DEVELOPMENT OF A METHOD TO MEASURE RHEOLOGICAL PROPERTIES OF CARROT CELL AND CELL-WALL MATERIALS

A method for measuring the rheological properties of carrot cells and cell-wall material during typical cooking conditions has been evaluated. A Bohlin controlled-stress rheometer with a rapid heating attachment was used. Because of potential slippage and sensitivity problems with cells and cell-wall material, different geometries (parallel plate, cone and plate, and concentric cylinder) were assessed. Initial experiments were designed to determine the linear viscoelastic regions for the cell and cell-wall materials at 25-degrees-C. Samples were subjected to varying shear stresses and oscillatory frequencies. For carrot cells, linearity was observed with cone and plate geometry and with concentric cylinder geometry over a fairly wide range of stresses and frequencies. Parallel plate geometry did not give linear results. Carrot cell-wall material behaved in a linear fashion with all three geometries over similar stress and frequency ranges as carrot cells. Using the appropriate conditions of geometry, frequency and stress, the influence of heat treatments on cell and cell-wall rheology was compared. A comparison of cone and plate and concentric cylinder geometry was performed using cells and cell-wall material heated from 25 to 90-degrees-C. Cone and plate geometry showed greater changes in rheological properties during heating than concentric cylinder geometry and also allowed a greater range of heating rates. Cone and plate geometry was used in further studies. Studies on the rheological behaviour of cells during heating showed that the complex shear modulus (G*) increased in the range 25-75-degrees-C then decreased. For cell-wall material, G* showed a steady increase from 25 to 90-degrees-C. Further experiments demonstrated that the increase in G* was not due to water loss during heating. The amount of cell damage during heating was measured using a conductivity meter to establish if there was a correlation between changes in G* and cell damage.