The glue that holds plant cells together is what sets your jam
Pectin cements neighbouring plant cells together. When fruit ripens, enzymes dissolve it and the flesh goes soft; when autumn leaves drop, the same breakdown frees their stalks. Extracted from citrus peel, that cellular glue becomes the powder that turns boiled fruit and sugar into a wobbly jelly.
The French chemist Henri Braconnot isolated pectin in 1825 and found it in the leaves, stems and fruit of many plants. Apples, pears, quince, plums, gooseberries and citrus are rich in it, while cherries, grapes and strawberries carry much less, which is why cooks often add it to soft-fruit jams. The name comes from an Ancient Greek word for congealed or curdled. Chemically it is a long, complex sugar chain built mostly from galacturonic acid, a relative of galactose, which forms roughly 65% of its repeating units.
Inside the plant it is the main ingredient of the middle lamella, the layer bonding one cell to the next, and it lets young cell walls stretch as the plant grows. It is packaged in the Golgi apparatus and shipped to the wall in small vesicles. Its structure alternates smooth, unbranched stretches with hairy regions where side chains of sugars such as arabinose and galactose sprout from occasional rhamnose units that kink the chain.
How pectin sets depends on how many of its acid groups carry a methyl tag. In nature around 80 percent do. High-methoxy pectins, with more than half tagged, gel only in acidic conditions with lots of sugar: above 60% dissolved solids and a pH between 2.8 and 3.6, sugar soaks up the water and forces the strands into a three-dimensional net. Low-methoxy pectins instead need calcium, which bridges the chains in what chemists call the egg box model, and can set across a wide pH range with far less sugar. A modified amidated form even re-sets after reheating, unlike ordinary pectin gels.
People eat about 5 g a day if they manage 500 g of fruit and vegetables. The small intestine cannot digest it, but bacteria in the large intestine ferment it into short-chain fatty acids, making it a soluble fibre. Studies report it trims LDL cholesterol slightly, by 3 to 7%, apparently by thickening gut contents, with apple and citrus pectin working better than orange pulp fibre. One oddity: breaking down fruit pectin in the colon can raise the body's methanol level by up to tenfold.
Source: Pectin
