Low α-Amylase Starch Digestibility of Cooked Sorghum Flours and the Effect of Protein

The comparably low starch digestibility of cooked sorghum flours was studied with reference to normal maize. Four sorghum cultivars that represent different types of endosperm were used. Starch digestibilities of 4% cooked sorghum flour suspensions, measured as reducing sugars liberated following α-amylase digestion, were 15–25% lower than for cooked maize flour, but there were no differences among the cooked pure starches. After the flours were predigested with pepsin to remove some proteins, the starch digestibility of cooked sorghum flours increased 7–14%, while there was only 2% increase in normal maize; however, there was no effect of pepsin treatment on starch digestibility if the flours were first cooked and then digested. After cooking with reducing agent, 100 mM sodium metabisulfite, starch digestibility of sorghum flours increased significantly while no significant effect was observed for maize. Also, starch solubility of sorghum flours at 85 and 100°C was lower than in maize, and sodium metabisulfite increased solubility much more in sorghum than in maize. Differential scanning calorimetry results of the flour residue after α-amylase digestion did not show any peaks over a temperature range of 20–120°C, indicating that sorghum starches had all undergone gelatinization. These findings indicate that the protein in cooked sorghum flour pastes plays an important role in making a slowly digesting starch.     

REVIEW: Developments in Our Understanding of Sorghum Polysaccharides and Their Health Benefits

The composition and structure of sorghum polysaccharides are remarkably similar to those in maize. Sorghum grain is rich in starch, cellulosic and noncellulosic polysaccharides (mainly glucuronoarabinoxylans [GAX]). Sorghum starch is similar to maize starch in terms of amylopectin, but the amylose may be more branched. This may account for sorghum starch having a generally slightly higher gelatinization temperature. The GAX in sorghum are highly substituted with glucuronic acid and arabinose, but the degree of these substitutions is lower when compared with maize GAX. Sorghum polysaccharides themselves are not sufficiently functional to allow the production of high‐quality baked goods. Sorghum has generally lower starch digestibility than maize. This is primarily due to the endosperm protein matrix, cell wall material, and tannins (if present) inhibiting enzymatic hydrolysis of the starch. Protein disulfide bond cross‐linking involving the kafirin prolamins in the protein matrix around the starch granules seems to be of major importance in reducing starch digestibility. It does not seem that sorghum polysaccharides, per se, have any unique health‐promoting effects. Any health‐promoting effects related to sorghum polysaccharides seem to be due to interactions between the polysaccharides and the endosperm matrix protein and phenolics.     

Characterization of polymeric proteins from vitreous and floury sorghum endosperm

Differences in protein content and composition between vitreous and floury endosperm were investigated using a number of different techniques. Differences in protein cross-linking between vitreous and floury endosperm were investigated using differential solubility, size exclusion chromatography (SEC), and analysis of sulfhydryl content and composition. Vitreous endosperm was found to have higher levels of total protein and kafirins, but floury endosperm had a higher proportion of gamma-kafirins than the vitreous. Floury endosperm was found to have higher levels of SDS-soluble proteins than SDS-insoluble proteins extracted using sonication than vitreous endosperm. Conversely, vitreous endosperm had a greater proportion of the insoluble proteins. SEC analysis of the polymeric proteins revealed that the insoluble proteins had more polymeric proteins than did the soluble proteins, indicating greater cross-linking and a larger Mw distribution. Vitreous endosperm was also found to have a greater percentage (i.e., a higher ratio of disulfide to total sulfhydryls) of disulfide bonds than floury endosperm. These results show that the proteins in vitreous endosperm have a higher degree of cross-linking and a greater Mw distribution than those found in floury endosperm.     

