Kinetics of softening of potato tissue by temperature fluctuations in frozen storage

Pre-packed and unpacked potato tissues were frozen, subjected to temperature fluctuations and then thawed. Temperature fluctuations ranged from -24 degrees C to -18 degrees C and from -18 degrees C to -6 degrees C. The number of fluctuacions ranged from 0 (that is to say, only freezing and thawing processes) to 32 at each above fluctuation range, simulating practical frozen storage conditions. Compression, shear and tension tests were carried out to measure the extent of structural damage caused to the potato tissue. Plots of log (rheological parameters and moisture content) versus number of temperature fluctuations showed two distinct regions; the first was a rectilinear plot with a steep negative slope up to four fluctuations. The second was also a rectilinear plot with a shallow negative slope beyond four fluctuations. For higher number of fluctuations, most of rheological parameters reached a value almost constant. These two-stage softening rate curves are consistent with the biphasic model and qualitatively similar to those for thermal softening of the vegetables. This study shows that two substrates S-a and S-b may be involved in providing firmness to potato tissue in freezing and frozen storage conditions. By analogy with earlier works, the term " frozen storage firmness'' can be proposed to describe the amount of firmness that is resistant to degradation by freezing with temperature fluctuations during frozen storage and final thawing of the product.