Impacts of Kafirin Allelic Diversity,Starch Content,and Protein Digestibility on Ethanol Conversion Efficiency in Grain Sorghum

Seed protein and starch composition determine the efficiency of the fermentation process in the production of grain‐based ethanol. Sorghum, a highly water‐ and nutrient‐efficient plant, provides an alternative to fuel crops with greater irrigation and fertilizer requirements, such as maize. However, sorghum grain is generally less digestible because of extensive disulfide cross‐linking among sulfur‐rich storage proteins in the protein– starch matrix. Thus, the fine structure and composition of the seed endosperm directly impact grain end use, including fermentation performance. To test the hypothesis that kafirin (prolamin) seed storage proteins specifically influence the efficiency of ethanol production from sorghum, 10 diverse genetic lines with allelic variation in the β‐, γ‐, and (δ‐kafirins, including three β‐kafirin null mutants, were tested for ethanol yield and fermentation efficiency. Our selected lines showed wide variation in grain biochemical features, including total protein (9.96–16.47%), starch (65.52–74.29%), and free amino nitrogen (FAN) (32.84–73.51 mg/L). Total ethanol yield (ranging from 384 to 426 L/metric ton), was positively correlated to starch content (R² = 0.74), and there was a slight positive correlation between protein digestibility and ethanol yield (R² = 0.52). Increases in FAN content enhanced fermentation efficiency (R² = 0.65). The highest ethanol producer was elite staygreen breeding line B923296, and the line with the highest fermentation efficiency at the 72 h time point was inbred BT×623. A large‐seeded genotype, KS115, carrying a novel γ‐kafirin allele, was rich in FAN and exhibited excellent short‐term fermentation efficiency at 85.68% at the 20 h time point. However, the overall ethanol yield from this line was comparatively low at 384 L/metric ton, because of insufficient starch, low digestibility, and high crude protein. Multivariate analysis indicated an association between the β‐kafirin allele and variation in grain digestibility (P = 0.042) and FAN (P = 0.036), with subsequent effects on ethanol yield. Reversed‐phase HPLC profiling of the alcohol‐soluble kafirin protein fraction revealed diversity in protein content and composition across the lines, with similarities in peak distribution profiles among β‐kafirin null mutants compared with normal lines.     

Characterization of Kafirins in Algerian Sorghum Cultivars

The prolamins in seven Algerian Sahara sorghum cultivars of varying seed shape and color were investigated. Protein contents ranged from 12 to 16%. Prolamins were the major protein fraction. They could be separated according to degree of disulfide cross‐linking. Kafirin monomers and low molecular weight polymers could be extracted with 70% ethanol, whereas highly cross‐linked kafirins additionally needed a reducing agent to become extractable. Kafirin monomers of α‐, β‐ and γ‐type were purified and N‐terminally sequenced. For the first time, δ‐kafirin was identified at the protein level. The study clearly revealed intercultivar differences between protein levels. The joint use of SDS‐PAGE, SE‐HPLC, and RP‐HPLC allowed discriminating among cultivars based on the differences in prolamin levels and composition.     

A Rapid Protein Digestibility Assay for Identifying Highly Digestible Sorghum Lines

Protein digestibility in sorghum (Sorghum bicolor (L.) Moench) lines was determined using two standard procedures (pepsin digestibility and pH‐stat) and compared with a newly developed, rapid electrophoresis‐based screening assay. The new assay was based on the rate of α‐kafirin disappearance after pepsin digestion. α‐Kafirin, the major sorghum storage protein, makes up ≈60–70% of the total protein in the grain. In the new assay, samples were first digested with pepsin for 1 hr, and undigested proteins were then analyzed by SDS‐PAGE. The intensitizes of the undigested α‐kafirin bands were measured. Higher band intensity indicated lower protein digestibility. The new assay was significantly correlated with the standard pepsin digestibility assay (r = −0.96, n = 16) after which it was patterned. The same was true of the pH‐stat procedure (r = −0.85, n = 16). This implies that the new assay is comparable to existing procedures and can be used for screening sorghum lines for protein digestibility. Two groups consisting of high‐protein digestibility and wild‐type sorghum lines were identified when the new assay was tested on 48 sorghum lines derived from crosses of wild‐type and mutant high protein digestibility lines, indicating that the new assay was efficient in differentiating between the two groups. Advantages of the new assay over the standard procedures include considerable reduction in analysis time and sample size required for the analysis. For example, analysis time was reduced by 20% and sample size by 10% when the new assay was used as compared with the pH‐stat procedure. We estimate that ≈60 sorghum lines can be screened in a day by a single operator using the new assay.     

