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.     

Increasing Slowly Digestible Starch Content of Normal and Waxy Maize Starches and Properties of Starch Products

Changes in the digestibility and the properties of the starch isolated from normal and waxy maize kernels after heat‐moisture treatment (HMT) followed by different temperature cycling (TC) or isothermal holding (IH) conditions were investigated. Moist maize kernels were heated at 80°C for 2 hr. The HMT maize kernels were subjected to various conditions designed to accelerate retrogradation of the starch within endosperm cells. Two methods were used to accelerate crystallization: TC with a low temperature of –24°C for 1 hr and a high temperature of 20, 30, or 50°C for 2, 4, or 24 hr for 1, 2, or 4 cycles, and IH at 4, 20, 30, or 50°C for 24 hr. The starch granules were then isolated from the treated kernels. The starch isolated from HMT normal maize kernels treated by TC using –24°C for 1 hr and 30°C for 2 hr for 2 cycles gave the greatest SDS content (24%) and starch yield (54%). The starch isolated from HMT waxy maize kernels treated by TC using –24°C for 1 hr and 30°C for 24 hr for 1 cycle had an SDS content of 19% and starch yield of 43%. The results suggest that TC after HMT changes the internal structure of maize starch granules in a way that results in the formation of SDS (and RS). They also suggest that thermal treatment of maize kernels is more effective in producing SDS than is the same treatment of isolated starch. All starch samples isolated from treated normal maize kernels exhibited lower peak viscosities, breakdown, and final viscosities and higher pasting temperatures than did the control (untreated normal maize starch). Although peak viscosities and breakdown of the starch isolated from treated waxy maize kernels were similar to those of the control (untreated waxy maize starch), their pasting temperatures were higher. The starch isolated from treated normal and waxy maize kernels with the highest SDS contents (described above) were further examined by DSC, X‐ray diffraction, and polarized light microscopy. Onset and peak temperatures of gelatinization of both samples were higher than those of the controls. Both retained the typical A‐type diffraction pattern of the parent starches. The relative crystallinity of the starch from the treated normal maize kernels was higher than that of the control, while the relative crystallinity of the starch from the treated waxy maize kernels was not significantly different from that of the control. Both treated starches exhibited birefringence, but the granule sizes of both starches, when placed in water, were slightly larger than those of the controls.     

Physicochemical and Nutritional Properties of Different Maize Races on Toasted Tortillas

Biodiversity contributes to nutrient production and, together with processing, is a critical factor in product quality. Physicochemical and nutritional properties of toasted tortillas (totopos) were evaluated in 1) maize samples from Oaxaca communities and 2) maize races of different endosperm texture. Texture profiles show that totopos elaborated from Zapalote Chico maize race showed the best performance (low breaking force) and higher crunchability, similar to commercial totopos. Quality of Zapalote Chico totopos was explained by flotation index (FI) and starch viscosity as well as thermal properties. FI was negatively correlated with texture that may related to end use. Zapalote Chico maize gelatinizes at higher (P < 0.05) pasting temperature (72.8–73.3°C) and it had higher (P > 0.05) peak viscosity (3,093–4,723 cP), suggesting a more organized starch structure. In hybrid and Tuxpeño samples, most of the starch granules (90%) were gelatinized and increased the hardness in totopos. The totopo samples increased the peak at 4.45 Å, a characteristic of type‐V diffraction of amylose‐lipid complexes (resistant starch). A small increase in resistant starch (0.6%) was found in totopos, which has important nutritional benefits for consumers. Our results support the preference of Oaxaca people for the totopos made from Zapalote Chico maize.     

Influence of Cooking Conditions on the Protein Matrix of Sorghum and Maize Endosperm Flours

