Changes in Creep Behaviour and Microstructure of Model Mozzarella Cheese During Working

The effect of shear work input on the microstructure, fat particle size and creep behavior of model Mozzarella type cheeses was studied. Cheese samples were prepared in a twin screw cooker at 70 °C by mixing protein and fat phases together with different amounts of shear work input. Major changes in cheese structure were observed while working at 150 rpm and 250 rpm screw speeds. Confocal microstructures plus macroscopic observations showed systematic changes in structure with increased shear work inputs with unmixed buttery liquid observed at kg−1, typical Mozzarella type microstructures (elongated fat-serum channels) at 6–15 kJ kg−1 and homogeneously distributed, small size fat droplets at \u3e58 kJ kg−1. At very high shear work inputs, \u3e 75 kJ kg−1, striations or anisotropy in the microstructures had disappeared and small micro-cracks were evident. A 4-element Burger\u27s model was found adequate for fitting the creep data of model cheese at 70 °C but a 6-element model was required at 20 °C. As shear work input increased retarded compliance decreased and zero shear viscosity increased indicating the more elastic behavior of the cheeses with higher shear work input. Changes in the protein matrix appear to be the main reason for increased elastic behavior

Effect of Shear Work Input on Steady Shear Rheology and Melt Functionality of Model Mozzarella Cheeses

Model Mozzarella cheeses with varied amounts of shear work input were prepared by working molten cheese mass at 70 °C in a twin screw cooker. Rheology and melt functionality were found to be strongly dependent on total shear work input. A non-linear increase in consistency coefficient (K from power law model) and apparent viscosity and decrease in flow behaviour index (n from power law model) were observed with increasing amounts of accumulated shear work, indicating work thickening behaviour. An exponential work thickening equation is proposed to describe this behaviour. Excessively worked cheese samples exhibited liquid exudation, poor melting and poor stretch. Nonfat cheese exhibited similar but smaller changes after excessive shear work input. We concluded that the dominant contributor to the changes in properties with increased shear work was shear induced structural changes to the protein matrix. A good correlation was found between the steady shear rheological properties and the melting properties of the cheeses

Strain Hardening and Anisotropy During Tensile Testing of Sheared Model Mozzarella Cheeses

We studied the tensile fracture properties of model Mozzarella cheeses with varying amounts of shear work input (3.3–73.7 kJ/kg). After manufacture, cheeses were elongated by manual rolling at 65°C followed by tensile testing at 21°C on dumbbell-shaped samples cut both parallel and perpendicular to the rolling direction. Strain hardening parameters were estimated from stress–strain curves using 3 different methods. Fracture stress and strain for longitudinal samples did not vary significantly with shear work input up to 26.3 kJ/kg and then decreased dramatically at 58.2 kJ/kg. Longitudinal samples with shear work input \u3c30 kJ/kg demonstrated significant strain hardening by all 3 estimation methods. At shear work inputs \u3c30 kJ/kg, strong anisotropy was observed in both fracture stress and strain. After a shear work input of 58.2 kJ/kg, anisotropy and strain hardening were absent. Perpendicular samples did not show strain hardening at any level of shear work input. Although the distortion of the fat drops in the cheese structure associated with the elongation could account for some of the anisotropy observed, the presence of anisotropy in the elongated nonfat samples reflected that shear work and rolling also aligned the protein structure

Tensile Testing to Quantitate the Anisotropy and Strain Hardening of Mozzarella Cheese

We explored anisotropy of mozzarella cheese: its presence is debated in the literature. Tensile testing proved a good method because the location and mode of failure were clear. Mozzarella cheese cut direct from the block showed no significant anisotropy, though confocal microscopy showed good structure alignment at a microscale. Deliberately elongated mozzarella cheese showed strong anisotropy with tensile strength in the elongation or fibre direction ∼3.5× that perpendicular to the fibres. Temperature of elongation had a marked impact on anisotropy with maximum anisotropy after elongation at 70 °C. We suggest the disagreement on anisotropy in the literature is related to the method of packing the mozzarella cheese into a block after the stretching stage of manufacture. Tensile stress/strain curves in the fibre direction showed marked strain hardening with modulus just before fracture ∼2.1× that of the initial sample, but no strain hardening was found perpendicular to the fibre direction

Measurement Techniques for Steady Shear Viscosity of Mozzarella-Type Cheeses at High Shear Rates and High Temperature

While measuring steady shear viscosity of Mozzarella-type cheeses in a rotational rheometer at 70 °C, three main difficulties were encountered; wall slip, structural failure during measurement and viscoelastic time dependent effects. Serrated plates were the most successful surface modification at eliminating wall slip. However, even with serrated plates shear banding occurred at higher shear rates. Because of the viscoelastic nature of the cheeses, a time dependent viscous response occurred at shear rates \u3c1 \u3es−1, requiring longer times to attain steady shear conditions. Prolonged continuous shearing altered the structure of the molten cheeses. The effects of structural change were greatly reduced by minimising the total accumulated strain exerted on the sample during flow curve determination. These techniques enabled successful measurement of steady shear viscosity of molten Mozzarella-type cheeses at 70 °C at shear rates up to 250 s−1

Impact of steaming conditions on the structure and on the properties of bread crust; in the case of a crispy roll

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