A Novel Modified Endosperm Texture in a Mutant High‐Protein Digestibility/High‐Lysine Grain Sorghum (Sorghum bicolor (L.) Moench)

Development of high‐protein digestibility (HPD)/high‐lysine (hl) sorghum mutant germplasm with good grain quality (i.e., hard endosperm texture) has been a major research objective at Purdue University. Progress toward achieving this objective, however, has been slow due to challenges posed by a combination of genetic and environmental factors. In this article, we report on the identification of a sorghum grain phenotype with a unique modified endosperm texture that has near‐normal hardness and possesses superior nutritional quality traits of high digestibility and enhanced lysine content. These modified endosperm lines were identified among F₆ families developed from crosses between hard endosperm, normal nutritional quality sorghum lines, and improved HPD/hl sorghum mutant P721Q‐derived lines. A novel vitreous endosperm formation originated in the central portion of the kernel endosperm with opaque portions appearing both centrally and peripherally surrounding the vitreous portion. Kernels exhibiting modification showed a range of vitreous content from a slight interior section to one that filled out to the kernel periphery. Microstructure of the vitreous endosperm fraction was dramatically different from that of vitreous normal kernels in sorghum and in other cereals, in that polygonal starch granules were densely packed but without the typically associated continuous protein matrix. We speculate that, due to the lack of protein matrix, such vitreous endosperm may have more available starch for animal nutrition, and possibly have improved wet‐milling and dry‐grind ethanol processing properties. The new modified endosperm selections produce a range that approaches the density of the vitreous parent, and have lysine content and protein digestibility comparable to the HPD/hl opaque mutant parent.     

Gluten Structural Evolution During Pasta Processing of Refined and Whole Wheat Pasta from Hard White Winter Wheat: The Influence of Mixing,Drying, and Cooking

An attempt was made to evaluate gluten structural changes in refined and whole wheat pasta from hard white winter wheat to elucidate the impact of whole wheat components on the formation and structure of the gluten network in pasta. Attenuated total reflectance–FTIR spectroscopy was used to track gluten secondary structure through most of the major steps in pasta processing: raw material, mixing, drying, and cooking. Protein solubility, accessible thiols, and SDS‐PAGE data were also collected to provide additional information on the nature of protein interactions and network composition. Few secondary structural differences were observed between refined and whole wheat flours from hard white wheat. However, mixing induced a significant shift to β‐sheet structures in refined dough that was not equally matched by whole wheat dough. Drying under both high temperature, short time (HT) and low temperature, long time (LT) conditions resulted in a reversion to structural distributions similar to those for flour in both pastas. However, greater protein denaturation in HT samples was indicated by lower protein solubility also in the presence of denaturants and disulfide reducing agents. Cooking generated a substantial increase in β‐sheet structures for both pasta systems. This structure was greatest in refined and LT samples. Thiol accessibility data indicate the presence of a highly aggregated, compact gluten network in refined pasta, mostly driven by hydrophobic association. Conversely, the network in whole wheat pasta was more loosely associated and dependent on disulfide bonding, both of which fit well with the secondary structural data.     

Protein Distribution Pattern in Floury and Vitreous Endosperm of Maize Grain

Alpha‐amino nitrogen compounds of floury and vitreous parts of hand‐dissected endosperm from eight maize (Zea mays L.) inbred lines, representing a broad range of vitreousness (42–95%), were isolated as nonprotein nitrogen, albumin‐globulins, zeins, and true glutelins. The three protein classes averaged, respectively, 13, 48, and 35% of total nitrogen in floury endosperm, and 4, 79, and 15% of that in vitreous endosperm. For six inbreds, floury endosperm was richer in 27 kDa γ‐zein than vitreous endosperm; the reverse was found for an Argentine flint inbred (ARGL 256), and only traces of 27 kDa γ‐zein occurred in both floury and vitreous endosperm of inbred F113. Results were compared with protein distribution patterns reported in the literature of whole endosperm of wild‐type and mutant genotypes of maize, and with wild relatives of maize, Tripsacum, and teosintes. When percentage of salt‐soluble nitrogen increased from 2% (Tripsacum) to 22% (in double mutant Oh43o2;bt2), zeins decreased from 87 to 22%, and true glutelins increased from 11 to 57%. The pattern of whole endosperm of Zea perennis was very similar to that of the vitreous endosperm of line ARGL 256. The mean pattern for whole endosperm of six o2 inbred lines was identical to that of floury endosperm of eight wild‐type lines, consistent with a lack of synthesis of α‐zeins due to the mutation in the O2 gene.     

