Research into methods for measuring sorghum and maize hardness

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Sorghum and maize kernel structures

The structure and chemistry of a kernel play a crucial role in determining the processing properties of a cereal grain. According to Kent and Evers (1994), the kernel characteristics of shape, size and mass are the most important in respect of cereal grain quality. Sorghum and maize kernels are similar in their structure, chemical composition and biochemical basis for hardness (reviewed by Chandrashekar and Mazhar 1999). However, the relative proportions of the pericarp, germ and floury and corneous endosperm in kernels vary among varieties. The structure of the sorghum kernel has been reviewed in depth by Rooney and Miller (1982). Sorghum is a naked caryopsis, comprising 8% pericarp, 10% germ and 82% endosperm. Serna-Saldivar and Rooney (1995) and Watson (2003) described the structure of the maize kernel. It is also a naked caryopsis and comprising about 85% endosperm, 10-14% germ and 5-6% tip cap and pericarp. The maize kernel is the largest of cereal grains and weighs about 350 mg compared to 30 mg of sorghum. Figs 2.1a and 2.1b show the longitudinal sections of the sorghum and maize kernels, respectively.

Pericarp

Rooney and Miller (1982) explained that the pericarp of sorghum grain has three sections, namely the epicarp, mesocarp and endocarp. The epicarp is the outer most layer with thick walled rectangular cells and pigments, which strongly influence kernel colour. Pericarp colour is genetically controlled by R and Y genes resulting in red (R Y), yellow (rrY) and white (Ryy or rryy) sorghum colours (Earp et al 2004). The endocarp is the innermost layer of the pericarp and consists of cross and tube cells. The sorghum mesocarp is several layers thick and seemingly determines pericarp thickness. The pericarp thickness varies among sorghum genotypes and within individual kernels with the thickest part at the crown and the thinnest area over the embryo. The study by Earp et al (2004) revealed that the pericarp thickness varied among sorghum varieties related to the quantity of starch granules in the mesocarp. Their study showed varieties with a thin pericarp had fewer starch granules than those with a thick pericarp. Fig 2.2 shows the sections through the pericarp of tannin sorghum with starch granules in the mesocarp, and the testa and aleurone layers. The testa layer is thicker in type II and type III sorghums, which are pigmented and contain condensed tannins.

Pasting

Workers have investigated whether there are relationships between sorghum and maize pasting properties and grain hardness. Almeida-Dominguez et al (1997) used a Rapid Visco Analyser (RVA) to distinguish maize kernels of varying hardness. The authors found that peak viscosity was correlated with kernel hardness values measured with a TADD, density by floatation and endosperm texture. In their study, endosperm texture and proteins were thought to affect the pasting behaviour of maize of different hardness levels. In floury kernels, hydration proceeds with ease as the starch granules are loosely packed.
However, in harder grains, the starch granules are compacted by the protein matrix and may require longer hydration times, thereby exhibiting lower peak viscosities than floury cultivars. Taylor et al (1997) observed lower peak viscosity in harder sorghum. Kafirins of sorghum are also thought to play a role in lowering viscosity of hard sorghum since they surround starch granules and their hydrophobicity and disulphide bonding presumably limit water penetration to the starch granules (Chandrashekar and Mazhar 1999). According to Chandrashekar and Kirleis (1988), higher levels of kafirin containing protein bodies in hard sorghum affects pasting by hindering starch gelatinisation (actually granule expansion). The protein bodies remain buried in the protein matrix even after cooking. Ezeogu et al (2008) showed that in hard sorghum (corneous endosperm), the protein matrix collapsed and matted extensively due to high levels of disulphide bonding between matrix proteins. However, this matting was lower in maize, due to limited disulphide bonding. Moreover, starch granules of hard sorghums appeared to be enclosed in protein matrix and cell wall and this packing also affected granule expansion (Ezeogu et al 2008).

Near infrared spectroscopy

The principle underlying near infrared spectroscopy (NIRS) is that light of a particular wavelength in the near infrared region is absorbed by some bonds such as C-H, O-H and N-H, which vibrate in proportion to their concentration in the grain. Samples can either reflect the light in Near Infrared Reflectance (NIR) or transmit light in Near Infrared Transmittance (NIT) spectroscopy. Williams (1979) used the NIR for screening wheat for protein and hardness. The equipment was calibrated for wheat hardness using the particle size index (PSI) test. Three types of mills were used to grind samples and the burr mill was considered the most suitable for NIR hardness testing as it could clearly screen wheat cultivars of different hardness. De Alencar Figueiredo et al (2006) tested sorghum for hardness with NIR calibrated using PSI. The authors also concluded that the nature of the sample affected the calibration and that ground samples gave better calibration equations than whole grain. Wehling et al (1996) used NIR spectroscopy to predict dry milling quality of dent maize calibrated to TADD AHI. The authors found a correlation coefficient of r = 0.87 between TADD and NIR. The authors recommended a wavelength of between 1100 and 1175 nm. The absorption band was thought to correspond to the –CH and –OH bonds due to carbohydrate, protein and lipids of the grain. Thus, interaction of the chemical bonds and the strength between them could be related to grain hardness, which is dependent on protein and starch interactions.

