Investigation of the Inherent and Processed Bitterness in Whole Wheat Foods
2016-11
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Investigation of the Inherent and Processed Bitterness in Whole Wheat Foods
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2016-11
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Although whole grain foods are considered to be a key component of a healthy diet, consumer acceptability is hindered by a number of factors including negative sensory attributes, such as excessive bitterness. This thesis investigates the compounds that contribute to bitterness in wheat bread. Ultimately this work aimed to provide a better understanding of the chemical basis of bitterness development in whole wheat foods, including characterization of the key bitter compounds and generation pathways. Chapter 1 includes an overview of aroma and taste research literature including the basic concepts and current flavor challenges in whole grain food, with a focus on the present knowledge of bitterness in whole grains. Chapters 2, 3, and 4 present the finding of this work regarding the chemical origins of bitterness in whole wheat foods. Whole wheat bread was studied as a food model. Crust and crumb were both examined as they have unique flavor profiles and pathways of flavor generation due to their difference in moisture content and temperature during baking. In Chapter 2 Maillard reaction product 5-(hydroxymethyl) furfural and 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one were identified as key chemical markers of the bitterness in bread crust. Using Pearson’s correlation analysis, significant correlations (a= 0.01) were observed between the perceived bitterness in the crust and the quantity of 5-(hydroxymethyl) furfural (r2 = 0.93) and 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (r2 = 0.95). Bitterness markers in bread crumb were also investigated (Chapter 3), and key bitter compounds were identified as L-tryptophan, apigenin-C-glycosides, and 9,12,13-trihydroxy-trans-10-octadecenoic acid (pinellic acid). Pinellic acid was confirmed as the most influential bitterant in bread crumb via sensory recombination experiments. The identified bitterants were quantified in flour and during different stages of bread making, in order to elucidate the origin of bitterness in whole wheat bread crumb. L-tryptophan and apigenin-C-glycosides were found to be intrinsic in whole wheat flour and found to degrade during the bread manufacture process, whereas pinellic acid was generated during bread production. These findings provide an improved knowledge of the origin of bitterness in whole wheat products and a basis to further develop breeding and processing strategies to reduce bitterness in whole wheat bread. Finally in Chapter 4, the mechanism of pinellic acid generation during production of whole wheat bread was additionally investigated. Isotope labelling analysis using [13C18]linoleic acid revealed that the natural free linoleic acid in flour was the major contributor/precursor for the formation of pinellic acid which occurred in two bread-making stages: dough kneading and baking. Results suggested that generation of pinellic acid involved both enzymatic and non-enzymatic mechanistic pathways. The initial step for both pathways involved the oxidation of linoleic acid by lipoxygenase to linoleic acid hydroperoxides (LOOH). The enzymatic pathway was the main quantitative source of pinellic acid that occurred first during the hydration of the flour, which resulted in the decomposition of LOOH into pinellic acid by epoxidation and subsequent hydrolysis step. In the second stage that occurred during baking, a non-enzymatic pathway, involving decomposition of LOOH via a free radical mechanism facilitated by transition metals was suggested to lead to formation of hydroxyoctadecadienoic acid that further react with hydroxyl radicals to form pinellic acid. The mechanistic understanding of bitter pinellic acid formation allows for targeting and controlling key factors that help minimizing bitterness generation during manufacturing of whole wheat products. The influence of wheat class and flour storage temperature on the enzymatic formation of pinellic acid in dough was also examined in order to provide insights and ultimately develop guidelines for improved flavor quality of whole wheat products via wheat varietal selection and flour storage practices (Chapter 4). Results showed that Hard Red Spring (HRS) wheat entries generally had higher linoleic acid content and generated higher level of pinellic acid during dough kneading when compared to Hard White Winter (HWW) wheat samples. Difference in pinellic acid generation between the two classes of wheat, HRS and HWW were further noted during thermal processing. Temperature of flour storage temperature was also shown to influence the linoleic acid content in flour and subsequently the generation of pinellic acid in dough. Ambient temperature had the greatest influence on the generation of pinellic acid by increasing the free linoleic acid while maintaining lipoxygenase activity during storage. It is suggested that generation of linoleic acid hydroperoxide as a precursor for pinellic acid occurred in flour stored at ambient temperature; therefore higher pinellic acid formation was observed in the dough. In summary, this work provided a basis to understand bitterness development in whole wheat products. The improved knowledge of key bitter markers, their origin, and pathways of generation in whole wheat foods can provide critical insights into grain selection, potential breeding and of course processing and storage ultimately facilitating the improvment of flavor quality of whole grain foods.
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University of Minnesota Ph.D. dissertation. November 2016. Major: Food Science. Advisor: Devin Peterson. 1 computer file (PDF); xvi, 107 pages.
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Bin, Qing. (2016). Investigation of the Inherent and Processed Bitterness in Whole Wheat Foods. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/185198.
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