Observations on the microstructure and heat-induced changes in the viscoelasticity of commercial cheeses

Commercial samples of low-moisture mozzarella (LMMC), cheddar (CC) and analogue pizza cheese (APC) were evaluated for microstructure and heat-induced changes in viscoelasticity. Major inter-variety differences in microstructure were noted. The casein in the LMMC was in the form of longitudinally aligned fibres with entrapped fat columns, consisting of discrete and coalesced fat globules, of similar orientation. Less orientation of the casein and fat phases was evident in the CC. The microstructure of the APC was distinctly different from that of the LMMC and CC, having a more uniformly distributed paracasein matrix (with no orientation) and an emulsion of discrete, rounded fat droplets of varying size. Increasing the temperature resulted in a decrease in the elastic shear modulus, G', and an increase in the phase angle, δ, indicating a transition from an unmelted cheese, largely elastic in rheological response, to a melted cheese, which is more viscous in character. There was a distinct phase change from elastic to viscous nature in the LMMC and CC cheeses, as indicated by an abrupt increase in δ to ∼55-60° in the temperature range 60-82°C. In contrast, the increase in δ for the APC was more gradual and the maximum value attained was much lower, at ∼38°, There was good agreement between the viscoelastic parameters and functional attributes of the baked cheese, with low values of δ max and high values of G' mir (as in the APC samples) coinciding with relatively low flowability and high apparent viscosity.