Abstract:
Manufacturers, researchers and consumers are all interested in the texture of soft solids, which is an essential characteristic to fulfilling the requirements of end-users. The texture of a soft solid and its deformations under external applied forces depend on its rheological, mechanical and microstructural properties. The work performed for this thesis involved examining the rheological, mechanical and microstructural properties of five types of edible fat and three types of pectin gels in order to gain insight into the performance of these systems in storage and consumer usage environments. Edible fats and pectin gels are soft solids with different structural properties. Two brands of butter (Pam's butter and Mainland butter), a spreadable blend (Anchor Spreadable blend) and two brands of spread (Sunrise spread and MeadowLea Original spread) were selected as the three food groups to be investigated. Two pectins with different degrees of methyl esterification as well as a low-methoxyl amidated pectin were also examined. A texture analyser was used to perform puncture tests in order to explore the rheological properties of the edible fats and the pectin gels at both 5° C and 20° C. The measured force-versus-displacement relationships were transformed into stress-versus-strain relationship by assuming incompressibility of the food products in the linear viscoelastic region (LVR). Young's modulus of each sample at each temperature was calculated from the obtained stress-versus-strain curves. A flow curve test, frequency sweep test, amplitude sweep test, creep test and a test of the effect of temperature on the edible fats and pectin gels were also performed. In addition, the fatty acid composition and micrographs of edible fats were obtained using gas chromatography and environmental scanning electron microscopy, respectively. The shear elastic moduli of edible fats in the LVR were compared with Young's modulus in the same region. Young's modulus was highest for butter, followed by the spreadable blend, spreads and pectin gels at both 5° C and 20° C. The flow curve test confirmed the pseudoplastic behaviour of both sample types. The shear elastic modulus can be used as an indicator of both structure and rheology. Creep curves were analysed and modelled according to the Burger model, which comprises a Maxwell element with Kelvin-Voigt elements in series. Moreover, creep and recovery curves were represented by the action of a time-dependent latch model using Weibull functions. The Weibull distribution function clearly showed the viscoelastic failure of elements in the soft solids. One or two Voigt-units were required to simulate the viscoelastic region of creep curves for all edible fats and pectin gels. The shape parameters (β) of both brands of butter investigated showed an approximate value of 1 for the creep test, indicating a constant failure rate at both 5° C and 20° C. The value of β was always less than 1 for Anchor Spreadable blend, Sunrise spread and MeadowLea Original spread, indicating that the fatcrystal network structures of these samples have decreasing failure rates. Butter at 5° C showed the highest value of the life parameter (α), indicating that the likelihood of their network structure failing was lower than for the spreadable blend or spreads. The results indicate that hydrogen bonding of high-methoxyl pectin with water was stronger than the bonding of low-methoxyl and low-methoxyl amide pectins with calcium ions and water. All of the samples exhibited breakdown of their structures when they were deformed beyond their respective yield points. The viscoelastic failure was dependent on the temperature and microstructural properties of the product.