Sorghum Bran as a Potential Source of Kafirin

Sorghum bran, a coproduct of sorghum dry milling, could be a source of protein for industrial applications. Condensed tannin‐free red and white sorghum samples were decorticated by abrasion until ≈10 or 25% grain by weight was removed. Kafirin was then extracted from the milling fractions using an aqueous ethanol based solvent system. The brans were darker and considerably higher in protein and fat compared with the whole grain flours and decorticated grain flours, with the 25% bran having higher protein than the 10% bran. This is due to increased contamination of the bran with protein‐dense, corneous endosperm. The protein extracted from all the milling fractions, including the brans, was pure kafirin. However, the yield of kafirin from the brans (15.9–26.7% of total protein present) was somewhat lower than that from whole grain and decorticated grain flours (45.0–57.9% of total protein present), due to the fact that kafirin is located solely in the endosperm. Also, the kafirin from bran was more contaminated with fat, polyphenols, and other substances, and more highly colored, particularly the kafirin from red sorghum. Thus, sorghum bran could be used as a source of kafirin but further purification steps may be necessary.     

Extraction and Film Properties of Kafirin from Coarse Sorghum and Sorghum DDGS by Percolation

The high cost of kafirin and zein restricts their use for bioplastic and food applications. Effective, simple, and rapid kafirin/zein isolation processes are required. Here a percolation‐type aqueous ethanol solvent extraction process from coarse meals (grits) and coarse sorghum distillers dried grains and solubles (DDGS) for kafirin and zein isolation employing a low ratio of extractant to meal (2.5:1) was investigated, which is potentially applicable in the grain bioethanol industry. Postextraction filtration times were more than twice as fast using coarse meals compared with fine flours. Washing the meals prior to extraction to remove starch improved protein preparation purity to 73–85% compared with 68–72% for unwashed meals. Hence, no subsequent filtration or centrifugation step is required to clean up the kafirin/zein solution prior to solvent evaporation. With a single extraction step, kafirin/zein yields were 48% (protein basis) for DDGS and 53–70% for washed sorghum/maize meals. Cast films were used as a model bioplastic system to evaluate extracted kafirin/zein functional properties. DDGS kafirin films had rough surfaces but had the lowest water uptake and in vitro digestibility, owing to heat‐induced disulfide crosslinking during DDGS processing. Extraction by percolation using coarse meal/DDGS has potential to improve kafirin/zein viability.     

Formulation and Characterization of Drug‐Loaded Microparticles Using Distillers Dried Grain Kafirin

Kafirin, a protein extracted from sorghum grain, has been formulated into microparticles and proposed for use as a delivery system owing to the resistance of kafirin to upper gastrointestinal digestion. However, extracting kafirin from sorghum distillers dried grains with solubles (DDGS) may be more efficient, because the carbohydrate component has been removed by fermentation. This study investigated the properties and use of kafirin extracted from DDGS to formulate microparticles. Prednisolone, an anti‐inflammatory drug that could benefit from a delayed and targeted delivery system to the colon, was loaded into DDGS kafirin microparticles by phase separation with sodium chloride. Scanning electron micrographs revealed that the empty and prednisolone‐loaded microparticles were round in shape and varied in size. Surface binding studies indicated prednisolone was loaded within the microparticles rather than being solely bound on the surface. These findings demonstrate DDGS kafirin can be formulated into microparticles and loaded with medication. Future studies could investigate the potential applications of DDGS kafirin microparticles as an orally administered targeted drug‐delivery system.     

