Effect of gum arabic-modified alginate on physicochemical properties, release kinetics, and storage stability of liquid-core hydrogel beads
Introduction
Much attention has been focused on hydrogel bead formed by food-grade biopolymers as a delivery system to protect and encapsulate some food ingredients, drugs, and bioactive compounds and/or control their release behavior (Gouin, 2004; Matalanis, Jones, & McClements, 2011).
Alginate is a natural linear biopolymer consisting of 1,4-linked β-d-mannuronic (M residues) and α-l-guluronic acids (G residues) and divided into homopolymeric blocks (G- and M-blocks) and heteropolymeric blocks (MG-blocks) (Pawar & Edgar, 2012). Normally, it is extracted from brown algae; however, in order to obtain a more flexible structure and better physical properties, alginate has been recently shown to be able to be produced by bacterial biosynthesis (Lee & Mooney, 2012). Alginate is widely used in the food, medical, and pharmaceutical industries due to its non-toxicity, relatively low cost, simple preparation, high compatibility, and biodegradability (Zeeb, Saberi, Weiss, & McClements, 2015). Alginate undergoes ionotropic gelation and forms egg-box dimers due to the interaction between G-blocks and some cations, such as Cu2+, Zn2+, Ca2+, Ba2+, and Al3+. Egg-box dimers further aggregate and compose egg-box multimers (Fang et al., 2007; Nayak, Das, & Maji, 2012). Ionotropically gelled alginate is a pH-sensitive polymer that shrinks in acidic conditions and swells in a high-pH environment (Wang, Zhang, & Wang, 2009). This characteristic makes alginate widely used for the delivery of proteins, drugs, and probiotics, protecting these compounds from destruction by stomach fluid (Cai et al., 2014; Mohy Eldin, Kamoun, Sofan, & Elbayomi, 2014).
Reverse spherification (RVS), one of the encapsulation methods, is used for preparing liquid-core hydrogel beads (LHB). The method is performed by dripping droplets that contain ions and bioactive compounds, into an ionotropic polymer solution. The most common materials are Ca2+ and alginate. RVS is divided into two steps, the first and secondary gelations (Tsai, Chuang, Kitamura, Kokawa, & Islam, 2017). The first gelation is a step of LHB formation, in which Ca2+ release from droplets forms a water-insoluble coating (calcium alginate). The thickness of the coating layer increases with time until the osmotic pressures are balanced. Next, these semifinished beads are transferred into a Ca2+ solution for secondary gelation, where Ca2+ permeates into the network of coating layer. Ca2+ fills into the G blocks that were not combined with Ca2+ in first gelation, and the stability and hardness of coating layer increases. Thus, secondary gelation is defined as a hardening step. In our previous study, the conditions of LHB formulation by response surface methodology (RSM) was optimized to investigate the effects of first and secondary gelation on different physical properties (Tsai, Kitamura, & Kokawa, 2017). Our studies demonstrated that LHB prevents the DPPH-scavenging ability of functional compounds from decreasing during storage (Tsai, Chiang, Kitamura, Kokawa, & Khalid, 2016) and examined the release profiles of functional compounds in simulated gastrointestinal fluid (in vitro) and during thermal treatments (Tsai, Chuang et al., 2017). These results indicated that alginate could be used as a potential delivery method; however, properties such as loading efficiency, hardness, and release characteristics in gastric fluid could be improved for more efficient delivery. Amine et al. (2014) indicated that ionotropically gelled alginate has a high permeability and entrapped compounds are released from alginate hydrogel beads rapidly due to their hydrophilic and porous structure.
Some studies have reported that improving physicochemical properties by adding other polymers as fillers, such as tapioca starch, chitosan and gum arabic (Chopra et al., 2015; Lozano-Vazquez et al., 2015; Mukhopadhyay, Chakraborty, Bhattacharya, Mishra, & Kundu, 2015). Gum arabic (GA), also known as gum acacia, is a highly branched natural polymer formulated from the tree sap of Acacia Senegal trees. The main chain of GA consists of β-d-galactopyranosyl units and side chains are formed of l-arabinose, l-rhamnose, d-galactose, and d-glucuronic acid (Chopra et al., 2015, Nayak et al., 2012). It is widely used as stabilizer, thickening agent, hydrocolloid emulsifier, and carrier in food, pharmaceutical, and cosmetic industries (Nami, Haghshenas, & Yari Khosroushahi, 2016).
Alginate and GA are both biodegradable and biocompatible polymers as well as generally regarded as safe (GRAS) by the United States Food and Drug Administration (USFDA). Fang et al. (2011) indicated that in the case of dry alginate beads, the addition of gum arabic reduced the side-by-side aggregation of the egg-box structure of the alginate. Side-by-side aggregation occurs when calcium alginate is dried. The egg-box junctions are drawn together due to the collapse of the alginate network, which results in further combining of the egg-box junctions by the presence of calcium ions. Side-by-side aggregation leads to a loss of the swelling capacity of calcium alginate. The combination of alginate and GA has attracted attention for the protection of probiotic bacteria and drugs during drying, storage, and in the gastric tract (Chopra et al., 2015, Nami et al., 2016, Nayak et al., 2012). However, to our knowledge, little or no information is currently available on the LHB prepared by alginate/GA matrix.
Preparing LHB by RSV has received increased attention in recent years. Materials which have high functionality and are suitable for RVS processing have been searched for. This work is the first paper to prepare LHB from alginate combined with GA by RVS. The objective of this work was to investigate a delivery of vegetable extract in an attempt to protect its functional compounds from being destroyed in gastrointestinal tracts and during storage. The first physical properties that were evaluated were the diameter, sphericity, and loading efficiency of the alginate/GA bead (AGB). The change of hardness, total phenolic compounds (TP) release behavior, and release kinetics in an in vitro system were also investigated. Finally, the stability of stored TP, including their antioxidant ability and degradation kinetics, were examined.
Section snippets
Materials
Radish (Raphanus sativus L.) is an important root vegetable crop worldwide because of its high nutritional and medicinal value. Furthermore, radish leaves have an abundance of minerals and the content of phenolic compounds and flavonoids in leaves are approximately 2.0-fold and 3.9-fold that of their content in roots, which are the parts which are normally consumed (Goyeneche et al., 2015). Radish leaves are seldomly consumed because of their bitter taste and strong flavor, despite containing
Characterization of AGB
In this section, the diameter, sphericity, and loading efficiency were used to evaluate the physicochemical properties of AGB (Table 2). The diameter of AGB ranged from 4.63 to 5.66 mm. There was no significant difference between AGB0 and AGB0.25 as well as between AGB0 and AGB0.5 (p < 0.05); however, AGB0.75 showed a relatively larger diameter. Our former study (Tsai et al., 2016) demonstrated that semifinished beads tended to shrink during secondary gelation because alginate in the outer layer
Conclusions
This study revealed that GA is a good material to improve the physicochemical properties of alginate hydrogel beads. All of the variations showed a small sphericity (lower than 0.05) and demonstrated that the deformation of AGB is not clearly visible by human eyes. Storage tests confirmed that the addition of GA can prevent TP from degradation, and the results were well fit by a first-order kinetic model, with R2 ranging from 0.957 to 0.988. GA can also maintain antioxidant activity during
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2022, Journal of Molecular LiquidsCitation Excerpt :Arabic gum is a natural amphiphilic polysaccharide that can also be utilized to form hydrogels [12,13]. However, these hydrogels are usually relatively soft and fragile, which limits their application [14,15]. We hypothesized that composite hydrogels with tunable properties could be created by using combinations of AG and okara cellulose to assemble them.