EFFECT OF HEAT AND REDUCING AGENTS ON IN VITRO DIGESTIBILITY OF SORGHUM PROTEINS (PEPSIN)

BRUCE RANKIN HAMAKER, Purdue University

Abstract

Studies were conducted with the goal of determining why sorghum proteins are considerably less digestible than other cereal proteins. It was shown that the pepsin or trypsin/chymotrypsin-indigestible proteins of either uncooked or cooked sorghum are primarily kafirin proteins. Using either enzyme system, cooking reduced the digestibility of sorghum kafirins but had no effect on the digestibility of maize zeins. The solubility of kafirins in aqueous alcohol was substantially decreased after cooking and more than for zeins. A variety of reducing agents were shown to significantly improve the in vitro digestibility of uncooked or cooked sorghum proteins. The addition of a reducing agent brought sorghum protein digestibility to the level of maize, barley, rice, and wheat. When sorghum flour was soaked in a 100 mM solution of 2-mercaptoethanol for 12 hr, pepsin solubilized over 70% of the sorghum proteins in the first 15 minutes compared to above 40% in untreated flour, and about 20% in sorghum that had been cooked. Scanning electron micrographs showed that the kafirin-containing protein bodies of the sorghum endosperm retained their structural integrity during cooking. Also, using this technique, the protein bodies seemed to be less susceptible to pepsin digestion than the matrix proteins. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration chromatography, sorghum proteins were shown to form intermolecular disulfide bonds during the cooking process. These disulfide-linked protein polymers occurred to a greater extent among the glutelin protein fraction, which make up the protein matrix, but also occurred to some extent with the kafirin proteins. Following cooking, an extremely large molecular weight complex was found in the void volume fraction from a Sepharose CL-2B column. When separated by SDS-PAGE, these proteins when reduced were found primarily to be non-kafirin proteins in the molecular weight range of 50,000-70,000 daltons. These proteins, however, were well digested by pepsin. Three glutelins (100,000-120,000 dalton monomers) were found to be less digestible and were solubilized by pepsin just prior to the start of kafirin digestion. These may be the proteins associated with the protein bodies and may be related to kafirin digestibility. Accordingly, we propose that the decrease in kafirin digestibility after cooking is due to two factors: (1) the formation of a disulfide-bound protein coat surrounding the protein bodies, and (2) polymerization of the kafirins themselves.

Degree

Ph.D.

Subject Area

Food science

COinS