Use of SDS to Extract Sorghum and Maize Proteins for Free Zone Capillary Electrophoresis (FZCE) Analysis

Two different extraction methods for extracting sorghum (Sorghum bicolor L. Moench.) storage proteins for free zone capillary electrophoresis (FZCE) analysis were compared. A traditional solvent based on 60% t‐butanol was compared with a pH 10 borate buffer containing the anionic detergent SDS followed by precipitation of nonkafirins using 60% t‐butanol. FZCE analysis of both types of extracts showed identical patterns, despite the fact that the SDS should have given all proteins equal charge‐to‐mass ratios. This methodology was also successfully applied to maize proteins. The use of t‐butanol to precipitate nonkafirins, combined with electrophoresis at low pH, is thought to have removed the SDS from the storage proteins. The SDS extraction procedure produced more stable extracts for FZCE analysis. These extracts could also be used directly for SDS capillary electrophoresis (SDS‐CE) separations. Kafirins from 15 genotypes were extracted with this procedure and analyzed by FZCE and SDS‐CE. Resolution of the kafirins by FZCE was much higher than the SDS‐CE, demonstrating that the kafirin proteins possessed a high level of charge density variability within a relatively small molecular size distribution. Two distinct groups of α‐kafirins could be seen in the FZCE electropherograms.     

Gene families encoding isoforms of two major sesame seed storage proteins, 11S globulin and 2S albumin

Sesame (Sesamum indicum L.) seed has been recognized as a nutritional protein source owing to its richness in methionine. Storage proteins have been implicated in allergenic responses to sesame consumption. Two abundant storage proteins, 11S globulin and 2S albumin, constitute 60-70 and 15-25% of total sesame proteins, respectively. Two gene families separately encoding four 11S globulin and three 2S albumin isoforms were identified in a database search of 3328 expressed sequence tag (EST) sequences from maturing sesame seeds. Full-length cDNA sequences derived from these two gene families were completed by PCR using a maturing sesame cDNA library as the template. The amino acid compositions of these deduced storage proteins revealed that the richness in methionine is attributed mainly to two 2S albumin isoforms and partly to one 11S globulin isoform. The presence of four 11S globulin and three 2S albumin isoforms resolved in SDS-PAGE was confirmed by MALDI-MS analyses. The abundance of these isoforms was in accord with the occurrence frequency of their EST sequences in the database. A comprehensive understanding of these storage proteins at the molecular level may also facilitate the identification of allergens in crude sesame products that have caused severe allergic reactions increasingly reported in the past decade.     

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.     

Protein Quality and Physical Characteristics of Kisra (Fermented Sorghum Pancake‐like Flatbread) Made from Tannin and Non‐Tannin Sorghum Cultivars

Kisra is a naturally lactic acid bacteria‐ and yeast‐fermented sorghum thin pancake‐like flatbread produced in Sudan. Kisra has considerable potential as the basis for development of a gluten‐free sandwich wrap. To help direct cultivar selection for commercial production of these products, two white, tan plant non‐tannin Type I, one white Type II tannin, and one red Type III tannin sorghum cultivars were evaluated with respect to kisra protein quality and physical characteristics. Kisra from the non‐tannin sorghums were flexible and had an open‐textured structure with many regular gas cells, whereas those from the tannin sorghums were more brittle, denser in structure, and contained far fewer and smaller gas cells. Kisra from the tannin sorghums had the lowest reactive lysine content, in vitro protein digestibility, and Protein Digestibility Corrected Amino Score (PDCAAS), with values being lowest for the Type III sorghum. PDCAAS of kisra from the Type III sorghum was only 0.12, less than half of that from the Type I sorghums. As the tannins in tannin sorghums adversely affect kisra protein quality and physical characteristics, white tan plant, non‐tannin sorghum cultivars are most suitable for kisra production and for development of wrap‐type sorghum‐based baked goods.     

