Characteristics of the bulk hydrogels made of the citric acid cross-linked whey protein microgels

A non-toxic, biocompatible and generally recognized as safe (GRAS) chemical, citric acid, was employed for pH adjustment and cross-linking of whey proteins either before or after their microgelification by heat. A conventionally-made microgel sample was also fabricated by using hydrochloric acid for pH adjustment on 5.8 and subsequent heating at 85°C. Size approximation of microgel particles from atomic force microscopy images indicated that chemical cross-linking resulted in smaller particles (80-130nm and 100-150nm for post- and pre-microgelification cross-linked samples, respectively) compared with conventional counterpart (150-300nm). Based on Fourier transform infrared spectroscopy results, it was concluded that prolonged citric acid cross-linking of whey proteins prior to microgelification caused extensive cross-linking of protein units and preserved proteins α-helical structure. The pre-cross-linked microgels formed firmer (higher fracture stress and complex modulus) calcium-induced cold-set bulk gel with higher water-holding capacity and denser microstructure compared with conventional and post-cross-linked microgels. These superiorities were attributed to existence of higher number of carboxyl residues in the structure of the pre-cross-linked microgels. The alteration in the properties of the final bulk hydrogels via cross-linking of the microgel building-units can be exploited for modulating and controlling the release behavior of loaded cargo within hydrogel, resulting in a more tuned application of gels as edible delivery systems.