Sorghum kernel structure and its relation to milling performance and porridge quality

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Sorghum kernel structure and its relation to milling performance and

porridge quality Detailed structural descriptions of the sorghum kernel are documented by Rooney and Miller (1982). In their descriptions, the sorghum kernel is considered to be a naked caryopsis, which varies widely in size and shape among the sorghum types. The kernel is generally spherical in shape, measuring 4 mm long, 2 mm wide and 2.5 mm thick. The kernel weight, volumetric weight and density can range between 25 and 35 mg, 708 and 760 kg / m3 , and 1.26 and 1.38 g/cm3 , respectively. Kernels of some sorghum types retain glumes after threshing (Rooney and Serna-Saldivar 1993). These glumes may be red, purple or tan (Rooney and Miller 1982), and may also be sienna (Prof G. Peterson, sorghum breeder, University of Texas A&M, personal communication). When the grain is exposed to damp conditions while in the field, intensely coloured pigments leach from the glumes and stain the kernel (Serna-Saldivar and Rooney 1995). The stained kernels subsequently result in the discolouration of the final sorghum products. Rooney and Miller (1982) also reviewed the structure of the sorghum caryopsis, which consists of three main parts, namely the pericarp, endosperm and germ (Fig. 2.1). The relative proportions of these components vary among sorghum types, influenced by genetic and environmental factors (Serna-Saldivar and Rooney 1995).

Pericarp

The sorghum pericarp (which originates from the ovary wall) is the outer covering adhering strongly to the endosperm, and is characterized by three distinctive layers: the epicarp, mesocarp and endocarp (Earp et al 2004). The epicarp is the outermost layer, and it is generally coated with wax and contains coloured pigments, which determine the colour of the grain (Serna-Saldivar and Rooney 1995). The mesocarp is the middle layer, which often contains starch granules, a characteristic unique to sorghum and pearl millet (Serna-Saldivar and Rooney 1995). According to Taylor and Dewar (2001) the presence of starch in the mesocarp could account for the high friability of the sorghum pericarp. A friable pericarp is disadvantageous for dry milling as it fragments into fine pieces and escape separation, thus contaminating the meal (Perten 1984). The thickness of the mesocarp determines the overall thickness of the pericarp. This is controlled by the Zgene, where the dominant Z gene produces a thin pericarp and the recessive gene combination (zz) codes for a thick mesocarp (Rooney and Miller 1982). Thin pericarps tend to adhere tightly to the kernel, while thick pericarps attach loosely (Bassey and Schmidt 1989). Consequently, the thickness of the pericarp is an important grain property because sorghum types with thick pericarps decorticate easily by hand pounding than sorghums with thin pericarps, and the latter types perform better under mechanical decortication (Maxson et al 1971, Scheuring et al 1983). Pericarp thickness is also important in that sorghum types with thick pericarps are prone to weathering (SernaSaldivar and Rooney 1995). These types of sorghums were shown to consistently contain higher amounts of phenolic compounds (Beta et al 1999), which if incorporated into derived food products could cause astringent taste (Drewnoski and Gomez-Carneros 2000). The mesocarp forms a mechanically weak area, which permits the pericarp to peel off in large flakes during milling (Shepherd 1981).

Germ

The germ, which consists of the embryonic axis (radicle and plumule) and the scutellum, is firmly embedded in the kernel, secured by strong cementing layer and interlinking glands between the scutellum and the endosperm (Rooney 1973). The germ is rich in lipids, protein and minerals, with most of the lipids contained in the scutellum (Rooney and Serna-Saldivar 1993). The size of the germ varies among sorghum types, but in all sorghums, it is proportionally large relative to the size of the endosperm (FAO 1995), making the sorghum kernel one of the highest in oil content among the cereals (Kent and Evers 1994). It is difficult to de-germ sorghum (FAO 1995), and the degree of difficulty varies among sorghum types (Rooney 1973). As a result sorghum meals inevitably become contaminated with oil (Gomez 1993), which often cause rancidity problems in sorghum products (Hoseney 1994).

Protein

Protein is the second major component of sorghum, making up approximately 12% of the whole grain weight (Serna-Saldivar and Rooney 1995). Taylor and Schüssler (1986) determined that about 80, 16, and 3% of the total sorghum protein is contained in the endosperm, germ, and pericarp, respectively. As such, the decortication process, which primarily removes the pericarp, also removes some of the germ and part of the outer endosperm, which contain some amounts of protein. Thus, in essence, decortication reduces the amount of protein available in the grain (Serna-Saldivar and Rooney 1995).