To understand the influence of the sorghum and maize endosperm protein matrix honeycomb structure on starch hydrolysis in flours, three‐dimensional fluorescence microscopy was applied to floury and vitreous endosperm flours cooked under various conditions. Cooking caused the collapse and matting of the sorghum and maize vitreous endosperm matrices, with the effect being greater in sorghum. The effect of cooking was rather different in the floury endosperm in that the protein matrices expanded and broke up to some extent. These effects were a consequence of expansion of the starch granules through water uptake during gelatinization. Cooking in the presence of 2‐mercaptoethanol caused an expansion of the vitreous endosperm matrix mesh due to breakage of disulfide bonds in the protein matrix. Mercaptoethanol also caused an increase in the proportion of β‐sheet structure relative to α‐helical structure of the endosperm proteins. Increased energy of cooking caused collapse of the sorghum matrix. Disulfide bonding and an increase in β‐sheet structure occurred with cooking, with the increase in disulfide bonding being greatest in sorghum vitreous endosperm. The tendency for the sorghum protein matrix to collapse and mat more with cooking than the maize matrix appears to be due to greater disulfide bonding. This is responsible for the observed low starch digestibility of cooked sorghum flour as a result of the more disulfide‐bonded protein matrix limiting the expansion of the starch granules and hence amylase access.     

A Study of Maize Endosperm Hardness in Relation to Amylose Content and Susceptibility to Damage

The relative amounts of amylose and amylopectin in maize starch were determined in samples representing hard and soft endosperm. Although differences were small, amylose content differed significantly (P < 0.001 and P < 0.05) between the two types of endosperm, with hard endosperm containing a higher percentage of amylose. Scanning electron microscopy was used to determine that the surface appearance of starch granules from hard and soft endosperm differed. Starch granules from soft endosperm had randomly distributed pores on their surfaces, which had a rough appearance. Few pores were observed on granules from hard endosperm. A fairly common occurrence with starch granules from soft endosperm was the development of wrinkles or fissures upon prolonged exposure to the beam of the electron microscope. Thus, a correlation existed between endosperm hardness, amylose content, and susceptibility to wrinkling and fissures. The granules of the soft endosperm of maize, presumably less mature than the granules of the hard endosperm, have a lower amylose content (20.5 ± 1.9% vs. 23.0 ± 1.0%), exhibit more surface pores, and are more susceptible to wrinkling in an electron beam, compared with granules of the hard endosperm. Results suggested that the composition and internal architecture of the starch granule differ depending on the hardness of the endosperm from which it was obtained.     

Amyloplast Formation and Starch Granule Development in Hard Red Winter Wheat

Plastids in the coenocytic endosperm of young wheat caryopses were mostly in the form of pleomorphic proplastids with a few of the plastids containing small starch granules. Following cellularization of the coenocytic cytoplasm, the outer one or two endosperm cell layers became meristematic and continued to divide until about 14 days after flowering (DAF). During the first week of endosperm development, newly divided cells had plastids that were pleomorphic in shape, while subaleurone cells interior to the meristematic region contained amyloplasts that contained a single‐size class of starch granules (incipient A‐type starch granules). The pleomorphic plastids exhibited tubular protrusions that extended a considerable distance through the cytoplasm. Amyloplasts in cells interior to the meristematic region did not exhibit protrusions. Both subaleurone and central endosperm cells had amyloplasts that exhibited protrusions at 10–12 DAF, and some of the protrusions contained small starch granules (incipient B‐type starch granules). Protrusions were not observed in endosperm amyloplasts at 14 DAF. Two sizes of starch, large A‐type and smaller B‐type granules were present within the cells, however. Amyloplast protrusions were numerous again at 17 DAF in both subaleurone and central endosperm cells; at 21 DAF, a third size class of small C‐type starch granules was observed in the cytoplasm. Amyloplasts in the endosperm of wheat apparently divided and increased in number through protrusions because binary fission typical of plastid division was never observed. Protrusions were observed in the coenocytic cytoplasm, in dividing cells, in subaleurone and central endosperm cells at 10–12 DAF, and in subaleurone and central endosperm cells at 17 DAF. The results suggest that there are three sizes of starch granules produced at specific times during wheat endosperm development.     

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.     

Single Kernel Sampling Method for Maize Starch Analysis While Maintaining Kernel Vitality

A nondestructive protocol for maize kernel starch sampling was developed, enabling starch preparation from a single kernel for analysis of starch structure while also maintaining the vitality of the seed. To develop the single kernel sampling (SKS) method, maize genotypes varying in starch structure including ae, wx, su2, du and normal in the W64A inbred line were used. Crude endosperm material was removed from the kernel crown, soaked, ground, washed, and dissolved in 90% DMSO. The sample represented ≈10% of the total kernel. Endosperm starch was also isolated from the same genotypes by a standard multikernel isolation (MKI) method. Starches isolated by the two methods were debranched and analyzed by high‐performance size‐exclusion chromatography (HPSEC) and fluorophore‐assisted carbohydrate electrophoresis (FACE). HPSEC and FACE showed similar results for the two sampling methods for degree of polymerization (DP) ≤ 50. We concluded that the material obtained by SKS could be used for identifying amylopectin structural differences among genotypes. Kernel sampling for SKS had no effect on germination, thus plants could be grown for subsequent genetic crosses and analysis. The SKS method may be useful for the screening of populations of maize kernels from genotypes producing novel amylopectin structure, and allow the growth of novel genotypes for further analysis.     

