Microgel particle formation: Influence of mechanical properties of pectin-based gels on microgel particle size distribution

Microgel particle formation usually requires a dispersing/emulsifiying step of macrogel into pieces, or of direct/gelled droplets. Either liquid or solid-like, yields and fails under given critical conditions. In this work we studied the influence of a gel's mechanical properties on the resulting particle/droplet sizes. As the structure of the polymer influences its mechanical properties, the viscoelastic and mechanical response of pectin gels towards shear, compressive and tensile stress were analysed. Gels were prepared with different pectin types (amidated pectin and citrus pectin) and pectinic acid, at a constant stoichiometric molar ratio R= 2 x [Ca2+]/[COO-] = 1. A dependency of storage modulus G', Young's modulus E, and critical breakup stresses and strains on the pectin type used for gelation was shown. Values of the storage modulus G' were influenced by the degree of methyl-esterification (DM), indicating an increase on the cross-linking density of gels. Gels prepared with pectinic acid (DM = 2) showed overall the highest values of G'. Values of the Young's modulus E as well as fracture stresses and strains depended on the type of strain applied to the gel and the pectin type. Concordant with the G' values, gels become stiffer and brittle with increasing DM, resulting in higher E values, lower fracture strains and higher fracture stresses. In addition, under compressive stress, gels investigated sustained lower stress values before structural failure. The findings of mechanical and rheological analyses were compared with the particle size distribution of microgels produced from the prepared gels at constant process conditions. Gels with high values for G' and E, and low fracture stresses and strains were shown to be dense cross-linked gels of brittle material characteristics which resulted in smaller particles compared to more elastic gels.