Effect of Grain Structure and Cooking on Sorghum and Maize in vitro Protein Digestibility

Uncooked and cooked sorghum showed improvement in in vitro protein digestibility as the structural complexity of the sample reduced from whole grain flour through endosperm flour to protein body-enriched samples. This was not the case for maize. Cooking reduced protein digestibility of sorghum but not maize. Treating cooked sorghum and maize whole grain and endosperm flours with alpha -amylase to reduce sample complexity before in vitro pepsin digestion slightly improved protein digestibility. The reduction in sorghum protein digestibility on cooking was not related to the total polyphenol content of samples. Pericarp components, germ, endosperm cell walls, and gelatinised starch were identified as possible factors limiting sorghum protein digestibility. Electrophoresis of uncooked and cooked protein-body-enriched samples of sorghum and maize, and prolamin fractions of sorghum under non-reducing conditions showed oligomeric proteins with molecular weights (M_r) 45, 66 and >66 kDa and monomeric kafirins and zeins. Protein-body-enriched samples of sorghum had more 45–50 kDa oligomers than those of maize. In cooked sorghum, some of these were resistant to reduction. Pepsin-indigestible residues from protein-body-enriched samples consisted mainly of α-zein (uncooked and cooked maize) or α-kafirin (uncooked sorghum), whilst cooked sorghum had in addition, β- and γ-kafirin and reduction-resistant 45–50 kDa oligomers. Cooking appears to lead to formation of disulphide-bonded oligomeric proteins that occurs to a greater extent in sorghum than in maize. This may explain the poorer protein digestibility of cooked sorghum.     

Particle size heterogeneity in milled barley and sorghum grains: Effects on physico-chemical properties and starch digestibility

The average particle size of ground grains is known to influence properties related to processing (e.g. water absorption and solubility) and nutritional value (e.g. starch digestion rate) of human foods and animal feeds. The purpose of this study was to identify the contributions made by individual size fractions of hammer-milled barley and sorghum grains to average bulk compositional, hydration, rheological, and enzyme susceptibility properties. Barley and sorghum grains were each hammer-milled through a 4 mm screen and subsequently fractionated on a set of eight sieves ranging from 0.125 mm to 2.8 mm. Individual fractions were characterised for (1) starch, aNDF, and water content, (2) water absorption index (WAI) and water solubility index (WSI), (3) viscosity profile during cooking and cooling in excess water, and (4) in vitro starch digestibility. Weighted average values based on fraction yields and property values for WAI, WSI, and starch digestibility were not significantly different from values obtained for non-fractionated ground grains of both barley and sorghum. Glucose yields from starch digestion varied about ten-fold between the smallest and largest particle fractions, and WAI and WSI had value ranges of 1.9–2.8 g/g (sorghum), 2.1–4.0 g/g (barley) and 1.3–4.5% (sorghum), 0.7–10.3% (barley), respectively. Viscosity profiles for milled sorghum grain fractions were dominated by starch swelling which became increasingly restricted as particle sizes increased. Viscosity profiles for milled barley grain fractions did not exhibit typical starch-based behaviour and were most likely dominated by soluble fibres. Taken together, the results show that there is considerable potential for designing combinations of hydration, rheological and digestibility properties of ground grains through informed selection of appropriate grains and particle size distributions.     

Effects of endosperm texture and cooking conditions on the in vitro starch digestibility of sorghum and maize flours

The effects of endosperm vitreousness, cooking time and temperature on sorghum and maize starch digestion in vitro were studied using floury and vitreous endosperm flours. Starch digestion was significantly higher in floury sorghum endosperm than vitreous endosperm, but similar floury and vitreous endosperm of maize. Cooking with 2-mercaptoethanol increased starch digestion in both sorghum and maize, but more with sorghum, and more with vitreous endosperm flours. Increasing cooking time progressively reduced starch digestion in vitreous sorghum endosperm but improved digestibility in the other flours. Pressure-cooking increased starch digestion in all flours, but markedly more in vitreous sorghum flour; probably through physical disruption of the protein matrix enveloping the starch. Irrespective of vitreousness or cooking condition, the alpha-amylase kinetic constant (k) for both sorghum and maize flours remained similar, indicating that differences in their starch digestion were due to factors extrinsic to the starches. SDS-PAGE indicated that the higher proportion of disulphide bond-cross-linked prolamin proteins and more extensive polymerisation of the prolamins on cooking, resulting in polymers of M_r>100k, were responsible for the lower starch digestibility of the vitreous sorghum endosperm flour.     

