Interactive lesson

Preserving basics, step by step

Interactive lesson: preserves that don't spoil and don't poison — acidity, hygiene, jars, water bath or pressure canner, seal checks and signs of spoilage.

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The whole lesson as text

1. Why preserves spoil

A jar is a closed space with no air, and that is exactly where the most dangerous bacterium in the kitchen feels at home.

Microbes from the soil — Moulds, yeasts and bacteria come from soil, air, hands and utensils. Botulism spores live in the soil and are on every vegetable that grows in or near it.
Moulds and yeasts — Mould makes white, green or black patches, and some moulds produce toxins. Yeasts make foam, bubbles and a smell of alcohol, so the lid lifts.
Botulism in the jar — In a closed jar, without oxygen, at room temperature and in low-acid food, the spores germinate. The bacteria then produce one of the most potent toxins known.
You see nothing — The toxin has no smell or colour, and the jar often looks completely normal. Spores survive boiling at 100 °C (212 °F) for hours.

Why: Vegetables are full of moulds, yeasts and bacteria from soil, air, hands and utensils. Preserving works when heat destroys them and they can't come back and multiply. Botulism is special because you often see nothing at all.

The old advice “boil the jars for three hours and they're safe” is not true for low-acid vegetables.
For low-acid vegetables there are only three safe routes: a pressure canner, acidifying to a tested recipe, or freezing.

2. Acid and low-acid

All preserves fall into two groups, and everything else depends on which one.

The pH 4.6 line — The lower the pH, the more acidic the food. Lemon juice is around pH 2.3, vinegar about 2.5–3, water 7.
Nearly all is low-acid — Green beans, peas and sweetcorn have a pH of 5.6–6.8. Only a pressure canner or the freezer keeps them safely.
Tomatoes on the line — Tomatoes have a pH of 4.2–4.9. They go into a boiling-water bath only with added acid, every time.
Measure the pH — Cool a sample, thin a thick product with a little distilled water and dip the strip. The target for acidified preserves is 4.2 or lower, so you keep a margin below 4.6.

Why: At pH 4.6 or below, spores cannot germinate even in a sealed jar. Nearly all vegetables are low-acid, and even tomatoes are often right on the line. So acidity comes from the recipe, not from an assumption.

When a recipe says 0.9 L of 5 % vinegar to 0.6 L of water, that is not a matter of taste: less vinegar may mean the jar is no longer safe.
Choose narrow-range strips (e.g. 2.8–4.6 or 4.0–7.0); broad 1–14 strips can't tell 4.2 from 4.8.