Oil

The sorghum kernel contains oil in the range of 2.1-5.0% (Hoseney 1994), where about 76%, 13% and 11% of the total oil is contained in the germ (scutellum), endosperm and pericarp, respectively (Serna-Saldivar and Rooney 1995). Consequently, some amount of oil is removed with the bran (pericarp and germ) upon decortication (FAO 1995), but certainly not all (Rooney 1973, Gomez 1993). Like other cereals, sorghum contains polar, non-polar and non-saponifiable lipids (Osagie 1987). Non-polar lipids (also called neutral lipids) are the most abundant, accounting for approximately 93% of the total lipid content (Serna-Saldivar and Rooney 1995). The fatty acid composition is dominated by the unsaturated linoleic, oleic, and palmitic acids (Osman et al 2000), which make up approximately 49%, 31% and 14%, respectively, of the total free fatty acid content (Rooney and Serna-Saldivar 1993). Because of their unsaturated molecular structure, these fatty acids are prone to hydrolytic and oxidation spoilage (Eskin and Przybylski 2001), and hence cause rancid off flavours in the meals (Hoseney 1994).

Minerals

The mineral content of sorghum is highly variable, influenced mainly by the environment than by genetics (FAO 1995). The minerals are concentrated in the pericarp, germ and the aleurone layer, and therefore their presence in the meal (determined as ash content) is indicative of the degree of bran contamination (and removal). Ash mainly contains salts of phosphorus and potassium (Serna-Saldivar and Rooney 1995). According to Pedersen and Eggum (1983), minerals such as iron, zinc, copper and phosphorus, decreased with extraction rates in sorghum flours. Phytic acid, a compound contained in the germ and the aleurone layer (Serna-Saldivar and Rooney 1995), binds with divalent dietary minerals, making them biologically unavailable (Rhou and Erdman 1995), if the bran is not adequately removed from the meal.

Dry milling processes for sorghum grain

In any sorghum dry-milling operation, the primary objectives are to (i) remove the outer less palatable and oil-rich tissues of the grain (pericarp and germ), thus retaining maximum yields of the starchy endosperm; and (ii) to reduce the endosperm into a meal (Munck et al 1982). Since the pericarp and the germ account for about 10-15% of the total kernel weight, an ideal milling process should yield from 85 to 90% of refined endosperm particles (Rao 1982), with minimal amounts of the pericarp and the germ. However, in practice this ideal endosperm yield is seldom achieved, even at the optimum conditions of the milling process, because of the intrinsic inefficiencies of the milling processes.

Roller Milling

In a conventional roller milling process, flour is produced by gradually reducing the particle size of the feed stock by a series of grindings (pairs of counter-rotating rolls), with intermediate separation of bran, germ and endosperm meal streams by sifters and purifiers (Posner and Hibbs 1997). In each pair, the rolls are separated by a small gap and are set to rotate at different speeds, such that the grain passing between the pair become subjected to shear and compressive forces imposed by corrugations on the roll surfaces and pressure exerted by the rolls as they revolve (Hague 1991, Posner and Hibbs 1997). The rolls used usually range from 225 to 300 mm in diameter and vary from 450 to 1500 mm in length. They either have corrugated surface (called break rolls) or smooth surface (called reduction rolls). For wheat flour milling, up to 16 roller milling operations may be used to achieve flour with minimal bran contamination and optimal extraction rates (Campbell and Webb 2000).

TABLE OF CONTENTS :