Turbidity Assay for Rapid and Efficient Identification of High Protein Digestibility Sorghum Lines

Recently, our laboratory reported a protein digestibility assay based on SDS‐PAGE that distinguishes mutant high protein digestibility from wild‐type sorghum lines. Using that assay, high protein digestibility sorghum lines were identified both qualitatively (visual observation) and quantitatively by measuring the SDS‐PAGE band intensity of the undigested α‐kafirin protein. Here, we report on a new turbidity assay that can be used for an even quicker quantitation of the undigested proteins with much higher throughput for screening purposes. Proteins remaining after 1 hr of pepsin digestion were extracted with a buffer of SDS, 2‐mercaptoethanol, and borate and an aliquot of the extract was precipitated using 72% trichloroacetic acid (TCA). Absorbance of the resulting turbid solution was then read at 562 nm. Lower readings corresponded to more digestible lines. The turbidity of the suspensions developed quickly and reached a plateau at ≈5 min for high protein digestibility lines and 10 min for wild‐type lines. The turbid solutions remained stable for at least 1 hr. Two distinct groups, wild‐type and high protein digestibility sorghum lines, were obtained when the assay was compared with a standard pepsin digestibility procedure and to our recently developed SDS‐PAGE assay. A comparison with the bicinchoninic acid (BCA) assay of protein quantitation indicated that the turbidity assay is more efficient in differentiating between wild‐type and high protein digestibility sorghum lines. We have further refined the turbidity assay for microtiter plate analysis making it possible for a single operator to analyze ≈200 sorghum lines per day, compared to 60 lines when using the SDS‐PAGE assay.     

Physical,Mechanical, and Barrier Properties of Kafirin Films from Red and White Sorghum Milling Fractions

Fractions from the sorghum dry milling industry, including bran, are a potential source of kafirin. Free‐standing plasticized cast films were prepared from defatted kafirin preparations from red and white sorghum flour and bran fractions, and from commercial zein. All the kafirin preparations were able to form films. However, there were differences in film thickness, clarity, flexibility, surface texture, odor, and color between the different kafirin films. Bran kafirin films were highly colored, less flexible with a less smooth surface texture compared with films from flour, probably due to higher levels of contaminants in the bran kafirins. The strong color of the bran films could limit their use in certain coating applications. The kafirin films had much higher tensile strength and lower extensibility than zein film, probably because of the presence of β‐ and γ‐kafirins in the kafirin, giving high levels of disulfide cross‐linking in the kafirin films. The kafirin films had poorer water barrier properties than zein film, possibly due to greater thickness or to poorer flexibility, which may have caused microcracks.     

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.     

Nitrogen lowers the sulfur amino acid content of soybean (Glycine max [L.] Merr.) by regulating the accumulation of Bowman-Birk protease inhibitor

Soybeans in general contain 35-40% protein. Efforts are underway to increase further this protein content, thus enhancing their nutritive value. Even though higher protein is a desirable characteristic, whether such an increase will be accompanied by enhanced protein quality is not known. Soybean protein quality could be significantly improved by increasing the concentration of the sulfur-containing amino acids, cysteine and methionine. To ascertain if a correlation existed between protein quantity and quality, a comparison of the amino acids of soybeans differing in protein content was made. Soybeans with higher protein content had a significantly lower percentage of sulfur amino acids, while those with lower protein exhibited a higher content of cysteine and methionine. Nitrogen application elevated the protein content but lowered that of the sulfur amino acids. Transmission electron microscopy examination of thin sections of low protein soybean seeds revealed several protein storage vacuoles that were partially filled with storage proteins. Fluorescence two-dimensional difference gel electrophoresis of soybean seed proteins revealed that nitrogen application favored the accumulation of the beta-subunit of beta-conglycinin while decreasing the accumulation of Bowman-Birk protease inhibitor (BBI), a protein rich in cysteine. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 60% 2-propanol-extracted proteins showed a drastic reduction in the accumulation of BBI with increasing protein content. Northern blot analysis indicated that nitrogen had a negative influence on the expression of the BBI gene. Our results indicate that the negative correlation between total protein and sulfur amino acid content is mostly mediated by the differential accumulation of BBI.     

