Relationship between gluten strength and free asparagine content of Canadian wheat varieties
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The free amino acid asparagine is the main precursor to formation of acrylamide, a probable carcinogen, in bakery products. Therefore, it is critical to understand if reducing free asparagine in wheat will be detrimental to dough viscoelasticity, and consequently, to the quality of the bread made from the dough. Thus, this study aimed to investigate how factors affecting free asparagine levels in wheat, i.e., environment, genotype, and fertilization treatments, affected dough rheological properties. Because good gluten strength is necessary for good bread quality, this study sought to understand the relationship between wheat free asparagine concentration and gluten strength. Eight commercial Canadian wheat varieties were grown at four site-years under four fertilization treatments, i.e., combinations of two levels of nitrogen and two levels of sulfur. Wheat grain samples were milled into whole-wheat and straight grade flour. Free asparagine concentration was quantified using whole-wheat flours and dough rheological properties were measured using straight grade flours. Empirical dough rheological properties were evaluated using the Brabender Farinograph and the Brabender Extensograph. Using a subset of 10 selected samples, shear rheometry and low-intensity ultrasound (LIU) tests were performed on yeasted and non-yeasted doughs. From empirical tests, it was observed that environment, genotype and their interaction were the main factors affecting gluten strength, with little effect from fertilization. Additionally, gluten strength parameters were negatively correlated to wheat free asparagine concentration. When evaluated under linear shear oscillatory tests, non-yeasted dough rheology significantly changed over time, while prominent dough softening was observed during fermentation of yeasted doughs. When tested under creep-recovery and stress relaxation, a strong negative correlation was observed between wheat free asparagine concentration and gluten strength. Finally, LIU provided meaningful information on dough rheological properties, with similar outcomes compared to shear tests. LIU results showed bubble disproportionation happened to a greater extent in samples with higher free asparagine concentration and protein content, samples that were hypothesized to have higher gliadin-to-glutenin ratios. In conclusion, selecting wheat varieties with low levels of free asparagine and applying commercial levels of nitrogen fertilization are good strategies to produce safe and high-quality wheat, with no detrimental effects on gluten strength.