EFFECT OF FREEZING RATE AND PROGRAMMED FREEZING ON RHEOLOGICAL PARAMETERS AND TISSUE STRUCTURE OF POTATO (CV MONALISA)

Compression, shear and tension tests were carried out to determine the effect on potato tissues of different freezing rates (0.5, 1.25 and 2 degrees C/min down to - 18 degrees C), thawing up to + 20 degrees C a t the same rates, and one, two, three or four successive freeze/thaw c ycles. The effect of freezing rate on the zone of maximum crystallizat ion was also examined, along with different combinations of programmed freezing, and the effect of prior cooling was assessed. Minimum alter ation of the rheological behaviour of slowly thawed tissues was achiev ed by pre-freezing (3 degrees C for 30 min), slow cooling phases (0.5 degrees C/min) before and after the phase of maximum ice crystallizati on and quick freezing (2 degrees C/min) in the same phase. The shear t est was found to be well suited to the study of these effects. Examina tion of the tissues by SEM revealed differing degrees of mechanical da mage to tissue structure, which accounted for the rheological behaviou r of the samples. Coefficients of softening per freeze/thaw cycle were determined for the various rheological parameters, the highest value being given by the modulus of rigidity (17.75%).