Jam is made from crushed or ground fruit. The end product is less firm than jelly, but still holds its shape. The fruit content must be at least 40%. In mixed fruit jams the first-named fruit must be at least 50% of the total fruit. The total sugar content must be no less than 68%. In tropical climates, 70% sugar is preferable.
Fruit furnishes the flavor and part of the needed pectin and acid. Some irregular and imperfect fruit can be used.
In the manufacture of jam, the deforested fruit is passed through a pulper that allows essentially all the berries with the seeds to pass through the strainer.
Sugar and water are then added the amount of sugar depending on whether the frozen product was packed with or without sugar. About 0.1 – 0.15% dispersed pectin is then added and the product is heated in open kettles to concentrate it to soluble solids content about 68%.
When a commercial pectin is added, fully ripe fruit can be used. The use of this added pectin also increases the yield from a recipe.
Citric acid solution is then added to adjust the pH to 3.3 and the fruit is packed in glass jars and vacuum capped. The jars should be clean and sterilised. The ideal temperature for pouring is 82-85°C. Hotter than this and condensation will form under the lid. This will drop down and dilute the jam, allowing mould to grow. Colder than this and the jam will be difficult to pour.
Manufacturing of fruit jam
