The effects and interactions of milk proteins on the properties of oat starch : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

Oats are minor cereal crop and widely used as breakfast cereals. Oat starch is a by-product derived from the β-glucan extraction process and is widely used in food and non-food products such as oat meal, soups, gravies, coating agents for tablets and gloves and cosmetic products. It is also used for oat meal and other novel oat based food products such as oat puddings and oat-based sports gel consisting of oat starch and milk protein ingredients. These products normally undergo thermal treatment, where starch-milk protein interactions could affect the product’s final rheological and sensory properties. Oat starch also exhibits unique properties including small granule size, a well developed granule surface and high lipid content. Hence, the overall aim of this thesis was to determine the effect of milk protein ingredients on oat starch and to advance our understanding of how starches and milk proteins interact with each other. The effect of substituition of oat starch and oat flour at various concentrations (5% and 10%, w/w) with three different readily available milk protein ingredients, namley whey protein concentrate (WPC), whey α-lactalbumin (WLAC) and skim milk powder (SMP) were investigated. All three ingredients decreased the pasting responses of oat starch. SMP showed a greater effect on oat starch characteristics in oat starch and flour systems in comaprison with WPC and WLAC. It was evident that even a small (5%) substituition of oat starch/flour with milk protein ingredients resulted in loss of native properties of oat starch/flour systems, where SMP affected these properties most. SMP increased the pasting temperature and decreased the starch swelling. The effect of the addition of different concentrations (25%, 50%, 75% and 100%) of pure protein fractions, whey protein isolate (WPI) and calcium caseinate (CaCN) on oat starch were also investigated. Studies on the thermal behaviour of oat starch/milk proteins mixtures by differential scanning calorimetry (DSC) showed that WPI and CaCN increased the peak temperature (TP), suggested delaying of starch gelatinisation, where WPI had a greater effect on oat starch than CACN. The X-ray diffraction and Fourier transform infrared spectroscopy studies revealed that oat starch/WPI mixtures increased the relative crytallinity suggesting an increase in ordered structure, while a decrease in IR bands at 1024 cm-1 and 1080 cm-1 indicated less gelatinisation of starch molecules. CaCN did not affect the relative crystallinity and showed an increase in amorphous structuration in FTIR analysis, which is an indication of complete gelatinisation. The qualitative viscoelastic behaviour of oat starch/milk proteins gels were studied in terms of steady state flow behaviour, amplitude strain sweep and thixotropic analyis. WPI and CaCN increased the shear thinning behaviour of oat starch/milk proteins gels, while WPI decreased the elastic modulus (G’) up to 75% concentration and increased at 100%. However, CaCN resulted in an increase in viscous modulus (G”), indicating weak viscous gels in comparison with WPI/ oat starch gels. WPI and CaCN at high concentration (100%) resulted in an increase in elastic nature, but gels were found to be brittle in nature. Finally, it was concluded that WPI illustrated high potency to affect oat starch gelatinisation in comparison with CaCN. This study is a first step to understand the oat starch- milk protein ingredients behaviour during thermal processing, and the trends obtained from the resultant work could predict the usage of oat starch/milk proteins in the development of oat-milk based foods.