Sugar preserving is the most forgiving method in this domain, which is why it is the one to start with. Unlike canning low-acid food, the failure modes are visible: a jam that has gone wrong looks wrong, smells wrong and grows mould, rather than being silently dangerous.
That said, it only works at the right concentration, and the commonest modern mistake is reducing the sugar because a full-sugar preserve seems excessive.
How sugar preserves
Sugar ties up the water. Micro-organisms need available water to grow, and at a high enough sugar concentration there is effectively none left for them. That is the whole mechanism, and it is the same reason honey and salt preserve.
The threshold is roughly 60% sugar in the finished preserve, which is what standard recipes achieve. Below that, keeping quality falls away quickly.
Which means sugar here is not a matter of taste. It is the preservative, in the same way that vinegar is in a pickle and salt is in a cure. See cure meat and fish, which makes the same argument about salt.
If you want a low-sugar preserve, that is entirely reasonable and it is a different product: use a recipe designed for it, and keep it in the fridge or the freezer, or heat process it and treat it as a short-keeping preserve.
Pectin, acid and sugar
A set needs all three, and a jam that will not set is short of one of them.
Pectin falls as fruit ripens, so a proportion of slightly underripe fruit improves the set. Overripe fruit makes better flavour and worse jam.
Boost a low-pectin fruit by adding cooking apple or crab apple, by adding the juice and pips of a lemon, by using preserving sugar with added pectin, or by adding commercial pectin.
Acid does two jobs: it is required for the pectin to gel, and it helps keeping. Add lemon juice to anything not already sharp, roughly the juice of one lemon per kilogram of fruit as a starting point.
A muslin bag of apple cores, peel and lemon pips simmered with the fruit and then squeezed out is the traditional free pectin source, and it works well.
Jars
- Wash in hot soapy water, rinse thoroughly, and check every rim for chips. A chipped rim will not seal.
- Sterilise. Oven at around 120 degrees for fifteen minutes, or a hot dishwasher cycle finishing as you need them, or boiling water. Whichever you use, the jars must be hot when you fill them, or they crack.
- New lids for anything with a rubber sealing compound, because that compound is single use.
- Fill to the brim for jam, which is the traditional method, and seal immediately while everything is hot.
- Waxed discs and cellophane are the older method and work; screw-top lids are simpler and give a better seal.
For long storage and reliability, a short water bath process after filling is the more rigorous route and is what modern preserving guidance recommends. See water bath canning, jar to seal and water bath or pressure: choosing correctly.
Jam is high in sugar and high in acid, which is why it is a water bath candidate rather than a pressure canning one. That distinction matters enormously elsewhere and is the subject of the choosing page.
Making jam
- Prepare the fruit. Hull, stone, chop. A proportion slightly underripe.
- Soften the fruit first, in a little water, before the sugar goes in. Sugar toughens skins, so anything with a skin must be soft before you sweeten it. This is the step people skip and it is why some jams have leathery fruit in them.
- Add the sugar and stir over a low heat until it has completely dissolved. Undissolved sugar crystallises later.
- Then boil hard. A rolling boil that cannot be stirred down. A simmer will not reach setting point in a sensible time and overcooks the flavour.
- Skim the foam near the end, or stir in a small knob of butter to disperse it.
- Test for set from about five minutes of hard boiling, and keep testing.
- Rest for ten minutes before potting anything with whole fruit or peel, so it does not all float to the top.
- Pot hot, seal at once, label with the contents and the date.
Use a wide pan, much larger than you think, because a rolling boil rises alarmingly. A preserving pan is wide and shallow for fast evaporation and has room to spare.
Testing for set
Three methods, and use two.
- Thermometer. Setting point is around 104 to 105 degrees C. Fast, reliable, and the easiest for a beginner. A sugar thermometer is inexpensive.
- Wrinkle test. Put two or three saucers in the freezer before you start. Take the pan off the heat, drop a teaspoon of jam onto a cold saucer, wait half a minute, and push it with a finger. If the surface wrinkles, it is set. Take the pan off the heat while you test, or it overshoots.
- Flake test. Lift a wooden spoon and let the jam run off. When drips join together and fall as a flake rather than as drops, it is close.
Overcooked jam is stiff, dark and tastes of caramel. Undercooked jam is syrup. Test early and often rather than trusting a time.
Jelly
The same chemistry, strained.
- Simmer the fruit with water until completely soft. Do not add sugar yet.
- Strain through a jelly bag or several layers of muslin, suspended over a bowl, and leave it for several hours or overnight.
- Do not squeeze the bag. Squeezing forces solids through and makes the jelly cloudy. This is the one rule of jelly making.
- Measure the juice, add sugar in the proportion the recipe gives, dissolve, and boil to setting point exactly as for jam.
Crab apple, quince, blackberry, redcurrant and rowan all make excellent jellies and are all high in pectin, which is why hedgerow jelly is the classic use for a foraged autumn. See hedgerow foraging by season.
Yield is low. A jelly uses far more fruit for the same number of jars, which is the price of the clarity.
The other sugar preserves
- Jam: crushed or chopped fruit, set.
- Jelly: strained juice, set, clear.
- Marmalade: citrus, including the peel, and the pith and pips supply the pectin.
- Conserve: whole fruit suspended in a lighter syrup, softer set.
- Fruit butter and cheese: fruit pulp cooked down long and slow with sugar until thick. Quince cheese is the classic and it keeps extremely well.
- Fruit in syrup: fruit bottled in sugar syrup, which needs heat processing to keep.
- Candied and crystallised fruit: fruit gradually saturated with syrup over days. Slow and long keeping.
- Fruit leather: puree dried in sheets. No sugar needed for the preservation itself. See dry food, and make jerky safely.
Storing and judging it
- Cool, dark and dry. Warmth and light both degrade colour and flavour.
- Label with the contents and the date. Everything looks the same in eighteen months.
- A properly made full-sugar jam keeps for a year or more unopened.
- Refrigerate after opening, and use within a few weeks.
- Check the seal before opening: a screw lid should not flex, and there should be no seepage.
When it goes wrong
- Will not set. Short of pectin, acid or sugar, or not boiled hard enough. Return it to the pan, add lemon juice, and boil harder. If it still will not set, bottle it as a syrup and use it as one.
- Too stiff. Overcooked. It is still edible and it will be dark. Warm it gently with a little water to loosen.
- Crystallised. Sugar not fully dissolved before boiling, or too little acid.
- Fruit floating at the top. Not rested for ten minutes before potting.
- Cloudy jelly. The bag was squeezed.
- Shrunk and dry at the top. A poor seal, letting moisture out.
- Foam. Cosmetic. Skim it, or stir in a knob of butter.