| Popis: |
The major use of sugar in preservation is in the manufacture of jam. This entails the boiling of fruit, sugar and water for the correct length of time so that a gel structure develops. The essential component for developing the gel structure is the pectin, either provided from the fruit itself or, in cases where there is insufficient in the fruit, added externally. The structure of a gel depends very much on the acidity, the optimum being pH 3. The best concentration of sugar is about 67.5 per cent although by adjusting the pectin composition it is possible to manufacture jam with as little as 60 per cent sugar. When pectin is added externally the usual concentration required is about 1 per cent by weight although this depends to some extent on the quality and origin of the pectin. The pectin level varies significantly for different fruits: blackcurrants, damsons and greengages have the highest content, approximately 1 per cent, whereas plums, gooseberries and apples have pectin contents varying between 0.75 per cent and 0.85 per cent and strawberries, raspberries, blackberries and redcurrants have contents varying between 0.5 per cent and 0.6 per cent. It is generally necessary to include commercial apple pectin in all jam formulations: usually 3–15 kg per 100 kg of manufactured jam is required. Commercial pectins are available in three basic forms: slow-setting, fast-setting and low-methoxy pectin. |
