Integrated extraction of phenolic-rich apple peel extracts and in silico evaluation of quercetin dairy protein interactionsAbstract
Apple peel (Malus sp.), a by-product of fruit processing, is rich in phenolic compounds, particularly quercetin, as well as other flavonoids and dietary fiber. This study integrated conventional maceration with Microwave-Assisted Extraction (MAE) to obtain phenolic- and flavonoid-rich apple peel extracts and to explore their potential interaction with major dairy proteins through in silico docking. Apple peel powders were first macerated and then subjected to MAE at irradiation times (3, 6, 9, and 12 minutes). The extracts were assessed for antioxidant activity (DPPH), total phenolic content (TPC), and total flavonoid content (TFC), with all experiments performed in triplicate and analyzed using one-way ANOVA followed by an appropriate post hoc test. Prolonged MAE treatment enhanced antioxidant activity (93.85–95.09%), phenolic content (13.52–14.73 mg GAE/g), and flavonoid content (23.76–29.62 ppm). Molecular docking indicated that quercetin showed comparable predicted binding affinity toward both milk proteins, with binding energy values of −6.7 kcal/mol for β-lactoglobulin and −6.9 kcal/mol for α-lactalbumin. These findings suggest that apple peel extract obtained through integrated extraction may have potential for future application in dairy-based functional systems, although formulation and stability studies are still required.
Keywords:
apple peel; quercetin; yogurt-based; functional foods
Resumo
A casca de maçã (Malus sp.), um subproduto do processamento da fruta, é rica em compostos fenólicos, particularmente quercetina, além de outros flavonoides e fibras alimentares. Este estudo integrou a maceração convencional com a Extração Assistida por Micro-ondas (Microwave-Assisted Extraction, MAE) para obter extratos de casca de maçã ricos em compostos fenólicos e flavonoides e para explorar sua potencial interação com as principais proteínas do leite por meio de docking molecular in silico. Os pós da casca de maçã foram inicialmente submetidos à maceração e, em seguida, à MAE em diferentes tempos de irradiação: 3, 6, 9 e 12 minutos. Os extratos foram avaliados quanto à atividade antioxidante pelo método DPPH, ao teor total de compostos fenólicos (TPC) e ao teor total de flavonoides (TFC), sendo todos os experimentos realizados em triplicata e analisados por ANOVA unidirecional, seguida de teste post hoc apropriado. O tratamento prolongado por MAE aumentou a atividade antioxidante (93,85–95,09%), o teor de compostos fenólicos (13,52–14,73 mg EAG/g) e o teor de flavonoides (23,76–29,62 ppm). O docking molecular indicou que a quercetina apresentou afinidade de ligação prevista comparável para ambas as proteínas do leite, com valores de energia de ligação de −6,7 kcal/mol para a β-lactoglobulina e −6,9 kcal/mol para a α-lactoalbumina. Esses achados sugerem que o extrato de casca de maçã obtido por meio de extração integrada pode apresentar potencial para futura aplicação em sistemas funcionais à base de lácteos, embora estudos de formulação e estabilidade ainda sejam necessários.
Palavras-chave:
casca de maçã; quercetina; à base de iogurte; alimentos funcionais
1. Introduction
Agro-industrial fruit processing generates large quantities of peel, pomace, and other by-products, representing a significant waste-management challenge and an underutilized source of valuable bioactive compounds. Among these residues, apple peel represents a major fraction discarded by juice, cider, and puree industries. Apple peel is rich in phenolic compounds, particularly quercetin, along with other flavonoids, phenolic acids, and dietary fiber, apple peel offers significant health-promoting properties (Massias et al., 2015; Suleria et al., 2020). Quercetin was selected as the focal compound because it is a well-recognized apple peel flavonol with strong antioxidant relevance and potential interaction with dairy proteins. (Zhang et al., 2021; Eisvand et al., 2022). These attributes highlight the potential of apple peel as a sustainable source of quercetin for use in functional foods and nutraceutical applications.
Efficient recovery of phenolic compounds, including quercetin, from plant matrices remains challenging because extraction efficiency is strongly influenced by solvent composition, extraction time, and temperature. Conventional extraction methods, such as Soxhlet extraction, maceration, and solvent reflux, are often labor-intensive, require high solvent volumes, and yield low extraction efficiencies (Pilařová et al., 2022). Moreover, quercetin and other may undergo thermal and oxidative degradation during extraction and processing, making it important to apply extraction techniques that help preserve compound stability while improving recovery efficiency. This underscores the need for greener and more carefully controlled extraction strategies.
