Ultrasound-assisted extraction of pomegranate peel phenolic compounds and investigation of bioaccessibility of pomegranate peel-propolis extracts mixture

This thesis aims to create added value by extracting phenolic compounds from pomegranate peel, a non-consumed waste product of the food industry, and to examine the bioaccessibility of propolis extracts, a valuable food supplement, and their mixtures. In the first part of the study, maceration and ultrasound-assisted extraction methods were tested for the extraction of phenolic compounds from pomegranate peel. In both methods, the independent variables were extraction time (hours for maceration, minutes for ultrasound-assisted extraction), extraction temperature (°C) and sample quantity/solvent volume (g/mL), while the dependent variables were total phenolic content (TPC) and total antioxidant capacity (TAC). Response surface methodology was applied for optimization in both methods. The optimum conditions for maceration were determined as follows: extraction time 3 h, extraction temperature 43°C and sample amount/solvent volume 1/30 g/mL. The responses obtained at these optimum conditions were 120.9 mg GAE/g dry weight (TPC), 60.6 mmol TE/kg dry weight (TACDPPH) and 1645.2 mmol TE/kg dry weight (TACCUPRAC). The optimum conditions for the ultrasound-assisted extraction method were determined as follows: extraction time 30 min, extraction temperature 41°C and sample amount/solvent volume 1/30 g/mL. The responses obtained at these these optimum conditions were 159.3 mg GAE/g dry weight (TPC), 64.3 mmol TE/kg dry weight (TACDPPH) and 4149.5 mmol TE/kg dry weight (TACCUPRAC). According to the results obtained from optimization studies, the ultrasound-assisted extraction method gave better results than maceration, especially in terms of duration and antioxidant capacity values. Therefore, in the second part of the study pomegranate peel extract obtained from ultrasound-assisted extraction method at optimum conditions was used in the in-vitro digestion application. TPC and TAC values obtained initial and during gastrointestinal digestion of 10% and 15% propolis extracts (70% v/v glycol) and 20% and 30% propolis extracts (70% v/v ethanol) samples were examined and 15% and 30% propolis extracts samples were selected to be used in mixtures according to the results. Pomegranate peel and propolis extracts were mixed in 1:1, 1:2 and 2:1 (v/v) ratios. TPC and TAC values of 10 samples, including pomegranate peel extract, 2 propolis (glycolic and ethanolic) extracts, 3 glycolic pomegranate-propolis extract mixtures, and 3 ethanolic pomegranate-propolis extract mixtures were determined initially and during in vitro digestion. The results were statistically evaluated using SPSS IBM-23. In general, it was determined that in most samples digested in-vitro, TPC and TAC values obtained after the stomach phase decreased, while both values increased after the intestinal phase. In the bioaccessibility study, it was observed that the use of pomegranate peel extract with propolis extract had a positive effect on bioaccessibility, and as the amount of pomegranate peel extract increased, the bioaccessibility of the mixtures increased.