Role of hydroxycinnamic acids on the generation of Maillard-type aroma compounds in whole grain wheat bread

The non-enzymatic reaction between sugars and amino acids, also known as the Maillard reaction, affects many aspects of food quality, including color, taste and aroma formation, nutritional value, and toxicity. While many parameters such as pH, temperature, and water activity/content can affect compounds formed by the Maillard reaction, the effect of phenolic compounds is a relatively recent discovery. Specifically, one class of phenolic compounds, hydroxycinnamic acids (HCAs), the predominant type of phenolic compounds in wheat, were further studied to determine their effects on aroma production in various bread model systems using gas chromatography-olfactometry-mass spectrometry (GC-O-MS), liquid chromatography-tandem mass spectrometry (LC-MS/MS), and sensory techniques.;Simple aqueous Maillard model systems were conducted to study the reactivity of HCAs on aroma development. The addition of HCAs to carbonyl-amine Maillard reaction systems showed a suppression in the formation of several key Maillard aroma compounds, such as various pyrazines (pyrazine, methylpyrazine, 2,5-dimethylpyrazine), 2,3-butanedione (diacetyl), and 2-acetyl-1-pyrroline. The mechanisms of this reaction were investigated using isotopically labeled glucose and glycine. In these models, HCAs were found to form adducts with transient Maillard reaction flavor precursors, such as sugar fragments, as well as amino acids and amino acid reaction products.;To further study the effect of HCAs on aroma development, the influence of whole wheat flour, in comparison to refined wheat flour, on bread crust aroma was investigated. Differences in the aroma profile between the two breads were characterized by both comparative gas chromatography-olfactometry-aroma extract dilution analysis (GC-O-AEDA) and quantitative gas chromatography-mass spectrometry-chemical ionization (GC-MS-CI) analysis utilizing stable isotope surrogate standards. For refined bread crust (versus the whole wheat crust) five compounds were reported to be higher in concentration, 2-acetyl-1-pyrroline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-phenylethanol, 2-acetyl-2-thiazoline and 2,4-dihydroxy-2,5-dimethyl-3(2H)-furanone by 4.0, 3.0, 2.1, 1.7, and 1.5-fold, respectively; whereas three compounds were at lower concentrations, 2-ethyl-3,5-dimethylpyrazine, (E,E)-2,4-decadienal, and (E)-2-nonenal by 6.1-, 2.1-, and 1.8-fold, respectively. Evaluation of the bread crust by a trained sensory panel reported the perceived aroma intensity of the characteristic ‘fresh refined bread crust’ aroma was significantly higher in the refined bread sample in comparison to the whole wheat sample; however when the five aroma compounds, that were higher in the refined bread crust, were added to the whole wheat crust at equivalent concentrations, no significant differences in the aroma intensity were observed.;Liberation of HCAs from the predominantly insoluble conjugate (bound) form in wheat bran, and their effect on aroma development in bread was further studied through the use of 13C6-benzene ring labeled ferulic acid. The addition of free ferulic acid to a refined wheat bread system, comparable to the amount released in whole wheat bread after fermentation and baking, resulted in an aroma profile similar to whole wheat bread crust, in that there was a reduction in the generation of five aroma compounds (2-acetyl-1-pyrroline, 2,4-dihydroxy-2,5-dimethyl-3(2H)-furanone, 2-acetyl-2-thiazoline, 2-phenylethanol and 4-hydroxy-2,5-dimethyl-3(2H)-furanone), similar to that observed in whole wheat bread. However, the concentrations of lipid oxidation products ((E)-2-nonenal and (E,E)-2,4-decadienal) remained consistent in concentration to that found in refined bread.;In conclusion, the addition of HCAs to both simple and complex model systems affects the aroma generation in thermally-treated systems. HCAs appear to trap transient sugar fragments and amino acid reaction products, with the largest effect observed for the key bread aroma compound 2-acetyl-1-pyrroline. In addition to HCAs altering aroma development in bread, a second mechanism of aroma generation, related to lipid oxidation, was also found to lead to differences in aroma observed between the two types of bread. This suggests a dual effect of the presence of the entire wheat seed on aroma generation in bread, which can provide new modes to control flavor development in whole wheat bread.