Microwave-Assisted Extraction (MAE) offers several advantages over traditional methods, including rapid heating, enhanced solvent penetration, reduced processing time, and lower solvent consumption (Routray and Orsat, 2018). MAE is considered a promising technique for recovering thermolabile compounds such as quercetin, particularly when extraction conditions are carefully optimized and controlled. When combined with pre-maceration, a process that softens tissues and increases solvent accessibility, the performance of MAE can be further enhanced (Vasco-Correa and Zapata, 2017). The novelty of this study lies in integrating pre-maceration with MAE for apple peel extraction and linking the resulting extract profile with preliminary in silico evaluation of quercetin–dairy protein interactions.
Beyond extraction, the incorporation of quercetin into dairy-based functional foods has gained increasing attention. Dairy proteins may serve as potential carriers for polyphenols because of their nutritional value and their ability to interact with bioactive compounds (Krisnaningsih et al., 2019). These protein–polyphenol interactions can influence the solubility, stability, sensory properties, and bioavailability of quercetin in dairy matrices (Gharibzahedi and Smith, 2021). In this context, in silico molecular docking offers a rapid and informative approach for predicting protein–ligand interactions and guiding formulation development. Therefore, this study aimed to: (i) optimize the extraction of phenolic-rich apple peel extracts using an integrated maceration–MAE approach, (ii) characterize their antioxidant-related properties, and (iii) evaluate the molecular interactions of quercetin with major dairy proteins through in silico docking. These efforts contribute to apple peel valorization and provide a preliminary basis for future dairy-based functional food applications.
2. Material and Methods
2.1. Materials
Fresh apple peels were used as the raw material in this study to provide a controlled and standardized starting material for evaluating the extraction process. Although the present work used fresh peel rather than post-industrial apple waste, the findings are intended as a proof-of-concept for the valorization of apple peel by-products in food-related applications. A solvent consisting of 50% (v/v) methanol in distilled water was prepared and employed for the extraction process. Several chemicals were utilized, including DPPH reagent, Folin-Ciocalteu reagent, aluminum chloride (AlCl3), gallic acid, and quercetin standard, which were applied in the analysis and characterization of the extracts.
2.2. Extraction procedure
Apple peels were oven-dried at 50 °C for 24 hours, ground, and sieved to 80 mesh. For each extraction, 3 g of apple peel powder was mixed with 100 mL of methanol:water (1:1, v/v), corresponding to a solid-to-solvent ratio of 1:33.3 (w/v), and macerated for 24 h at 4–10 °C. The extract was then subjected to Microwave-Assisted Extraction (MAE) using a microwave system operated at 450 W and 2450 MHz, with an extraction temperature of 70 °C and irradiation times of 3, 6, 9, and 12 min. The extracts were subsequently filtered and concentrated using a microwave evaporator. All treatments were performed in four replicates. Maceration-only and MAE-only treatments were also included as control comparisons under the same solvent conditions.
2.3. Determination of antioxidant activity
Antioxidant activity was determined using the DPPH radical scavenging assay following Molyneux (2004). The extract solution was reacted with DPPH solution, and the absorbance was measured at 517 nm using a UV–Vis spectrophotometer. Radical scavenging activity was expressed as percentage inhibition, calculated from the difference between the absorbance of the DPPH control and that of the sample. IC50 determination and ascorbic acid positive-control analysis were not included in the original experimental design. Total phenolic content (TPC) was determined by the Folin–Ciocalteu method following Singleton et al. (1999) and expressed as mg gallic acid equivalent (GAE)/g extract. Total flavonoid content (TFC) was determined by the AlCl3 colorimetric method following Chang et al. (2002) and expressed as [mg quercetin equivalent (QE)/g extract] or, where recalculation was not possible, as ppm quercetin equivalent in the analyzed assay solution. FTIR analysis was used to characterize the functional groups of the extract, while SEM was used to observe microstructural morphology.
