Obtained by hydrolysis of pectin where it is present as polygalacturonic acid: Ehrlich, Chem. Ztg. 41, 197 (1917); Ehrlich, Guttmann, Biochem. Z. 259, 100 (1933); Ber. 66, 220 (1933); Niemann, Link, J. Biol. Chem. 95, 203 (1932); 104, 743 (1934); Morell, Link, ibid. 100, 385 (1933); Anderson, King, J. Chem. Soc. 1961, 5333. Isoln from mustard seeds: Goering, US 2987448 (1961 to Oil Seed Prod.).