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Influence of Cooking Conditions on the Protein Matrix of Sorghum and Maize Endosperm Flours

¹University of Pretoria, Department of Food Science, Pretoria 0002, South Africa. ²University of Nigeria, Department of Microbiology, Nsukka, Nigeria. ³Corresponding author. Phone: +27 12 4204296. Fax: +27 12 4202839. E-mail: jtaylor@postino.up.ac.za

To understand the influence of the sorghum and maize endosperm protein matrix honeycomb structure on starch hydrolysis in flours, three-dimensional fluorescence microscopy was applied to floury and vitreous endosperm flours cooked under various conditions. Cooking caused the collapse and matting of the sorghum and maize vitreous endosperm matrices, with the effect being greater in sorghum. The effect of cooking was rather different in the floury endosperm in that the protein matrices expanded and broke up to some extent. These effects were a consequence of expansion of the starch granules through water uptake during gelatinization. Cooking in the presence of 2-mercaptoethanol caused an expansion of the vitreous endosperm matrix mesh due to breakage of disulfide bonds in the protein matrix. Mercaptoethanol also caused an increase in the proportion of β-sheet structure relative to α-helical structure of the endosperm proteins. Increased energy of cooking caused collapse of the sorghum matrix. Disulfide bonding and an increase in β-sheet structure occurred with cooking, with the increase in disulfide bonding being greatest in sorghum vitreous endosperm. The tendency for the sorghum protein matrix to collapse and mat more with cooking than the maize matrix appears to be due to greater disulfide bonding. This is responsible for the observed low starch digestibility of cooked sorghum flour as a result of the more disulfide-bonded protein matrix limiting the expansion of the starch granules and hence amylase access.