How long it keeps once opened, and what actually decides that

Two phrases sit on packaging and mean different things. «Use by» marks a date past which the food may not be sold, and applies to highly perishable products. «Best before» marks the minimum durability date: until that day the producer guarantees the declared characteristics, colour, aroma, texture.

Both share an assumption almost nobody reads: they hold for an unopened package kept in the stated conditions. The moment you open it, that date stops describing what you are holding, and something else starts counting.

That something else is composition, and you can read it without instruments.

First, water. Microorganisms need available water to multiply, which is why mozzarella and dried pasta belong to different worlds despite both being food. The wetter a product, the more alive it is.

Second, salt, which works directly against the first: it binds water and makes it unavailable. Prosciutto crudo keeps for months not because it was sterilised but because salt took the water out of play, and the same logic governs Pecorino Romano or salt-packed anchovies.

Third, fat, which behaves unlike everything else. It does not fear microorganisms, it fears oxygen: it goes rancid. That is why olive oil follows rules of its own, and why the cut face of a prosciutto is sealed with its own fat.

Fourth, acidity. Below a certain pH many things simply do not grow: it is the principle behind vinegar, giardiniera and pickles generally, and partly behind cheese too, where lactic fermentation lowers pH while the paste dries.

Line the four criteria up and the keeping of almost anything becomes predictable without looking it up. The table below gives no number of days, and that is not an omission: the criteria transfer, while the number shifts with your fridge temperature, the humidity of your kitchen, and how fresh the thing was when you bought it.

A hard cheese is alive and still breathing. Wrapping it in cling film against the paste stops it exchanging moisture, and the surface turns tacky before growing mould where it should not.

The paper good counters use, the paper-and-polyethylene laminate, exists for exactly this: it lets a little air through and holds enough moisture back. Failing that, the domestic answer is baking parchment inside a closed container, which is a reasonable compromise between drying out and suffocating.

On hard pastes there is a second rule you can see by eye: the rind protects, the cut face does not. A piece with more cut surface dries faster than a compact piece of the same weight.

The difference between a whole prosciutto and a tray of slices is not convenience, it is geometry. Slicing multiplies the surface exposed to air many times over, and everything that happens to a cured meat happens at the surface.

Hence the practice at the counter, which is not laziness: the cut face of a whole ham is covered with its own sugna, the fat taken off the previous cut. It acts as a plug, keeps air away and leaves the next slice soft.

On pre-sliced trays the opposite of the cheese rule applies: there, film in contact makes sense, because the enemy is air rather than moisture.

Extra virgin olive oil has no microorganisms to fear. It has oxygen. Over time it oxidises, loses its fruity notes and develops rancidity, and that process does not run backwards.

What speeds it up, in order: light, heat, and the air that enters at every opening. Three practical consequences follow. The dark bottle is not styling. The spot beside the hob is the worst place in the kitchen. And a five-litre tin bought to save money, in a household of two, ends up costing more than two one-litre bottles, because it will sit half full of air for months.

And it is the reverse of wine: olive oil does not improve with age, and the best-before date exists precisely so you can work out how recent it is.

This is about how products change over time: how much they dry, how much they oxidise, how far a ripening carries on. You will not find whether something is still safe to eat.

That is not formal caution. The judgement depends on the individual package, on how it was kept, on the real temperature of your fridge and on who is eating. No web page can know those things. Food safety rules are published by health authorities, and they are the reference.

What we can say, and what the table sets out, is what governs each product's behaviour. That is the information you need in order to decide how much to buy and how to keep it, which is a different question.