Increasing protein content and digestibility in sorghum grain with a synthetic biology approach

Despite great genetic diversity, sorghum grain consistently suffers from poor protein digestibility. The physicochemical packaging of protein bodies which consist of protease-resistant β- and γ-kafirin is considered a major obstacle. A synthetic β-kafirin gene, which shares the endosperm-specific promoter and signal peptide with the native β-kafirin gene (Sobic.009G001600.1), was transformed into sorghum inbred line Tx430. The gene was modified with ten additional proteolytic sites. These sites were designed to be amenable to cleavage by pepsin and/or chymotrypsin proteinases. Five independent transgenic lines were regenerated by microprojectile transformation. Notably, considerably more protein was observed in the peripheral endosperm of transgenic lines under scanning electron microscopy. Microscopy revealed invaginated or irregularly shaped protein bodies in the endosperm of transgenic lines. Grains of transgenic lines contained 11–37% more protein, which was 11–21% more pepsin digestible and 7–25% more chymotrypsin digestible than Tx430. Additionally, the abundant synthetic β-kafirin protein (5.6% of total protein) was detected by mass spectrometry data analysis in the transgenic line 9-1. Field-grown homozygous transgenics retained higher protein content, larger seed size and no reduction in grain number per plant. The results illustrated that plant synthetic biology could play an important role in improving sorghum nutritional value.     

Kinetics of starch digestion and functional properties of twin-screw extruded sorghum

The time-course of starch digestion in twin-screw extruded milled sorghum grain was investigated using an in-vitro procedure based on glucometry. The sorghum grains were hammer-milled, and extruded at three levels each of moisture and screw speed. Irrespective of the extrusion conditions, extruded and non-extruded milled sorghum grain exhibited monophasic digestograms, and the modified first-order kinetic and Peleg models adequately described the digestograms. Extrusion increased the rate of digestion by about ten times compared with non-extrudates. Starch gelatinisation varied in the extrudates, and microscopy revealed a mixture of raw, gelatinised and destructured starch and protein components in the extrudates. Starch digestion parameters significantly (p < 0.05) correlated with extruder response and various functional properties of the extrudates. Extrusion conditions for maximum starch gelatinisation in milled sorghum grain for fastest digestion as an efficient animal feed were interpolated, as well as the conditions for directly-expanded extrudates with potential for human food, where minimum starch digestion is desired.     

Effect of suppressing the synthesis of different kafirin sub-classes on grain endosperm texture, protein body structure and protein nutritional quality in improved sorghum lines

To improve sorghum grain protein nutritional quality, improved sorghum lines were transformed to suppress the synthesis of different kafirin sub-classes, or backcrossed into transgenic lines with improved protein quality. Co-suppression of the alpha-, gamma- and delta-kafirin sub-classes and removal of the tannin trait resulted in transgenic sorghum lines with high cooked protein digestibility (±80%), improved Amino Acid Score (0.8) and Protein Digestibility Corrected Amino Acid Score (0.7) compared to the non-transgenic null controls (±50%, 0.4 and 0.2, respectively). These high protein quality lines had a floury endosperm. They also had modified protein body structure, where the protein bodies were irregular shaped with few to numerous invaginations and were less densely packed, with a dense protein matrix visible around the protein bodies. When fewer sub-classes were suppressed, i.e. gamma 1 and delta 2, the endosperm was corneous with normal protein body structure but the improvement in cooked protein digestibility appeared to be less. Apparently, co-suppression of several kafirin sub-classes is required to obtain high protein nutritional quality sorghum lines, but this seems to result in floury-type grain endosperm texture.     

Tef: The Rising Ancient Cereal: What do we know about its Nutritional and Health Benefits?

This review covers the nutritional significance of tef cereal as compared to other common cereals with emphasis on carbohydrate content and starch digestibility, protein content, iron and zinc bioavailability and antioxidant potentials. Tef is a gluten free cereal and contains the highest iron and calcium among other cereals. It has high micro- and macro- nutritional profile and is becoming globally popular in the healthy grain food chain. Tef starch has a high gelatinization temperature, an essential precondition in the preparation of low glycemic index foods. There are significantly conflicting reports of iron content of tef ranging from 5 to 150 mg/100 g dm. The traditional fermentation of injera reduced majority of the phytic acid but no significant change to mineral bioavailability was observed. This review indicated that studies on starch digestibility, protein characterization, amylase and protease inhibitors, mineral bioavailability and antioxidant potentials are needed to further explore the nutritional and health benefits of tef.     

