Please explain pectin and why sugar and acid are needed when making jelly.
The molecules of pectin contain enormous chains of atoms, often hundreds or even thousands of atoms long.. Such chains are also found in cellulose and starch, and are used by plants to give them strength and structure. These chain-like molecules are naturally occurring polymers or plastics. The giant molecules in pectin are based on small molecular units of D-galacturonic acid that have joined together like strings of paper dolls. The presence of acid groups on the pectin molecules help to make pectins very water soluble and also sensitive to the acid-base balance of their environment. I am not an expert in the exact structure and chemistry of pectin, or in the proper pH needed for jellymaking, so I can’t give you an exact explanation for how to control the jelling process with acids. But the jell forms because these giant molecules spread out in the viscous solution of sugar and fruit juice, and form a tangled network of filaments that span the entire container. At high temperatures, there is enough mobility in the molecular chains to allow the mixture to flow, but at room temperature, the tangle of molecular filaments prevents flow. In the language of polymer or plastic science, the mixture goes from a liquid flow regime at high temperature to an elastic plateau regime at low temperature. When you deform cold jelly, you are pulling the filaments tight but they can’t disentangle themselves enough to allow the jelly to actually flow. When you deform the cold jelly too far, the filaments begin to break and the jelly tears into fragments. However, when you warm the jelly, thermal energy allows the filaments to move past one another and the jelly begins to flow like a thick (or viscous) liquid.