Why food spoils
Bacteria and moulds need three things to multiply: available water, mild temperatures and a food that isn't very acidic. A fridge works on temperature. Traditional methods work on the other two: they take the water away from microbes, or make the food so acidic they can't grow.
None of these methods sterilises food: they don't kill every microbe, they only stop them growing. That's why it matters so much to start with fresh, clean food and to follow tested recipes instead of guessing amounts.
Five methods that work without electricity
1. Drying
Without available water, bacteria can't reproduce. That's why dried beans, rice, dried fruit or dried mushrooms keep for months in a well-closed jar. Fruit can be sun-dried in hot, dry climates, sliced thin and protected from insects.
Meat is the tricky exception. The USDA recommends heating it to 160 °F (71 °C), or 165 °F (74 °C) for poultry, before drying it, because drying alone doesn't reach the temperature that kills bacteria, and once dried they become more heat resistant.
2. Salting
Salt pulls water out of the food and out of the microbes themselves, so they can no longer grow. It's how salt cod, cured beef or ham are made. It takes a lot of salt and time, and salted foods need soaking in water before you eat them.
3. Sugar
Sugar does the same as salt: it holds the water so microbes can't use it. That's why a jam with enough sugar lasts far longer than fresh fruit. If you cut the sugar in a recipe, it no longer keeps as well: store it in the fridge once opened.
4. Vinegar
Pickles work through acidity: dangerous bacteria, including the one behind botulism, can't grow in a very acidic food. The key is vinegar of at least 5 % acidity, which is what every tested recipe uses, and not adding more water than the recipe says, because diluting the vinegar lowers the acidity and the pickle is no longer safe.
5. Fermenting
In sauerkraut or olives, salt lets good bacteria take over and produce lactic acid. That acid is what preserves the food. Sauerkraut usually uses around 2 % salt by weight of the vegetables. If mould appears, it smells bad or the texture turns slimy, throw it out.
Oil doesn't preserve
Covering raw vegetables, garlic or chillies with oil doesn't preserve them: it takes the air away, and botulism bacteria grow precisely without air. Homemade chillies in oil appear in botulism outbreaks. If you make garlic or herb oils, keep them in the fridge and use them within a few days.
Home canning: what you need to know
Food safety agencies such as Spain's AESAN advise avoiding home canning whenever possible, because without the right technique it can cause botulism, a serious poisoning. If you do it, the acidity of the food decides the method.
| Type of food | Examples | Safe method |
|---|---|---|
| High-acid | Jams, fruit, vinegar pickles, tomatoes with added acid | Boiling water bath, with clean jars and times from a tested recipe |
| Low-acid | Vegetables, beans, mushrooms, meat, fish | Only in a pressure canner that reaches about 240 °F (116 °C). A water bath isn't safe |
Boiling water can't go above 212 °F (100 °C), and botulism spores survive that: that's why low-acid foods need pressure. A kitchen pressure cooker isn't designed for this: canning guides advise against using one for low-acid foods.
When to throw a jar out
If the lid bulges, there's rust, the liquid leaks or looks cloudy, or it smells odd when opened, throw it out without tasting it. Botulism doesn't always change the taste. And never give honey to babies under 12 months.
What to do in a power outage
If the outage lasts hours or a few days, you don't need any of this: keep the fridge closed, eat the perishables first and then the pantry. The details are in the food storage guide. These methods are for the long term and for making the most of what you have without relying on electricity.