Gluten protein composition and aggregation properties as predictors for bread volume of common wheat, spelt, durum wheat, emmer and einkorn
Introduction
In the 21st century, the production area of the “ancient” (hulled) wheat species einkorn (Triticum monococcum L., diploid), emmer (T. dicoccum L., tetraploid) and spelt (T. spelta L., hexaploid) is negligibly small compared to that of the “modern” (naked) wheat species common wheat (T. aestivum L., hexaploid) and durum wheat (T. durum L., tetraploid). Especially in the last decades ancient wheat species were replaced by modern wheat species due to higher grain yields (spelt 37%, emmer 55% and einkorn 62% lower yield compared to common wheat). Ancient species are hulled wheats with a though glume, which has to be separated from the grain in the mill (Longin et al., 2015). Because some consumers associate the consumption of ancient wheats with health benefits, ancient wheat species have been attracting attention in the last 20 years and special products such as bread, pasta and beer have been developed (Longin et al., 2015). Studies on the contents of bioactive components (e.g. dietary fiber components, phenolic acids, folates) in ancient and modern wheats revealed only small differences between modern and ancient wheat species. For example, even though emmer and einkorn contained more of the carotenoid lutein than common wheat, durum wheat had comparable contents of lutein due to its yellow color (Shewry and Hey, 2015). More studies on a wider range of genotypes of ancient and modern wheats grown under standardized conditions are currently needed to assess possible health benefits (Shewry, 2018). Further advantages of ancient wheats include their disease tolerance, adaptation to different climatic conditions, low requirement of fertilizers and potential to increase biodiversity (Longin et al., 2015). In addition, tetraploid and diploid wheat species may contain lower amounts of immunoreactive proteins and peptides compared to hexaploid species. For example, the celiac disease-active 33-mer peptide was not detected in emmer, durum wheat and einkorn samples due to absence of the D-genome, but spelt and common wheat had comparable contents of the 33-mer (Schalk et al., 2017).
The baking quality of wheat flours is mostly determined by gluten quality and quantity. Gluten proteins are storage proteins and divided into gliadins (GLIA) soluble in aqueous alcohol and glutenins (GLUT) soluble in aqueous alcohol only after reduction of disulfide bonds. Contents and composition of GLIA (ω5-, ω1,2-, α- and γ-GLIA) and GLUT (ωb-gliadins, high- (HMW-GS) and low-molecular-weight glutenin subunits (LMW-GS)) are typically analyzed by modified Osborne fractionation followed by reversed-phase high-performance liquid chromatography (RP-HPLC) (Wieser et al., 1998). One glutenin subfraction with particular importance for baking quality is the polymeric glutenin macropolymer (GMP) that is insoluble in aqueous sodium dodecyl sulfate (SDS) solution. GMP is one of the largest protein-based biopolymers (Don et al., 2003; Weegels et al., 1996) and can be quantitated by gel-permeation (GP-)HPLC (Cinco-Moroyoqui and MacRitchie, 2008). There are only few studies on the content and composition of gluten proteins of the different wheat species. White flours of common wheat had GLIA/GLUT ratios between 1.7–3.1 (Wieser and Kieffer, 2001) and 1.4–2.1 (Thanhaeuser et al., 2014), whereas those of spelt (2.2–9.0) (Koenig et al., 2015), durum wheat (3.1–5.0) (Wieser, 2000; Wieser et al., 2003), emmer (3.5–7.6) (Wieser and Koehler, 2009) and einkorn (4.0–14.0) (Wieser et al., 2009) were higher compared to common wheat. The contents of GLIA (r = 0.80), GLUT (r = 0.76) and GMP (r = 0.80) were suitable to predict the bread volume of common wheat, because of higher correlation coefficients compared to the prediction using crude protein contents (r = 0.71) (Thanhaeuser et al., 2014). The GLUT content and especially that of HMW-GS was correlated to the bread volume (Wieser et al., 2009). Regarding GMP, common wheats contained 8–18 mg GMP/g of flour, but there are no studies available for spelt, durum wheat, emmer and einkorn.
