Orange peel by-products (OPB) represent a large, underutilized fraction of citrus production and contain valuable components such as pectin, carotenoids, flavonoids and dietary fiber. Because these residues can account for up to 55% of fruit weight and are commonly landfilled, their valorization addresses environmental impacts and fits circular economy objectives. Dietary fiber (DF) is a recognized functional ingredient with health benefits; based on solubility it is classified as soluble dietary fiber (SDF) or insoluble dietary fiber (IDF). SDF can modulate glycemic response, cholesterol, and the gut microbiota, while IDF affects transit time and fecal bulk.
The study evaluated extraction approaches that combine physical and biological techniques to recover SDF from OPB. Conventional chemical methods (acid/alkaline) are inexpensive but can use harsh conditions and high temperatures that risk polysaccharide degradation. Ultrasound‑assisted extraction (UAE) delivers acoustic cavitation that disrupts cell walls, improving mass transfer at lower temperatures and solvent use. Enzymatic extraction (EE) employs protease and amylolytic enzymes to remove impurities and obtain higher‑quality fiber under milder conditions. Applying ultrasound as a pretreatment before enzymatic hydrolysis aims to preserve enzyme activity while improving yields and enabling lower solid:liquid ratios such as 1:25 to reduce solvent consumption and enhance scalability.
Four extraction workflows were compared: alkaline extraction (ALE), alkaline extraction with ultrasound pretreatment (UAEAL), enzymatic extraction (EE), and ultrasound‑pretreated enzymatic extraction (UAEE). The UAEE protocol implemented ultrasound pretreatment under temperature‑controlled sonication prior to enzymatic hydrolysis to avoid simultaneous high‑temperature sonication that can compromise enzyme performance. The enzymatic sequence used is intended to hydrolyze proteins and starch, enriching the fiber fraction.
The ultrasound‑pretreated enzymatic extraction (UAEE) achieved the highest reported SDF recovery in this work: 26.04 ± 0.81% with approximately 90% purity. This yield is substantially higher than yields typically reported for conventional chemical methods and aligns with prior reports that ultrasound and enzymatic combination methods increase extraction efficiency. The use of a low mass‑to‑volume ratio (1:25) during sonication is highlighted as a process advantage for reducing solvent use and facilitating scale‑up.
Phenolic compounds bound to the recovered SDF were quantified and ranged from 3.23 ± 0.03 to 12.15 ± 0.21 mg GAE g⁻¹ across extraction conditions. These retained phenolics indicate that SDF fractions may contribute antioxidant‑related bioactivity and that extraction methods preserve some bound phenolic content. Total flavonoid content values were assessed in the original study; specific numeric values beyond the phenolic range above are reported in the source but are not restated here.
Functional properties differed by extraction method. SDF obtained by alkaline extraction displayed the highest water‑holding capacity (WHC) measured at 9.16 ± 0.08 g g⁻¹. Enzymatic extraction induced structural changes and partial depolymerization that reduced fiber porosity and water affinity in some samples. Oil‑holding capacity (OHC) ranged narrowly from 1.08 ± 0.12 to 1.24 ± 0.05 g g⁻¹ with no significant differences among methods, suggesting oil retention is less sensitive to the variations in extraction technique employed here.
Microscopy and spectroscopic analyses showed method‑dependent structural outcomes. UAEE‑recovered SDF presented a more amorphous structure and lower crystallinity by X‑ray diffraction compared with other preparations, together with greater surface disruption observed by scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) indicated higher thermal stability for UAEE SDF. Enzymatic action generally promoted partial depolymerization and altered porosity, which correlates with the observed changes in water affinity and textural properties.
Beyond physicochemical characterization, the study evaluated the ability of recovered SDF to serve as a carbon source for intestinal bacteria. UAEE SDF was a suitable substrate for growth of mammalian intestinal symbionts including Faecalibaculum rodentium and Bacteroides thetaiotaomicron, species recognized for benefits to host health. This functional validation supports potential prebiotic or microbiota‑modulating applications of the recovered SDF.
Ultrasound pretreatment prior to enzymatic extraction is presented as a promising, greener strategy to recover high‑value soluble dietary fiber from orange peel by‑products at improved yield (≈26%) and high purity (~90%), while retaining bound phenolics and producing fibers with altered structural and thermal profiles. The recovered SDF exhibited functional techno‑properties relevant to food formulation and supported growth of beneficial gut microbes, indicating potential as a tailored functional ingredient. The study emphasizes process‑level advantages such as low mass‑to‑volume ratios (1:25) and temperature‑controlled sonication for scalability. Detailed experimental parameters, full compositional tables and statistical analyses are provided in the source article for readers seeking replication or scale‑up data.