Physical and mechanical properties of alginate composite gels

Abstract

Alginate is one of the most widely used biopolymers in encapsulation systems for food and pharmaceutical substances due to its superior ability to create a gel matrix in the presence Ca2+ ions and biocompatibility. However, despite having all good characteristics for immobilization system, alginate has also several limitations related to the poor mechanical property, high porosity and syneresis occurring during the formation of the gel and when it is stored in an acid medium. Those drawbacks reduce its effectiveness for encapsulating water soluble materials.This study was conducted to circumvent those drawbacks through combining alginate with biopolymers from hydrocolloid, protein and starch groups. However, due to limitation of prior published data, this present research firstly systematically evaluated the gel strength and syneresis of alginate gels as a function of gelling time, CaCl2 solution concentrations (0.05 M - 0.20 M) and pH (4-11). The results showed that an increase in the CaCl2 concentration resulted in a faster gelling time, higher gel strength and higher syneresis degree. In contrast, as pH of the alginate solutions increased, the gel strength and the syneresis degree decreased. It was observed that in the presence of excess CaCl2, the syneresis degree gradually increased until a stable point (equilibrium) of syneresis was reached, regardless of CaCl2 concentration.Next, the effects of combining three hydrocolloids (low methoxy pectin, carboxymethyl cellulose (CMC), and guar gum) and five ratios of alginate-hydrocolloid (1.8:0.2, 1.6:0.4, 1.4:0.6, 1.2:0.8, 1:1) on the physical properties of alginate composite gels were investigated. Evaluations were conducted on the syneresis and swelling behaviour, hardness, and surface morphology of dried gels. The results showed that combining of hydrocolloids could reduce the syneresis degree of the alginate composite gels and increased their swelling degree compared to alginate gel. However, only pectin and CMC increased the hardness (by >90 % and 15%, respectively) of composite gels. A maximum hardness reached at the alginate-hydrocolloid ratio of 1.2 : 0.8 (alginate : pectin) and 1.6 : 0.4 (alginate : CMC).Next, the effects of milk protein concentrate (MPC85) (untreated, cold-renneted, and pre-heated (80 0C, 30 minutes)) addition on the properties of sodium alginate composite gels were evaluated. The results showed that the addition of both untreated and treated MPCs reduced the hardness of the gels. However, only cold-renneted MPC could reduce the syneresis degree of composite gels by 16.4 % after incubation in simulated gastric fluid (SGF) at pH 1.55. Moreover, only cold-renneted MPC produced freeze-dried gels with lower shrinkage and wrinkles on the surface of the gels. Therefore, it was implied that cold-rennetting of MPC could increase the effectiveness of MPC as a composite material for alginate gels.Next, three different amylose/amylopectin content of corn starch (0/100 (waxy), 23/77 (maize), 50/50 (Gelose50), and 80/20 (Gelose80)) in ungelatinized and gelatinized forms were used to change the syneresis and swelling behavior as well as the hardness of sodium alginate (2% w/w) composite gels. The results showed that the addition of gelatinized starches increased the hardness and decreased the syneresis degree of alginate gels. The higher the amylopectin content, the greater the effect of gelatinized starches on increasing the strength of the gel, and on reducing syneresis. Gelatinized waxy starches reduced the syneresis degree of alginate gel by 32.6%. Moreover, gelatinized-high amylopectin starch was able to control the shrinkage and to maintain the shape of the gels during lyophilisation. The results implied that the addition of gelatinized-high amylopectin starches could provide better mechanical properties to alginate gel.Further, the best substance from each above biopolymers (low methoxy pectin, cold-renneted milk protein concentrate (MPC), and gelatinized waxy starch) were used to encapsulate water-soluble material (green tea polyphenols). The results showed that the addition of pectin, MPC and waxy starch into alginate gels increased the particle size, increased the encapsulation efficiency (%EE) and reduced the release of polyphenol in water and simulated gastric fluid (SGF). Among the three biopolymers, cold-renneted MPC provided the best protection for polyphenols encapsulated in alginate microbeads. It increased %EE from 63% to 68%, reduced the release percentage in water from 72% to 62%, reduced the release percentage in SGF from 76% to 67%.Overall, this study provides a better understanding of the effects of combining hydrocolloid, protein and starch materials on the mechanical and physical properties of alginate gels. This work clearly indicates that combining hydrocolloids, proteins and starches in correct proportions to alginate gel can provide better protection, increase entrapment and prolong the release of the entrapped substance since the added biopolymers can influence the density, porosity and strength of alginate gels.