Adding Humic Acids to Gelatin Hydrogels: A Way to Tune Gelation

Abstract

Exploring the chance to convert biowaste into a valuable resource, this study tests the potential role of humic acids (HA), a class of multifunctional compounds obtained by oxidative decomposition of biomass, as physical agents to improve gelatin’s mechanical and thermal properties. To this purpose, gelatin–HA aqueous samples were prepared at increasing HA content. HA/gelatin concentrations changed in the range 2.67–26.67 (wt/wt)%. Multiple techniques were employed to assess the influence of HA content on the gel properties and to unveil the underlying mechanisms. HAs increased gel strength up to a concentration of 13.33 (wt/wt)% and led to a weaker gel at higher concentrations. FT-IR and DSC results proved that HAs can establish noncovalent interactions through H-bonding with gelatin. Coagulation phenomena occur because of HA–gelatin interactions, and at concentrations greater than 13.33 (wt/wt)%, HAs established preferential bonds with water molecules, preventing them from coordinating with gelatin chains. These features were accompanied by a change in the secondary structure of gelatin, which lost the triple helix structure and exhibited an increase in the random coil conformation. Besides, higher HA weight content caused swelling phenomena due to HA water absorption, contributing to a weaker gel. The current findings may be useful to enable a better control of gelatin structures modified with composted biowaste, extending their exploitation for a large set of technological applications.

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“…On the other hand, the FTIR spectra of HScontaining samples show a slight increase in the absorption bands in the region between 2900−3500 and 1500−1900 cm −1 , which could be better appreciated at higher HS contents (Gel HS 8 and Gel HS 16 samples) and should be straightly related to the presence of HSs, whose main featuring bands fall in the same wavenumber range. 49 The spectra of gels treated with EDC are similar to those of neat gelatin (Figure 5), thus suggesting that cross-linking with EDC/NHS did not provoke any marked change in the molecular structure, in accordance with previous studies. 55 At a closer look, the spectra of gelatin films modified with EDC/NHS evidenced a slight increase in the intensity of the amide A, amide I, amide II, and amide III band regions.…”

“…As a result of HS ability to absorb water, its addition resulted in an increase in swelling ratio. Indeed, HS exhibits a distinct hydrophilic behavior, as shown by the relevant swelling phenomena in water, and demonstrated in our previous work, and this feature could improve the water sorption and swelling behavior of gelatin . On the other hand, a non-monotonic trend was observed for the swelling ratio of chemical gels (Figure B).…”

“…This evidence suggests that HSs affect the protein secondary structure, preventing gelatin chains from organizing into triple-helix domains and causing their assembly according to a more disordered organization. As for similar systems reported in the literature, this effect could be ascribed to the high hydrophilic nature of HS moieties, which could preferentially attract the available water molecules and subtract them from H-bond bridging interactions required to form a triple-helix structure . In addition, more marked changes can be appreciated in the XRD patterns of Gel HS EDC gels.…”

“…It is possible that, as published elsewhere on a similar system, 49 at higher concentrations, HS established preferential bonds with water molecules, hindering the triplehelix domains, thus requiring a slower kinetics for their growth. 49 Figure 2 displays the dynamic frequency sweep response at 30 °C for gelatin samples with chemical cross-linkers at various HS concentrations. Figure 2 shows the linear viscoelastic behaviors for each sample.…”

“…Similar results have been previously achieved on the gelatin/HS system in the absence of chemical cross-linkers. 49 Physicochemical Investigation. Assessment of Carboxylic Group Content.…”