upon cooking. Secondly, the higher level of sugar produced will produce a chemical, Acrylamide, if the vegetable is fried or baked at high temperature. Acrylamide is potentially a cause of cancer in humans. Potatoes should be stored in a cool, dry area outside the fridge. If you keep them in a bag, use a breathable bag: one with holes, mesh or a paper bag, for example.
Onions should be stored in a breathable bag (such as of mesh) in a dry area of your kitchen or pantry. Keeping them in the fridge will, over time, cause them to become moist and soggy. It is true that an onion taken out of the fridge will induce less tearing from your eyes when you cut into it, so feel free to refrigerate an onion a little while before you cook with it, just for this reason. Once you have cut into the onion, you can refrigerate it for keeping, but moisture is an issue and you are best served to use a crisper drawer to reduce the humidity level around the vegetable.
As with onions, garlic will turn to moist mush if kept in the fridge. Keep it on a counter top or shelf at room temperature and in a dry location. Storing a bulb of garlic in a paper bag works quite well.
Quite delicate, not only in their internal structure, but also in terms of flavor and aroma, the preservation state of a tomato can make a world of difference in its quality. Store bought tomatoes are often already rather compromised in flavor because they are picked early when not fully ripe. This is because they are more durable in this state, given that they must be picked, packed and shipped over long distances.
Keeping a tomato in the fridge is particularly harmful to it because temperatures below 55 degrees cause its flesh to break down and become mushy. Additionally, tomatoes produce volatile compounds which are responsible for flavor and aroma. Low temperatures destroy these. Keep your tomatoes in a dry environment on your counter to allow them to ripen naturally. Their flavor will be enriched over time as these volatile compounds increase. Once the tomato is properly ripe you may refrigerate it for a few days to prolong its prime period before it degrades. However, storing it for a period of say four days of more, in this state, then produces a degrading vegetable which becomes watery and mushy. To mitigate this, store them in a brown paper bag in the crisper drawer.
As delicate as tomatoes are, lettuce and a number of other greens are even more so. Wilted, browned, soggy lettuce leaves is an experience so common that it leaves one wondering if it is something one just has to accept. Should you lower the temperature of the fridge? Should you not store in plastic bags? Should you be buying a different type of lettuce?
Lettuce is very perishable, so it’s a question of preserving it for several days, not weeks. Do not wash your lettuce until you are about to use it. The dampness will only accelerate the wilting time. Keep lettuce in a loose plastic bag in your crisper. Isolate it from apples, pears and other fruits that release ethylene as they ripen. Ethylene will cause your leafy greens to wilt quickly.