The effect of blanching and freezing on the distribution and changes of pectic substances in carrot

Blanching prior to freezing inactivates enzymatic reaction and improves the textural qualities of frozen fruits and vegetables. Freezing changes the textural qualities of fruits and vegetables by causing excessive softening of tissues due to the degradation of middle lamella pectins and other cell wall polysaccharides. Total galacturonic acid and total sugars in cell walls of carrot tissue after blanching and freezing were analyzed colorimetrically. Methylated pectin and neutral sugars were determined by gas chromatography. Changes in location of pectins in the cell wall of carrots were determined by JIM 7 immunogold labeling. Faster freezing rates decreased the galacturonic acid and total sugar content in the alcohol-insoluble residue (AIR) of carrots less than slow freezing rates. Carrots that underwent high-temperature-short-time blanching (HTST-100°C/0.58 min) retained higher galacturonic acid and total sugars than those subjected to a low-temperature-long-time (LTLT-70°C/71.10 min) treatment. More pectin was found in the hydrochloric acid extract of the AIR than in the water extract. Freezing rates affected pectin and neutral sugar content of the hydrochloric acid and water fractions similarly to that observed for total pectin and total sugars in the AIR. Blanching decreased the degree of methylation of pectins more than did freezing, with the LTLT treatment having a greater effect than the HTST treatment. Based on JIM 7 labeling, more of the remaining methylated pectin was found in the straight sections of the middle lamella than in the three way junctions, and some separation of the three way junctions in processed carrots was observed. The suspension cultured cell model was used to study the distribution of pectins in the cell wall, but results were not as clear as those observed in the intact organ. These two observations indicate that texture deterioration of processed carrots initiates in the three way junctions between carrot cells rather than in the straight sections. The integrity and quality of carrots is best maintained using HTST blanching and fast freezing rates.