The influence of extraction pH on the chemical compositions, macromolecular characteristics, and rheological properties of polysaccharide: The case of okra polysaccharide

Extraction pH plays an essential role in influencing the characteristics and functions of polysaccharides. However, the systematic study on the influence under a broad range and diverse gradient of pH is limited. In this study, polysaccharides from okra pods (Abelmoschus esculentus L.) were prepared using six extraction solutions with pH of 2.0, 4.0, 5.7, 8.0, 10.0, and 12.0. Physicochemical properties of the okra polysaccharides were then investigated. The respective results of the yield, protein content, total carbohydrate content, and degree of esterification of the polysaccharide samples were in the range of 7.68–20.15%, 1.38–3.79%, 67.86–88.77%, and 56.42–73.85% depending on the extraction pH. All polysaccharide samples, with a RG-I pectic structure subunit, were mainly composed of galactose, galacturonic acid, rhamnose, glucose, and arabinose. The molar ratio of the monosaccharides varied depending on the extraction pH and the side chain length of the RG-I backbone for acidic-extracted polysaccharides was larger than that for alkaline-extracted polysaccharides. In addition, the molecular weight increased with increasing extraction pH under acidic extraction conditions, whereas decreased when the pH rose to the alkaline range. Furthermore, rheological measurements revealed that all polysaccharide samples behaved as pseudoplastic fluids over a range of shear rates (0.1–100 s−1) and exhibited different viscosity and viscoelastic behaviors due to the variation in chemical compositions and macromolecular characteristics. The results indicate that the preparation of polysaccharides with specific characteristics and functions could be controlled by adjusting extraction pH.