TempoLife › Features › Home preserving: freezing, pickling, fermenting and jam › plum

Plum

Plums ripen all at once and do not wait, which makes them the classic weekend-preserving fruit. They are high in acid and reasonably high in pectin — much of it in the skins — so they set into jam without help, dry into prunes without sugar, and can be bottled in syrup with no acidification at all.

The season

Late August into September, and the whole crop tends to ripen within about a fortnight, which is why plum weekends are a thing.

Acidity decides the method

Plums are strongly acid at roughly pH 2.8–3.4, so every acid-dependent method is open and jam sets readily.

This is a reference guide, not a recipe authority and not medical advice. The one organism the safety rules here are written against is Clostridium botulinum, and the one way to be protected against it is a tested recipe followed exactly — substitutions of the acid, the sugar or the salt are not cosmetic. Where the national food authority publishes advice for a specific food (Baltic fish, wild mushrooms), that advice outranks this page.

Method by method

The classic preparation: Plum jam and prunes

For jam: halve and stone the plums, cook them with a splash of water until the skins have softened completely — this matters, because plum skins added to sugar early stay tough all the way to the jar — then add sugar, usually between 700 g and 1 kg per kilo of fruit, boil hard and test for the set at 104–105 °C. Plum jam sets easily and often needs less sugar and less boiling than the recipe suggests. For prunes: halve and stone, lay the halves cut side up on the trays so the juice does not run, and dry at 57–63 °C until leathery and pliable. Condition the batch for a week and store airtight.

Preparation that matters

Pick when the fruit gives slightly to a thumb and has lost its hard sheen. Do not preserve plums with brown soft patches; the rot spreads through the flesh faster than it shows on the skin. Stones come out easily from freestone varieties and fight you in clingstone ones — for the latter, cook and pass through a sieve rather than trying to halve them neatly. The kernels inside the stones are not food: like other stone-fruit kernels they contain amygdalin, and there is no reason to add them to anything.

What the preserve is worth

Prunes are the one dried fruit with a genuine, well-documented functional effect: they are high in fibre and in sorbitol, which is why they have been used for constipation for as long as anyone has dried plums. That is a food property, not a treatment plan, and any persistent digestive problem is a conversation with a clinician. Nutritionally, drying concentrates everything except vitamin C, which is largely lost. Plum jam, like every jam, is mostly sugar by weight.

The ways this goes wrong

• Adding sugar before the skins have softened, which leaves tough shreds through the jam.
• Freezing plums whole and then trying to stone them.
• Drying with the cut side down, so the juice runs into the tray.
• Preserving fruit that has begun to rot at one point.

Safety rules, temperatures, blanching times, headspace and the pH 4.6 line: USDA Complete Guide to Home Canning and the National Center for Home Food Preservation guidance (US Government / land-grant publications, public domain). Acidity classes: the FDA's published approximate-pH tables. Jam composition: Directive 2001/113/EC. Fish freezing before raw or lightly cured use: Regulation (EC) No 853/2004, Annex III. Checked 2026-09-06.

Sources for this food

• USDA Complete Guide to Home Canning (public domain)

Frequently asked questions

Related pages