The Two Faces of Wheat

Wheat-based foods have been staple foods since about 10,000 years and constitute a major source of energy, dietary fiber, and micronutrients for the world population. The role of wheat in our diet, however, has recently been scrutinized by pseudoscientific books and media reports promoting the overall impression that wheat consumption makes people sick, stupid, fat, and addicted. Consequently, numerous consumers in Western countries have started to question their dietary habits related to wheat consumption and voluntarily decided to adopt a wheat-free diet without a medical diagnosis of any wheat-related disorder (WRD), such as celiac disease, wheat allergy, or non-celiac gluten sensitivity. The aim of this review is to achieve an objective judgment of the positive aspects of wheat consumption as well as adverse effects for individuals suffering from WRDs. The first part presents wheat constituents and their positive nutritional value, in particular, the consumption of products from whole-grain flours. The second part is focused on WRDs that affect predisposed individuals and can be treated with a gluten-free or -reduced diet. Based on all available scientific knowledge, wheat consumption is safe and healthy for the vast majority of people. There is no scientific evidence to support that the general population would benefit from a wheat-free diet

Chemistry of wheat gluten proteins: Quantitative composition

Background and Objectives Wheat is essential to secure nutrition for the world\u27s population. Its unique processing properties are largely determined by gluten protein content and composition. Findings Gluten proteins are subdivided into gluten protein types, α-, γ-, ω1,2-, and ω5-gliadins and high-molecular-weight glutenin subunits and low-molecular-weight glutenin subunits. The overall content and relative proportions of these types vary considerably depending on different genetic and environmental factors and mutual interactions. Conclusion This review summarizes the latest developments related to the chemistry of gluten and how species and variety, as well as soil type, weather conditions, atmospheric CO2 concentration, diseases, and fertilization with nitrogen, sulfur, phosphorus, potassium, and other minerals affect wheat gluten protein composition. Significance and Novelty Significant progress has been made to study the effect of different factors on gluten composition. However, comparisons between studies are almost impossible, because of the huge variability in experimental setups, environmental conditions and varieties studied. This calls for a need to develop common guidelines on how to set up experiments, on which parameters to investigate and on which procedure to use to improve comparability and reproducibility of the results

Chemistry of wheat gluten proteins: Qualitative composition

Background and Objectives Wheat gluten proteins make up one of the most complex protein aggregates in nature. Their qualitative and quantitative composition is determined by genetic and environmental factors as well as technological processes. Findings Gluten proteins comprise ω5-, ω1,2-, α-, and γ-gliadins as well as high-molecular-weight glutenin subunits (HMW-GS) and low-molecular-weight (LMW) GS. About 50% of gluten proteins are monomeric gliadins with MWs from 28,000 to 55,000, while about 15% are present as disulfide-linked oligomeric proteins with MWs between 70,000 and 700,000, called HMW-gliadins. The remaining 35% are disulfide-linked polymeric glutenins with MWs from 700,000 to more than 10 million. Intrachain disulfide bonds, present in all types except ω-gliadins, stabilize the three-dimensional structure, while interchain disulfide bonds, mainly linking HMW-GS and LMW-GS, generate oligomers and polymers. Conclusions In this review, we provide an updated and detailed insight into the chemistry of wheat gluten proteins with a focus on the qualitative composition. Significance and Novelty An enhanced understanding of gluten protein structure and how it is affected will be essential to select and breed more resilient wheat varieties with favorable processing properties to help ensure nutrition and food security worldwide

Characterization and relative quantitation of wheat, rye, and barley gluten protein types by liquid chromatography–tandem mass spectrometry

