Optimization of free and bound phenolic compound extraction from onion skin, investigation of effect of model foods on phenolic compound bioavailability In Vitro gastrointestinal system

This study aimed to optimize the extraction of free and bound phenolic compounds from onion skin using classical and ultrasound-assisted extraction (CE and UAE) techniques. Additionally, the study focused on determining the phytochemical and antimicrobial activities as well as the bioavailability of onion skin phenolic extracts (OSPEs) in in vitro gastrointestinal systems using model systems prepared with various food items (sunflower oil, cream, milk, lactose-free milk, apple vinegar, mashed potatoes, honey) and food components (oleic acid, sucrose, casein). Furthermore, it aimed to evaluate the production of bread and kefir supplemented with OSPEs, as well as the in vitro bioavailability and various properties of these products during refrigerated storage (+4 °C) on days 1, 7, 14, and 21. For free phenolic compound extraction from onion skin via CE, total phenolic content (TPC) and total flavonoid content (TFC) were found to be 75.10 and 59.23 mg GAE/g dry sample, respectively, under conditions of 47.94% EtOH concentration, 149.88 min., and 64.37 °C. For bound phenolic compounds, the values were 26.06 and 17.34 mg QE/g dry sample, respectively, under conditions of 4.98 M NaOH concentration, 119.34 min., and 75.80 °C. Using UAE, TPC and TFC for free phenolic compound extraction were determined as 78.25 and 69.70 mg GAE/g dry sample, respectively, under conditions of 45.33% EtOH concentration, 86.42 min., and 63.84 °C. For bound phenolic compounds, the values were 20.20 and 15.16 mg QE/g dry sample, respectively, under conditions of 4.83 M NaOH concentration, 87.32 min., and 61.58 °C. It was found that approximately 26% of the TPC in onion skin consists of bound phenolic compounds. The ABTS (16.36–77.18 mg TE/g dry sample), DPPH (5.22–29.11 mg TE/g dry sample), and FRAP (26.92–82.72 mg TE/g dry sample) activities of the extracts obtained under optimal conditions were determined. Additionally, the quercetin (1.49–9.61 mg/g dry sample), quercetin 3-β-D glucoside (5.29–6.57 mg/g dry sample), and protocatechuic acid (3.18–3.87 mg/g dry sample) contents of the extracts were quantified. The minimum inhibitory concentration (MIC) values of the extracts against pathogenic and starter microorganisms ranged from 1.563 to 6.25 mg/mL and 3.125 to 12.5 mg/mL, respectively. An increase in phenolic content and antioxidant activity was observed with higher OSPE supplementation levels in bread and kefir. However, increased OSPE levels led to a reduction in bread volume and total volume index, while crust thickness and hardness increased. For kefir, an increase in pH was observed with higher OSPE levels. In the serum dialyzable (IN) phase evaluation, an enhancing effect on TPC and TFC was detected in samples with milk, cream, and sucrose. For FRAP, an enhancing effect was observed in combinations with milk, lactose-free milk, cream, sunflower oil, honey and mashed potatoes. However, no enhancing effect was detected for ABTS in any combination. Among individual phenolic compounds, an enhancing effect for protocatechuic acid was observed in combinations with milk, lactose-free milk, cream, sunflower oil, honey, vinegar, and oleic acid, whereas no enhancing effect was detected for quercetin and quercetin 3-β-D glucoside in any combination.