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

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 imp eller) to follow changes in dynamic properties. The number of freeze-thaw c ycles ranged from zero (that is. thawing only) to four and were applied to three types of commercial product (broccoli-potato, carrot-potato and celer y-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 propertie s as a result of an apparent heavy water loss during heating. The storage m odulus (G') was the rheological property most affected by either effect in both broccoli- and carrot-potato purees, showing that their behaviour is pr edominantly elastic, with phase angles in the region of 5-12'degrees. After one or two cycles (depending on thawing conditions), broccoli-potato prese nted much greater elasticity and apparent freeze-thaw stability than the ot hers. Celery-potato puree presented the highest fluidity, possibly because its initial higher water content meant that loss modulus values were more s ignificantly affected by thawing conditions. Results showed a complex depen dence of the dynamic properties on structural factors of the purees as well as on processing parameters.