Influence of Cooking and Freeze-Thawing Cycles on Viscoelastic Properties of Vegetable Purees

A study was undertaken to ascertain the changes occurring in dynamic properties of frozen vegetable purees during heating (20-90 °C) and cooling to 20 °C. The effect on the rheological properties of freeze-thaw cycles (up to six) prior to cooking was analysed. A helical ribbon impeller was used to follow the changes in complex modulus (G*) and phase angle (). On heating, both dynamic properties indicated greater elasticity in broccoli puree. In all three purees the onset temperature of G*increase on heating seemed to be related to the onset temperature of gelatinization of the starch grains. The most substantial effects on viscoelastic properties were observed on cooling. The increase in G*and decrease in of the purees on cooling appear to be due to either water loss or retrogradation of the starch grains. Repeated cycles led to concentration of dissolved starch components and changes in the cell and tissue structure of the vegetable components, although cycling effect on rheological behaviour depends on the measurement temperature.