Canning and bottling

Read this before anything else

This page explains how canning works. It does not give processing times, temperatures or pressures, and you must not improvise them.

The reason is specific rather than general caution. Clostridium botulinum forms spores that survive boiling water. In a low-acid food in a sealed jar — no oxygen, no acid, room temperature — those spores can germinate and produce a toxin. The food gives no reliable warning: it can look, smell and taste completely normal. There is no domestic test for it, and the usual defence of trusting your senses does not apply.

What protects against it is either enough acidity to prevent germination, or a temperature high enough to destroy the spores — which water boiling in an open pan cannot reach. Which of those applies, and the exact schedule for it, comes from tested figures. FoodHQ does not publish salt percentages, acidity figures, processing times, curing quantities or keeping windows. Those are safety numbers set by a food safety authority rather than by a cookery writer. In the UK the Food Standards Agency is the authority — take the figures from them or from a body of equivalent standing, and use this page to understand what those figures are doing.

The division that governs everything

Every canning recipe is one of these two, and they are not interchangeable.

High-acid foods

Most fruit, jams, jellies, chutneys, properly acidified pickles, tomatoes with acid added. Their acidity prevents botulinum spores germinating, so a boiling water bath is sufficient — it deals with moulds, yeasts and vegetative bacteria, which is all that is left to deal with.

Low-acid foods

Vegetables, meat, fish, beans, stock, soup, most sauces. Nothing prevents the spores germinating, so the spores themselves have to be destroyed — which needs a temperature above boiling, which needs a pressure canner. A water bath cannot do it at any duration.

The borderline is not obvious

Tomatoes sit close to the line and modern varieties vary, which is why tested recipes specify added acid. Figs, some melons and low-acid fruit varieties are similar. This is exactly why the judgement belongs to a tested recipe rather than to a taste test.

Adding acid changes the category

Acidifying a low-acid food to a specified figure can bring it into water-bath territory. The figure is the whole point, and "a good splash of vinegar" is not one.

How the seal works

Once the contents are heated, the jar has to stay closed to whatever is outside it.

A jar filled hot and processed hot contains air and steam. As it cools, that gas contracts and the pressure inside falls below atmospheric pressure, pulling the lid down hard against the rim. That vacuum is what holds the seal and what keeps new organisms out. The characteristic ping as a jar cools is the lid dishing inward.

This is why headspace matters — too little and the contents push out and break the seal, too much and there is more air to expel than the process can manage — and why the rim must be wiped clean, because a fragment of food across the sealing surface leaves a channel. It is also why a lid that flexes when you press it has not sealed, whatever it looks like.

A properly sealed jar is not sterile in perpetuity. It is a container whose contents were made safe and which nothing new can get into.

What goes wrong, and what it means

• A lid that flexes or pops when pressed. No vacuum. The jar did not seal. Refrigerate and eat it soon, or reprocess with a new lid.
• Liquid loss during processing. Usually too little headspace, air bubbles left in, or a violent boil. Cosmetic if the jar sealed, provided the food is still covered.
• Cloudy liquid. Can be starch, hard water, or table salt with anti-caking agents — often cosmetic. Can also be spoilage. If in doubt, do not eat it.
• A bulging lid, leaking, or spurting when opened. Gas produced inside. Discard it without tasting.
• Mould on the surface of a jam. The jar was not sealed properly or was contaminated. Discard the jar.
• Anything that looks or smells wrong. Discard without tasting. And note that the reverse does not follow: looking and smelling fine is not evidence of safety in a low-acid canned food, which is the whole reason the process matters.

Common questions