Optimization of low-temperature blanching for retention of potato firmness:: Effect of previous storage time on compression properties

Low-temperature blanching (LTB) of potatoes (cv. Kennebec), both without further processing and prior to cooking or freezing + cooking, significantly increased firmness retention as measured from compression parameters. The increase in firmness with respect to that of unblanched potatoes diminished in the order: blanched at 60 degrees C for 60 min and cooked > blanched at 60 degrees C for 60 min frozen and cooked > blanched at 60 degrees C for 60 min. Potato tubers were kept in refrigerated storage, and firmness, PME activity and dry matter (DM) content were periodically sampled over a period of 80 days. In the early stages of storage, PME activity lost 40% of its original value after 60 min at 60 degrees C, indicating that the contribution of starch breakdown products to the firmness of cooked and frozen cooked potatoes predominated over the effect of enzyme activity. With increasing time in storage, PME activity measured in the fresh tissue increased by 95% of its original value after 35 days; this resulted in changes in the pectic polymers which made for a firmer texture and different PME behaviour versus LTB temperature and time. A central composite rotatable design was used to study the effects of variation in levels of temperature (52.93-67.07 degrees C) and time (31.72-88.28 min) on compression parameters and PME activity. Stationary points showing maximum mechanical resistance had critical temperatures and times in the ranges of temperature (58-60 degrees C) and time (66-75 min) used for each independent variable. Results show a high correlation between PME activity and tissue firmness, suggesting that the contribution of the changes in the composition of the cell wall to the firmness of frozen cooked potatoes increased with increasing time in storage and reached a maximum in the intermediate stages of storage (35 days). Engineering stress (sigma(u)) proved to be the most appropriate compression parameter for detecting the firming effect that the PME activity produced on the frozen-cooked potato tissues as a consequence of LTB under these conditions.