The endosperm of a sorghum mutant cultivar, with high in vitro uncooked and cooked protein digestibilities, was examined by transmission electron microscopy and ␣-, ␤-, and ␥-kafirins (storage proteins) were localized within its protein bodies. Transmission electron microscopy micrographs revealed that these protein bodies had a unique microstructure related to high protein digestibility. They were irregular in shape and had numerous invaginations, often reaching to the central area of the protein body. Protein bodies from normal cultivars, such as P721N studied here, with much lower uncooked and cooked digestibilities are spherical and contain no invaginations. Immunocytochemistry results showed that the relative location of ␣-and ␤-kafirins within the protein bodies of the highly digestible genotype were similar to the normal cultivar, P721N. ␥-Kafirin, however, was concentrated in darkstaining regions at the base of the folds instead of at the protein body periphery, as is typical of normal cultivars. The resulting easy accessibility of digestive enzymes to ␣-kafirin, the major storage protein, in addition to the increased surface area of the protein bodies of the highly digestible cultivar appear to account for its high in vitro protein digestibility.

Particle size associated with accessible surface area has a significant impact on the saccharification of plant cell walls by cellulolytic enzymes. Small particle sizes of untreated cellulosic substrate are more readily hydrolyzed than large ones because of higher specific surface area. Pretreatment enlarges accessible and susceptible surface area leading to enhanced cellulose hydrolysis. These hypotheses were tested using ground corn stover in the size ranges of 425-710 and 53-75 mm. Ultrastructural changes in these particles were imaged after treatment with cellulolytic enzymes before and after liquid hot water pretreatment. The smaller 53-75 mm corn stover particles are 1.5Â more susceptible to hydrolysis than 425-710 mm corn stover particles. This difference between the two particle size ranges is eliminated when the stover is pretreated with liquid hot water pretreatment at 1908C for 15 min, at pH between 4.3 and 6.2. This pretreatment causes ultrastructural changes and formation of micron-sized pores that make the cellulose more accessible to hydrolytic enzymes.

Lignin content, composition, distribution as well as cell wall thickness, structures, and type of tissue have a measurable effect on enzymatic hydrolysis of cellulose in lignocellulosic feedstocks. The first part of our work combined compositional analysis, pretreatment and enzyme hydrolysis for fractionated pith, rind, and leaf tissues from a hybrid stay-green corn, in order to identify the role of structural characteristics on enzyme hydrolysis of cell walls. The extent of enzyme hydrolysis follows the sequence rind < leaves < pith with 90% conversion of cellulose to glucose in 24 h in the best cases. Physical fractionation of corn stalks or other C(4) grasses into soft and hard tissue types could reduce cost of cellulose conversion by enabling reduced enzyme loadings to hydrolyze soft tissue, and directing the hard tissue to other uses such as thermal processing, combustion, or recycle to the land from which the corn was harvested.

Development of high‐protein digestibility (HPD)/high‐lysine (hl) sorghum mutant germplasm with good grain quality (i.e., hard endosperm texture) has been a major research objective at Purdue University. Progress toward achieving this objective, however, has been slow due to challenges posed by a combination of genetic and environmental factors. In this article, we report on the identification of a sorghum grain phenotype with a unique modified endosperm texture that has near‐normal hardness and possesses superior nutritional quality traits of high digestibility and enhanced lysine content. These modified endosperm lines were identified among F6 families developed from crosses between hard endosperm, normal nutritional quality sorghum lines, and improved HPD/hl sorghum mutant P721Q‐derived lines. A novel vitreous endosperm formation originated in the central portion of the kernel endosperm with opaque portions appearing both centrally and peripherally surrounding the vitreous portion. Kernels exhibiting modification showed a range of vitreous content from a slight interior section to one that filled out to the kernel periphery. Microstructure of the vitreous endosperm fraction was dramatically different from that of vitreous normal kernels in sorghum and in other cereals, in that polygonal starch granules were densely packed but without the typically associated continuous protein matrix. We speculate that, due to the lack of protein matrix, such vitreous endosperm may have more available starch for animal nutrition, and possibly have improved wet‐milling and dry‐grind ethanol processing properties. The new modified endosperm selections produce a range that approaches the density of the vitreous parent, and have lysine content and protein digestibility comparable to the HPD/hl opaque mutant parent.

Cereal Chem. 78(2): [160][161][162][163][164][165] Protein digestibility in sorghum (Sorghum bicolor (L.) Moench) lines was determined using two standard procedures (pepsin digestibility and pH-stat) and compared with a newly developed, rapid electrophoresisbased screening assay. The new assay was based on the rate of α-kafirin disappearance after pepsin digestion. α-Kafirin, the major sorghum storage protein, makes up ≈60-70% of the total protein in the grain. In the new assay, samples were first digested with pepsin for 1 hr, and undigested proteins were then analyzed by SDS-PAGE. The intensities of the undigested α-kafirin bands were measured. Higher band intensity indicated lower protein digestibility. The new assay was significantly correlated with the standard pepsin digestibility assay (r = -0.96, n = 16) after which it was patterned. The same was true of the pH-stat procedure (r = -0.85, n = 16). This implies that the new assay is comparable to existing procedures and can be used for screening sorghum lines for protein digestibility. Two groups consisting of high-protein digestibility and wild-type sorghum lines were identified when the new assay was tested on 48 sorghum lines derived from crosses of wild-type and mutant high protein digestibility lines, indicating that the new assay was efficient in differentiating between the two groups. Advantages of the new assay over the standard procedures include considerable reduction in analysis time and sample size required for the analysis. For example, analysis time was reduced by 20% and sample size by 10% when the new assay was used as compared with the pH-stat procedure. We estimate that ≈60 sorghum lines can be screened in a day by a single operator using the new assay.Sorghum (Sorghum bicolor (L.) Moench) grain ranks fifth among the world cereal grains. It is a major staple food in many parts of Africa and India. In the United States, it is mainly used as an animal feed grain. A significant problem with sorghum is its comparably poor nutritional quality. Protein digestibility in cooked sorghum was considerably lower when compared with other cereals (MacLean et al 1981; Hamaker et al 1986). The cooking process as used for human consumption reduces protein digestibility of sorghum due to rearrangement and formation of disulfide bonds (Hamaker et al 1987;Oria et al 1995b). This was demonstrated by the significant increase in protein digestibility observed when sorghum was cooked in the presence of 2-mercaptoethanol and other reducing agents that cleave disulfide bonds. Increasing protein digestibility in sorghum would benefit the undernourished poor. who rely heavily on sorghum as their main source of protein and energy. Proteins in sorghum grain used for animal feed are also somewhat less available than other grain proteins, and may further inhibit starch digestibility (Bramel-Cox et al 1995).The main proteins in sorghum are the aqueous alcohol-soluble storage prolamins known as kafirins. Sorghum prolamins are classified as α-, β-, and γ-kafirins based on m...