Genetic Analysis of Kafirins and Their Phenotypic Correlations with Feed Quality Traits,In Vitro Digestibility,and Seed Weight in Grain Sorghum

Twenty‐three entries of grain sorghum (Sorghum bicolor (L.) Moench), including eight inbred lines (five males and three females) and 15 hybrids, were evaluated to determine the proportion of γ, αII, and β‐αI‐kafirins and their association with contents of crude protein, fat, and starch; protein digestibility; in vitro dry matter disappearance; and seed weight. The male lines included three normal‐seeded lines (TX2737, TX435, and P954063) and two large‐seeded lines (Eastin1 and PL‐1). Female lines consisted of three common U.S. seed parent lines (Wheatland, Redlan, and SA3042). The lines and their hybrids were grown under dryland conditions at two locations in Kansas using a randomized complete block design. The effects of genotype, location, and males were significant for all kafirins. Wide variations in composition and general combining ability (GCA) for kafirin content were noted among parent lines and hybrids, with TX2737, Eastin1, and PL1 having the largest GCA values for γ (1.37), αII (1.99), and β‐αI (2.57), respectively. Correlations among kafirins ranged from ‐0.89 to 0, whereas those of kafirins with feed quality traits, digestibility, and seed weight ranged from ‐0.45 to 0.48.     

Effects of water‐soluble constituents of plant residues on water uptake by seeds

Abstract

There has been strong support for the hypothesis that the adverse effects of plant residues on crop yields are due to phytotoxic compounds derived from these residues. This hypothesis is based largely on studies showing that, when compared with distilled water, aqueous extracts of plant residues have an adverse effect on seed germination and seedling growth. Because seed germination and seedling growth are reduced by a delay in germination resulting from slow uptake of water by seeds, we studied the possibility that the adverse effects of aqueous extracts of plant residues on seed germination and seedling growth might be at least partly due to water uptake by seeds being retarded by water‐soluble constituents of these residues. To test this possibility, we compared the rates of water uptake and germination of seeds of corn (Zea mays L.), soybean [Glycine max. (L.) Merrill], and wheat (Triticum aestivum L.) when these seeds were treated with distilled water and with aqueous extracts of corn, sorghum [Sorghum bicolor (L.) Moench], and wheat residues. We found that the rates of water uptake and germination of seeds treated with aqueous extracts of plant residues were appreciably slower than the corresponding rates for seeds treated with distilled water. This may be due to the water potentials of these extracts (ca. ‐50 kPa) because when seeds of corn, sorghum, and wheat were treated with a solution of polyethylene glycol 8000 having a water potential similar to that of the extracts of plant residues tested, the rates of water uptake and germination were also slower than the corresponding rates for seeds treated with distilled water. These observations suggest that the adverse effects of aqueous extracts of plant residues on seed germination and seedling growth when compared with distilled water may be partly due to constituents of these extracts inducing water potential effects that reduce water uptake by germinating seeds.     

Evaluation of Novel Precast SDS‐PAGE Gels for Separation of Sorghum Proteins

Precast polyacrylamide gels using novel buffer chemistry for enhanced resolution and shelf life stability of the gels were evaluated for separating sorghum proteins. Two gels with different acrylamide concentrations, 12 and 4–12%, were tested with two different buffer systems. Gels were evaluated for separation resolution, as well as protein solubility problems such as streaking. High‐resolution separations were obtained for all the major classes of kernel proteins using these gels. Run times were typically 45–60 min, producing relatively rapid separations. Resolution was significantly affected by the buffer system used. The use of precast gradient gels eliminates the need for casting gradient gels for routine analysis of sorghum proteins and avoids handling the toxic acrylamide monomer. This system will be useful for routine separations of sorghum proteins as well as for research programs using SDS‐PAGE to screen sorghum lines for digestibility or for other protein‐related quality factors.     