The consumption of wheat, rye, and barley may cause adverse reactions to wheat such as celiac disease, non-celiac gluten/wheat sensitivity, or wheat allergy. The storage proteins (gluten) are known as major triggers, but also other functional protein groups such as α-amylase/trypsin-inhibitors or enzymes are possibly harmful for people suffering of adverse reactions to wheat. Gluten is widely used as a collective term for the complex protein mixture of wheat, rye or barley and can be subdivided into the following gluten protein types (GPTs): α-gliadins, γ-gliadins, ω5-gliadins, ω1,2-gliadins, high- and low-molecular-weight glutenin subunits of wheat, ω-secalins, high-molecular-weight secalins, γ-75k-secalins and γ-40k-secalins of rye, and C-hordeins, γ-hordeins, B-hordeins, and D-hordeins of barley. GPTs isolated from the flours are useful as reference materials for clinical studies, diagnostics or in food analyses and to elucidate disease mechanisms. A combined strategy of protein separation according to solubility followed by preparative reversed-phase high-performance liquid chromatography was employed to purify the GPTs according to hydrophobicity. Due to the heterogeneity of gluten proteins and their partly polymeric nature, it is a challenge to obtain highly purified GPTs with only one protein group. Therefore, it is essential to characterize and identify the proteins and their proportions in each GPT. In this study, the complexity of gluten from wheat, rye, and barley was demonstrated by identification of the individual proteins employing an undirected proteomics strategy involving liquid chromatography–tandem mass spectrometry of tryptic and chymotryptic hydrolysates of the GPTs. Different protein groups were obtained and the relative composition of the GPTs was revealed. Multiple reaction monitoring liquid chromatography–tandem mass spectrometry was used for the relative quantitation of the most abundant gluten proteins. These analyses also allowed the identification of known wheat allergens and celiac disease-active peptides. Combined with functional assays, these findings may shed light on the mechanisms of gluten/wheat-related disorders and may be useful to characterize reference materials for analytical or diagnostic assays more precisely

Comparative Study on Gluten Protein Composition of Ancient (Einkorn, Emmer and Spelt) and Modern Wheat Species (Durum and Common Wheat)

The spectrophotometric Bradford assay was adapted for the analysis of gluten protein contents (gliadins and glutenins) of spelt, durum wheat, emmer and einkorn. The assay was applied to a set of 300 samples, including 15 cultivars each of common wheat, spelt, durum wheat, emmer and einkorn cultivated at four locations in Germany in the same year. The total protein content was equally influenced by location and wheat species, however, gliadin, glutenin and gluten contents were influenced more strongly by wheat species than location. Einkorn, emmer and spelt had higher protein and gluten contents than common wheat at all four locations. However, common wheat had higher glutenin contents than einkorn, emmer and spelt resulting in increasing ratios of gliadins to glutenins from common wheat (< 3.8) to spelt, emmer and einkorn (up to 12.1). With the knowledge that glutenin contents are suitable predictors for high baking volume, cultivars of einkorn, emmer and spelt with good predicted baking performance were identified. Finally, spelt, emmer and einkorn were found to have a higher nitrogen partial factor productivity than common and durum wheat making them promising crops for a more sustainable agriculture

Fingerprinting of wheat protein profiles for improved distinction between wheat cultivars and species

Background and objectives Wheat protein composition is commonly characterized by reversed‐phase (RP)‐HPLC‐UV after extraction of albumins/globulins, gliadins (ω5‐, ω1,2‐, α‐, and γ‐gliadins), and glutenins (high‐ and low‐molecular‐weight glutenin subunits). However, this traditional classification does not consider the individual distribution of peaks, resulting in loss of information on protein fingerprints. We developed a new approach to peak integration and evaluated its suitability to differentiate between wheat cultivars and species. Findings Integration events were performed every 20 s, and the relative proportions of the peaks were calculated. We compared the traditional and new integration methods on two sample sets, the first comprising 60 common wheat cultivars from 1891 to 2010 and the second comprising 40 common wheat, spelt, durum wheat, emmer, and einkorn cultivars. The new integration method performed better in differentiating old and modern common wheat cultivars and was also applicable to different wheat species. Conclusions Unique cultivars were identified that stood out because of their protein composition. Four samples warrant further research to identify the specific proteins that are responsible for the differences. Significance and novelty The new integration allowed us to map the cultivar‐ and species‐specific fingerprints, identify cultivars with exceptional protein composition, and group similar cultivars

No correlation between amylase/trypsin-inhibitor content and amylase inhibitory activity in hexaploid and tetraploid wheat species