Effect of β‐Glucan on Technological,Sensory, and Structural Properties of Durum Wheat Pasta

β‐Glucan is known to have valuable properties for preventative health and is finding widespread use in foods. This study investigated the benefit of adding a commercial source of β‐glucan, Barley Balance (BB) flour, as a functional ingredient in spaghetti. Durum wheat semolina was substituted with BB at levels of 7.5, 15, and 20%, from which spaghetti was prepared on a laboratory scale. The substitution of BB increased the β‐glucan content of semolina from 0.3 to 6% in uncooked and 8% in cooked pasta. Antioxidant activity (measured by 2,2‐diphenyl‐1‐picrylhydrazyl) increased with BB and did not decline significantly on processing and cooking. Compared with the control, 7.5% BB had no or minimal effect on pasta cooking loss, stickiness, water absorption, aroma, and sensory texture. However, at higher doses, pasta became less yellow and more brown, firmer, of inferior aroma, more rubbery, and chewy, but less floury to the mouth. The extent of starch digestion decreased with increasing quantities of BB, suggesting that BB may lower glycemic index, with microscopy data suggesting that this decrease was mediated through the development of a more intensive fiber or fiber/protein matrix retarding enzymatic access to starch granules.     

Fibrillar microfilaments associated with a high-amylose rice, goami 2, a mutant of ilpumbyeo, a high-quality japonica rice

The ultrastructure of cooked and malt-treated cooked rice of Ilpumbyeo (IP) and its mutant Goami 2 (G2), which have extreme contrasts in physicochemical properties, cooking quality, and ultrastructural characteristics in raw grains (1, 2), was compared. In cooked rice of IP, starch granules in endosperm cells were evenly coalesced, appearing as homogeneously smooth sheetlike matrix and/or globules, whereas those in G2 were a heterogeneously coarse matrix in which a novel structural feature, the microfilaments, was embedded. In malt-treated cooked rice of IP, most starch was hydrolyzed by the malt enzymes, appearing as empty vacuoles surrounded by the cell wall, whereas that in G2 was highly resistant to malt treatment, remaining as distinct structural features, the malt-resistant compound starch granules. The property of G2's compound starch granules, which are tolerant of mechanical and chemical treatments thereby retaining their structural integrity (2) and of cooking and malt treatment thereby retaining their physical hardness, appears to play a major role in determining the quality of cooked rice of G2.     

Relationships Between the Microstructure,Physical Features,and Chemical Composition of Different Maize Accessions from Latin America

Chemical composition (moisture, total lipids, protein, and apparent amylose) and some physical features (1,000 kernel weight, hardness, and anatomical composition) were determined in 71 accessions representing races of maize from Latin America. Their microstructural characteristics (size and compaction of endosperm cell bodies, pericarp thickness, horny‐floury endosperm ratio, and morphology and size of starch granules) were also evaluated using environmental scanning electron microscopy (ESEM). Compaction was the most important microstructural feature of the maize kernels, representing kernel hardness. Highly compact kernels tended to be hard, with high protein, pericarp, and hard‐endosperm content and high pericarp thickness, but with low moisture, amylose content, and kernel weight and size. The opposite was observed in the least compact kernels. Highly compact kernels tended to have small, polygonal starch granules (<10 μm), while the least compact kernels contained large, spherical granules (>10 μm). These results suggest that microstructure is responsible for the physical features of maize kernels and that microstructure is related to chemical composition.     

Sequential in vitro pepsin digestion of uncooked and cooked sorghum and maize samples

An in vitro protein digestion study, using pepsin, was carried out in uncooked and cooked sorghum and maize flour samples. The digestibility values from the uncooked samples showed that sorghum presents digestibility values similar to those of maize. In the case of the cooked samples, it was found that a wet cooking procedure promotes a decrease in sorghum protein digestibility when compared to maize. Electrophoresis was used to follow the in vitro pepsin sequential digestion procedure, and infrared spectroscopy was applied to establish its efficiency. SDS-PAGE results showed that both uncooked samples (sorghum and maize) behave in a similar way. The wet cooking procedure increases the amount of high molecular weight aggregates and promotes the appearance of two nonreducible and nondigestible 45 and 47 kDa proteins. These two protein fractions are directly related to the loss of digestibility. It was also shown that in cooked sorghum the monomers (gamma-, alpha-, and beta-) are more resistant to digestion than the corresponding uncooked samples.     

