Exploring hydrodynamic cavitation for citrus waste valorisation in Malta: from beverage enhancement to potato sprouting suppression and water remediation

Abstract

Introduction: The endorsement of circular economy, zero-waste, and sustainable development by the EU and UN has promoted non-thermal technologies in agro-food and health industries. While northern European countries rapidly integrate these technologies, their implementation in Mediterranean food-supply chains remains uncertain. Aims: We evaluated the usefulness of hydrodynamic cavitation (HC) for valorizing orange peel waste in the fresh orange juice supply chain of the Maltese Islands. Method: We assessed: a) the effectiveness of HC in extracting bioactive compounds from orange peels (Citrus sinensis) in water (35°C) and 70% (v/v) ethanol (-10°C) over time, compared to conventional maceration, and b) the potato sprouting-suppression and biosorbent potential of the processed peel for copper, nitrate, and nitrite binding. Results: Prolonged HC-assisted extractions in water (high cavitation numbers), damaged and/or oxidized bioactive compounds, with flavonoids and ascorbic acid being more sensitive, whereas cold ethanolic extractions preserved the compounds involved in radical scavenging. HC-processing adequately modified the peel, enabling its use as a potato suppressant and biosorbent for copper, nitrate, and nitrite. Conclusion: Coupling HC-assisted bioactive compound extractions with using leftover peel for potato-sprouting prevention and as biosorbent for water pollutant removal offers a straightforward approach to promoting circular economic practices and sustainable agriculture in Malta.

Keywords: citrus waste; green solvents; heavy metals; hydrodynamic cavitation; nitrates; nitrites; potato sprouting; water remediation.

FIGURE 1

Orange peel waste extract contents and antioxidant activities. Top-to-Bottom: total reducing sugars, total flavonoids, total phenolics, DPPH, ABTS^•+, and H₂O₂ scavenging. HC-assisted extractions were conducted in water (35°C, left column) and 70% (v/v) ethanol (−10°C, right column). White histograms; maceration in 70% (v/v) ethanol, blue histograms; HC at 50% rotational frequency, orange histograms; HC at 75% rotational frequency, and magenta histograms; HC at 100% rotational frequency. pH changes with time for the corresponding treatments are indicated. Error bars denote the STDEV of triplicate measurements.

FIGURE 2

Non-liner Langmuir isotherm following adsorption of Cu²⁺ to untreated and HC-treated (water, at 75% rotational frequency, for 2 h) peel. q_e and c_e, refer to the adsorption capacity of the peel and the concentration of ions used at equilibrium, respectively (A). Non-liner Langmuir isotherm following adsorption of nitrates to water-macerated and HC-treated (water, at the specified rotational frequencies, for 2 h) peel (B). Non-liner Langmuir isotherm following adsorption of nitrites to water-macerated and HC-treated (water, at the specified rotational frequencies, for 2 h) peel (C). Error bars denote the STDEV of triplicate measurements.