Ultrasound-assisted extraction of carotenoids from citrus peel by olive oil and its application in functional emulsions

Citrus fruit is one of the most popular fruits in the world. The increasing production of citrus fruit globally leads to substantial amounts of peel waste generated during processing, which is often discarded and adds the burden on the environment. However, citrus peel is rich in phytochemicals and bioactive compounds, which can be used as an excellent source of natural additives and antioxidants. In the current study, olive oil was used to extract carotenoids from the peels of 11 citrus varieties under ultrasonic conditions. Among them, the content of carotenoids in the peel of Dahongpao red mandarin (DHPHJ) was the highest, which was 933 µg/g dry weight (DW). The optimum extraction process of carotenoids using olive oil with ultrasound-assisted involved extraction temperature of 55 °C, solid-oil ratio of 6:10, and extraction time of 70 min, resulting in the carotenoid yield of 157.07 µg/g. Stability tests and physicochemical indexes showed that the prepared carotenoid emulsion exhibited stability without water-oil separation and maintained a homogeneous state. Furthermore, the moisturizing properties of the emulsion indicated that carotenoid concentrations of 4 %, 6 %, and 8 % had 8-hour moisturizing properties, with the 8 % carotenoid emulsion keeping the highest moisture level. Additionally, antioxidant experiments showed that with the increase of carotenoid concentration in emulsion, its free radical scavenging ability was enhanced. Meanwhile, the antibacterial experiments demonstrated that the inhibitory effect on both Staphylococcus aureus and Escherichia coli was augmented with higher carotenoid concentrations in the emulsion. Specifically, 8 % carotenoid emulsion exhibited notable antioxidant and antibacterial ability. Finally, sensory evaluation results indicated that the carotenoid emulsion received a high acceptance score, comparable to commercially available moisturizing emulsion. Therefore, carotenoids extracted from citrus peel waste using olive oil can be used as natural additives for emulsion products and improve their efficacy. These findings are of great significance to the utilization of citrus by-products, as well as the sustainable development of cosmetics industry.