Among cereals common wheat is most suitable for bread making because the flour forms a viscoelastic dough when it is mixed with water. In comparison to common wheat the flours of ancient wheat species yield softer doughs with low elasticity and high extensibility because of the poor gluten quality (Sobczyk et al., 2017; Longin et al., 2015; Wieser et al., 2009). The baking quality is usually determined by baking tests which are very time-consuming and labor-intensive. Instead, so-called quality parameters such as crude protein (CP) content (ICC method 167), wet gluten content (ICC method 137/1), micro-scale extension tests (Wieser and Kieffer, 2001) or the Zeleny sedimentation test (ICC method 116/1) are used to predict the baking quality of wheat flours. Another fast and easy method to determine quality-related parameters of wheat flour is the GlutoPeak test (GPT) that registers gluten aggregation properties (torque) during high-speed mixing for a short time (6 min). Parameters such as maximum torque (MT), peak maximum time (PMT) and aggregation time (AGT) are calculated from the respective curve. These parameters were used to predict the gluten and baking quality in white flours of common wheat (Bouachra et al., 2017; Huen et al., 2017; Marti et al., 2015a, 2015b) and durum wheat (Marti et al., 2014). However, no GPT data are available for spelt, emmer and einkorn so far.
Although gluten content and composition of a variety of common wheat, spelt and einkorn cultivars were already characterized, these studies are difficult to compare, because the samples were cultivated in different areas and harvest years, fertilized differently and the grains were milled to white or wholemeal flours. Therefore, the aim of this study was to establish suitable quality parameters to predict the baking quality of wholemeal flours of common wheat, spelt, durum wheat, emmer and einkorn. For this purpose, the results for gluten content and composition and gluten aggregation properties were correlated with the results of baking experiments using eight cultivars of each wheat species grown at the same location (Seligenstadt, Germany) and in the same year (2013).
Section snippets
Chemicals
All chemicals were of analytical or higher grade and purchased from VWR Merck (Darmstadt, Germany), Serva (Heidelberg, Germany), LECO (Kirchheim, Germany) or Sigma-Aldrich (Steinheim, Germany). Water was deionized by a water purification system Arium 611VF (Sartorius, Goettingen, Germany).
Wheat samples
Eight cultivars per wheat species of common wheat, spelt, durum wheat, emmer and einkorn were cultivated by the State Plant Breeding Institute (SPBI), University of Hohenheim (Stuttgart, Germany) at
Results
All determinations were performed in triplicate per sample (eight cultivars of five wheat species, n = 40). The mean values for every cultivar [mg/g flour] are displayed in Supplementary Tables 2, 3 and 4 and the distribution of Osborne and SDSS/GMP fractions in Supplementary Figs. 1 and 2. Further, the mean coefficient of variation (CV) of each analysis for each group of wheat species is shown. Table 1 shows a summary of Supplementary Tables 2, 3 and 4 with the mean value and the range (the
Discussion
Protein content and composition, gluten aggregation properties as well as dough and bread properties were determined in a unique set of eight cultivars each of common wheat, spelt, durum wheat, emmer and einkorn. Environmental factors typically account for up to two-thirds of the variation in wheat protein content and composition with the remaining one-third being controlled by genetic factors with additional interaction between the two factors (Shewry, 2009). Grown under standardized
Conclusion
Five wheat species (eight cultivars each) grown under the same geographic and climatic conditions (year and area) were analyzed and characterized with various analytical and rheological techniques. Because of the standardized growing conditions (except for species-specific fertilization), significant differences in protein content and composition were due to species and cultivar. These differences significantly affected the baking quality (bread volume, dough stability). Different protein
Notes
The authors declare no competing financial interest.
Acknowledgement
This research project was supported by the German Ministry of Economics and Energy (via AiF) and the FEI (Forschungskreis der Ernährungsindustrie e.V., Bonn). Project AiF 18355 N. The authors thank Alexandra Axthelm, Anneliese Köhler, Stefanie Hackl and Katharina Schiesser for excellent technical assistance. Special thanks go to Friedrich Longin from the SPBI for cultivating and providing the wheat samples used in this study.
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