Pectic substances are polymers of D-galacturonic acid with varying proportions of the carboxyl groups esterified with methyl groups. These polysaccharides are found in the middle lamella and in the cell walls of plant tissue. Pectin used for making jelly is obtained from slightly immature or just-ripe fruits. Most commercial pectin is derived from lemon and lime peels. A small amount is extracted from apple pomace. Sugar and acid are required for pectin to form a gel. Sufficient pectin must also be present for gel formation. The amount of pectin present in fruit juice can be determined by measurement of the viscosity of the juice or by precipitation with alcohol.