2.4. In silico docking
Molecular docking was used to predict ligand–protein interactions based on binding affinity, in which more negative docking energy values indicate stronger predicted interactions, following standard molecular docking principles (Trott and Olson, 2010)
2.5. Protein & ligand preparation
The three-dimensional structure of the ligand was prepared and energy-minimized using the Universal Force Field (UFF) implemented in Open Babel within the PyRx platform (Rappé et al., 1992; O’Boyle et al., 2011; Dallakyan and Olson, 2015). After minimization, the ligand was converted from MOL2 to PDBQT format for docking analysis using Open Babel in PyRx (O’Boyle et al., 2011; Dallakyan and Olson, 2015). The protein structures used in this study were bovine β-lactoglobulin (PDB ID: 3NPO) and bovine α-lactalbumin (PDB ID: 1HFZ), obtained from the Protein Data Bank.
2.6. Molecular docking & molecular docking analysis
Molecular docking was performed to predict the interaction of quercetin with selected dairy proteins. The three-dimensional structure of quercetin was obtained from (https://prankweb.cz/) and energy-minimized using the UFF force field in Open Babel within PyRx. The crystal structures of bovine β-lactoglobulin (PDB ID: 3NPO) and bovine α-lactalbumin (PDB ID: 1HFZ) were retrieved from the Protein Data Bank. Potential ligand-binding pockets were predicted using PrankWeb, and the resulting coordinates were used only to define the docking grid box. Thus, PrankWeb probability values were treated as pocket-prediction scores rather than as docking affinity measurements (https://prankweb.cz/), in Table 1. Docking simulations were then carried out using AutoDock Vina implemented in PyRx 1.0, and the best binding pose for each complex was ranked based on binding affinity (kcal/mol). Ligand–protein interactions were visualized in Discovery Studio 2021 to inspect hydrogen-bond and hydrophobic contacts. Because casein proteins are structurally flexible and contain intrinsically disordered regions, they were not emphasized in the final structure-based docking interpretation. In addition, redocking validation and positive ligand control were not performed in the present study; therefore, the docking results should be considered preliminary and supportive rather than definitive.
PrankWeb Prediction Results and PyRx Input Dimension (X, Y, Z): 20 Angstrom Exhaustiveness: 50.
3. Results
3.1. Bioactive content of apple peel
Based on literature screening, several major bioactive compounds were identified in Malus sylvestris apple peel (Table 2). These compounds include phenolic acids (e.g., chlorogenic acid), flavan-3-ols (epicatechin and catechin), dihydrochalcones (phloridzin), flavonol glycosides (hyperoside and quercitrin), and the flavonol aglycone quercetin. Their molecular weights ranged from approximately 290 to 466 g/mol, except for pectin, which is a high-molecular-weight polysaccharide (889 g/mol). Flavonoids were among the predominant compounds, highlighting the strong antioxidant potential of apple peel. Chlorogenic acid and catechin derivatives also contribute significantly to the peel’s bioactive profile. The presence of these diverse phytochemicals confirms that apple peel is a rich natural source of antioxidants and health-promoting compounds.
3.2. Antioxidant Activity (DPPH Assay)
The antioxidant activity of apple peel extracts obtained through the combined maceration and Microwave-Assisted Extraction (MAE) was evaluated using the DPPH radical scavenging method at different irradiation times (3, 6, 9, and 12 minutes). The percentage of inhibition values is presented in Table 3. Statistical analysis showed a significant effect of extraction time on antioxidant activity (P < 0.05), with inhibition values ranging from 93.85% to 95.09%.
The results of antioxidant activity, total phenolic content, and total flavonoid content from each treatment.
Although a gradual increase in antioxidant activity was observed with longer MAE duration, the differences between the highest treatments (T3 and T4) were minimal, suggesting a possible plateau effect rather than a substantial improvement. Therefore, the practical significance of this increase should be interpreted cautiously.
The relatively high inhibition values may be influenced by the concentration of extract used in the assay. However, IC50 values were not determined in this study, which represents a limitation. The observed antioxidant activity is likely associated with the presence of phenolic and flavonoid compounds released from the apple peel matrix during extraction. Nevertheless, the results reflect overall antioxidant capacity and do not specifically quantify individual compounds such as quercetin.
