Rheological properties of frozen vegetable purees. Effect of freeze-thaw cycles and thawing conditions

Abstract

Results are presented on the effect of the number of successive freeze–thaw cycles and different final thawing conditions on rheological properties of frozen vegetable purees using a specific mixing device (helical ribbon impeller) to follow changes in dynamic properties. The number of freeze–thaw cycles ranged from zero (that is, thawing only) to four and were applied to three types of commercial product (broccoli–potato, carrot–potato and celery–potato). After freeze–thaw cycles, purees were thawed at room temperature, by microwave at three different settings, and in a saucepan. Rheological properties were affected more by the thawing conditions than by the number of cycles applied. Saucepan thawing increased the values of these properties as a result of an apparent heavy water loss during heating. The storage modulus (G') was the rheological property most affected by either effect in both broccoli– and carrot–potato purees, showing that their behaviour is predominantly elastic, with phase angles in the region of 5–12°. After one or two cycles (depending on thawing conditions), broccoli–potato presented much greater elasticity and apparent freeze–thaw stability than the others. Celery–potato puree presented the highest fluidity, possibly because its initial higher water content meant that loss modulus values were more significantly affected by thawing conditions. Results showed a complex dependence of the dynamic properties on structural factors of the purees as well as on processing parameters.