Development and characterization of pH-sensing films based on red pitaya (Hylocereus polyrhizus) peel treated by high-pressure homogenization for monitoring fish freshness

High pressure homogenization (HPH) was used to physically modify red pitaya (Hylocereus polyrhizus) peel (RPP) dispersions to develop pH-sensing films for monitoring fish freshness. Effects of HPH treatment at various pressures of 0 (control), 10, 20, 30, 40, 60 and 80 MPa on microstructure and properties of RPP film- forming dispersions (FFDs) and their resultant films were systematically investigated. Results showed that due to the structure destructing effect of HPH, the mean particle size of RPP dispersion treated at 80 MPa decreased significantly from 129.17 to 37.41 μm and its total soluble solids content increased from 7.64 to 8.62 °Bx. Rheological analysis manifested that all FFDs exhibited a shear thinning behavior with higher apparent viscosities as well as storage modulus and loss modulus with increasing HPH pressure. Besides, the HPH treated FFDs had a higher total betacyanins content and showed an obvious color response to pH variation. Consequently, the resultant films had not only smoother surfaces and more transparent appearance, but also better water vapor barrier and mechanical properties than the control. Particularly, film obtained from RPP treated at 60 MPa showed a highest tensile strength of 21.87 MPa and elongation at break of 22.09%, together with a lowest water vapor permeability of 11.41 g⋅mm⋅m-2⋅d-1⋅kPa-1. Notably, this film also exhibited high sensitivity to volatile ammonia and showed a remarkable color variation when applied for monitoring fish spoilage at 25°C. Thus, the pH-sensing films developed from HPH treated RPP had desirable properties to be potentially used in intelligent food packaging.