Describing the firmness, springiness and rubberiness of food gels using fractional calculus:Part I: Theoretical framework

Constitutive models for soft solids that quantitatively relate the state of the stress in the material to the deformation history have the potential to be used in a structure-texture engineering context, but successful examples are scarce. In the present work we define equations for the firmness F, springiness S, and rubberiness R, of semi-soft food gels such as cheeses that exhibit broad power-law stress relaxation over a wide range of timescales. The equations contain only two material properties, which have their origin in the food microstructure: a fractional exponent, which quantifies the frequency and temporal response and secondly a scale factor or “quasi-property”, which sets the magnitude of the stress in the material. Together they form a constitutive element, known as the ‘springpot’ or Scott Blair element which can accurately capture the viscoelastic properties of food gels such as semi-hard cheeses. Using this model it becomes possible to provide clear and unambiguous definitions of textural parameters such as firmness, springiness and rubberiness, and to quantify their time-dependence and interrelationship. The magnitude of the firmness and springiness are inversely related through the fractional constitutive model. Our FSR-equations can be used in a texture engineering context to guide effective product reformulation of soft-solid, hydrocolloidal gels.