Factors Governing Pasting Properties of Waxy Wheat Flours

Waxy wheat (Triticum aestivum L.) contains endosperm starch lacking in amylose. To realize the full potential of waxy wheat, the pasting properties of hard waxy wheat flours as well as factors governing the pasting properties were investigated and compared with normal and partial waxy wheat flours. Starches isolated from six hard waxy wheat flours had similar pasting properties, yet their corresponding flours had very different pasting properties. The differences in pasting properties were narrowed after endogenous α‐amylase activity in waxy wheat flours was inhibited by silver nitrate. Upon treatment with protease, the extent of protein digestibility influenced the viscosity profile in waxy wheat flours. Waxy wheat starch granules swelled extensively when heated in water and exhibited a high peak viscosity, but they fragmented at high temperatures, resulting in more rapid breakdown in viscosity. The extensively swelled and fragmented waxy wheat starch granules were more susceptible to α‐amylase degradation than normal wheat starch. A combination of endogenous α‐amylase activity and protein matrix contributed to a large variation in pasting properties of waxy wheat flours.     

Discovery of Grain Sorghum Germ Plasm with High Uncooked and Cooked In Vitro Protein Digestibilities

Grain sorghum has been documented to have low protein digestibility relative to other cereal grains. Low protein digestibility of sorghum is most pronounced in cooked foods and is ranked slightly lower than corn as a feed grain. In this article, sorghum germ plasm is identified that has substantially higher uncooked and cooked flour in vitro protein digestibility than normal cultivars. Sorghum lines were found within a high-lysine opulation derived from the mutant P721Q that have ≈10–15% higher uncooked and ≈25% higher cooked protein digestibilities using a pepsin assay. Highly digestible sorghum grain showed little reduction in digestibility after cooking, compared to the large reduction that is typical of normal sorghum cultivars. Using the three-enzyme pH-stat method, we showed that the highly digestible lines had the same degree of peptide bond hydrolysis in ≈5 min, as was found in 60 min in the normal cultivar, P721N. Differences in protein digestibility were related to enyzme susceptibility of the major storage prolamin, α-kafirin, that comprises ≈50–60% of the total sorghum grain protein. Using the enzyme-linked immunosorbent assay (ELISA) technique to track the pepsin digestion of α-kafirin, the highly digestible lines had ≈90–95% α-kafirin digested in 60 min compared to 45–60% for two normal cultivars. γ-Kafirin, a minor structural prolamin found mainly at the periphery of protein bodies, was also somewhat more digestible in the highly digestible sorghums. Highly digestible grain was of a floury kernel type, though recently this trait has been found in a modified background. More digestible protein from sorghum grain, that additionally is high in lysine content and has a fairly hard endosperm, could be of important benefit to populations who lack adequate protein in their diets, and may, pending further studies, prove to increase the value of sorghum as a feed grain.     

Significance of Amylose Content of Wheat Starch on Processing and Textural Properties of Instant Noodles

The effect of amylose content of starch on processing and textural properties of instant noodles was determined using waxy, partial waxy, and regular wheat flours and reconstituted flours with starches of various amylose content (3.0–26.5). Optimum water absorption of instant noodle dough increased with the decrease of amylose content. Instant noodles prepared from waxy and reconstituted wheat flours with ≤12.4% amylose content exhibited thicker strands and higher free lipids content than wheat flours with ≥17.1% amylose content. Instant noodles of ≤12.4% amylose content of starch exhibited numerous bubbles on the surface and stuck together during frying. Lightness of instant noodles increased from 77.3 to 81.4 with the increase of amylose content of starch in reconstituted flours. Cooking time of instant noodles was 4.0–8.0 min in wheat flours and 6.0–12.0 min in reconstituted flours, and constantly increased with the increase in amylose content of starch. Hardness of cooked instant noodles positively correlated with amylose content of starch. Reconstituted flours with ≤12.4% amylose content of starch were higher in cohesiveness than those of wheat flours of wild‐type and partial waxy starches and reconstituted flours with ≥17.1% amylose content. Instant fried noodles prepared from double null partial waxy wheat flour exhibited shorter cooking time, softer texture, and higher fat absorption (1.2%) but similar color and appearance compared with noodles prepared from wheat flour of wild‐type starch.     

