| Creation of desirable food textures begins with an understanding of structure, which impacts the mechanical, sensorial, and oral processing characteristics of the food product. Structural information may be obtained by measuring mechanical properties of foods under small- and large-strain testing conditions. While large-strain mechanical properties have been correlated with sensory and oral processing characteristics, small-strain properties are typically not well-correlated. However, most large-strain testing in food research involves determination of fracture properties rather than examination of nonlinear viscoelastic behavior. Viscoelastic behavior is traditionally measured using oscillatory shear or creep tests. In food research, these tests are generally performed in the linear viscoelastic region (LVR). Although these tests may be performed beyond the LVR, nonlinear oscillatory shear testing is unusual in food research due to the lack of physical meaning of the viscoelastic moduli and the breakdown of the relationships between creep compliance and viscoelastic moduli. Previous work in the polymer industry has developed an analysis protocol for large-strain oscillatory shear (LAOS) data that gives physical meaning to nonlinear viscoelastic moduli. However, this protocol was not experimentally validated with model elastic and viscous systems. To that end, a model elastic solid and viscous liquid were tested under LAOS to compare the protocol to standard rheometer output and the known linear and nonlinear properties of both systems. Upon successful validation of the protocol with experimental data, this analysis method was used to study whey protein isolate (WPI)/kappa-carrageenan gels and several commercial cheeses to determine the impact of structure on nonlinear viscoelastic behavior, and the relationships between nonlinear viscoelastic properties and rheological, sensory, and oral processing characteristics. To accomplish this objective, the nonlinear viscoelastic properties of these systems were determined and correlated to mechanical properties and sensory and oral processing characteristics. Additionally, WPI/kappa-carrageenan gel creep parameters were measured, fit to 4-element Burgers models, and correlated to nonlinear viscoelastic properties. Three gel structural types were evaluated: a homogeneous protein (WPI) gel, a carrageenan-continuous gel, and a bicontinuous gel, in which both WPI and kappa-carrageenan exhibited continuous network structures. Previous study found significant differences in rheological, sensory, and oral processing characteristics for all three structural types. Cheeses studied comprised Cheddar, Mozzarella, and American, as each cheese had distinct structural and textural properties. Carrageenan-continuous gels showed the greatest degree of nonlinearity of all three gels under LAOS (25% strain), while homogeneous gels displayed the least. Of the cheeses, Cheddar displayed the greatest amount of nonlinear behavior under LAOS (25% and 50% strain); American cheese displayed the least. For both the WPI/kappa-carrageenan gels and cheeses, nonlinear viscoelastic properties correlated (0.5≥R²≥0.9, p<0.05) to rheological, sensory, and oral processing characteristics. Correlations to sensory and oral processing characteristics included aspects evaluated after several chews as well as first bite. In addition, predicted creep compliance values for the WPI/kappa-carrageenan gels were found to agree with experimental data (R²≥0.90), and nonlinear viscoelastic properties correlated (R²>0.7, p<0.05) with parameters used in the 4-element Burgers model. For all materials tested, structure appeared to impact the nonlinear viscoelastic properties of the material. Overall, determination of the nonlinear viscoelastic properties of food, and how those properties relate to rheological, sensory, and oral processing characteristics, allows a better understanding of food structure and structural deformation, and how that structure impacts food texture. While nonlinear viscoelastic properties may be evaluated by both creep and LAOS tests, LAOS testing yields a quantitative measure of the type and extent of nonlinear behavior, while creep parameters indicate only the presence of nonlinear behavior. Thus, nonlinear viscoelastic properties derived from LAOS data provide a more complete picture of structural deformation and breakdown beyond the LVR. |
