POMEGRANATE PEEL EXTRACT ADDITION IMPROVES THE RHEOLOGICAL AND ANTIOXIDANT PROPERTIES OF CHITOSAN/GELATIN GELS FOR FOOD PACKAGING

Chitosan/gelatin is a versatile polyelectrolyte complex that can be used for several applications, including food coating. Chitosan is a biodegradable, biocompatible and non-toxic polymer with ability to form gels. When associated with gelatin, a protein obtained from collagen denaturation, the formed mixture can incorporate bioactive compounds. Pomegranate (Punica granatum L.) is a highly nutritious fruit and a rich source of bioactive compounds with antioxidant capacity (phenolics, flavonoids and anthocyanidins) even in the inedible parts, including peel and seeds. The aim of this study was to evaluate the effects of pomegranate peel extract addition on the rheological and antioxidant properties of chitosan/gelatin gels. The peel use in value added applications also minimizes the agricultural waste associated with pomegranatebased processing. Chitosan (6.7% degree of acetylation, MW = 3.3x105 Da) was extracted from squid pens and dissolved in 1% acetic acid to obtain a 1% chitosan solution. A 1% gelatin (Sigma-Aldrich®) solution was prepared by gelatinization at 60ºC for 30 min. Pomegranate peels were macerated and during the extraction process their powder was placed in a hydroethanolic solution (60%, v/v). The range of extract solutions to prepare the gels was 50 to 200 mg mL-1. Total phenolic content (TPC) of pomegranate peel extract was determined by the Folin-Ciocalteu method and expressed as gallic acid equivalent (GAE). The ABTS radical method (expressed as Trolox equivalent) was used to determine the antioxidant capacity of the extract and gels. Rheological properties were investigated using a stress-controlled rheometer (AR – 1000N – TA Instruments), with a stainless-steel cone/plate geometry of 20 mm of diameter, angle of 2º and a gap of 69 µm. To determine the viscoelastic linear region of the gels, a stress sweep was performed between 0.05 and 500 Pa. The main parameter studied was the critical strain, defined as the higher deformation value that can be applied to the system before the viscoelastic linear region ends. The critical strain increased linearly from 25% to 78% in the gel containing 200 mg of pomegranate peel extract. Consequently, viscoelastic linear region of the gels became more extensive, what implies that pomegranate peel extract made the mixture more resistant to the deformation applied. Regarding the TPC, it was found the value of 492 mg GAE/g extract, which decreased about 2.5 times after its incorporation into the system. This may be related to the chemical interaction between phenolic hydroxyls from extract with amine, amide and hydroxyls groups from chitosan; amines and carboxyl groups from gelatin. Despite the lower TPC value, the antioxidant capacity of the mixture increased 183% compared to the chitosan gel without the pomegranate peel extract (760 µmol and 268 µmol Trolox equivalent/g gel, respectively). The results clearly show extract addition changed the rheological properties of the gels, by making them more resistant to the deformation applied, and improved their antioxidant capacity. This study opens new perspectives for pomegranate processing by-products in innovative bio-based products with several applications in food sector such as coating protection against oxidation.