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Translucency

According to Hoseney (1994), the appearance of the endosperm is as a result of the packing of the starch granules. In translucent (corneous) endosperm, starch granules are tightly packed without airspaces and allow light to diffuse through the kernel. In the floury endosperm there are air spaces, which diffract light because of the loosely packed structure. The air voids give the endosperm an opaque or chalky appearance (Serna-Saldivar and Rooney 1995). A light box to estimate endosperm translucency in maize is commonly used to estimate grain hardness. Erasmus and Taylor (2004) refined the light box technique by developing a digital image analysis procedure to measure maize kernel translucency. This involved placing a whole kernel on top of an illuminated surface, which was smaller than the kernel to eliminate light from external sources. The light was allowed to pass through the kernel creating a contrast between the vitreous and opaque endosperms.

TABLE OF CONTENTS :

DECLARATION
DEDICATION
ACKNOWLEDGEMENTS
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
1 INTRODUCTION
2 LITERATURE REVIEW
- 2.1 Sorghum and maize kernel structures
- 2.1.1 Endosperm
- 2.1.2 Pericarp
- 2.1.3 Germ
- 2.2 Research into methods for measuring sorghum and maize hardness
- 2.2.1 Destructive methods
- 2.2.1.1 Abrasive milling
- 2.2.1.2 Pasting
- 2.2.1.3 Endosperm texture
- 2.2.2 Non-destructive methods
- 2.2.2.1 Near infrared spectroscopy
- 2.2.2.2 Translucency
- 2.2.2.3 Test weight
- 2.2.2.4 Kernel size
- 2.3 Sorghum and maize proteins and their influence on grain hardness
- 2.4 The influence of grain hardness on porridge quality
- 2.5 Changes in sorghum and maize starch as they relate to grain hardness
- 2.6 Grain modification during malting and the effect of hardness on malt quality
- 2.7 Sorghum and maize phenolic acids and their role in grain hardness
- 2.7.1 Mechanisms of cross linking of phenolic acids to cell walls and their influence on grain hardness
- 2.8 CONCLUSIONS
3 HYPOTHESES AND OBJECTIVES
- 3.1 HYPOTHESES
- 3.2 OBJECTIVES
4 RESEARCH
- 4.1 Relationships between Simple Grain Quality Parameters for the Estimation of Sorghum and Maize Hardness in Commercial Hybrid Cultivars
- 4.1.1 INTRODUCTION
- 4.1.2 MATERIALS AND METHODS
- 4.1.2.1 Materials
- 4.1.2.2 Methods
- 4.1.2.3 Statistical analyses
- 4.1.3 RESULTS AND DISCUSSION
- 4.1.3.1 Physical and hardness properties of sorghum and maize cultivars with a wide range of properties
- 4.1.3.2 Commercial sorghum physical and hardness properties
- 4.1.3.4 Physical and hardness properties of commercial maize cultivars
- 4.1.4 CONCLUSIONS
- 4.1.5 LITERATURE CITED
- 4.2 Relationship between sorghum and maize grain hardness, porridges and sorghum malt modification
- 4.2.1 INTRODUCTION
- 4.2.2 MATERIALS AND METHODS
- 4.2.2.1 Samples
- 4.2.2.2 Malting
- 4.2.2.3 Physical sorghum and maize grain characteristics
- 4.2.2.4 Viscosity
- 4.2.2.4 Porridge texture measurements
- 4.2.2.5 Scanning Electron Microscopy (SEM)
- 4.2.2.6 Statistical analyses
- 4.2.3 RESULTS AND DISCUSSION
- 4.2.3.1 Pasting properties of sorghum grain flours and textural properties of their porridges
- 4.2.3.2 Pasting Properties of Maize Flours and Texture of their Porridges
- 4.2.3.3 Changes in grain hardness during sorghum malting
- 4.2.3.4 Modification of the sorghum kernel during malting
- 4.2.3.5 The effect of sorghum grain hardness on malt modification
- 4.2.3.6 The effect of malting on pasting properties of sorghum malt flours and on the texture of malt porridges made from cultivars varying in hardness
- 4.2.4 CONCLUSIONS
- 4.2.5 LITERATURE CITED
- 4.3 Phenolic acid content composition of sorghum and maize cultivars varying in hardness
- 4.3.1 INTRODUCTION
- 4.3.2 MATERIALS AND METHODS
- 4.3.2.1 Samples
- 4.3.2.2 Physical and hardness tests
- 4.3.2.3 Sample preparation
- 4.3.2.4 Total phenolic content (TPC)
- 4.3.2.5 Extraction of bound phenolic acids
- 4.3.2.6 HPLC-MS/MS analysis
- 4.3.2.7 Statistical analyses
- 4.3.3 RESULTS AND DISCUSSION
- 4.3.3.1 Physical and hardness characteristics of sorghum and maize cultivars
- 4.3.3.2 Total phenolic content of sorghum and maize bran and flour methanolic extracts
- 4.3.3.3 Phenolic acid composition of sorghum and maize cultivars
- 4.3.3.4 Bound phenolic acids of sorghum bran and flour fractions
- 4.3.3.5 Bound phenolic acids of maize bran and flour fractions
- 4.3.3.6 Identification and quantification of sorghum and maize diferulic acids
- 4.3.3.7 Relationship between phenolic acids of sorghum and maize with grain hardness parameters
- 4.3.4 CONCLUSIONS
- 4.3.5 LITERATURE CITED
5 GENERAL DISCUSSION
- 5.1 METHODOLOGIES
- 5.2 RESEARCH FINDINGS
6 CONCLUSIONS AND RECOMMENDATIONS
7 LITERATURE CITED
8 APPENDIX