橙柚果汁中水溶性果胶结构差异及其对浊度稳定性的影响机制研究

本研究系统比较了非浓缩还原（NFC）橙汁与柚汁中水溶性果胶（WSP）的理化特性，发现橙汁WSP具有更高分支化RG-I结构及聚集倾向，其较强的界面活性反而导致浊度稳定性降低。成果为精准调控果汁品质提供了新视角。

　　

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WSP compositions

Table 1 shows the chemical compositions of the WSPs extracted from orange and pomelo juices. The total WSP concentrations in the orange and pomelo juices were 0.73 ± 0.0 and 0.43 ± 0.1 mg/mL, respectively. Thus, orange juice contained a significantly higher amount of WSP compared to pomelo juice (p < 0.05). As shown in Table 1, the neutral sugar and uronic acid contents of WSP isolated from orange juice were 29.3 ± 0.3 and 34.5 ± 1.1 wt%, respectively, while those of WSP isolated from pomelo juice were 28.1 ± 0.3 and 36.1 ± 0.8 wt%, respectively. The protein content in O-WSP (3.8 ± 0.2 wt%) was significantly higher than that in P-WSP (2.1 ± 0.1 wt%). Similarly, the total phenolic content in O-WSP (1.9 ± 0.1 mg GAE/g) was significantly higher than that in P-WSP (1.2 ± 0.1 mg GAE/g). The monosaccharide composition analysis revealed that both WSPs were primarily composed of galacturonic acid (GalA), galactose (Gal), arabinose (Ara), and rhamnose (Rha). However, the molar ratios of Rha/GalA and (Ara+Gal)/Rha in O-WSP were significantly higher than those in P-WSP, suggesting that O-WSP possessed more abundant and highly branched rhamnogalacturonan I (RG-I) regions.

Molecular properties of WSP

The weight-average molecular weight (M_w) of O-WSP was determined to be 678 kDa, which was significantly higher than that of P-WSP (385 kDa). The radius of gyration (R_g) values for O-WSP and P-WSP were 42.1 nm and 31.5 nm, respectively. The higher R_g value of O-WSP indicated a larger molecular size in solution. The conformational parameter (ρ = R_g/R_h) for O-WSP was 1.52, suggesting a branched chain conformation, while the ρ value for P-WSP was 1.21, indicating a relatively compact spherical structure. Atomic force microscopy (AFM) images visually confirmed that O-WSP molecules exhibited a more aggregated and network-like structure, whereas P-WSP molecules appeared as individual spherical chains. Small-angle X-ray scattering (SAXS) analysis further supported these findings, showing that O-WSP had a higher fractal dimension, indicative of a more compact and aggregated structure in solution.

Interfacial properties of WSP

The interfacial tension at the oil-water interface was measured to evaluate the surface activity of the WSPs. O-WSP demonstrated a stronger ability to reduce the interfacial tension compared to P-WSP. The critical aggregation concentration (CAC) of O-WSP was 0.15 mg/mL, which was lower than that of P-WSP (0.25 mg/mL). This lower CAC value indicated that O-WSP molecules began to aggregate and form micelle-like structures at a lower concentration, consistent with its higher tendency for self-association observed in the structural analyses.

Cloud stability of WSP solutions and NFC juices

The stability of WSP solutions and NFC juices was assessed by monitoring turbidity loss and sediment formation over time. The O-WSP solution showed a faster rate of turbidity decrease and formed more sediment compared to the P-WSP solution. This trend directly correlated with the cloud stability of the corresponding NFC juices. NFC orange juice, containing O-WSP, exhibited poorer cloud stability than NFC pomelo juice containing P-WSP. The larger aggregate size and higher surface activity of O-WSP were identified as key factors promoting particle flocculation and sedimentation, leading to cloud loss.

Conclusion

In this study, we extracted water-soluble pectin from fresh orange and pomelo juices and then characterized their compositional and structural properties. This analysis showed that O-WSP contained more protein, phenolic substances, and branches (RG-I regions) than P-WSP. These differences in composition and structure led to differences in the tendency for the pectin molecules to associate with each other and form clouds. O-WSP was more surface active and prone to aggregation than P-WSP, which ultimately resulted in reduced cloud stability in NFC orange juice. These findings provide new insights into the relationship between pectin structure and juice quality, which could be used to develop strategies for improving the shelf-life of citrus juices.