Pressurized hot water crosslinking of gelatin-alginate for the enhancement of spent coffee oil emulsion stability

In this study, subcritical water was used to crosslink gelatin and alginate (G/A) at different temperatures (120 °C, 130 °C, 140 °C, 150 °C, and 160 °C) under constant pressure and reaction time (3 MPa, 30 min). Structural and morphological characterization was performed using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (¹H NMR), and scanning electron microscopy (SEM). The coffee oil-filled hydrogels were prepared using G/A blend by ultrasound sonication. The emulsions revealed a unimodal distribution of particles with varying diameters (654 ± 98) nm, (509 ± 44) nm, and (412 ± 68) nm for G/A at 120 °C, 130 °C and 140 °C, respectively. However, the treatment of G/A at 150 °C and 160 °C resulted in bimodal size distribution and declined viscosity, and indicated the molecular degradation of hydrogels leading to weak repulsion force against droplet aggregation. Highly stable emulsions were observed from G/A at (120–140) °C, while at 150 °C and 160 °C, the emulsion exhibited poor stability during 7 weeks of storage at 5 °C and 25 °C.