Infra-red analysis and the intermolecular interactions of ingredients during baking: enzymatic hydrolysis of starch, protein secondary structure and product characteristics

摘要

Wheat flour doughs are complex systems with many intermolecular interactions determining end product quality and consumer appeal. The primary aim of this study has been to investigate these interactions for selected baking ingredients during the three primary stages of bread making process. An approach based on attenuated total reflectance Fourier transform infrared (ATR/FT-IR) spectroscopy has been evaluated. Baking tests were used to study the impact of ingredient interactions on the quality attributes of the end product. The paramters evaluated included texture and bread staling measured using texture analysis, bread colour using a chromameter as well as bread volume. The investigation using ATR/FT-IR measured the relative contributions of the different types of protein secondary structure, specifically α-helix, β-pleated sheet and β-turns. The results showed that proteins are the primary determinant of the dough development process. The relative proportion of the β-sheet content increased upon addition of the oxidising agent ascorbic acid. The resultant changes in dough strength reflect the increased contribution of the β-sheet structure. Since the glutenin components of the gluten matrix are known to be rich in β -sheet, the dough strength has been interpreted in relation to the role and involvement of the glutenin within the matrix. Conversely, the incorporation of the reducing agent L-cysteine weakens the dough structure. The oxidising agent also increases inter-molecular interactions within the same protein molecule by enhancing the formation of disulfide bonds. The action of α-amylase on starch in the doughs was also monitored, using ATR/FT-IR in the region of 800-1300 cm -1 corresponding with vibration associated with C-C and C-O bonds. In the spectra, peaks corresponding to the various sugars were detected, the relative areas of each were calculated and the amount of the sugars was observed to increase during the reaction. The rate of cleavage of the glycosidic linkage in starch by the action of α-amylase was monitored over the incubation time. The changes in starch were also estimated using the peak at 1045 cm -1 indicating crystallinity of the starch. This decreased while increases were seen in the peak at around 1022 cm -1 , corresponding with the amorphous structure of starch. In addition, it was possible to quantitate the peak appearing at 976 cm -1 , identified as damaged starch. Varing levels of the oxidising agent ascorbic acid increased bread volume and enhanced bread texture while the reducing agent L-cysteine increased bread volume to certain extent, but with elevated levels of incorporation, marked decreases in volume occurred, whilst at all levels bread crust color was improved. The addition of α-amylase was found to enhance bread volume, texture and colour as well as slowing the rate of bread staling. The current study enhances our knowledge of the baking process, confirms the value of deconvolution software for the evaluation of infra-red spectra as well as demonstrating the value of infra-red analysis as a means of monitoring the changes taking place and the influence of improver ingredients.