• TITLE PAGE
• DECLARATION
• ACKNOWLEDGEMENTS
• ABSTRACT
• TABLE OF CONTENTS
• LIST OF TABLES
• LIST OF FIGURES
• 1 INTRODUCTION
• 2 LITERATURE REVIEW
  • 2.1 Sorghum kernel structure and its relation to milling performance and porridge quality
  • 2.1.1 Pericarp
  • 2.1.2 Endosperm
  • 2.1.3 Germ
  • 2.2 Sorghum chemical composition as related to milling and product quality
  • 2.2.1 Starch
  • 2.2.2 Protein
  • 2.2.3 Oil
  • 2.2.4 Dietary fibre
  • 2.2.5 Minerals
  • 2.3 Dry milling processes for sorghum grain
  • 2.3.1 Traditional hand pounding
  • 2.3.2 Dry Abrasive decortication
  • 2.3.3 Roller Milling
  • 2.4 Quality evaluation procedures
  • 2.4.1 Grain evaluation and quality standards
  • 2.4.2 Meal (flour) evaluation and quality standards
  • 2.5 Conclusions
  • 2.6 Hypotheses
  • 2.7 Objectives
• 3 RESEARCH
  • 3.1 Effects of Hand Pounding, Abrasive Decortication-Hammer Milling, Roller Milling and Sorghum Type on Sorghum Meal Extraction and Quality
  • ABSTRACT
  • 3.1.1 INTRODUCTION
  • 3.1.2 MATERIALS AND METHODS
  • 3.1.2.1 Grain
  • 3.1.2.2 Grain characterisation
  • 3.1.2.3. Hand pounding
  • 3.1.2.4. Abrasive decortication and hammer milling
  • 3.1.2.5. Optimising the roller milling process for sorghum
  • 3.1.2.6. Roller milling
  • 3.1.2.7. Estimation of the amount of endosperm in ‘‘bran’’
  • 3.1.2.8. Analysis of meals
  • 3.1.2.9. Statistical analysis
  • 3.1.3. RESULTS AND DISCUSSION
  • 3.1.3.1. Grain characterisation
  • 3.1.3.2. Optimisation of roller milling
  • 3.1.3.3. Comparative evaluation of hand pounding (HP), abrasive decorticationhammer milling (ADHM), and roller milling (RM)
  • 3.1.3.3.1. Extraction rates
  • 3.1.3.3.2. Colour of the meals
  • 3.1.3.3.3. Ash content of the meals
  • 3.1.3.3.4. Oil content of the meals
  • 3.1.3.3.5. Protein content of the meals
  • 3.1.3.3.6. Particle size distribution of the meals
  • 3.1.4. CONCLUSIONS
  • 3.1.5 LITERATURE CITED
  • 3.2 Effects of Sorghum Type and Milling Process on the Sensory Characteristics of Sorghum Porridge
  • ABSTRACT
  • 3.2.1 INTRODUCTION
  • 3.2.2 MATERIALS AND METHODS
  • 3.2.2.1 Sorghum Meal Samples
  • 3.2.2.2 Porridge Preparation
  • 3.2.2.3 Descriptive Sensory Analysis
  • 3.2.2.4 Statistical analyses
  • 3.2.3 RESULTS AND DISCUSSION
  • 3.2.3.1 Differences Between the Attributes
  • 3.2.3.2 Panelist and Session Effects
  • 3.2.3.3 Sorghum Type and Milling Process Effects
  • 3.2.3.4 Principal Component Analysis
  • 3.2.4 CONCLUSIONS
  • 3.2.5 LITERATURE CITED
  • 3.3 Influence of Sorghum Grain Characteristics and Milling Process on the
  • Textural Properties of Sorghum Porridge
  • ABSTRACT
  • 3.3.1 INTRODUCTION
  • 3.3.2 MATERIALS AND METHODS
  • 3.3.2.1 Samples
  • 3.3.2.2 Meal particle size index (PSI)
  • 3.3.2.3 Water Absorption Index (WAI)
  • 3.3.2.4 Water Solubility Index (WSI)
  • 3.3.2.5 Damaged starch and amylose content
  • 3.3.2.6 Pasting properties
  • 3.3.2.7 Textural properties
  • 3.3.2.8 Scanning Electron Microscopy (SEM)
  • 3.3.2.9 Wide angle X-ray diffraction
  • 3.3.2.10 Statistical analysis
  • 3.3.3 RESULTS AND DISCUSSION
  • 3.3.3.1 Damaged starch
  • 3.3.3.2 Water absorption index (WAI)
  • 3.3.3.3 Water solubility Index
  • 3.3.3.4 Effects of milling method and sorghum type on pasting properties of sorghum meals
  • 3.3.3.5 Effects of milling method and sorghum type on porridge texture (firmness and stickiness)
  • 3.3.3.6 Principal Component Analysis (PCA)
  • 3.3.4 CONCLUSIONS
  • 3.3.5 LITERATURE CITED
• 4 GENERAL DISCUSSION
  • 4.1 Methodologies: A critical review
  • 4.1.1 Sampling of sorghum types
  • 4.1.2 Selected milling processes
  • 4.1.3 Descriptive sensory analysis
  • 4.1.4 Differences in the solids concentrations of porridges for sensory and instrumental texture analysis
  • 4.1.5 Porridge microstructure
  • 4.2 Comparative performance of the milling processes
  • 4.2.1 Extraction rates
  • 4.2.2. Throughput
  • 4.2.3. Energy efficiency
  • 4.2.4 Comparison of meal and porridge quality
  • 4.2.4.1 Effects of the milling process
  • 4.2.4.3 Effects of the sorghum type
  • 4.3 Suggested improvements for sorghum milling processes
  • 4.3.1 Small-scale service and/or semi-commercial milling
  • 4.3.2 Commercial milling plant
  • 4.4 Recommendations for future research
• 5 CONCLUSIONS AND RECOMMENDATIONS
• 6 LITERATURE CITED

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Sorghum dry-milling processes and their influence on meal and porridge quality