Two beliefs do most of the damage in domestic food handling. The first is that your nose is a reliable safety instrument. The second is that mayonnaise is the usual villain. Both are wrong in ways that matter, and both lead people to take risks with packed food that they would not take if they knew better.
Smell tells you about spoilage, not safety
The organisms that make food smell bad and the ones that make people ill are not the same. Spoilage changes taste, texture and odour; it is your food going off in an obvious way. The organisms of concern for illness can be present in quantity with no smell, no slime and no change in appearance whatsoever. A sandwich that seems perfectly normal after five warm hours is exactly the case in question.
What to use instead of your nose
Time and temperature, not sensory checks. Track how long the food has been out of proper chilling and how warm it has been, and make the decision on that. If a lunch has been in a hot car all day, it does not matter how appetising it looks. Keeping a bag genuinely cold, with a proper pack in a bag such as those at https://mexsus.com/, is what protects you rather than a sniff test.
The mayonnaise myth
Commercial mayonnaise is quite acidic, which does not make it a hospitable place for much to grow. In practice it is the things it is mixed with that carry the risk: cooked chicken, egg, prawns, potato and pasta. Blaming the mayonnaise means people confidently pack a chicken salad without mayonnaise and think they have solved something.
The foods that genuinely need care
Cooked meat and poultry, cooked rice and pasta, eggs, fish and shellfish, soft cheese, milk-based products and cut fruit and vegetables are the group that need proper chilling. Notice how many of those are the ingredients of a normal packed lunch. That is why the cold source matters so much.
- Judge by time and temperature, never by smell or appearance
- Home-made egg-based dressings are the risky ones, not shop mayonnaise
- Cooked rice and cooked chicken deserve the most care
- Reheat only to genuinely steaming, and only once
- When in doubt about a lunch, bin it rather than taste it
Tasting a small bit to check
This is a common habit and a poor one. A small quantity is enough to cause a problem, and tasting tells you nothing that looking did not. If you are uncertain enough to want to test it, you already have your answer.
The other half of the problem is cross-contamination
A great deal of domestic illness comes not from food that sat too warm but from raw juices reaching something that will not be cooked. A leaking tray of chicken above a bag of leaves in the same cooler is the classic case. Sealed containers, separate bags for raw items, and washing the bag afterwards deal with it.
Being sensibly cautious rather than anxious
None of this requires paranoia. Chill overnight, pack cold with a solid pack, keep the bag shut and out of the sun, eat within a sensible window, and wash the bag when something leaks. That routine handles almost everything, and it frees you from having to make judgement calls with a nose that was never designed for the job.