Intermediate wheatgrass (Thinopyrum intermedium) proteins and their network formation : a breadmaking perspective

Summary

The development of a viscoelastic protein network in wheat (Triticum aestivum) dough is the reason behind the unmatched suitability of wheat flour to be used in baked goods, especially leavened products like bread. This viscoelastic network is assumed to be unique for Triticum species, i.e., proteins from other crops exhibit different behavior and therefore their dough systems do not have comparable properties. Despite the importance of wheat for human nutrition, the cultivation of an annual grain like wheat can be demanding on the environment. Thus, efforts are ongoing to develop perennial grains for a more environmentally friendly food production system. However, how easily such perennial grains can be used for food production depends on their functional properties. The perennial grain intermediate wheatgrass (Thinopyrum intermedium, IWG) has been suggested as a novel crop, but its food properties have scarcely been studied. Little is therefore known about what contributes to its functional properties and how these may be changed. The premise of this thesis was that, for the use in leavened baked goods, particularly bread, it would be beneficial if IWG exhibited network-forming ability. This thesis investigated whether IWG proteins can form a viscoelastic protein network, and how this may relate to dough and bread properties. IWG was compared to other cereal grains (rye(Secale cereale) and ancient wheat species) to evaluate the protein composition and properties. IWG contained more protein and less starch than einkorn (Triticum monococcum), emmer (Triticum turgidum ssp. dicoccum), and wheat. Moreover, proteins with a similar molecular weight to high-molecular-weight glutenin subunits in wheat were present in IWG. However, the ratio between the high and low molecular weight glutenin subunit-like proteins in IWG was lower than the ratio found for wheat and emmer. The proportion of unextractable polymeric proteins of total polymeric protein (%UPP) in dough was similar between IWG and wheat. Although IWG formed a weak dough, some rheological properties were similar to those of both wheat and emmer. Moreover, hearth loaves could be produced with 100% IWG, and their specific volume was similar to emmer and form ratio (height/width) similar to wheat. The similarities of IWG dough to emmer and wheat extended to certain responses to the addition of sodium chloride (NaCl) and ascorbic acid (AA), whereas rye dough properties were not affected by them. IWG dough was strengthened by the addition of AA, similar to emmer and wheat. However, whereas NaCl also strengthened emmer and wheat dough, IWG dough did not respond similarly. Evaluation of sulfhydryl groups and glutathione content in dough also revealed different responses to NaCl addition between IWG and wheat. The change in %UPP in IWG dough upon AA and NaCl addition followed a similar pattern as for wheat, but with smaller changes. Taken together, the results suggest that it is possible to modify the viscoelastic network of IWG by AA and NaCl, although the latter may affect IWG proteins differently than gluten proteins. A viscoelastic gluten-like material with high protein content was isolated from IWG dough, however, the maximum resistance to extension was lower than for wheat gluten. A similar pattern of monomeric and polymeric proteins in IWG and wheat was observed for proteins extracted from dough. Additionally, the IWG and wheat proteins had relatively similar amino acid compositions. The microstructure of proteins in dough showed that IWG proteins formed a highly homogeneous honeycomb-like network, previously described for weakened types of wheat dough. Thus, the microstructure of the protein network differed between IWG and wheat, with the latter having a strong gluten network. Findings from all three papers support that IWG proteins can form a viscoelastic protein network resulting in dough with gas-holding properties. This protein network in IWG was weaker than that of wheat, likely due to its more homogeneous microstructure. However, IWG dough still exhibited rheological properties similar to those of wheat and emmer dough, suggesting some similarity in viscoelastic properties. Furthermore, breadmaking with 100% IWG confirmed that IWG dough was able to retain its shape during proofing and baking, likely due to the viscoelastic nature of its protein network. This study has increased the understanding of IWG proteins and may facilitate the use of IWG as a food ingredient in the future.