The nutritive value of some selected Tanzanian plant food sources

The chemical composition of different varieties of field beans (Vicia faba L.), cowpeas (Vigna unguiculata), groundnuts (Voandzeia subterranea), and sorghum (Sorghum vulgare) were determined. Nutrients analysed in these materials included amino acids, starch, sugar, fibre, minerals and antinutritional factors. All the materials were further tested in balance trials with rats; true protein digestibility (TD), biological value (BV), net protein utilization (NPU) and digestible energy (DE) were evaluated. Based on the chemical analyses as well as on the biological data, the nutritive value of plant materials between species as well as between varieties within species differed considerably. Therefore, more quality evaluation studies along with plant breeding programmes are needed to select the most promising varieties from the nutritional point of view.     

Digestibility of proteins and starch (in vitro) of amphidiploids (black gram×mung bean) as affected by domestic processing and cooking

The effects of different domestic processing and cooking methods on starch digestibility (in vitro) and protein digestibility (in vitro) of four strains of amphidiploids (black gram×mung bean) were investigated. An increase of 35 to 48% and 22 to 25% was observed in starch digestibility and protein digestibility, respectively, when the seed of amphidiploids were soaked for 18 h. Cooking (both of unsoaked and soaked seeds) and germination improved significantly the starch digestibility and protein digestibility of all the varieties.     

In vitro starch digestibility and pasting properties of germinated brown rice after hydrothermal treatments

Germinated brown rice (GBR) recently has received renewed attention due to its enhanced nutritional value. Pasting properties and in vitro starch digestibility of GBR were examined before and after hydrothermal treatments. Steeping in water (30 °C, 24 h) raised the moisture content and germination percentage of brown rice. Pasting viscosity was substantially decreased but gelatinization temperatures and enthalpy were decreased only marginally by germination (30 °C, 48 h). However, annealing (50 °C, 24 h) and heat-moisture treatment (100 °C, 1 h at 30% moisture) after germination resulted in increased pasting viscosity and gelatinization temperatures. The hydrothermal treatments, however, induced browning reactions to darken the flour of GBR. The digestibility of starch in brown rice was increased by germination. The contents of rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) in the cooked brown rice were 47.3%, 40.8%, and 11.9%, respectively, but changed to 57.7%, 39.1%, and 3.2%, respectively upon germination. The hydrothermal treatments, however, decreased the digestibility of starch in GBR. The heat-moisture treatment decreased the RDS content in GBR near to that of native brown rice. The digestibility and physical properties of brown rice can be controlled by germination and hydrothermal treatments.     

Ethanol production from supercritical-fluid-extrusion cooked sorghum

Sorghum (Sorghum bicolor (L.) Moench) is a starch-rich grain similar to maize (Zea mays L.), but sorghum has been underutilized for biobased products and bioenergy. This study was designed to investigate the effects of supercritical-fluid-extrusion (SCFX) of sorghum on ethanol production. Morphology, chemical composition, and thermal properties of extruded sorghum were characterized. Analysis of extruded sorghum showed increased measurable starch content, free sugar content, and high levels of gelatinized starch. SCFX cooked and non-extruded sorghum were further liquefied, saccharified, and fermented to ethanol by using Saccharomyces cervisiae. The ethanol yield increased as sorghum concentration increased from 20 to 40% for both extruded and non-extruded sorghum. Ethanol yields from SCFX cooked sorghum were significantly greater than that from non-extruded sorghum (>5%).     

Compositional variation amongst sorghum hybrids: Effect of kafirin concentration on metabolizable energy

Kafirins are stored proteins that negatively affect the nutritional quality of sorghum grain. Kafirin concentration and other chemical characteristics were determined in 12 sorghum hybrids and varied significantly, from 58% (HB1) to 42% (HB12) as percent total protein. Kafirin concentration correlated negatively with crude protein (CP) (−0.49), with acid detergent fiber (−0.40), apparent metabolizable energy (−0.61), and true metabolizable energy corrected for N (−0.63). HB12 was the hybrid with the lowest content of kafirins, amylose and tannins, and the highest content of apparent metabolizable energy. No differences were observed in the concentration of starch, but differences were found in apparent metabolizable energy (3325–2944 kcal kg⁻¹) probably due to a greater availability of starch, related to differences in kafirin concentration.     

