We have shown previously that sorghum is highly digestible in the rat. However, other workers have shown that sorghum is much less digestible than wheat, maize, and rice in young children. Because the rat does not show these digestibility differences, we developed an empirical pepsin digestion method, furst reported in 1981, which simulates the digestion values found in children. In this report the method has been improved and used to analyze wheat, maize, rice, millet, and sorghum and certain processed samples of millet and sorghum. The pepsin digestion values parallel those found in children for wheat, maize, rice, and sorghum. In addition, a processed sorghum product that gave a high digestion value in children also gave a high value with the in vitro pepsin method.Sorghum [Sorghum bicolor (L.) Moench] is an important source of calories and protein for a large segment of the human population in the semiarid tropics.In a previous publication (1), the apparent digestibility of the proteins in four (cooked) sorghum gruels varied from 76.5% to 83.0% (average, 80.4%) in young rats. The same four sorghum flours, when cooked and fed to children of age 6-27 months, gave an average apparent digestibility value of only 46%, compared with values of 81%, 73%, and 66% for wheat, maize, and rice, respectively (2). Because the rat gave values for cooked sorghum that were the same as those MacLean et al. (2) found for cooked wheat in the child, we developed an empirical in vitro pepsin method (1) that more closely simulates MacLean's values in children. In this report we present improvements in our original method and data on the pepsin digestibility of the major cereals. We also include data on processed sorghum and millet products [sorghum and millet nasha (fermented baby foods) and extruded decorticated sorghum].MATERIALS AND METHODS found on the grain in Sudan), was added, and the mixture was incubated at 30'C for 12 hr. The slurry was fed to a laboratory drum dryer or was freeze-dried, and the dry residue was used for digestibility studies.Pepsin. The pepsin used was porcine pepsin 1:10,000 (Sigma). It had an activity of 1,200-2,000 units/mg of protein (A28) (3).Modified Pepsin Method (Residue Method). Ground cereal samples (200 mg) prepared with a Udy grinder and 0.4-mm screen were suspended in 35 ml of a solution of pepsin (1.5 mg/ml) in 0.1 M phosphate buffer (pH 2.0); the mixture was incubated with gentle shaking at 370C for 2 hr. Incubation for 3 hr raises all values about 10% above the 2-hr values; incubation for 1 hr lowers all values about 10% below the 2-hr values. Therefore, time of incubation is critical in the assay. After incubation the suspension was centrifuged (12,000 x g for 15 min at 40C), and the residue was suspended in 10 ml of 0.1 M phosphate buffer (pH 2.0) and centrifuged as before. The low-pepsin moist residue was freed from the walls of the centrifuge tube with a spatula and dumped in the center of Whatman filter paper no. 3 on a 43-mm Buchner funnel. Suction (aspirator) was applied, an...

We have shown in previous reports that cooked sorghum protein is less digestible than other cooked cereal proteins. The pepsin-indigestible proteins in sorghum were found to be mainly prolamin proteins. Cooking sorghum in the presence of 2-mercaptoethanol increased protein digestibility (in vitro with pepsin or trypsin/chymnotrypsin) to a level comparable with other cereals. At a concentration of 100 mM, other reducing agents (dithiothreitol, sodium bisulfite, and L-cysteine) were equally effective in improving sorghum digestibility. When maize was cooked in the. presence of 2-mercaptoethanol, protein digestibility increased 5% compared to 25% for sorghum. Cooking barley, rice, and wheat with 2-mercaptoethanol had no significant effect on protein digestibility. The addition of reducing agents appears to prevent the formation of protein polymers linked by disulfide bonds.Sorghum [Sorghum bicolor (L.) Moench] is one of the major food crops of the semiarid regions of Africa and Asia and a major animal feed crop in the Americas and Europe. Sorghum is considered to be poor people's food and is generally consumed in rural areas, where it may comprise >70% of the food intake (1). Because sorghum is eaten in areas where the populations are frequently undernourished, it is important to consider the quality, quantity, and availability of the nutrients in the grain.Nitrogen-balance studies conducted on children or young adults using sorghum cooked to a gruel or a bread have shown that the proteins are poorly digested relative to the proteins of other cereals (2-4). In studies with Peruvian children, MacLean et al. (4) reported that sorghum gruels from four tannin-free cultivars had an apparent protein digestibility mean value of 46%, while rice, maize, and wheat gruels gave values of 66%, 73%, and 81%, respectively.Axtell et al. (5), using an in vitro pepsin digestibility method to estimate protein digestibility of the sorghums supplied to MacLean, reported that digestibility values decreased significantly after cooking, indicating that cooking may alter the sorghum proteins and make them less digestible. Cooked sorghum proteins digested with trypsin/chymotrypsin or pepsin followed by trypsin/chymotrypsin were also poorly digested compared to those of the uncooked flour (6). On the other hand, cooked maize flour was either as digestible or slightly more digestible than the uncooked flour when pepsin and/or trypsin-chymotrypsin were used.Animal feeding studies also support the premise that cooking causes a reduction in digestibility of sorghum proteins. Mitaru et al. (7) found that chickens digested 31.5% less protein from cooked, whole-grain, low-tannin sorghum than from the uncooked grain. Rats fed with low-tannin sorghum that was cooked to a gruel digested 7% less protein than those fed the uncooked flour (8).Recent findings (6) showed that cooking sorghum preferentially decreased the pepsin digestibility of the alcoholsoluble prolamin proteins, the kafirins. Cooking also substantially altered the solubility pr...

Uncooked sorghum was almost as digestible as uncooked maize, when assayed in vitro with pepsin and/or a trypsin-chymotrypsin mixture. However, after cooking, sorghum protein digestibility was significantly decreased. In contrast, cooking maize had no effect on pepsin digestibility and increased trypsinchymotrypsin and pepsin-trypsin-chymotrypsin digestibility. After cooking, sorghum prolamins, measured by the Landry-Moureaux procedure and SDS-PAGE, became much less soluble and less pepsin digestible than maize prolamins. These findings demonstrate that sorghum and maize proteins behave differently when cooked and may explain why cooked sorghum has a lower digestibility than maize, wheat, and rice in children.

The reduction of zein synthesis in the maize (Zea mays L.) opaque-2 mutant is associated with an increased percentage of lysine in the endosperm protein. When expressed on an endosperm basis, we found that W64A opaque-2 contains 490 micrograms of lysine compared with 350 micrograms in W64A normal. SDS-PAGE analysis of endosperm proteins indicated that several non-zein proteins are more abundant in the mutant than in normal genotype. To determine the subcellular origin of these proteins, we separated an endosperm homogenate from developing kernels by sucrose density gradient centrifugation and used marker enzyme assays and immunoblot analyses to identify cellular components. Amino acid analysis of proteins in the gradient fractions showed that the majority of the lysine occurs in soluble proteins at the top of the gradient. To identify these proteins, we prepared a complex antiserum against the entire soluble protein fraction and used it to immunoscreen an endosperm cDNA expression library. Sequence analysis of clones identified mRNAs involved in carbohydrate metabolism, amino acid biosynthesis, and protein synthesis. RNA dot blot hybridization analysis with these clones revealed significant variation in the levels of transcripts between normal and opaque-2 endosperm, but we identified several mRNAs that are elevated in opaque-2 and that may encode proteins responsible for the enhanced lysine content.

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