3.3. Total phenolic content
The total phenolic content of the apple peel extract obtained using combined maceration and MAE showed significant differences among the treatment times (P<0.05). As shown in Table 3, phenolic content increased with longer MAE durations, ranging from 13.52 to 14.73 mg GAE/g. The lowest value was recorded at 3 minutes (T1), while the highest phenolic content was observed at 12 minutes (T4). Treatments T3 and T4 exhibited significantly higher phenolic concentrations compared to the shorter extraction times. These results indicate that extending the MAE process enhances the release of phenolic compounds from apple peel, thereby improving the overall extraction efficiency.
3.4. Total Flavonoids of apple peel extract
The total flavonoid content of apple peel extracts obtained through combined maceration and MAE varied significantly across the different extraction times (P<0.05). As shown in Table 3, flavonoid content increased as the MAE duration was extended, with values ranging from 23.76 to 29.62 ppm. The lowest flavonoid content was detected at 3 minutes (T1), whereas the greatest was observed at 12 minutes (T4). In comparison to the shorter extraction durations, treatments T3 and T4 demonstrated noticeably higher quantities of flavonoids. These data imply that longer extraction durations lead to a higher release of flavonoid chemicals from the apple peel matrix, hence boosting total extraction efficiency.
3.5. FTIR spectral analysis
The FTIR spectra of apple peel extracts obtained at different extraction times (3, 6, 9, and 12 minutes) are presented in Figure 1. The spectra showed similar overall absorption patterns across all treatments, indicating the presence of comparable functional groups in the extracts.
The FTIR result of quercetin identification on different time extraction (blue = 3 minutes; black = 6 minutes; red = 9 minutes; green = 12 minutes).
Characteristic absorption bands were observed, including a broad peak in the range of 3200–3500 cm−1 corresponding to O–H stretching vibrations, peaks around 1600–1650 cm−1 associated with C=O and aromatic C=C stretching, and bands in the region of 1200–1300 cm−1 attributed to C–O–C and phenolic C–O bonds. These features are generally consistent with the presence of phenolic compounds in the extracts.
Minor variations in peak intensity were observed among different extraction times, which may be related to differences in extract concentration rather than changes in chemical structure. However, it should be noted that FTIR analysis alone cannot confirm the specific presence of quercetin, as it only provides information on functional groups. Therefore, further analytical techniques, such as HPLC or comparison with standard spectra, would be required for definitive compound identification. .
3.6. Molecular docking and in silico interaction with milk proteins
The results of the analysis of quercetin compounds in apple skin by-products through pharmacokinetic tests are listed in Table 4 and Figure 2. These results were obtained by analyzing 38 secondary metabolites using the SWISSADME web server (http://www.swissadme.ch/). Molecular docking analysis showed that quercetin interacted favorably with the two evaluated whey proteins. The predicted binding affinity was −6.7 kcal/mol for β-lactoglobulin (3NPO) and −6.9 kcal/mol for α-lactalbumin (1HFZ), indicating comparable interaction strength with both proteins. These results suggest that whey proteins may serve as potential carriers for apple peel bioactives in dairy-based systems (Table 5). β-casein showed only weak binding (0.026), κ-casein had low interaction (0.1), and no meaningful interaction was detected for α-casein. The structural modeling (Figure 3) further supported these findings, confirming that quercetin fits more effectively within the binding regions of whey proteins, especially β-Lg, compared to casein. Overall, these results suggest that quercetin has a stronger affinity toward whey proteins, which may facilitate the formation of more stable whey–quercetin complexes, potentially beneficial in dairy-based functional food applications such as yogurt formulation. All three ligands form hydrophobic surface interactions. The figure also presents a surface hydrophobicity map, where the protein surface is color-coded from brown (hydrophobic), through white (intermediate), to blue (hydrophilic). Greater hydrophobic interactions are associated with increased stability of the complex conformation.
The visualization results indicate that bioactive compounds from apple peel can form relatively stable interactions with bovine β-lactoglobulin. Chlorogenic acid and epicatechin exhibit binding affinities of −7.0 kcal/mol, while hyperoside shows a binding affinity of −6.9 kcal/mol with bovine β-lactoglobulin (Figure 4-5, Table 5-6).
Interaction of bovine β-lactoglobulin with (A) chlorogenic acid, (B) epicatechin, and (C) hyperoside.