Separation and Characterization of A- and B-Type Starch Granules in Wheat Endosperm

Mature wheat (Triticum aestivum L.) endosperm contains two types of starch granules: large A-type and small B-type. Two methods, microsieving or centrifugal sedimentation through aqueous solutions of sucrose, maltose, or Percoll were used to separate A- and B-type starch granules. Microsieving could not completely separate the two types of starch granules, while centrifuging through maltose and sucrose solutions gave a homogenous population for B-type starch granules only. Centrifuging through two Percoll solutions (70 and 100%, v/v) produced purified populations of both the A- and B-type starch granules. Analysis of starch granule size distribution in the purified A- and B-type granule populations and in the whole-starch granule population obtained directly from wheat endosperm confirmed that the purified A- and B-type starch granule populations represented their counterparts in mature wheat endosperm. Centrifugations through two Percoll solutions were used to purify A- and B-type starch granule populations from six wheat cultivars. The amylose concentrations and gelatinization properties of these populations were analyzed. All of the A-type starch granules contained higher amylose concentrations and had higher gelatinization enthalpies than did B-type starch granules. Although A- and B-type starch granules started to gelatinize at a similar temperature, B-type starch granules had higher gelatinization peak and completion temperatures than did A-type starch granules     

Detection of Proteins in Starch Granule Channels

Proteins were detected in channels of commercial starches of normal maize, waxy maize, sorghum, and wheat through labeling with a protein‐specific dye and examination using confocal laser scanning microscopy (CLSM). The dye, specifically 3‐(4‐carboxybenzoyl)quinoline‐2‐carboxaldehyde (CBQCA), fluoresces only after it reacts with primary amines in proteins, and CLSM detects fluorescence‐labeled protein distribution in an optical section of a starch granule while it is still in an intact state. Starch granules in thin sections of maize kernels also had channel proteins, indicating that proteins are native to the channels and not artifacts of isolation. Incubation of maize starch with protease (thermolysin) removed channel proteins, showing that channels are open to the external environment. SDS‐PAGE analysis of total protein from gelatinized commercial waxy maize starch revealed two major proteins of about M_r 38,000 and 40,000, both of which disappeared after thermolysin digestion of raw starch. Commercial waxy maize starch granule surface and channel proteins were extracted by SDS‐PAGE sample buffer without gelatinization of the granules. The major M_r 40,000 band was identified by MALDI‐TOF‐MS and N‐terminal sequence analysis as brittle‐1 (bt1) protein.     

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.     

Evaluation of Quality Factors in Argentine Maize Races

Samples of Argentine maize from 12 landraces were analyzed for starch and amylose content and were evaluated for hardness parameters. Amylose contents of these Argentine landraces were generally higher than for typical dent hybrids grown in the United States. Hardness, as estimated by near‐infrared reflectance; from wavelength shift of the near‐infrared transmittance spectrum absorbance maximum (λ_max) in the 610–680 nm region; and by percentage of γ‐zein in zein‐2 (determined by reversed‐phase HPLC), correlated well with classical definitions of endosperm type and with amylose content. Starch content correlated negatively with hardness. Flint landraces varied substantially in amylose and starch content. The strong correlation between amylose content and maize endosperm hardness confirms and strengthens previous observations, and may provide a compositional basis for endosperm quality.     

