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Abstract

In this study, the effects of calcium chloride (CaCl2) addition on the physical and oxidative stabilities of filled hydrogel were investigated. The results revealed that CaCl2 significantly enhanced the particle size, interfacial layer thickness, apparent viscosity, and viscoelastic behavior of filled hydrogels and decreased their light and whiteness values (p < 0.05). This phenomenon was mainly attributed to the strong binding ability between Ca2+ and protein/pectin mixtures, which were present in the interfacial area or aqueous phase, as verified by cryo-scanning electron microscopy results. Moreover, lower levels of CaCl2 (2 or 4 mM) significantly enhanced the oxidative stability of filled hydrogels (p < 0.05), particularly at a concentration of 4 mM. However, a higher level of CaCl2 (6 or 8 mM) resulted in an electrostatic shielding effect, which resulted in the aggregation of multiple droplets and the flocculation of the filled hydrogels, which negatively affected the oxidative stability of filled hydrogels. The findings of this study indicated that appropriate Ca2+ levels (4 mM) improved the physical and oxidative stability of filled hydrogel, and this finding may provide useful insights for the development of effective delivery systems for specific applications.

2.4. Fabrication of the Filled Hydrogel

Filled hydrogels were prepared based on the method by Matalanis and McClements [] with some modifications, which are shown in . Briefly, the as-prepared O/W emulsion, the lower phase, and upper phase were mixed at a mass ratio of 1:1:18, after which the mixture was stirred at a speed of 10,000 rpm for 45 min, while maintaining a pH of 5.0. CaCl2 solution (50 mM, pH 5.0) was added to as-prepared sample to achieve final Ca2+ levels of 0, 2, 4, 6, and 8 mM in the final systems. Lastly, sodium azide (0.02%, w/w) was added to the filled hydrogels to inhibit microbial growth.

2.5. Particle Size and Zeta-Potential

The particle size and surface charge of freshly prepared and 10-days-stored filled hydrogels were analyzed by using a Nano ZS dynamic light scattering instrument (Malvern instruments Ltd., Worcestershire, UK) based on the method proposed by Hinderink, Schröder, Sagis, and Berton-Carabin [].