Rheological characterization of fresh and cooked potato tissues (cv. Monalisa)

The rheological properties of fresh and cooked (15 min in boiling water) potato tissue which had been deformed to a lessor or greater extent by uniaxial compression, shear, uniaxial tension, successive cycles of stress relaxation, creep compliance and texture profile analysis were evaluated. Structural failure of fresh tissue under compression always occurred along a single plane of maximum shear stress, while fresh tensile specimens failed under tension. Tensile tests proved better methods by which to determine the failure parameters of cooked specimens since it was possible to observe two differents modes of failure with this technique. Equivalent modes of failure (causing damage to the cell wall or cell separation) and changes in structural components caused by cooking proved easier to idetify with tensile tests. The six element Burger's model incorporating two discrete Voigt-Kelvin units was the most suitable for defining tissue creep behaviour. The instantaneous elastic modulus could be related to the internal cell pressure, and gelatinization of starch and viscoelastic units appeared to reflect the viscoelastic properties of pectic sustances and hemicelluloses, respectively.