Effects of Flour Particle Size and Starch Damage on Processing and Quality of White Salted Noodles

Several reduction grinding conditions were used on a Canadian Western Red Spring (CWRS) farina to yield flours of comparable protein content within three specific particle size ranges (132–193, 110–132, 85–110 μm) at three starch damage levels (3.0, 3.9, 7.0 Megazyme units). White salted noodles (1% w/w NaCl) were initially processed at a fixed absorption (32%). Dynamic oscillatory and large deformation creep measurements indicated that doughs with lower starch damage, thick or thin, exhibited lower G′ (storage modulus), higher tan δ (G″ [loss modulus]/G′) values, and greater maximum strain during creep than doughs with higher starch damage. There were no clear trends between work input during sheeting and either starch damage or particle size. Instrumental texture analysis of raw noodles showed no significant differences due to either starch damage or flour particle size. Flours with fine particle size gave cooked noodles with the best textural attributes, whereas starch damage exhibited no consistent relationship with cooked noodle texture. Cooking loss was greatest in samples with highest starch damage and coarsest particle size; water uptake was inversely related to starch damage and particle size. Experiments were repeated at adjusted water absorptions (32–36.5%) for fine and coarse flours with highest and lowest starch damage. Differences in raw noodle dough rheological properties were largely eliminated, confirming that differences noted at constant absorption were primarily due to flour water absorption. Work input during sheeting was inversely related to starch damage and was higher for fine particle size. Cooking losses were highest for higher starch damage and fine particle size. Water uptake was highest for fine particle size, but in contrast to cooking loss, was higher at lower starch damage. Textural parameters indicated superior cooking quality when particle size was finer and starch damage was lower. Flour particle size and starch damage (as indicated by water absorption) are both primary quality determinants of white salted noodle properties and, to some extent, exert their influence independently.     

Effect of processing conditions on phytic acid, calcium, iron, and zinc contents of lime-cooked maize

Tortillas are made by cooking maize in a lime solution during variable times and temperatures, steeping the grain for up to 12 h, washing and grinding it to a fine dough, and cooking portions as flat cakes for up to 6 min. The effects of the main processing steps on the chemical composition, nutritive value, and functional and physicochemical characteristics have been areas of research. The present work evaluates the effect of lime concentration (0, 1.2, 2.4, and 3.6%) and cooking times (45, 60, and 75 min) on phytic acid retention of whole maize, its endosperm, and germ, as well as on the content of calcium, iron, and zinc on the same samples. The effects of steeping time and temperature and steeping medium on the phytic acid of lime-cooked maize were also studied. Finally, phytic acid changes from raw maize to tortilla were also measured. The results indicated that lime concentration and cooking time reduce phytic acid content in whole grain (17.4%), in endosperm (45.8%), and in germ (17.0%). Statistical analyses suggested higher phytic acid loss with 1.2% lime and 75 min of cooking. Cooking with the lime solution is more effective in reducing phytic acid than cooking with water. Steeping maize in lime solution at 50 degrees C during 8 h reduced phytic acid an additional 8%. The total loss of phytic acid from maize to tortilla was 22%. Calcium content increased in whole maize, endosperm, and germ with lime concentration and cooking and steeping times. The increase was higher in the germ than in the endosperm. The level, however, can be controlled if steeping of the cooked grain is conducted in water. Iron and zinc contents were not affected by nixtamalization processing variables but were affected in steeping.     

Visualization of Channels and Cavities of Corn and Sorghum Starch Granules

Evidence is presented that corn (maize) and sorghum starch granules have channels that connect a central cavity to the external environment. A mutant sorghum starch with numerous, large surface pores was treated with a methanolic solution of merbromin and sectioned. Light, fluorescence, and compositional backscattering electron microscopy revealed channels connecting an internal cavity to the external surface in most granules. Cavities and channels could also be seen in whole corn and sorghum starch granules treated with merbromin in methanol and viewed by fluorescence microscopy. Treatment of sorghum starch granules with an aqueous solution of merbromin revealed that the molecule penetrated the granule matrix under even slightly swelling conditions. Light microscopy showed cavities in unstained, whole, dry corn and sorghum starch granules mounted in immersion oil.     

