Protein nutritional value of sorghum and maize

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Localisation of proteins in the various anatomical parts of sorghum and maize

Protein distribution is uneven between the different anatomical portions of sorghum and maize. The most comprehensive investigations into protein compositions of the anatomical parts of maize and sorghum include those of Landry and Moureaux (1980) on maize and Taylor and Schussler (1986) on sorghum. These studies indicate that sorghum and maize are very similar with regard to localisation of proteins in the various parts of the grains. Approximately 3% of the total gram nitrogen is found in sorghum pencarp (Taylor & Schussler, 1986) but most of this pericarp protein was not extractable using the modified Osborne fractionation procedure of Landry and Moureaux (1970), possibly due to association with cell walls. Landry and Moureaux (1980) reported a similar content of protein in maize peri carp, 25% of which could be extracted with water and saline. The remaining protein was not subjected to further extraction with alcohol. Sorghum pericarp protein is rich in glycine, lysine and arginine. Small quantities of protein extracted with alcohol from sorghum pericarp had relatively low quantities of glutamic acid and rich in lysine compared to similarly extracted endosperm proteins, suggesting that they were not kafirins (Taylor & Schussler, 1986).
Sorghum germ contains approximately 16% of grain nitrogen (Taylor & Schussler, 1986) whilst two maize varieties studied had protein concentrations of20.1 % and 14.9% in the germ (Landry & Moureaux, 1980). Most of the germ protein occurs as low molecular weight nitrogen and albumin and globulin proteins and were rich in essential amino acids, especially lysine (Landry & Moureaux, 1980; Taylor & Schussler, 1986). Sorghum endosperm contains the highest proportion of grain nitrogen, approximately 80% and more than 60% of this protein is prolamin, rich in glutamic acid, proline, alanine and leucine but poor in lysine (Taylor & Schussler, 1986). Maize endosperm had a similar protein profile (Landry & Moureaux, 1980). The kafirins and zeins are the most abundant proteins of sorghum and maize grains and they are endosperm-specific (Landry & Moureaux, 1980;
Taylor & Schussler, 1986).
The G3-glutelin protein (extracted with buffer, reducing agent and detergent) was the second most important fraction in sorghum endosperm. It was poor in glutamic acid and rich in lysine compared to the kafirins. Taylor and Schussler (1986) suggest that the G3-glutelins may comprise the glutelin matrix surrounding the protein bodies in sorghum endosperm. The protein compositions of the vitreous (horny) and opaque (floury) portions of the endosperm are different. Work on sorghum revealed that vitreous endosperm contains 1.5-2 times more total protein than opaque endosperm (Watterson, Shull & Kirleis, 1993). Opaque endosperm also contained less kafirin (2.0-2.4%) compared to vitreous endosperm (5.8-8.5%). In contrast, opaque endosperm had higher levels of albumin and globulin proteins whilst the amount of glutelin protein was similar in both vitreous and opaque endosperm (Watterson et at., 1993).

Racemization and isopeptide formation

The amino acids of proteins are members of the L-series. Whilst D-amino acids occur in nature, they are not constituents of proteins (Coultate, 1990). The process whereby L-amino acids are converted to the D form is known as racemization. This conversion is of importance nutritionally because D-amino acids are absorbed much slower than the corresponding L form and even if digested and absorbed, most D isomers of essential amino acids are not utilised by man (Liardon & Hurrell, 1983). In addition, L-D, D-L and D-D peptide bonds introduced during the racemization process would resist attack by proteolytic enzymes which function best with L-L bonds (Friedman, Zahnley & Masters, 1981). Amino acid racemization occurs most readily after alkaline treatments (Masters & Friedman. 1979; Liardon & Hurrell, 1983; Jenkins, Tovar, Schwass, Liardon & Carpenter, 1984), but can also occur to a lesser extent in acid conditions (Ikawa, 1964; Manning, 1970; Jacobsen, Willson & Rapoport, 1974), and during severe heat treatment and roasting of proteins (Hayase, Kato & Fujimaki, 1975; Liardon & Hurrell, 1983).
Racemization of amino acids is believed to be a prelude to the formation of isopeptide bonds in proteins (Friedman et at., 1981). The racemized amino acid forms a dehydroprotein (also called a dehydroalanyl residue) by elimination of nucleophilic species like the disulphide group of cystine or hydroxyl group of serine (Friedman et aI., 1981; Erbersdobler, 1989; Orterburn, 1989). The isopeptide linkage is then formed when the dehydroprotein reacts with other amino acids. These amino acids may include cystine to form a lanthionine crosslink, lysine to form a lysinoalanine crosslink, arginine to form an ornithinoalanine crosslink and histidine to form a histidinoalanine crosslink (Friedman et aI., 1981; Erbersdobler, 1989; Otterburn, 1989). Isopeptide crosslinks can impair the nutritional quality of foods by decreasing the amount of essential L-amino acids and decreasing digestibility and bioavailability of proteins (Friedman et aI., 1981; Erbersdobler, 1989; Otterburn, 1989).

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CHAPTER 1: INTRODUCTION
CHAPTER 2: LITERATURE REVIEW
2.1 Sorghum and maize: Origin, physical characteristics and chemical composition
2.2 Food uses of sorghum and maize
2.3 Protein nutritional value of sorghum and maize
2.4 Factors affecting protein digestibility of sorghum and maize
2.5 Analytical methods for protein digestibility and protein conformation
2.6 Gaps in knowledge
2.7 Objectives and hypotheses
CHAPTER 3: MATERIALS AND METHODS
3.1 Grain samples
3.2 Sample preparation
3.3 Analytical methods
3.4 Statistical analyses
CHAPTER 4: RESULTS
4.1 Total protein and polyphenol contents of sorghum and maize samples
4.2 Ultrastructure of protein body-enriched samples
4.3 In vitro protein digestibility of whole grain, endosperm and protein body enriched samples and enzyme inhibition by whole grain
4.4 In vitro protein digestibility of reduced/alkylated and reduced/non-alkylated kafirin and zein
4.5 SDS-PAGE of protein body-enriched samples of sorghum and maize under non-reducing and reducing conditions
4.6 FTIR and HC NMR spectroscopy of uncooked and cooked protein body enriched samples of sorghum and maize
4.7 In vitro protein digestibility and FTIR spectroscopy of popped sorghum and maize
CHAPTER 5: DISCUSSION 
CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS 
CHAPTER 7: REFERENCES

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