玉米应力裂纹的显微分析

观察了３种应力裂纹类型（单裂、双裂、龟裂）玉米的内部显微结构和应力裂纹产生的位置、数量、宽度，分析了应力裂纹在胚乳中扩展的情况以及应力裂纹对胚乳结构产生的影响。裂纹的类型不同，裂纹的形态、数量及大小亦不同。应力裂纹生成在粉质胚乳的中心部位，沿淀粉颗粒的边缘向外扩展。应力裂纹是内部裂纹，只能扩展到种皮附近，接近糊粉层。     

Comparison of Flour Starch Properties in Half‐Sib Families of Opaque‐2 Maize (Zea mays L.) from Argentina

Since the discovery of the o2 mutation in maize, many studies have reported the characterization of the protein quality of opaque‐2 genotypes. However, few have reported the properties of their starch. The objective of this study was to characterize flour starch properties of 12 half‐sib families of opaque‐2 maize from Argentina. Chemical composition and thermal and pasting properties of whole grain flour were determined. Nonopaque genotypes were used as a control. Starch content of opaque‐2 genotypes did not show significant differences compared with nonopaque genotypes, yet amylose content was significantly lower. A high variability in pasting and thermal properties was observed in genotypes. Opaque samples showed a significantly higher peak viscosity and a lower pasting temperature compared with nonopaque samples, probably owing to larger and less compact starch granules in the floury endosperm. The higher the gelatinization enthalpy of opaque‐2 genotypes was, the lower the amylose content in relation to nonopaque varieties. Two retrogradation endotherms were observed in DSC analysis: one corresponding to amylopectin crystallization and the other to melting of amylose‐lipid complex. Both enthalpies were considered total starch retrogradation (ΔH _RT). A wide range of variation was obtained in ΔH _RT in opaque‐2 genotypes, but no significant differences between opaque and nonopaque genotypes were observed. The differences in starch properties found in this study would make it possible to identify opaque‐2 families with particular characteristics for the development of starchy food items adapted to specific processing traits.     

Morphological,Physical, and Chemical Properties of Grain and Flour from Chalky Rice Mutants

Morphological, physical, and chemical properties of grain and flour of 20 chalky rice mutants were compared with those of a translucent parental cultivar, Koshihikari. Compound starch granules were loosely packed and single starch granules were observed in chalky parts of mutant endosperm. Chalky mutant lines were classified into milky‐white and white‐core lines based on the degree of endosperm chalkiness. Physical and chemical characteristics also suggest a division of chalky mutant lines into two groups, consistent with the classification made based on morphological characteristics. Milky‐white mutant lines showed significantly lower grain weight, decreased starch content, and lower grain hardness than white‐core mutants and Koshihikari. Rice flour prepared from milky‐white mutants by dry milling showed less starch damage and finer mean particle size than white‐core mutants and the parent. These results indicated that a loosely packed structure in chalky endosperm was responsible for fragile grain and that it yielded fine flour with lower damaged starch contents. Decreased starch contents of chalky mutant lines correlated to lower RVA viscosities. Milky‐white mutant lines were expected to be useful to produce fine flour, which undergoes less starch damage during dry milling processing.     

Morphological Changes of Granules of Different Starches by Surface Gelatinization with Calcium Chloride

Native starch granules of 11 selected cultivars (potato, waxy potato, sweet potato, normal maize, high‐amylose maize, waxy maize, wheat, normal barley, high‐amylose barley, waxy barley, and rice) were treated with a calcium chloride solution (4M) for surface gelatinization. The surface‐gelatinized starch granules were investigated using light microscopy and scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). In general, those starches with larger granule sizes required longer treatment time to complete the gelatinization. The salt solution treatment of starch was monitored by light microscopy and stopped when the outer layer of the granule was gelatinized. The surface gelatinized starch granules were studied using scanning electron microscopy. On the basis of the gelatinization pattern from calcium chloride treatments, the starches could be divided into three groups: 1) starches with evenly gelatinized granule surface, such as normal potato, waxy potato, sweet potato, maize, and high‐amylose maize; 2) starches with salt gelatinization concentrated on specific sites of the granule (i.e., equatorial groove), such as wheat, barley, and high‐amylose barley; and 3) starches that, after surface gelatinization, can no longer be separated to individual granules for SEM studies, such as waxy barley, waxy maize, and normal rice. The morphology of the surface gelatinized starch resembled that of enzyme‐hydrolyzed starch granules.     