Effect of Extrusion Cooking on the Primary Structure and Water Solubility of β‐Glucans from Regular and Waxy Barley

Water‐soluble β‐glucan from native and extrusion‐cooked barley flours of two barley cultivars, Candle (a waxy starch barley) and Phoenix (a regular starch barley), was isolated and purified. The purity of β‐glucan samples was 85–93% (w/w, dry weight basis) for Candle and 77–86% (w/w, dry weight basis) for Phoenix. The water solubility of β‐glucan (at room temperature, 25°C) in the native and extruded flours (primary solubility) was different from that of the purified β‐glucan samples (secondary solubility). The solubility of β‐glucan in the native and extruded Candle flour was substantially higher than that of β‐glucan in Phoenix. For both cultivars, β‐glucan in the extruded flours had solubility (primary solubility) values higher than in their native counterparts. The solubility of β‐glucan in the purified β‐glucan samples differed depending on the barley cultivar and the extrusion conditions employed. The glycosidic linkage profiles of purified soluble β‐glucan from native and extruded barley flours were determined in order to understand the changes in the primary structure of β‐glucan and the effect of extrusion on the β‐glucan structure‐solubility relationship.     

Hardness Changes and Endosperm Modification During Sorghum Malting in Grains of Varying Hardness and Malt Quality

This study examined the interaction between sorghum grain hardness and sorghum malt quality in terms of diastatic power and free amino nitrogen with endosperm modification during malting. The changes in kernel hardness during malting of four commercial sorghum cultivars of differing quality in terms of endosperm texture and potential malt quality were measured using tests for hardness and density, and endosperm modification was followed by scanning electron microscopy. The general pattern of modification during sorghum malting was confirmed to start at the endosperm–scutellum interface and then continue into the floury endosperm toward the kernel distal end. Significantly, a cultivar of intermediate hardness and low malting quality remained harder and modified more slowly than a harder cultivar of high malting quality. It appeared that intrinsic grain hardness and malt amylase and protease activity both affected malt hardness and endosperm modification, but amylase and protease activity had a greater effect because of their degradation of endosperm starch and protein. Of the hardness and density tests studied, the tangential abrasive dehulling device (TADD) gave the best measure of hardness throughout malting; maximum range was 24–100% kernel removed over five days of malting. Also, the data agreed with the observed malt modification rates. Thus, the TADD may have application as a simple and rapid test for estimating sorghum malt quality.     

Kernel Microstructure of Latin American Races of Maize and Their Thermal and Rheological Properties

Seventy‐one races of maize representing races from Latin America were analyzed for microstructural features such as the degree of compaction of the endosperm cell bodies, starch granule size and morphology, and hard‐soft endosperm relationship. Flours were analyzed using rapid visco analysis and differential scanning calorimetry. Compaction grade was the most important microstructural feature of the maize kernels that related to thermal and rheological properties. Highly compact kernels developed low peak and final viscosities; small, polygonal starch granules; and required more time and higher temperature to gelatinize. The opposite was the case for less compact kernels. This indicates that the characteristic protein matrix of highly compact kernels represents a physical barrier to water migration into the granules, retarding the gelatinization process.     

Effects of Brans from Specialty Sorghum Varieties on In Vitro Starch Digestibility of Soft and Hard Sorghum Endosperm Porridges

Brans of specialty sorghum varieties (high tannin, black, and black with tannin) were used to investigate the effects of sorghum phenolic compounds on starch digestibility of soft and hard sorghum endosperm porridges. Endosperms of varieties with the highest and lowest grain hardness index were mixed with brans of specialty sorghum varieties in the ratio of 85:15 and cooked into porridges with distilled water using a Rapid Visco Analyzer. Brans of condensed tannin containing sorghum varieties (high‐tannin and black with tannin sorghums) significantly (P < 0.05) decreased starch digestibility and estimated glycemic index (EGI) and increased resistant starch (RS) content of endosperm porridges. However, the addition of phenolic‐rich tannin‐free (mostly anthocyanins) black sorghum bran significantly (P < 0.05) increased starch digestibility and EGI but did not affect RS content of endosperm porridges. The disparate effects with black bran may, in part, result from its larger particle size and different bran structure compared with other sorghum varieties evaluated. Thus, our study showed that not only presence of phenolic compounds in the brans but also structural differences of specialty sorghum brans can have significant effects on starch digestibility.