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    A comprehensive study on the relative importance of disulphide and non-covalent interactions between proteins on the heat-induced aggregation and funcitonal property of acid milk gels : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Albany, New Zealand
    (Massey University, 2014) Nguyen, Nguyen Hong Anh
    Understanding the interactions between the milk proteins during heat treatment of milk can be employed to manipulate the functional properties of dairy products. The ability to control the functional properties can be beneficial to the dairy industry. When being heated, milk proteins interact via two main types of bonding: disulphide bonds and non-covalent interactions. They are both considered to be important in the properties of heated milks and the resulting milk products. This research aimed to investigate the relative importance of each interaction type on the heat-induced aggregation between the proteins in milk and the functional properties of a milk product in a model food system. Experiments involved adding low concentrations of a disulphide-bond reducing agent or a thiol blocking reagent to milk systems to either enhance or inhibit the thiol-disulphide exchange reactions between the proteins. The reagent was added to unheated milks, heated milks and unheated milks followed by heating. The effect of modifying the extent of thiol-disulphide exchange reactions between the proteins on the level of proteins participating in intermolecular disulphide bonds, on the degree of interactions between the casein micelles/casein proteins and the whey proteins were investigated. The treated milks were acidified to form acid milk gels of which the rheological properties and the microstructure were examined. Results demonstrated that the proportion of proteins participating in intermolecular disulphide bonds can be controlled by systematically modifying the thiol-disulphide exchange reactions between the milk proteins. It was shown that the initial interactions between the proteins in milk upon heating were non-covalent and disulphide bonds were subsequently formed to strengthen the bonding between the proteins in the heat-induced aggregates. When the milks were made to acid gels, both types of protein interactions in the milk were equally important in influencing the storage modulus (Gʹ) values of the resulting gels with the higher the degree of connections, the higher the Gʹ values. On the other hand, disulphide bonds played a more important role than non-covalent interactions in determining the yield properties of the acid gels. The yield stress values can be increased by increasing the proportion of disulphide bonds in the milk system before acidification or by enhancing the formation of disulphide bonds between the particles during the formation of acid gels.
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    Investigations of the behaviour of pectin in casein micelle systems and their analogues : thesis presented for the degree of Doctor of Philosophy in Physics
    (Massey University, 2009) Cucheval, Aurelie Suzanne Bernadette
    Firstly, the effect of pectin on acid milk gels in concentrated, quiescent systems was investigated by passive microrheology using two complementary techniques: diffusive wave spectroscopy (DWS) and multiple particle tracking (MPT). DWS, by allowing probing the mechanical properties of the network at high frequency, gave information on its microstructure. The addition of high methoxyl pectins was shown to change the network structure which has been explained by bridging of the casein micelles by the polymer as the system was undergoing acidification. On the other hand, the presence of low methoxyl pectin in the acid milk gel was shown to have no effect on the microstructure of the network at low concentration of polymer (0.1%w/w) which has been attributed to the sensitivity of this low DM pectin to calcium: LM pectin are trapped by calcium and not able to interact with casein micelles anymore. Multiple particle tracking was used to probe the effect of pectin on the heterogeneity of the system by following the distribution of the displacements of added micro beads at a given time lag during the gelation using the Van Hove distribution. Furthermore, the surface chemistry of the probes was modified in an attempt to control their location in the system. Finally, the mean square displacements of the casein micelles obtained by DWS and, of k-casein coated particles obtained by MPT were shown to give good agreement for the same acid milk system. Having established that the interaction between casein micelles and low methoxyl pectin is prevented by the pectin sensitivity to calcium, the effect of the pectin fine structure was investigated on the interaction between k-casein and pectin by surface plasmon resonance (SPR). The amount of pectin binding on a k-casein coated gold surface was shown to be strongly dependant on the pectin fine structure. It was concluded that small negative patches on the pectin backbone, likely to comprise of around two consecutive unmethylesterified galacturonic acid, are the most effective for pectin binding to k-casein. The effect of the direct interaction between pectin and k- casein on ‘calcium-free casein micelle mimics’ in pectin solution was then investigated using coated latex beads. A pectin structure with a limited number of negative patches on its backbone was also shown to limit the potential for destabilization via bridging.
