Eur Food Res Technol (2001) 213:30–37 DOI 10.1007/s002170100314 Abstract Results are presented on the effect of the num- ber 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 proper- ties. 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, pu- rees 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 pu- rees, showing that their behaviour is predominantly elas- tic, with phase angles in the region of 5–12°. After one or two cycles (depending on thawing conditions), broc- coli–potato presented much greater elasticity and appar- ent freeze–thaw stability than the others. Celery–potato puree presented the highest fluidity, possibly because its initial higher water content meant that loss modulus val- ues were more significantly affected by thawing condi- tions. Results showed a complex dependence of the dy- namic properties on structural factors of the purees as well as on processing parameters. Keywords Dynamic properties · Cyclic freezing and thawing · Microwave · Starch concentration · Helical ribbon Introduction Frozen purees made from mixtures of vegetables are a relatively new kind of high-quality product with a good potential market in Europe. As with any other frozen vegetable products, their textural quality is particularly affected by the type of product and the range and number of temperature fluctuations to which the product is sub- ject [1, 2, 3,4]. It is well known that textural properties of fresh vege- table structures are determined by the relative proportions of the different tissues and their structural arrangement [5]. How cellular structures of many fruits and vegetables are affected by freezing and/or thermal fluctuations can be found in the literature. For example, small-cellular vegetables such as peas and carrots are successfully fro- zen and thawed without significant loss of textural quality [6]. In contrast, more frequent temperature fluctuations reduce the mechanical strength of frozen potatoes, even in narrow ranges of fluctuation and especially if the pota- toes are packed [2, 3,4]. Alvarez and Canet [7] showed that the structural rigidity of potato tissues subjected to up to four freeze–thaw cycles was less than half that of tissues subjected to only one freeze–thaw cycle. Changes in the viscoelastic properties of fruit and vegetable purees that have undergone freezing or freeze– thaw treatments are of practical significance for their ac- ceptance and consumption. For instance, the phase tran- sition temperatures of apple puree-type products change after freezing because of structural breakdown due to the crystallization of water [8]. Cycled freezing and thawing generally improves the consistency of these products. There have been numerous studies on changes in rheo- logical properties of hydroxypropyl potato starch pastes during freeze–thaw treatments [9, 10,11]. Frozen vegetable puree structures consist of cells severely damaged by the successive steps in the manufac- turing process. This process includes a cooking stage in which tissues are softened by loss of cell turgor pressure due to physical breaking of the cells or to chemical chang- es in the cell wall or cell membrane, which allow leakage M.D. Alvarez Torres ( ✉ ) · W. Canet Centre de Génie Chimique des Milieux Rhéologiquement Complexes (GEMICO), 1 Rue Grandville, B.P. 451, 54001 Nancy Cedex, France Present address: M.D. Alvarez Torres, W. Canet, Department of Plant Foods Science and Technology, Instituto del Frío (CSIC), Ciudad Universitaria s/n, 28040 Madrid, Spain ORIGINAL PAPER María Dolores Alvarez Torres · Wenceslao Canet Rheological properties of frozen vegetable purees. Effect of freeze–thaw cycles and thawing conditions Received: 5 October 2000 / Revised version: 29 December 2000 / Published online: 16 May 2001 © Springer-Verlag 2001