Barley genotype expressing “stay-green”-like characteristics maintains starch quality of the grain during water stress condition

The identification of “stay-green” in sorghum and its positive correlation with yield increases has encouraged attempts to incorporate “stay-green”-like traits into the genomes of other commercially important cereal crops. However, knowledge on the effects of “stay-green” expression on grain quality under extreme physiological stress is limited. This study examines impacts of “stay-green”-like expression on starch biosynthesis in barley (Hordeum vulgare L.) grain under mild, severe, and acute water stress conditions induced at anthesis. The proportions of long amylopectin branches and amylose branches in the grain of Flagship (a cultivar without “stay-green”-like characteristics) were higher at low water stress, suggesting that water stress affects starch biosynthesis in grain, probably due to early termination of grain fill. The changes in long branches can affect starch properties, such as the rates of enzymatic degradation, and hence its nutritional value. By contrast, grain from the “stay-green”-like cultivar (ND24260) did not show variation in starch molecular structure under the different water stress levels. The results indicate that the cultivar with “stay-green”-like traits has a greater potential to maintain starch biosynthesis and quality in grain during drought conditions, making the “stay-green”-like traits potentially useful in ensuring food security.     

Effect of a gibberellin-biosynthesis inhibitor treatment on the physicochemical properties of sorghum starch

Inhibition of plant growth by Trinexapac-ethyl, TE, a gibberellin-biosynthesis inhibitor, can produce a shorter stemmed plant, requiring less nutrients and water to grow, while maintaining grain yield. Although TE and other plant growth regulators are commonly used in grain crops, their effects on starch biosynthesis in the grains have not been systematically examined. The changes in the structural and functional properties of starch in grains harvested from TE-treated sorghum (Sorghum bicolor (L.) Moench) were examined, and the results compared with those from the untreated controls. TE treatment had little or no effects on the molecular structures of starch, starch granule morphology, and starch and amylose contents, but increased the protein content of the grains significantly. Consistent with the lack of change in the molecular structure, there were no significant effects on the thermal properties of the starch. The pasting properties of TE-treated sorghum flours, however, showed lower peak viscosity, trough, and final viscosity, which were attributed to their higher protein contents. The TE treatment thus does not have an appreciable effect on the biosynthesis of starch during grain development in sorghum.     

Nutritional evaluation of genetically modified rice expressing human lactoferrin gene

The nutritional quality of a new strain of genetically modified rice (Oryza sativa L.) expressing human lactoferrin gene (hLF rice) was evaluated on the basis of components, nutrient digestibility in pigs, protein availability in rats and protein digestibility corrected amino acid scores (PDCAAS), and compared to its parental rice variety (PR rice). Although exogenous human lactoferrin gene was introduced, it did not interfere with the digestibility of protein, carbohydrates, fat and crude fiber. The revised protein efficiency ratio of hLF rice was increased to 2.50, which was significantly higher than that of PR rice. The PDCAAS of PR rice was 52.66 and its first limiting amino acid was lysine, while the PDCAAS of hLF rice was improved to 54.06 and its first limiting amino acid was tryptophan. Thus, it can be concluded that the nutritional quality of hLF rice is superior to PR rice according to the results of availability experiments and PDCAAS, and the hLF rice would be a superior strain of rice based on protein composition of the grain.     

Compositional variation amongst sorghum hybrids: Effect of kafirin concentration on metabolizable energy

Kafirins are stored proteins that negatively affect the nutritional quality of sorghum grain. Kafirin concentration and other chemical characteristics were determined in 12 sorghum hybrids and varied significantly, from 58% (HB1) to 42% (HB12) as percent total protein. Kafirin concentration correlated negatively with crude protein (CP) (−0.49), with acid detergent fiber (−0.40), apparent metabolizable energy (−0.61), and true metabolizable energy corrected for N (−0.63). HB12 was the hybrid with the lowest content of kafirins, amylose and tannins, and the highest content of apparent metabolizable energy. No differences were observed in the concentration of starch, but differences were found in apparent metabolizable energy (3325–2944 kcal kg⁻¹) probably due to a greater availability of starch, related to differences in kafirin concentration.     

