The commonest cause of home-made jam spoiling is not the recipe but the jar. And more often than the jar, the lid.
Sterilisation or pasteurisation?
The two terms get confused but describe different processes.
Pasteurisation happens between 60 and 90 degrees and kills vegetative microorganisms.
Spores survive.
Sterilisation happens above 100 degrees, usually under pressure, and kills everything including spores.
True sterilisation is difficult at home; it requires pressurised equipment.
But jam does not need it.
Jam is high in sugar and low in pH, and those two prevent spores developing.
The aim with jam is therefore killing vegetative microorganisms and preventing recontamination.
That can be achieved at boiling-water level.
Despite being commonly called sterilisation, the process is really an intensive pasteurisation.
How are the jars prepared?
Three methods are used.
The first is boiling. The jars go into a pan, covered with water, and boil for ten minutes.
The commonest method; it needs no equipment.
A cloth should go under the jars; glass touching the base of the pan directly can crack.
The second is the oven. Washed jars are held at 140 degrees for fifteen minutes.
Dry heat; the jars come out hot and ready for filling.
But lids must not go into the oven; the plastic in the seal degrades.
The third is steam. The jars are held over steam for ten minutes.
A dishwasher's highest-temperature programme can be used too, though it is not considered sufficient on its own.
| Method | Temperature | Time | Suitable for lids |
|---|---|---|---|
| Boiling | 100 °C | 10 minutes | Yes |
| Oven | 140 °C | 15 minutes | No |
| Steam | 100 °C | 10 minutes | Yes |
| Dishwasher | 70-75 °C | Full cycle | Partly |
| Microwave | Variable | — | Not advised |
Why is the lid the real problem?
In spoiled jam the source is usually here.
Metal lids carry a plastic or rubber seal on the inside.
That seal lets the lid sit airtight against the jar.
But the same seal is also a refuge for microorganisms.
Its surface is uneven and difficult to clean.
The lid cannot go into the oven; high dry heat deforms the seal and the airtight fit is lost.
The correct method is boiling the lids for five minutes in a separate pan.
Afterwards they should sit on a clean cloth with the inner face upward.
The inner face should not be touched by hand.
Previously used lids are best avoided; the seal loses its elasticity and the vacuum does not form fully.
A lid that is deformed, scratched or rusted should never be used.
Why does the filling temperature matter?
Jam should be filled hot; that is the system's second line of defence.
Filled into the jar above 85 degrees, the air inside heats and expands.
After the lid is closed the jar cools, the air contracts and a vacuum forms inside.
That vacuum pulls the lid inward and strengthens the seal.
Oxygen in the headspace also falls, making mould growth harder.
Hot jam should go into a hot jar; boiling jam poured into a cold jar cracks the glass.
The jar should not be filled to the brim; a centimetre of headspace should be left.
That space is necessary for the vacuum to form.
The rim should be wiped with a clean cloth after filling; jam left on the rim stops the seal seating properly.
How do you know the vacuum formed?
Three checks apply.
The first is sound. A "click" from the lid as the jars cool indicates the vacuum has formed.
That sound usually comes anywhere from fifteen minutes to an hour after filling.
The second is appearance. The centre of the lid should be dipped inward.
A flat or domed lid indicates no vacuum.
The third is a pressure test. Pressing the centre of the lid with a finger should produce no movement.
If it clicks, there is no vacuum.
Jars without a vacuum should be eaten within a week or kept in the fridge.
Alternatively the lid can be replaced and the process repeated, but the jam must be reboiled.
What mistakes are common with jars?
Six recur.
The first is not sterilising the lid; the commonest of all.
The second is inverting the jar. That method is widespread but debated; the jam touches the seal and the airtight fit is put at risk.
The third is filling to the brim, leaving no headspace for a vacuum.
The fourth is letting sterilised jars stand. A prepared jar should be filled immediately; left open it becomes contaminated again.
The fifth is drying with a cloth. A sterilised jar should not be dried; the cloth is a source of recontamination.
The jar dries on its own residual heat.
The sixth is insufficient sugar. Jam low in sugar is less protected; those products belong in the fridge and should be eaten quickly.
How is it stored and when should you be suspicious?
Sealed jars should be kept somewhere cool, dark and dry.
Light fades the colour and reduces vitamin C.
Under suitable conditions, high-sugar jam keeps one to two years.
Reduced-sugar jam keeps six months.
Once opened it belongs in the fridge and should be eaten within two to four weeks.
A clean, dry spoon should be used; a knife that has touched bread contaminates the jar.
There are five signs of spoilage.
Mould at the surface, bubbles, a sour smell, a lid domed outward and marked darkening of colour.
Even where mould appears only at the surface the product should be discarded; mould threads reach depths invisible to the eye.
A domed lid is the most serious sign; the jar should be discarded unopened.
Crystallisation, by contrast, is not spoilage; it is normal behaviour in sugar and the product is safe to eat.
Which jars should be used?
The jar itself affects the result.
Thick-walled jars withstand heat treatment better; thin glass cracks under a temperature difference.
Wide-mouthed jars are easier to fill, but the sealing surface is larger and the risk of a poor seal rises.
Smaller jars are preferable; they are used up quickly once opened and the risk of spoilage falls.
Two hundred and fifty to four hundred grams is a practical range.
Jars that are scratched or chipped at the rim should not be used; the lid will not seat properly.
Jars that previously held pickles or spiced products can retain odours.
