Effect of steam cooking on the residual enzymatic activity of potatoes cv. Agria

The aim of this work was to study the influence of steam cooking on pectin methylesterase (PME) and endogenous alpha- and beta-amylase activities in different tissues (cortex and pith) of raw and heat-treated potatoes cv. Agria. Three different cooking temperatures were chosen (55, 70 and 85 degrees C). For each cooking trial, time-temperature profiles were recorded and the degree of cooking was expressed in terms of cooking factor. RESULTS: Steam cooking contributed to significantly activate PME at 55 degrees C and to reduce its activity at the final processing temperature (85 degrees C), with the highest amount in the cortex (0.3745 +/- 0.0007 mu mol galacturonic acid (GA) g(-1) fresh weight (FW) min(-1)) compared with the pith (0.2617 +/- 0.0012 mu mol GA g(-1) FW min(-1)). The presence of heat-labile and heat-stable isoforms of PME in the considered potato tissues was also assumed. Heat treatment by steam resulted in a significant decrease in endogenous alpha- and beta-amylase activities in both tissues compared with the raw potato, though without complete deactivation. Starch-degrading enzymes were also found to be differently distributed in the raw tuber. CONCLUSION: Steam cooking affected in different ways the assessed residual enzymatic activity in the considered tissues of potatoes cv. Agria. Further research is needed to confirm the results obtained.