How jam is made
A factory can make jam at 65 to 80 °C, well below boiling, by cooking it under a vacuum.
- stations
- 6
- things upstream
- 19
- raw sources
- 10
- steps from the ground
- 7
The short version
Crushed fruit is cooked with about its own weight in sugar until pectin from the fruit links up and the mixture will set when cool. Factories boil in open pans or in a vacuum vessel, which lowers the boiling point and saves flavor. The jam is heated briefly for safety and filled hot into jars.
-
Crush the fruit
Fruit is mashed or chopped into pulp. The fruit also supplies pectin, which is the gelling agent. Tart apples, currants and quinces carry plenty, while strawberries and peaches are low in it.
-
Cook with sugar
The fruit is boiled with sugar and water. A common starting point is equal weights of fruit and sugar. Sugar both sweetens and preserves.
Goes in -
Top up the pectin
For low-pectin fruit, makers add commercial pectin or high-pectin fruit, which also cuts cooking time. Pectin chains link into a network that holds the jam in a gel.
-
Boil down to the set point
The batch cooks until the acid and pectin react with the sugar and it will set on cooling. In an open pan this happens at 104 °C. Under a vacuum the boiling point falls so far that water leaves the jam at a lower temperature, which keeps more fruit aroma and avoids burnt sugar.
104 °C open pan; 65 to 80 °C under vacuum
-
Pasteurize briefly
A short heating kills pathogens that would otherwise grow.
95 to 100 °C
-
Fill jars while hot
A flame sterilizes the rim and lid of each jar, and steam is injected just before lidding. When that steam condenses, it makes a vacuum that pulls down the lid's safety button.
Goes in
Worth repeating
- Jam making became a great industry in 19th-century England, once sugar duties were repealed in 1874.
- Marmalade is jam made with citrus peel, and Seville oranges are prized for it because they carry so much pectin.
Back to the ground
Follow every input upstream and jam starts as these 10 raw sources.
Everything it is made of See it on the map
Open the whole family tree
Keep walking
Sources
Cite this page: "How jam is made." Assembly Line, 2026-11-03. https://assemblyline.org/make/jam