Pectin gels

Pectin is a polysaccharide naturally contained in the cell walls of plants. Coupled with sugar and acid, pectin is what transforms cooked fruits into jams and jellies.

Brief history

The Ancient Greeks discovered that fruit cooked with honey developed a texture unlike any of its counterparts. By the 7th century there were several recipes for making delicate jellies made by boiling the juice of a quince with honey (quince is especially rich in pectin). The introduction of the sugar cane from Asia transformed jelly production. Unlike honey, sugar did not have excess moisture that needed to be evaporated. The Arab world used sugar to make fruit preserves through the Middle Ages, and took them to Europe in the 13th century. Still, sugar jams and jellies did not become common until after the 19th century, when sugar became cheaply available.

Transforming a jelly

Pectin, sugar, and acid are needed to make a fruit preserve, jam, or jelly. When fruit is cut up and heated near the boil, the pectin chains come off of the cell walls and dissolve into the released cell fluids and added water. Pectin molecules have the ability to bond with each other and form a meshwork that gives the jam a gel-like texture. However, pectin cannot accomplish this alone. When in solution, pectin molecules obtain a negative electrical charge that repels them from each other and they are too dilute to form a continuous network. To form the gel, sugar is added to absorb water molecules away from pectin, leaving the long chains exposed for bonding. In addition, boiling concentrates the pectin molecules as excessive water evaporates. Finally, acid provides the needed H+ that neutralize the negative electrical charge of pectin and allows the chains to come in close-enough proximity to bond.  The ideal conditions for pectin gelation are: a pectin concentration of 0.5 - 1.0%; a sugar concentration of 60 -65%; and a pH between 2.8 - 3.5, about the acidity of orange juice.