Sodium alginate (NaAlg) isolated from brown algae has been recommended as a dietary fiber, but its inhibitory effect on the calcium absorption is still a concern for its application. To resolve this problem, the present study prepared calcium alginate (CaAlg) gels with varying Ca contents, and compared their structural characteristics and effects on Ca release during digestion. As characterized by rheometer, low-field nuclear magnetic resonance (LF-NMR) and cold-field scanning electron microscope (cryo-SEM), the Ca addition enhanced the viscoelastic properties, water-holding capability and stabilized the network structure. Fourier transform infrared spectroscopy (FT-IR) demonstrated that Ca bonded electrostatically with the carboxylate groups (-COO-) and formed non-covalent crosslinks with the hydroxyl groups (-OH) in guluronic acid of NaAlg. Moreover, NaAlg without Ca reduced 21 % and 25 % of the exogenous Ca level during simulated gastric and intestinal digestion, respectively, while the addition of Ca ≥ 1.08 mg/mL in NaAlg could reverse this effect. Notably, CaAlg gel prepared with Ca content of 1.62 mg/mL could release 0.54 mg/mL and 0.57 mg/mL Ca during simulated gastric and intestinal digestion, respectively. Further FT-IR and LF-NMR analysis revealed the protonation of carboxyl group of CaAlg gels in the gastric environment, which led to gel shrink and released Ca into the gastric fluid. Then in the subsequently intestinal fluid, the CaAlg gels underwent structural disintegration, further releasing Ca from the gel network structure. Therefore, increasing the Ca content in CaAlg gel is an effective strategy to counteract the inhibitory effects of NaAlg on calcium absorption, and the alginate gel with higher Ca content ≥1.08 mg/mL is potential for applications in the food industry.
Keywords: Alginate gel; Calcium alginate; High calcium gel; In vitro digestion; Sodium alginate.
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