The refrigerator door shelves are the warmest part of the fridge and have the most temperature fluctuations; they are the first place exposed to room air every time the door opens. That is why foods whose salt, acid, or sugar acts as a preservative can go in the door, while perishable foods such as milk, eggs, raw meat, and cooked dishes belong on the interior shelves.
The door shelf is farthest from the back wall, where cold air blows, and only the thickness of the door separates it from the outside of the refrigerator. Each time the door opens, room air hits that area first. In a refrigerator opened 15 times a day, the door shelves stay a few °C warmer than the interior shelves, and the difference fluctuates rather than staying constant: food warms, cools, and warms again. For perishable food, this repeated rise and fall is more damaging than the average temperature. Even while the refrigerator display shows the correct temperature, the door shelf can stay above it.
Put foods with their own preservatives in the door. The salt in pickles and olive brine, the vinegar in ketchup and mustard, the sugar in jam and marmalade, and the acid in lemon juice already inhibit bacterial growth; these foods are not affected by fluctuations of a few °C. Juice cartons and water bottles also fit easily in the door. Accessibility is another advantage: when foods taken out and put back several times a day are kept there, the refrigerator can stay closed longer. You do not have to open the door all the way to reach a frequently used jar; it can be opened halfway and closed again.
Perishable foods should not go in the door. Milk, eggs, and raw meat top this list; all three are sensitive to temperature fluctuations, and all three keep measurably less time in the door. Cooked dishes and leftovers do not belong there either. The door shelf is also a poor place for eggs because they jostle every time the door opens, which can damage the thin membrane inside the shell; keep eggs in their carton on an interior shelf. Milk, too, belongs on an interior shelf rather than in the door, however quickly you use it.
A few foods fall between the two groups, and the decision depends on how quickly they will be used. Butter can go in the covered butter compartment in the door; because it is the warmest part of the refrigerator, the butter stays spreadable, but this is appropriate only if it will be used within 1–2 weeks. Butter that will be stored longer belongs on an interior shelf or in the freezer. Sliced cheese and opened cream follow the same rule: if they will be used soon, keep them in the door; if they will be kept longer, put them on an interior shelf. The covered butter compartment in the door is slightly better insulated, so it has a small advantage over the shelf itself for these foods.
You can tell whether the door shelves really are warmer by measuring, not guessing. Put a thermometer in a glass of water, place it on the middle shelf, then on the door shelf, and leave it in each spot for several hours; the difference between the two readings shows how that particular refrigerator’s door behaves. If the difference is large, arrange the food accordingly. The gap grows even wider in a refrigerator with a worn door seal, because air keeps leaking in around the edges even while the door is closed. Check the seal by seeing whether a piece of paper slipped into it pulls out easily.