Interaction of bovine α-lactalbumin with (A) quercitrin, (B) phloridzin, and (C) chlorogenic acid.
4. Discussion
This study confirmed that apple peel, a major by-product of fruit processing, contains significant amounts of quercetin and other phenolic compounds with strong antioxidant activity. The use of combined maceration and Microwave-Assisted Extraction (MAE) effectively enhanced the yield of bioactive compounds. MAE has been widely reported as a green extraction technique that improves mass transfer, disrupts cell structures, and increases solvent penetration, resulting in higher recovery of phenolic compounds compared to conventional methods (Delazar et al., 2012; Dhanani et al., 2017). The increasing extraction time in this study was positively correlated with higher antioxidant activity, total phenolic content, and flavonoid content, demonstrating the efficiency of MAE in quercetin recovery.
The antioxidant activity values obtained (93.85–95.09%) were relatively high compared to some reported values for other fruit by-products, such as lemon peel (52.64%) and avocado seeds (80.32%) (Verdiana et al., 2018; Weremfo et al., 2020). However, such comparisons should be interpreted with caution due to differences in extraction methods, sample preparation, concentrations, and assay conditions across studies. The high antioxidant activity observed in this study may be associated with the presence of phenolic and flavonoid compounds in apple peel extracts. Previous studies have also reported that apple peel contains higher levels of antioxidant compounds than the flesh (Huber and Rupasinghe, 2009; Wolfe et al., 2003). Overall, these findings support the potential of apple peel as a source of natural antioxidants. However, further studies are required to evaluate its effectiveness under different conditions and to validate its practical application in food systems. .
In addition to antioxidant potential, the apple peel extract demonstrated substantial total phenolic (13.52–14.73 mg GAE/g) and flavonoid content (23.76–29.62 ppm). Such values are higher compared to other fruit peels, confirming that apple peel is a rich reservoir of phytochemicals. Polyphenolic recovery is influenced by factors such as solvent composition, extraction duration, and temperature, which explains the higher yield observed with the combined maceration–MAE method (Ranjha et al., 2020; Utami et al., 2020). These findings align with previous reports, which indicate that wild apple varieties contain high levels of flavonoids and phenolic compounds with strong antioxidant potential (Karaman et al., 2013; Mihailović et al., 2018).
The in silico molecular docking results provided further insight into the functional applicability of quercetin. Quercetin showed comparable predicted binding affinity toward β-lactoglobulin and α-lactalbumin, suggesting that whey proteins may serve as favorable carriers for apple peel bioactives in dairy-based systems. Previous studies have also shown that flavonoids can interact with milk proteins to form stable complexes, enhancing both bioavailability and functional stability during processing (Pawlikowska-Pawlęga et al., 2014; Buljeta et al., 2022; Husen et al., 2025). This supports the potential incorporation of quercetin into dairy products such as yogurt, which may enhance nutritional and therapeutic benefits (Asma et al., 2023).
Overall, the findings of this study highlight the dual significance of apple peel utilization. First, valorizing apple peel by-products contributes to sustainability and circular economy practices by transforming agro-industrial waste into valuable functional ingredients. Second, the quercetin-rich extracts obtained have promising applications in the development of functional foods, particularly in antioxidant-enriched dairy formulations. These results underscore the relevance of apple peel quercetin as both a nutraceutical compound and a sustainable ingredient for future food innovations (Massias et al., 2015; Suleria et al., 2020).
5. Conclusion
This study demonstrated that the combined maceration and Microwave-Assisted Extraction (MAE) approach can produce apple peel extracts with high antioxidant activity and substantial phenolic and flavonoid content. The FTIR analysis indicated the presence of functional groups consistent with phenolic compounds, while molecular docking suggested potential interactions between quercetin and milk proteins, particularly β-lactoglobulin , supporting its incorporation into dairy-based functional foods. These findings emphasize the dual significance of apple peel valorization: enhancing human health through antioxidant-rich functional products while promoting sustainable use of agro-industrial by-products. Future research should focus on in vivo validation and formulation studies to advance the application of apple peel-derived quercetin in commercial yogurt-based food systems.
Acknowledgements
This research project was supported by a grant (DRTPM Kemdikbudristek, Indonesia).
Data Availability Statement
All data were available upon request to the corresponding author.
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Editor:
Takako Matsumura Tundisi
