Title : A Engineering Approach to Preserve Long-Term Texture Quality and Authenticity of Certified Hard Cheeses: The Parmigiano Reggiano Cheese perspective
The Parmigiano Reggiano Cheese (PR) is an Italian artisanal cheese famous worldwide. Its commercial quality and designation of origin are regulated and protected by a law disciplinary, which imposes requirements about making technology, time of ripening and product properties including the “struttura finemente granulosa” and “frattura a scaglie” among the main typical structure-related properties which can be considered a fingerprinting of cheese quality and authenticity. The latter two requirements are specified without a clear definition and analytical method for their quantitative measurement.The aim of this work was performing a multi-scale and non-destructive investigation of this precious cheese to individuate the fundamental material properties underpinning cheese quality law requirements. Cheeses samples provided by PR’s Consortium with different ripening ages were firstly investigated by Electron Scanning Environmental Microscopy and X-Ray Microtomography with the aim to virtualize non-destructively the bulk and crack interface structure. Temperature and frequency sweep, and isothermal creep tests were performed to evaluate the effect of temperature and shear stress on cheese deformation and fracture. Differential scanning calorimetry was carried out to evaluate the temperature effect alone. Crack extent, load and load line displacement were video recorded under three-point bending conditions and then analyzed using the J-Integral theory to determine the fracture toughness, the mechanical property related to the crack-initiation, crack-growth propagation, and kinetics. A computer implementation in visual basic language was performed to decouple the elastic and inelastic strain energy in single-edge bending test specimens under strain-controlled fracture according to the elasto-plastic fracture mechanics theory.
Results suggested that the crack initiation and propagation in PR cheese are strictly related to the structural heterogeneity over a large range of scale as well as to the elastic and inelastic energy release under strain. The applied stresses concentrated around the tips of sub-micron discontinuities, this latter arises from partially fused curd junctions originated during milk clotting and cutting. Fat escapes from casein network and crack propagates along with interfaces between fat and casein matrices due to their different relaxation times, while new surfaces originate with an intergranular pattern. The extent of crack propagation within the cheese bulk follows different kinetic regimes ranging from unstable and spontaneous to stable and strain-rate driving one. The spontaneous fracture was attributed to the ability of casein network to release the minimum elastic strain energy. Whereas the stable and strain-rate driving fracture was attributed to the plastic and viscous structural elements, mainly represented by the micro-discontinuities, free fat, globular fats and aqueous solution physically linked to the casein network.
Two apparent creep activation energies from 10 to 80J/Kmol were found suggesting at least two main mechanisms for steady-state structure relaxation. The first relaxation mechanism has been mainly attributed to the dislocation/coalescence of macro-discontinuities and micro-porosity up a softening transition temperature, with minor temperature dependence of relaxation time. These results have been attributed to the counterbalancing plasticizing effect of water and free fatty acids, both acting as plasticizers. The longest the time of ripening, the more extent of water evaporation and the naturally occurring lipolysis, i.e. triglycerides hydrolysis degradation to a low molecular weight compounds. The second relaxation mechanism observed beyond the transition temperatures has been attributed to the sliding of the interfaces between caseins (harder regions) and free fat (soft regions) without significant differences with respect to the time of ripening.