Abstract
Simple SummaryExhausted olive pomace (EOP) is the main residue of the pomace oil extraction industry, which is generated in large quantities and has limited applications. Thus, this study aimed to obtain bioactive compounds from EOP using ultrasound-assisted extraction as a potential first valorization step. Two types of devices were tested: bath- and probe-type UAE. The operational parameters were studied and optimized to maximize the antioxidant compounds. In particular, hydroxytyrosol was the main phenolic compound identified and its content was 5.16 mg/g EOP (bath-type UAE) and 4.96 mg/g EOP (probe-type UAE). Mannitol was also detected in the extract, 59.53 mg/g EOP (bath-type UAE) and 69.73 mg/g EOP (probe-type UAE). The results highlight the great potential EOP has as a source of bioactive compounds, with applicability in several sectors. Moreover, the probe-type UAE shows potential to be applied for obtaining these bioactive compounds in a continuous and faster manner.Exhausted olive pomace (EOP) is the main agro-industrial waste of the olive pomace extracting industries. It contains phenolic compounds and mannitol, so the extraction of these bioactive compounds should be considered as a first valorization step, especially if EOP is used as biofuel. Therefore, EOP was subjected to bath-type ultrasound-assisted extraction (UAE), and the effects of the acetone concentration (20–80%, v/v), solid load (2–15%, w/v), and extraction time (10–60 min) on the extraction of antioxidant compounds were evaluated according to a Box–Behnken experimental design. By means of the response surface methodology, the optimum conditions were obtained: 40% acetone, 8.6% solids, and 43 min. For all the extracts, the total phenolic content (TPC), flavonoid content (TFC), and antioxidant activity (DPPH, ABTS, and FRAP) were determined. With the aim of shortening the extraction time, a two-level factorial experiment design was also carried out using a probe-type UAE, keeping the solid load at 8.6% (w/v) and the acetone concentration at 40% (v/v), while the amplitude (30–70%) and the extraction time (2–12 min) were varied to maximize the aforementioned parameters. Finally, a maximum of phenolic compounds was reached (45.41 mg GAE/g EOP) at 12 min and 70% amplitude. It was comparable to that value obtained in the ultrasonic bath (42.05 mg GAE/g EOP), but, remarkably, the extraction time was shortened, which translates into lower costs at industrial scale. Moreover, the bioactive compound hydroxytyrosol was found to be the major phenolic compound in the extract, i.e., 5.16 mg/g EOP (bath-type UAE) and 4.96 mg/g EOP (probe-type UAE). Other minor phenolic compounds could be detected by capillary zone electrophoresis and liquid-chromatography–mass spectrometry. The sugar alcohol mannitol, another bioactive compound, was also found in the extract, and its content was determined. Thus, the use of this technology can support the valorization of this waste to obtain bioactive compounds, including mannitol, hydroxytyrosol, and other derivatives, before being applied for other uses.
Highlights
Exhausted olive pomace (EOP) is the residual solid of the olive pomace oil industry obtained after subjecting the olive pomace to a drying process and a solid–liquid extraction with hexane [1]
Acetone–water was selected as the extraction solvent since previous results showed that richer extracts in phenolic compounds can be obtained with this solvent compared to water [32]
Acetone can be used in the manufacturing process to obtain food ingredients [33] because it is generally recognized as safe (GRAS) and complies with good manufacturing practices
Summary
Exhausted olive pomace (EOP) is the residual solid of the olive pomace oil industry obtained after subjecting the olive pomace to a drying process and a solid–liquid extraction with hexane [1]. EOP has a highcalorific value, and it is nowadays used as renewable low-cost fuel, while the ashes that are generated can be used for the production of ceramic materials [4] Both activities produce emissions of dangerous particles and gases during their combustion, causing an environmental problem [5]. Other alternatives for EOP valorization have been published, e.g., the obtainment of sugars, xylitol, and ethanol from their polymeric sugar fraction and lignin [2,3,6,7], as well as the production of xylanases [8] None of these ways are opposed to the extraction of bioactive compounds, such as phenolic compounds, as a first step in a biorefinery concept, instead of being lost without any revenue. Hydroxytyrosol-containing extracts can be added to foods such as vegetable oils to increase their oxidative stability [15,16]
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