Okra polysaccharides reduced the gelling-required sucrose content in its synergistic gel with high-methoxyl pectin by microphase separation effect
2019
Li, Xiao | Dong, Yi | Guo, Yan | Zhang, Zihan | Jia, Lirong | Gao, Hong | Xing, Zhihua | Duan, Feixia
In this work, we extracted, purified and structurally analysed okra polysaccharides (OPs), which were found to contain mainly rhamnogalacturonan I with long galactan side branches. The results of the line spread test and rheological measurement confirmed that although the high-methoxyl pectin (HMP) solution remained in a liquid state with less than 55% sucrose, HMP and OPs synergistically formed stable gels with only 40% sucrose at ratios from 9:1 to 2:8 (total polysaccharide concentration 2.0 wt%) at pH 2.0–3.5. Similar to the HMP gel, the HMP/OPs blend gels were spreadable, shear thinning and heat-irreversible but exhibited no obvious thermal annealing behaviour and higher strain capacity. The critical strain, fracture strain, storage modulus G′ and complex viscosity η* of the HMP/OPs blending (ratio 8:2, 40% sucrose, 0.1 rad/s, 20 °C) were 7.56%, 56.6%, 177.49 Pa and 1846.25 Pa s, respectively, while those of the HMP sample (55% sucrose) were 1.79%, 42.5%, 565.81 Pa and 5982.5 Pa s. The blend gels also showed smoother gel surfaces with lower porosity in the FESEM observations. The CLSM observation revealed that the uniformly distributed OPs molecules and locally aggregated HMP chain blocks formed a ratio-dependent microphase separation network in blend gels. The FT-IR spectra showed the decrease of sucrose free hydroxyl in the blend gel, indicating an enhancement of sucrose hydration. The sucrose hydration enhancement and strong water-binding capacity of galactan side branches in OPs molecules could compensate for the insufficient quantity of sucrose, which could promote HMP chain-chain connection and form microphase separation structures in blend gels.
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