Protein composition and in vitro protein digestibility of developing Sorghum bicolor (L.) Moench kernels with different endosperm texture

The overall objective of the study was to determine if there are differences in the protein composition of the sorghum bicolor varieties with soft, intermediate and hard endosperm texture as the kernel develops. The sorghum kernels were separated into three fractions, free protein bodies, matrix bound protein bodies, and residue. The protein composition of free protein bodies and the faster sedimenting matrix bound protein bodies was studied as the kernel develops. The protein composition of the soft, intermediate, and hard varieties was compared. The difference in vitro pepsin digestibility of free and matrix bound protein bodies was also studied. The amount of protein decreased from 10 to 40 DAHB in free protein body fraction and increased in the matrix bound protein body fraction. Gel electrophoresis showed that the $\alpha$- and the $\beta$-kafirins developed at earlier stages of kernel development in the free protein bodies. the $\gamma$-kafirin is completely absent in the free protein bodies fraction even at 40 DAHB. In the matrix bound protein bodies kafirin synthesis begins at later stages of kernel development and the $\gamma$-kafirin is a major component. HPLC showed that there were qualitative differences in kafirins from free and matrix bound protein bodies. Amino acid analysis of purified $\gamma$-kafirin showed that higher amounts of proline, histidine and cysteine were present compared to other kafirins. The softer variety had higher amount of protein in the free protein body fraction and lower amount of protein in the residue than the hard variety at all stages of kernel development. The amount of low molecular weight nitrogen, and albumin/globulin proteins was higher and glutelin proteins was lower in the soft variety compared to the hard variety throughout development. Gel electrophoresis revealed qualitative differences in the albumin/globulin proteins and the glutelin proteins of the hard and soft variety. HPLC of the free protein bodies fraction showed that there were ten kafirin protein components in the hard and only seven in the soft variety. The matrix bound protein bodies and the residue protein also differed qualitatively in the hard and soft varieties. Cytoplasmic condensation and matrix formation was found to occur at earlier stages of kernel development in the hard variety than in the soft variety. Both free and matrix bound protein bodies were more digestible before cooking than after cooking. Pepsin digested 97% of the protein in the uncooked free protein bodies and 90% of the protein in the uncooked matrix bound protein bodies. However, after cooking 83% of the protein in the free protein bodies was digested whereas only 59% of the protein in the matrix bound protein bodies was digested. SDS-PAGE showed that the undigested residue contained kafirin proteins