Most advice about packed lunches is about what to include. The more useful list is the opposite one. Certain foods simply do not survive several hours in a bag with a thawing ice pack, and packing them is how people end up with an unpleasant afternoon and a suspicion that their bag is faulty.
Home-made dressings and sauces
A shop-bought mayonnaise is acidic and stable; a home-made one built on raw egg is a different proposition entirely, and so is a yoghurt or cream dressing already mixed through a salad at seven in the morning. Carry dressings separately in a small sealed pot and add them when you eat.
Cooked rice that has been handled casually
Rice deserves more respect than it gets. Cooked rice left at room temperature for hours before being packed, then carried warm in a bag, is a genuinely poor idea. Cool it quickly after cooking, refrigerate it overnight and pack it cold, or leave it out of the rotation. Insulated bags with room for a proper cold pack, as sold at https://mexsus.com/, make the cold-packed version workable.
Anything still warm from the hob
Warm food inside insulation stays warm, which is precisely what you do not want. A portion of stew ladled into a box at breakfast time and zipped into a bag will spend the entire morning in the temperature range you are trying to avoid. Either send it genuinely hot in a good vacuum flask, or send it properly cold.
Cream cakes and custard-filled pastries
Fresh cream and custard fillings lose their margin quickly outside a fridge, and the packed lunch bag is not a fridge. A dry cake, flapjack, biscuit or fruit-based bake does the same job for morale and asks nothing of your ice pack.
- Dressings travel separately, never pre-mixed through leaves
- Rice goes in cold from the fridge or not at all
- Nothing warm ever goes into an insulated bag
- Cream and custard fillings stay at home
- Cut melon and prepared fruit needs the same care as meat
Prepared fruit left loose
Whole fruit is one of the most robust things you can pack. Cut fruit is not: once the skin is broken, a melon or a mango behaves much more like a perishable food than like an apple. Sealed box, ice pack, same treatment as anything else.
Leftover takeaway of unknown history
Last night’s delivery went from a restaurant kitchen through a courier bag to your table and sat there while you ate. By the time it reached the fridge it had already used a good part of its margin. Reheating it thoroughly at home is one thing; carrying it around in a bag for six hours the next day is another.
Soft cheeses and pâté on a hot day
These are fine in cool weather with a solid pack and a shaded bag. On a genuinely hot day with a long gap before lunch, they are among the first things to suffer. Hard cheeses, cured meats and pickled items are the summer versions of the same flavours.
The rule underneath all seven
Ask one question about each item: would I be happy leaving this on the kitchen worktop for the length of my morning? If the answer is no, it needs a solid ice pack next to it and a bag that stays shut. If it needs more than that, it belongs to a day when you can keep it in a fridge.