Cereal Chem. 71:476-482   |  VIEW ARTICLE


Effects of Minerals and Apparent Phytase Activity in the Development of the Hard-to-Cook State of Beans.

O. L. Kilmer, P. A. Seib, and R. C. Hoseney. Copyright 1994 by the American Association of Cereal Chemists, Inc. 

Orthophosphate, potassium, magnesium, and calcium ions were leached from pinto, red kidney, and Great Northern white beans during 16 hr of soaking in water at 25 C. The extent of leaching of the ions increased concurrently with the hard-to-cook (HTC) condition of the beans as they were aged at warm temperature and high humidity (WT/HH). Beans also developed the HTC defect during soaking at 41 C in acetate buffer of pH 4.8, but not in 0.05M fluoride ion. The inhibition by fluoride was reversed when fluoride- impregnated beans were soaked in an excess of tris-buffer at pH 6.8 and 25 C. Those results are consistent with fluoride ion inhibiting the hardening of beans through competitive inhibition of phytase, as opposed to its interference in the formation of magnesium and calcium pectinate through precipitation of magnesium and calcium ions as insoluble fluoride salts. Metaphosphate ion also inhibited the HTC condition in pinto beans, apparently by chelating the calcium and magnesium ions released during soaking at pH 4.8 and 41 C. A colorimetric method was developed to measure apparent phytase activity in ground cotyledons. Fifteen samples of beans from red, black, and white cultivars were found to have phytase activities that varied by a factor of three, whereas the half-lives for development of the HTC state at 46 C and 100% rh varied by a factor of 10.