Protein Quality in Wheat: Changes in Protein Polymer Composition during Grain Development and Dough Processing

Kuktaite, R. 2004. Protein Quality in Wheat: Changes in Protein Polymer Composition During Grain Development and Dough Processing. Doctoral dissertation. ISSN 1401-6249, ISBN 91-576-6778-0 The overall objective of the present studies was to increase the understanding about protein quality in wheat, i.e. to make it possible to improve the production of wheat with desired quality for different end-uses. Studies of changes in protein polymer composition during grain development and dough processing were used as one track to investigate the protein quality in the wheat material. Accumulation and build-up of the proteins in developing wheat grain was found to be a predetermined process and independent of variation in temperature and nitrogen regimes. For determination of amount and size distribution of polymeric proteins, the length of maturation time together with availability of nutrients during the latter half of the grain development are of highest importance. By the ultracentrifugation of dough specific low molecular weight (LMW) glutenins were found only in the gluten phase, indicating the importance of these proteins for breadmaking. Also, specific water soluble proteins were found in the gluten phase. The genetically and environmentally determined amount and size distribution of polymeric proteins form the basis for the build-up (cross-linking) of protein polymers during dough formation. The omega-gliadins interact with the glutenins during dough processing through non-covalent interactions. Prolonged mixing time caused an increase in the water content of gluten, changed the rheological properties of dough and made the gluten surface smoother. The storage modulus (G’, representing the gluten network density) was uninfluenced by prolonged mixing. Overmixing (from optimum to overmixing) had a smaller effect on strong flour compared to weaker flours. Differences in extractability behaviour between the SDS-extractable and –unextractable proteins indicated a more branched and complex structure in the SDS-unextractable protein. Omega gliadins showed a tight link with the glutenins in the gluten macropolymer. The use of proteomics, for investigating the whole protein composition and structure of the gluten polymer, are still rather limiting. One possible use of this technique is to investigate presence of disulphide bonds through the comparison of digested and digested+reduced samples. Taken together, the results presented in this thesis provide new insights into polymeric gluten protein structure specificity and changes from grain development to dough processing in general.