These compounds found in food are closely allied to the carbohydrates, but are neither starchy, saccharine, nor oily. Gum exists in the juices of almost all plants, coming from the stems, branches, and fruits. Examples: Gum arabic, gum tragacanth, and mucilage. Pectose exists in the fleshy pulp of unripe fruit; during the process of ripening it changes to pectin; by cooking, pectin is changed to pectosic acid, and by longer cooking to pectic acid. Pectosic acid is jelly-like when cold; pectic acid is jelly-like when hot or cold. Cellulose constitutes the cell-walls of vegetable life; in very young vegetables it is possible that it can be acted upon by the digestive ferments; in older vegetables it becomes woody and completely indigestible.