Jam

What it is

Jam is a fruit preserve made by cooking fruit with sugar until it sets into a spreadable mass. The fruit keeps its pulp in a jam, so the texture is soft and particulate. Jelly is the clear form, made from clarified juice, and a firmer fruit paste is the most concentrated form, cooked until it loses still more moisture.

All three depend on the same basic control: a high concentration of sugar and a low level of available water. The forms differ in how much of the fruit they keep and how far they are cooked.

How the keeping works

Sugar and cooking reduce the water that microbes can use. A preserve with enough dissolved sugar and a low level of free water resists the growth of many spoilage organisms. Acid helps as well: fruit supplies natural acid, and the record notes that acid is usually present, either from the fruit or added.

The cooking itself also changes the fruit. Pectin, the natural setting agent in many fruits, helps a jam or jelly set when sugar and acid are in balance. The record does not give the pectin levels or setting points, so this page does not state them.

Jam, jelly and paste

A jam keeps the pulp of the fruit and so has body and colour from the whole fruit. A jelly is made from clarified juice, so it is clear and firm. A fruit paste is cooked further to remove still more water, and it is the firmest of the three. These differences change what the cook does: whether the fruit is pulped, strained or reduced until it can be cut.

Limits of the record

The record gives no proportions of fruit to sugar, no cooking times and no storage limit. A jam is not safe on the strength of appearance or smell. A jar that bubbles, shows mould or has a sour, alcoholic smell has spoiled, and the fermenting or mouldy portion should not be eaten.