The potential of subcritical water as a “green” method for the extraction and modification of pectin: A critical review

Subcritical water is a "green" method of extraction and modification of pectin being explored in recent times. While the conventional acid extraction degrades the side chains and produces homogalacturonan (HG)-rich pectic polysaccharides, subcritical water extraction preserved the hairy region, namely the rhamnogalacturonan-I (RG-I) region of the pectin. However, higher temperatures (usually greater than 160 °C) degraded the RG-I and HG motifs, producing pectic oligosaccharides. A high selectivity towards pectic polysaccharides with a low protein content was observed during extraction by subcritical water. This can be majorly attributed to the heat-induced denaturation of proteins. Although the bioactive and emulsifying properties were more remarkable for subcritical water-extracted pectin, the rheological properties such as elasticity were negatively impacted. Apart from extraction, subcritical water can also be employed to aid the breakdown of pectic polysaccharides into oligosaccharides. The addition of several organic acids in subcritical water can help form pectic fragments, which are otherwise possible only by adding a cocktail of enzymes. For instance, carboxylic acids in subcritical water media can have a similar action to endo-polygalacturonase on the homogalacturonan backbone. It is worthwhile to note that intense extraction or modification conditions can form advanced glycation end products, which are undesirable and should be monitored throughout the modification process. Several thermodynamic and kinetic models can be employed to predict the breakdown of the pectin structure in subcritical conditions. Finally, this study suggests a strategy for obtaining the optimum process parameters, namely, temperature, duration, and the liquid:solid ratio for achieving maximum yield and the desired structure of the pectic polysaccharide.