Transgenic sorghum with altered kafirin synthesis: kafirin solubility, polymerization, and protein digestion

Transgenic sorghum (TG) lines with altered kafirin synthesis, particularly suppression of γ-kafirin synthesis, and improved protein quality have been developed. The proportion of kafirin extracted with 60% tert-butyl alcohol alone was greatly increased in the TG lines. However, the total amount of kafirin remained unchanged. Further, in the TG lines, the kafirin was much less polymerized by disulfide bonding. There was also evidence of compensatory synthesis of other kafirin proteins. Cooked protein digestibility was increased in the TG form, even after removal of interfering starch. The TG protein bodies were intermediate in appearance between the normal type and the invaginated high digestibility mutants. Hence, the increased protein digestibility of these TG lines is probably related to their lower levels of disulfide-bonded kafirin polymerization, allowing better access of proteases. This work appears to confirm that disulfide bond formation in kafirin is responsible for the reduced protein digestibility of cooked sorghum.     

Development of New Method for Extraction of α‐Zein from Corn Gluten Meal Using Different Solvents

A modified procedure for the extraction of α‐zein from corn gluten meal was developed and compared against a commercial extraction method. The modification involved raising the concentration of alcohol in solvent and removing the precipitate by centrifugation. Five organic solvent mixtures were compared using the modified extraction procedure developed along with the reductant sodium bisulfite and NaOH. The modified procedure precipitated most of the non‐α‐zein protein solids by increasing the concentration of alcohol. The supernatant had α‐zein‐rich fraction, resulting in higher yield of α‐zein than the commercial method when cold precipitated. The commercial extraction procedure had a zein yield of 23% and protein purity of 28% using 88% 2‐propanol solvent. The three best solvents, 70% 2‐propanol, 55% 2‐propanol, and 70% ethanol, yielded ≈35% of zein at protein purity of 44% using the modified extraction procedure. Zeins extracted using the novel method were lighter in color than the commercial method. Densitometry scans of SDS‐PAGE of α‐zein‐rich solids showed relatively large quantities of α‐zein with apparent molecular weights of 19,000 and 22,000 Da. The α‐zein‐rich solids also had small amounts of δ‐zein (10,000 Da) because it shares similar solubility properties to α‐zein. A solvent mixture with 70% 2‐propanol, 22.5% glycerol, and 7.5% water extracted significantly less zein (≈33%) compared to all other solvents and had α‐zein bands that differed in appearance and contained little to no δ‐zein.     

Natural Variation and Genome‐Wide Association Study of Antioxidants in a Diverse Sorghum Collection

Consumption of polyphenol‐rich food is associated with decreased risk of oxidative stress‐related chronic diseases. Sorghum, a major food and feed cereal crop, has many polyphenol‐containing accessions with high antioxidant activity in the grain. However, many of these polyphenol‐containing accessions are not high‐yielding or food‐grade varieties. The natural variation in sorghum grain polyphenols can be used to develop high‐yielding, health‐promoting specialty food types through marker‐assisted breeding. To identify new antioxidant‐rich germplasm, we quantified antioxidant activity, total polyphenols, and condensed tannins in whole kernel flour in 266 accessions from the genetically and phenotypically diverse Sorghum Association Panel. Antioxidant activity, total polyphenols, and condensed tannins were in the ranges of 9.6–325.1 μmol of trolox equivalents (TE)/g, 0.8–18.8 g of gallic acid equivalents/kg, and 0–65.5 g of catechin equivalents/kg, respectively. Twenty‐three accessions were rich sources of antioxidant activity, with PI534144 (SC84; 325.1 μmol of TE/g) and PI534117 (SC991; 237.0 μmol of TE/g) possessing the highest values. To identify quantitative trait loci associated with sorghum grain antioxidant traits, we conducted a genome‐wide association study with 404,628 single nucleotide polymorphism markers. Many significant associations were identified, including two homologs of Arabidopsis (Arabidopsis thaliana) transparent testa (TT ) genes TT10 and TT4. This study provides information that can help breeders incorporate health‐promoting traits into elite breeding lines.