Evaluation of drying methods and optimization of phenolic compound extraction and antioxidant evaluation of different soursop residue fractions

Agro-industrial residues are sources of bioactive compounds and may serve as sustainable and functional food ingredients. During pulp extraction, soursop generates large amounts of residues, such as pomace, seeds, and peels, which have high nutritional and functional potential but are frequently discarded. The objective was to compare oven drying and freeze-drying of soursop pomace, seeds, and peel, and evaluate phenolic extraction using water and alcoholic solvents by analyzing total phenolic contents and antioxidant capacity of the extracts. Soursop fruits were pulped, separating pomace, seeds, and peel. Samples were oven-dried (50 °C/24 h) or freeze-dried, ground, and subjected to phenolic extraction for 2 h using water or ethanol (50%, 70%, and 80%) with agitation and ultrasound pretreatment. Total phenolic contents were determined by Folin–Ciocalteu, and antioxidant capacity was evaluated by DPPH and ABTS assays. The three soursop fractions evaluated showed better preservation of color characteristics after freeze-drying compared to oven drying. For soursop pomace, freeze-drying resulted in higher total phenolic compound contents compared to oven drying. In the peel, extractions performed with alcoholic solutions showed higher phenolic contents in freeze-dried samples. For the seeds, higher total phenolic contents were observed in freeze-dried samples when water, 50% ethanol, and 80% ethanol were used as solvents, with 50% ethanol being the most efficient solvent for extraction for the three fractions (pomace, seed and peel). In the ABTS assay, pomace and peel showed higher antioxidant activity in freeze-dried samples, especially when extracted with 50% or 70% ethanol. For the seeds, antioxidant activity varied according to the drying method and solvent, with freeze-dried samples showing better results for water, 70%, and 80% ethanol extractions. In the DPPH assay, pomace showed higher antioxidant capacity in freeze-dried samples, especially with 70% and 80% ethanol. For the peel, oven-dried samples showed better results for aqueous extraction, no differences between drying methods for 50% and 70% ethanol, and higher activity in freeze-dried samples for 80% ethanol. In freeze-dried samples, 50% ethanol showed the highest antioxidant activity. For the seeds, freeze-dried samples showed better results for water and 80% ethanol extractions, while oven-dried samples performed better with 50% ethanol. No differences were observed for 70% ethanol. The results suggest that freeze-drying is a more suitable drying method for preserving and recovering bioactive compounds, likely due to the lower temperatures employed during the process, which minimize the degradation and oxidation of phenolic compounds. The superior performance of 50% ethanol in most extractions may be associated with its intermediate polarity, enhancing the solubilization of a broader range of phenolic compounds. Thus, freeze-drying combined with 50% ethanol appeared to be a favorable condition for obtaining soursop residue extracts with higher bioactive potential. We thank FAPEMIG, CAPES, and CNPq for financial support.