Inclusion of dietary fibre in model liquid and solid systems using chemistry and structure to probe potential functionality
Creators: name: Alqahtani, Nashi Khalid
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300602
subject: Food chemistry and food sensory science

description: Oat grain, being one of the major cereals in Australia, contains various macromolecules and microconstituents including protein, lipid, dietary fibre and phytochemicals that are beneficial to human health. Despite its advantageous attributes, inclusion of sufficient amount of finely milled oat powder in commercial food formulations is still limited due to undesirable organoleptic properties that might develop during storage prior to consumption. Therefore, the objective of this study is to probe the influence of oat in model liquid and solid systems through an armoury of analytical and physicochemical techniques, i.e. chromatography, spectroscopy, rheology, calorimetry, microscopy and sensory evaluation.<br><br>The first experimental chapter deals with the development of various analytical protocols to probe the presence of microconstituents in model liquid oat systems following UHT processing and subsequent storage. Compounds of interest included phenolic acids (ferulic and para-coumaric acids), avenanthramides, free fatty acids and volatile lipid oxidation products. Sample preparation involved a UHT treatment at 145 °C for 5 s followed by a twelve-week storage at 22 and 30 °C. Soluble and insoluble dietary fibre fractions (SDF and IDF, respectively) were extracted from defatted oat samples through sequential enzymatic digestion. In order to extract ester bound phenolic acids, alkaline hydrolysis and liquid–liquid partitioning steps were applied to SDF and IDF. Moderate enzymatic digestion temperature (50 °C) and saponification (2M NaOH for 6 hr) were found to produce the best separation of phenolic compounds analysed by high performance liquid chromatography (HPLC).<br><br>Chromatograms showed prominent ferulic and para-coumaric acids in both fractions, which were located mainly in IDF, and that was confirmed by the total phenolic content results obtained from Folin-Ciocalteau assay. Alkaline based extraction with methanol and an HPLC method were developed to analyse the avenanthramide content in oat samples. Results demonstrated that the protocols were able to detect the primary avenanthramides, i.e. A, B and C. In addition to phenolic compounds, the work examined oat lipids and their oxidation products which are the main culprit of rancid flavour in liquid oat systems. Free fatty acid determination involved fatty acid methyl ester (FAME) analysis coupled with a comprehensive gas chromatography (GC×GC) technique. Chromatograms recorded three main fatty acids, i.e. palmitic, linoleic and oleic acids, which accounted for nearly 98% of the total free fatty acids. In the case of secondary lipid oxidation compounds, head space solid phase micro-extraction (SPME) coupled with gas chromatography-mass spectroscopy (GC-MS) analysis was developed to detect the presence of volatiles, which are predominantly hexanal and 2-pentyl furan.<br><br>Once the required analytical protocols were established, the focus in this work turned to the evolution of the aforementioned microconstituents during extended storage at 22 and 30 °C. It was observed that ferulic acid decreases over time at both temperatures while para-coumaric acid remains more or less stable but at a much lower content. Reduction in ferulic acid suggested conversion into another compound called paravinyl guaiacol (PVG), and this is known as the process of thermal decarboxylation. The content of avenanthramides also remained unchanged over time at both experimental temperatures. In the case of free fatty acids, unsaturated moieties (oleic and linoleic acids) decreased with temperature indicating the occurrence of lipid oxidation. That was confirmed by an increase in the levels of volatiles (hexanal and 2-pentyl furan) during storage, with the oxidation process being enhanced with increasing storage temperature.<br><br>The second experimental chapter looked at off-flavour producing compounds, in particular PVG and hexanal, in relation to the sensory quality of model UHT liquid systems enriched with wholegrain oat during storage. In doing so, finely milled oat powder was mixed with skim milk powder and sucrose followed by UHT treatment and storage at ambient temperature. It was found that trained panellists were capable of differentiating the odours generated by the two compounds and identifying their intensity in the UHT samples as a function of storage time. Furthermore, a 9-point hedonic test was conducted with semi-trained panellists to assess the acceptability levels of both odours in the UHT samples. Results suggest that hexanal is the main contributor to off-flavour development in oat based liquid systems, hence arguing for the importance of avoiding lipid oxidation in these materials.<br><br>Following the study on model UHT liquid systems, the third experimental chapter focuses on UHT beverage formulations that closely resemble commercial oat products. Results unveil the importance of the particle size distribution of oat fibre on the overall acceptability of liquid foods, as well as on their consistency. Increasing the particle size distribution and concentration of these particles yields higher steady shear viscosity. Furthermore, prolonged storage at 22 °C enhanced the beverages’ viscosity, whereas storage at 30 °C produces the opposite trend. The former event is attributed to the swelling of fibre particles, whereas the latter is due to the hydration of starch molecules, which dissolve into the beverage matrix (particle erosion beyond swelling), hence being unable to suspend in solution the insoluble parts of the oat particle.<br><br>Given the above effect of oat-particle addition on the consistency of beverages as a major ingredient in formulations, the study was expanded into a model solid system containing gelatin as the supporting matrix. Protein concentration was fixed at 2% (w/w), with the level of oat particles varying from 0 to 4% (w/w). Findings demonstrate phase separated systems, with gelatin and oat particles serving as the continuous and dispersed phases, respectively. Incorporation of oat particles weakened the mechanical strength of the protein gels, which is a new finding in the field of hydrocolloid gels. Besides concentration, the work explores the effect of different size distribution of oat particles on the composite gels and argues that smaller particles are able to disrupt swiftly the protein network compared to the larger counterparts.<br><br> Outcomes from this PhD thesis provide valuable knowledge in understanding the causes of sensory and textural profiles that might be experienced in oat based food systems. Hence, the research can be used as a basis to develop palatable processed foods, over prolonged storage, which are enriched with high levels of wholegrain powders.