Corn Kernel Structural Integrity: Analysis Using Solvent and Heat Treatments

Most corn (Zea mays, L.) processing is accomplished by causing a structural change to the kernel. Associations between corn endosperm structural components were characterized using textural analysis after solvent and heat treating kernels. Intact Asgrow 405W and B73xMo17 kernels were incubated and treated at 20, 40, 55, and 90°C for 1, 24, and 48 hr in static air, in acetone, and in aqueous solutions of water, calcium chloride, sodium chloride, sodium bisulfite, lactic acid, lime, lye, ethanol urea, and sodium dodecyl sulfate (SDS). After treatment, kernels were compressed between flat platens. Acetone did not significantly soften endosperm structure. Ethanol reduced kernel fracturability by weakening cell‐to‐cell (wall) bonds, but ethanol did not effectively reduce kernel hardness. Water and aqueous solvents swelled and softened kernels by plasticizing structural components. Bisulfite and SDS softened kernels more than water only soaks because they denatured matrix proteins. Alkaline soaks reduced fracturability and softened the kernel by dissociating both cell‐to‐cell and intracellular (starch‐protein) bonds. Soaking for longer periods and at higher temperatures increased aqueous‐based solvent softening effect. Urea imbibition into the kernel and its softening effects were highly dependent on time and temperature of soak. Endosperm structural integrity is the governed by a combination of cell‐to‐cell bonds and intra‐cellular (starch‐protein) bonds. Reagents that denatured the endosperm matrix proteins and disrupted hydrogen bonds resulted in the greatest alterations to kernel structural integrity. Ultimately a better understanding of kernel structural integrity will lead to the development of improved hybrids and process technologies designed to facilitate desirable structural changes.     

A Radioisotopic Study of the Entry of Calcium Ion into the Maize Kernel During Nixtamalization

The entry of calcium ions from the nixtamalization solution into maize kernels over time was followed in model experiments using radiolabeled calcium ions, with autoradiographic evaluation of the kernels after different cooking and steeping times. Calcium ions immediately entered the pericarp and were rapidly fixed at the outer boundary of the endosperm, especially at the external surface of the germ. Entry of calcium into the endosperm occurred gradually after long steeping times, except in the case of broken kernels, for which massive invasion by calcium was observed. After extended steeping times, a moderate amount of calcium‐45 was evident in the germ. Specific perforation of the outer layers of the grains provided a defined route of facilitated entry of calcium into the endosperm. No fundamental difference with respect to penetrability by calcium ion was seen in a comparison between flint‐type grains and grains containing only floury endosperm.     

Comparison of the physicochemical properties and ultrastructure of japonica and indica rice grains

The physicochemical properties and ultrastructures of japonica vs indica rice varieties and waxy vs nonwaxy rice varieties were compared. The viscogram values of the indica varieties were significantly higher than those of the japonica varieties. The gelatinization temperatures, breakdown, and setback were significantly lower for waxy than for nonwaxy rice varieties. Japonica rice exhibited lower hardness but higher adhesiveness than indica rice. The air space between individual starch granules was larger for waxy than for nonwaxy rice. The starch granules were compact in japonica rice, while the compound starch granules of indica rice were much smaller than those of japonica rice and were scattered widely in the endosperm. The protein bodies in japonica rice were concentrated near the cell wall, whereas those in indica rice were scattered around amyloplasts. These results suggest that the ultrastructure of rice affects the texture of the cooked product.     

Associations of Starch Gel Hardness,Granule Size,Waxy Allelic Expression,Thermal Pasting,Milling Quality,and Kernel Texture of 12 Soft Wheat Cultivars

Starches were isolated from 12 soft wheat (Triticum aestivum L.) cultivars and were characterized for waxy (Wx) allelic expression, thermal pasting characteristics, and starch granule size. Gels were produced from the thermally degraded starches and were evaluated using large deformation rheological measurements. Data were compared with cultivar kernel texture, milling characteristics, starch chemical analyses, and flour pasting characteristics. Larger flour yields were produced from cultivars that had larger starch granules. Flour yield also was correlated with lower amylose content and greater starch content. Harder starch gels were correlated with higher levels of amylose content and softer kernel texture. The cultivar Fillmore, which had a partial waxy mutation at the B locus, produced the highest peak pasting viscosity and the lowest gel hardness. Softer textured wheats had greater lipid‐complexed amylose and starch phosphorus contents and had less total starch content. Among these wheats of the soft market class, softer textured wheats had larger starch granules and harder textured wheats had smaller starch granules. In part, this may explain why soft wheats vary in texture. The smaller granules have larger surface area available for noncovalent bonding with the endosperm protein matrix and they also may pack more efficiently, producing harder endosperm.