Optimized Extraction of Polyphenols from Kiwifruit Peels and Their Biological Activities

(1) Background: Kiwifruit is a globally valued fruit. Its industrial processing produces a substantial amount of waste, particularly peels, which present an appealing potential source of bioactive compounds. This study focuses on optimizing the extraction of phenolics from kiwi peels using a water bath (WB) and infrared irradiation (IR) and assessing their biological activities. (2) Methods: Optimal conditions for polyphenol extraction from kiwifruit peels, in terms of temperature and time, were determined using Response Surface Methodology. Total phenolic content (TPC) was measured by the Folin-Ciocalteu method, and antioxidant activity was assessed utilizing the DPPH method. Antibacterial activities against Bacillus cereus, Staphylococcus aureus, Escherichia coli, and Salmonella Typhimurium were determined using the minimum inhibitory concentration (MIC). The lyophilized extract was tested for its anticancer effect on A549 lung cancer cell lines. The phytochemical profiles of the WB and IR extracts were analyzed through High-Performance Liquid Chromatography (HPLC). (3) Results: The optimal extraction conditions for the WB method were found to be 75 °C for 35 min, and 90 °C for 5 min for IR. The corresponding TPC obtained by IR was 21 mg GAE/g DM, which was 4.4 times higher than that obtained by WB (4.8 mg GAE/g DM). This indicates that IR was more efficient in extracting phenolics from kiwi peels. The antioxidant activity under the optimum conditions of WB and IR was 14 mg TE/g DM and 16 mg TE/g DM, respectively. Both the WB and IR extracts demonstrated antibacterial activity against B. cereus with an MIC value of 25 mg/mL. Additionally, the IR extract displayed an antibacterial effect against S. aureus, with an MIC value of 50 mg/mL. The WB and IR kiwi peel extracts were effective in significantly reducing A549 cell viability at 50 µg/mL and 100 µg/mL, respectively. Caffeic acid (0.54 ppm) and catechin (5.44 ppm) were the major polyphenols identified in WB and IR extracts, as determined by HPLC. (4) Conclusions: IR was more efficient in extracting phenolics from kiwi peels than WB. The findings also suggest that kiwi peel can be effectively utilized as an antioxidant, antibacterial, and anticancer agent.