Shelf life: salt and sugar bind water
Also searched as: preserving, making food keep, water activity, why honey does not go mouldy, salting
In short: Germs need free water. Salt, sugar and drying bind it up, which is why honey, jam, salami and dried fruit spoil so slowly.
Like an oasis where every drop is already taken: newcomers find nothing left to drink.
Simplified model
lots of free water: meat, milk, bread
What happens
What counts is not just the water content, but how much water is freely available (water activity). and bind water to themselves; drying removes it. Most dangerous bacteria need a lot of free water; mould and some yeasts manage with less. That is why jam can go mouldy, but honey hardly ever does.
Why it matters
Explains why jam needs enough sugar, why salami keeps without a fridge and why reduced-sugar jam belongs in the fridge.
Rules of thumb
- Keep reduced-sugar jam in the fridge once opened.
- Store dried fruit and nuts dry and sealed.
- Salt, sugar and drying do not replace hygiene; they only slow germs down.
Levers
3
In practice
2
Desserts and sweets
Jam with lots of sugar keeps for a long time unopened, honey almost indefinitely.
Meat
Cured ham and salami keep thanks to salt and drying.
Sources · 2
- Clemson University: Available moisture in foods
- Harold McGee: On Food and Cooking (2004)