Oranges contain fibre, which is mainly in the form of pectin. Pectin is a common fibre in fruit and it gives fruit health properties that are not present in pectin-free foods. One aspect of pectin that is useful is the ability of pectin to gelitanise. This is taken advantage of in the making of orange marmalade and other preserves. The juices of oranges contain about 4.5 % pectin with lesser amounts of other fibres such as hemicellulose (2 %) and cellulose (1.5 %). Pectin methylesterase is an enzyme found in the cell walls of oranges, and when the juice is released the enzyme enters solution. The enzyme causes the removal of methoxy groups which leads to the formation of carboxylic radicals. Once esterification becomes established calcium ions allow the crosslinking of adjacent pectin molecules causing the formation of insoluble macropolymers that can cause the process of gelation. This allows the juice to set, in effect making marmalade. Inactivation of the enzyme prevents gelation and the juice remains fluid.
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