The composition of floury and vitreous endosperm affects starch digestibility kinetics of the whole maize kernel

Maize grain starch is the major energy source in animal nutrition, and its high digestion and utilization largely depend on endosperm traits and the structure of the starch-lipoprotein matrix. The aim of this work was to determine floury and vitreous endosperm traits and its relation to starch digestibility rate. In total, kernels of 30 hybrids were manually dissected, and amylose, total zein and starch and non-starch lipids were determined in both vitreous and floury endosperm. Starch digestibility of the whole kernel was determined based on glucose released during a two-step in vitro pig model of enzymatic digestion, and starch digestibility rate was calculated according to the first-order kinetics. The vitreous endosperm of tested hybrids had higher contents of amylose (204.6 vs 190.4 g/kg), zein (63.2 vs 40.4 k/kg) and starch lipids (5.6 vs 4.9 g/kg), and lower content of non-starch lipids (7.3 vs 9.6 g/kg) than floury endosperm. Digestibility coefficients varied among hybrids, and starch digestibility rate varied from 0.73 to 1.63 1/h. Lipids in both vitreous and floury endosperm negatively correlated with the most of digestion coefficients, whereas zein correlated in vitreous and amylose in the floury endosperm (P < 0.05). Starch digestibility rate negatively correlated with all traits, except amylose content in vitreous endosperm. As a result, a linear regression model with four variables including contents of zein and starch lipids in vitreous and zein and amylose in floury endosperm can predict more than 65% variability of starch digestibility rate of tested hybrids.     

Effect of germination time and temperature on the functionality and protein solubility of sorghum flour

Sorghum was germinated for different time (12, 24, 36 and 48 h) at different temperatures (25, 30 and 35 °C) and the changes in their nutritional and functional properties of germinated sorghum flour were assessed and compared with native sorghum flour. Germination inversely affects the crude protein, fat, fibre and ash content. A decrease in water absorption and swelling power and increase in oil absorption capacity was observed due to enzymatic starch modification as the germination duration progressed. Germination of sorghum increased the gel consistency while paste clarity was decreased as compared to native flour. Proteins were modified by action of enzymes during higher germination time and temperature conditions, which results in significantly higher protein solubility of germinated sorghum flour, which also result in enhancing the foaming and emulsifying properties of the flour. Lowest % synersis value and least gelation concentrations were observed in native sorghum has, which increased during germination and were highest in sorghum germinated for 48 h at 35 °C. Germination in overall can be used as low cost natural bio-processing technique for the preparation of modified flour with enhanced function properties without chemical modification or genetic engineering.     

Preferential binding of sorghum tannins with γ-kafirin and the influence of tannin binding on kafirin digestibility and biodegradation

Kafirins, sorghum prolamins bind with sorghum condensed tannins (CTs). The binding of different kafirin species with sorghum CTs was investigated. Analysis by chemical assay and by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE), reversed-phase high-performance liquid chromatography (RP-HPLC), and free zone capillary electrophoresis (FZCE), showed that γ-kafirin bound more CTs than the other kafirin species. SDS–PAGE suggested that the γ-kafirin-bound tannins were in the form of aggregates of molecular size >200k. RP-HPLC and FZCE revealed that sample preparation and drying the kafirins prior to the binding assays had a significant impact on γ-kafirin solubility. The effect of tannin binding on kafirin and kafirin film digestibility and film biodegradation was determined. Kafirins bound to tannins had lower digestibilities than unbound kafirins. Films made from tannin-bound kafirin had much lower digestibility and were less biodegradable than films made from unbound kafirin. The increase in kafirin film life by tannin modification appears to be due to a decrease in protein digestibility caused by kafirin–tannin binding. These findings suggest that γ-kafirin content in sorghum may be manipulated to either reduce or increase tannin binding in order to change the functionality of the kafirin in food, feed or film applications.     

Transgenic sorghum with suppressed synthesis of kafirin subclasses: Effects on flour and dough rheological characteristics

Arising from work showing that conventionally bred high protein digestibility sorghum types have improved flour and dough functionality, the flour and dough properties of transgenic biofortified sorghum lines with increased protein digestibility and high lysine content (TG-HD) resulting from suppressed synthesis of several kafirin subclasses, especially the cysteine-rich γ-kafirin, were studied. TG-HD sorghums had higher flour water solubility at 30 °C (p < 0.05) and much higher paste viscosity (41% higher) than their null controls (NC). TG-HD doughs were twice as strong as their NC and dynamic rheological analysis indicated that the TG doughs were somewhat more elastic up to 90 °C. CLSM of doughs and pastes indicated that TG-HD had a less compact endosperm protein matrix surround the starch compared to their NC. The improved flour and dough functional properties of the TG-HD sorghums seem to be caused by reduced endosperm compactness resulting from suppression of synthesis of several kafirin subclasses which modifies protein body and protein matrix structure, and to improved protein-starch interaction through hydrogen bonding specifically caused by reduction in the level of the hydrophobic γ-kafirin. The improved flour functionality of these transgenic biofortified sorghums can increase their commercial utility by complementing their improved nutritional quality.