Keep the harvest: the ten methods, and what each one costs

It is the acidity below which Clostridium botulinum cannot grow and produce its toxin. Foods that are naturally that acidic — most fruit, and vegetables once a tested recipe has acidified them — may be processed in a boiling-water bath. Foods that are not — vegetables, meat, fish — need a pressure canner, because boiling water cannot push a jar above 100 °C and botulinum spores survive that. The line is the single most useful fact in this whole subject, and most of the safety rules on these pages exist to keep the food on the right side of it.

No, and this is the fact that organises everything else. Freezing locks water up as ice and stops growth, but the organisms are still there and they restart the moment the food thaws. A freezer is a pause button, not a steriliser, which is why thawed food is handled like fresh food with the clock already running — and why anything that will be eaten raw still needs the treatment (usually freezing to a published figure) that deals with parasites before it is "raw".

Three things together: the acid already in the fruit, enough sugar to bind the water so nothing can grow in it, and a boiled, sealed jar. The composition rules for jam sold as jam are set in EU law — around 60 % soluble solids for the standard definition — but a home preserve does not have to meet that number to be safe, only to be thick, set and eaten promptly. The danger case is the opposite one: cutting the sugar far down and calling it jam while processing like one.

Only high-acid foods: most fruit, tomatoes with added acid, and vegetables made acidic by a tested pickling recipe. Everything low-acid — green beans, beets, carrots, meat, fish — needs a pressure canner, and several of the foods on these pages (fish especially) are among the least forgiving things anyone ever puts in one. Each food page says which side of the line the food sits on.

It depends entirely on the method, and the honest answer is always a range rather than a date: frozen food is safe indefinitely at a steady −18 °C but loses quality on a schedule, a fermented jar keeps half a year and beyond under its brine in the cold, and a properly processed high-acid jar keeps about a year at best quality in the dark. The sign of spoilage outranks every date: a domed lid, a hiss on opening, fuzz or a smell that is wrong — the jar goes, whatever the calendar says.