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.
Highlights
Low cost, biocompatibility, biodegradability, and poor antigenicity make it suitable for a wide number of applications in food as well as biomedical and pharmaceutical fields.[3−5] Gelatin is soluble in water at temperatures above 30 °C, and a thermoreversible physical gel can be obtained by cooling gelatin aqueous solutions,[6−9] as they undergo a sol−gel transition upon cooling, whose characteristics depend on various parameters.[10−13] Gelation of gelatin aqueous solutions has been widely studied in the literature.[14,15]
The viscoelastic moduli are reported as a function of temperature for both solutions (Figure 2a, gelatin; Figure 2b, gelatin−humic acids (HAs) 16)
Bare gelatin exhibits a smooth surface (Figure 10a,d), whereas the introduction of HA determines a significant change in gelatin structure, producing a rougher surface (Figure 10b,c,e,f) due to the presence of submicrometric particle aggregates. These results suggest that the interaction between gelatin and HAs induces a partial coagulation of the protein, preventing the formation of the ordered structure.[40,62]
Summary
Gelatin properties can be improved by adding cosolutes, which interact through either chemical or physical junctions in a way to increase, on the one side, rheological and mechanical strength, or to confer, on the other side, peculiar properties, such as water resistance or thermal stability.[19−21]. Despite the great efficacy of the commonly used aldehyde cross-linkers (formaldehyde and glutaraldehyde), their toxicity poses health and safety issues and strongly limits their application, in the biomedical field and food industry.[3,4] there is a growing interest to find more sustainable and safe cross-linking choices based on natural moieties.[22] Among these, polyphenols are known to interact with proteins, through physical and chemical conjugation, providing higher thermal stability as well as antioxidant features.[23] For such reasons, polyphenols have been explored as additives for gelatin, yet with contrasting results. Gelatin benefits from their addition, exhibiting improved mechanical properties as well as higher thermal stability.[22,24] In other cases, polyphenol addition results in lower tensile strength of gelatin films.[24]
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