Effect of high pressure/high temperature processing on cell wall pectic substances in relation to firmness of carrot tissue

Thermal processing for food preservation results in undesired softening of fruits and vegetables. To explore the potential of high pressure sterilization in food processing, the effects of combined high pressure/high temperature (HP/HT) treatments on carrot pectic polysaccharides and the related textural properties were investigated and compared with that of samples thermally processed at atmospheric pressure. Disks of fresh carrot (Daucus carota var. Yukon) tissue were subjected to three different treatments (80 °C–0.1 MPa, 100 °C–0.1 MPa and 80 °C–600 MPa) for varying time intervals. Subsequently, the residual texture and microstructural changes of the carrots were evaluated. Alcohol-insoluble residues were prepared from the samples and sequentially fractionated with water, cyclohexane-trans-1,2-diamine tetra-acetic acid (CDTA) and Na₂CO₃ solutions. Thermal treatments at 0.1 MPa caused extensive tissue softening. This was marked by increased cell separation, an increase in water soluble pectin (WSP) paralleled by a decrease in chelator (CSP) and sodium carbonate (NSP) soluble pectin. HP/HT treated carrots showed minimal softening and negligible changes in intercellular adhesion. This was accompanied by a significant reduction in the degree of methyl esterification of pectin, low WSP in contrast to the high CSP and NSP fractions, minor changes in the different pectin fractions during treatment, and a substantial amount of pectin in the fractionation residue. There was a clear difference between HP/HT and thermally processed carrot pectin; HP/HT showing pronounced texture preservation.