Wheat amylase/trypsin-inhibitors (ATI) are known triggers for wheat-related disorders. The aims of our study were to determine (1) the inhibitory activity against different α-amylases, (2) the content of albumins and globulins (ALGL) and total ATI and (3) to correlate these parameters in wholegrain flour of hexaploid, tetraploid and diploid wheat species. The amount of ATI within the ALGL fraction varied from 0.8% in einkorn to 20% in spelt. ATI contents measured with reversed-phase high-performance liquid chromatography (RP-HPLC) revealed similar contents (1.2–4.2 mg/g) compared to the results determined by LC-MS/MS (0.2–5.2 mg/g) for all wheat species except einkorn. No correlation was found between ALGL content and inhibitory activity. In general, hexaploid cultivars of spelt and common wheat had the highest inhibitory activities, showing values between 897 and 3564 AIU/g against human salivary α-amylase. Tetraploid wheat species durum and emmer had lower activities (170–1461 AIU/g), although a few emmer cultivars showed similar activities at one location. In einkorn, no inhibitory activity was found. No correlation was observed between the ATI content and the inhibitory activity against the used α-amylases, highlighting that it is very important to look at the parameters separately

Determination of soluble wheat protein fractions using the Bradford assay

Background and objectives Determination of different grain protein fractions in wheat cultivars is an important task in analyzing bread baking quality. In many laboratories, the Bradford assay is used to determine protein concentrations in solutions. In any protein assay (including Bradford), the ideal protein to use as a standard is the purified protein being assayed. In the absence of such an absolute reference, protein another protein must be selected as a relative standard such as bovine serum albumin (BSA) which is widely used. The aim of this work was to find conversion factors for BSA to determine correct albumin–globulin, gliadin, and glutenin concentrations, because these purified wheat grain protein fractions are mostly not available to be used for calibration purposes. Findings In case of BSA calibration, gluten concentration was underestimated (50%–54%) compared to calibration with the respective purified wheat proteins (65%–70%) in extracts of wheat grain samples. This result is explained with the different amino acid composition of BSA and wheat protein fractions leading to a more intense signal with BSA in the Bradford assay. Calibration of the Bradford assay using BSA as well as purified wheat protein fractions allowed to calculate the conversion factors of 2.11 for BSA/albumin–globulin, 4.25 for BSA/gliadin, and 3.42 for BSA/glutenin. Application of these conversion factors proved to accurately adjust protein concentrations of wheat fractions originating from ten cultivars, determined with BSA calibration of the Bradford assay. Conclusions BSA calibration of the Bradford assay in combination with the conversion factors can be used to determine protein concentration of wheat grain fractions. Significance and novelty Findings of this study make a contribution toward the correction of a common method, to provide a basis for better comparability of research results from different laboratories

Development of a barley reference material for gluten analysis

Celiac disease (CD) can be triggered in susceptible individuals by the consumption of gluten, a complex storage protein mixture present in wheat, rye and barley. There is no specific reference material (RM) available for barley and this leads to inaccurate quantitation of barley gluten in supposedly gluten-free foods. Therefore, the aim was to select representative barley cultivars to establish a new barley RM. The relative protein composition of the 35 barley cultivars averaged 25% albumins and globulins, 11% d-hordeins, 19% C-hordeins, and 45% B/γ-hordeins. The mean gluten and protein content was 7.2 g/100 g and 11.2 g/100 g, respectively. The prolamin/glutelin ratio (1:1) commonly used in ELISAs to calculate the gluten content was found to be inappropriate for barley (1.6 ± 0.6). Eight cultivars suitable as potential RMs were selected to ensure a typical barley protein composition and improve food safety for CD patients

Thermally induced gluten modification observed with rheology and spectroscopies

The protein vital gluten is mainly used for food while interest for non-food applications, like biodegradable materials, increases. In general, the structure and functionality of proteins is highly dependent on thermal treatments during production or modification. This study presents conformational changes and corresponding rheological effects of vital wheat gluten depending on temperature. Dry samples analyzed by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and thermalgravimetric analysis coupled with mass spectrometry (TGA-MS) show surface compositions and conformational changes from 25 to 250 °C. Above 170 °C, XPS reveals a decreased N content at the surface while FTIR band characteristics for β-sheets prove structural changes. At 250 °C, protein denaturation accompanied by a significant mass loss due to dehydration and decarbonylation reactions is observed. Oscillatory measurements of optimally hydrated vital gluten describing network properties of the material show two structural changes along a temperature ramp from 25 to 90 °C: at 56–64 °C, the temperature necessary to trigger structural changes increases with the ratio of gliadin to total protein mass, determined by reversed-phase high performance liquid chromatography (RP-HPLC). At a temperature of 79–81 °C, complete protein denaturation occurs. FTIR confirms the denaturation process by showing band shifts with both temperature steps