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    High-pressure-induced starch gelatinisation and its application in a dairy system : a thesis presented in partial fulfilment of the requirements for the Doctor of Philosophy in Food Science at Massey University, Auckland, New Zealand
    (Massey University, 2009) Oh, Hyunah Eustina
    This study investigated pressure-induced starch gelatinisation in water and milk suspensions. A rheological method, termed ‘pasting curves’, provided an objective and analytical means to determine the degree of pressure-induced starch gelatinisation. In addition, a polarised light microscope was used to observe birefringence of the starch granules and the degree of starch swelling was measured. The preliminary investigation into pressure-induced gelatinisation of six different starches showed that potato starch was the most pressure resistant and was not gelatinised after a pressure treatment of 600 MPa for 30 min at 20 °C. Waxy rice, waxy corn and tapioca starches showed complete gelatinisation after the same treatment while normal rice and normal corn starches were only partially gelatinised despite the disappearance of birefringence. Based on the preliminary study, two starches (normal and waxy rice starches) were selected for more detailed studies. The effects of treatment conditions (pressure, temperature and duration) on the gelatinisation were investigated with these selected starches. The degree of gelatinisation was dependent on the type of starch and the treatment conditions. The results also indicated that different combinations of the treatment conditions (e.g. high treatment pressure for a short time and low treatment pressure for a longer time) could result in the same degree of gelatinisation. Both starch types exhibited sigmoidal-shaped pressure-induced gelatinisation curves and there was a linear correlation between the degree of swelling and the apparent viscosity of the starch suspension. After treatments at =500 MPa for 30 min at 20 °C, both starches lost all birefringence although the apparent viscosity and the degree of swelling of normal rice starch did not increase to the same extent as observed in waxy rice starch. Pressure-induced gelatinisation of starch was retarded when starch was suspended in skim milk. This was attributed to the effect of soluble milk minerals and lactose present in the milk whereas milk proteins (casein and whey) did not affect the degree of gelatinisation at the levels present in 10% total solids skim milk. The presence of soluble milk and/or lactose may lead to less effective plasticising of starch chains by the suspension medium. Interactions between milk components and starch molecules may also play a role in retarding gelatinisation by reducing the mobility of starch chains. The functionality of starch in a dairy application was tested using acid milk gels as a model system. Skim milk with added starch (waxy rice or potato starch) was either pressure treated (500 MPa, 20°C, 30 min) or heat treated (80°C, 30 min) and subsequently acidified to form acid milk gels. The addition of waxy rice starch resulted in firmer acid milk gels, and increasing the amount of starch caused an increase in the firmness of both pressure-treated and heat-treated samples. However, pressure-treated samples with added potato starch did not show significant changes in the firmness whereas the heat-treated counterparts showed a marked increase in the firmness as the level of potato starch increased. The difference between the effects of the two different starches can be explained by the extent of starch gelatinisation in skim milk. Starch granules absorb water during gelatinisation whether induced by pressure or heat which effectively increases milk protein concentration in the aqueous phase to form a denser protein gel network on acidification. The firmness of acid milk gels can be increased by adjusting the pH at pressure or heat treatment to higher than the natural pH of milk. The effect of pH at pressure or heat treatment and addition of starch on the acid milk gel firmness was additive and independent of each other up to a starch addition level of 1%. This study provided an insight into pressure-induced gelatinisation of starch by showing gelatinisation properties of starches of different botanical origins and the effects of the treatment conditions (treatment pressure, treatment temperature and duration) on the degree of gelatinisation. Furthermore, the results from the pressure treatments of starch in dairy-based suspensions showed that pressure-induced gelatinisation was affected by other components in the system. These results demonstrate the importance of understanding the gelatinisation properties of starch in complicated food systems in which a number of other components are present. In terms of the application of starch in dairy systems, when starch was added to milk and gelatinised by pressure treatment, the acid milk gel produced by subsequent acidification was firmer than the acid milk gel made from skim milk alone.