Interactive lesson

Vegetables in oil and brine, step by step

Interactive lesson: why oil doesn't preserve, five safe routes, marinated peppers, aubergine, garlic, pickled green beans and the pressure canner.

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The whole lesson as text

1. Why oil doesn't preserve

Oil is not a preservative: it only separates the vegetables from the air.

Oil only seals — Oil separates the vegetables from the air. That is exactly when, without air, Clostridium botulinum can grow.
Spores from the soil — The spores come from the soil, especially on garlic. Garlic in oil is one of the best-known causes of botulism from home kitchens.
Three protections — The bacterium doesn't grow when it is acidic (pH 4.6 and below) and when it is cold (4 °C / 40 °F or below). The spores are killed by 116 °C (240 °F) in a pressure canner.
The toxin can't be seen — The oil needn't be cloudy and the lid needn't bulge. Safety isn't checked by looking, but by the method.

Why: Without air, at room temperature and in low-acid food, botulism spores germinate and produce toxin. Vegetables under oil in a jar on a shelf give the bacterium everything it wants.

Don't make garlic, fresh basil, chilli or mushroom oil at home for the larder.
Commercial oils like these are safe because they contain added acid or other protections that we can't measure at home.

2. Five safe routes

Every jar must belong to exactly one of five routes.

A: larder — Acidified to a tested recipe and processed in a water bath. Keeps in the larder for up to 12 months.
B: fridge — Vegetables cooked or soaked in vinegar, then into oil, with no bath. Fridge at 4 °C or below, 2–4 weeks, checked with a thermometer.
C: fresh in oil — Fresh garlic, herbs or vegetables in oil, with no acid. Fridge at 4 °C or below and 4 days at most.
D and E — Any vegetables in oil in the freezer at −18 °C (0 °F), up to 6 months. Low-acid vegetables in water or brine processed under pressure, in the larder for up to 12 months.

Why: Anything that doesn't fit exactly one route doesn't belong in a jar for the winter. The route decides where the jar is kept and for how long.

The simplest way: keep sun-dried tomatoes, roasted peppers and aubergine without oil, and add oil, garlic and herbs on the plate.
A fridge at 8 °C is not a fridge for this purpose. Set it to 4 °C (40 °F) or below.