Protective Seed Structures as Food Coproducts: Nutritional and Bioactive Diversity and Valorization Potential

Seed protective tissues removed during processing may contain concentrated levels of specialized fibers and metabolites, but differ in terms of their potential for value-added applications. The hypothesis of this study is that characterizing these seed coats will reveal chemical and nutritional profiles of interest for use as food ingredients or supplements, particularly due to the presence of methylxanthines, polyphenols, flavonoids, fiber, and other nutrient matrices. Peanut skins, cocoa shells, coffee silver skins, and guarana shells were compared in terms of centesimal composition, water activity (aw), hygroscopicity after 168 h, total extractives, total phenolics, and antioxidant activity as measured by DPPH, ABTS, FRAP, and ORAC. Hydroethanolic extracts were also profiled by LC–ESI–IT3D–MS/MS to investigate chemical markers associated with functional differences. All matrices were rich in dietary fiber (55.96–62.87 g/100g dry weight), but exhibited distinct nutritional profiles. The peanut skin had the highest protein (19.25 g/100g) and lipid (11.81 g/100g) contents, while the cocoa husk had the highest ash content (8.12 g/100g) and the highest soluble fiber fraction (16.65 g/100g). Coffee husk also had a high protein content (18.67 g/100g). Total extractives ranged from 21.14 g/100 g in guarana husk to 39.15 g/100g in peanut husk. The water activity (aw) ranged from 0.364 to 0.576, and hygroscopicity at 168 h ranged from 4.44 to 6.96 g/100g of powder, indicating distinct technological behavior. The peanut hull exhibited the highest phenolic content (38.18 mg EAG/g of dry biomass) and the highest responses in FRAP (95.44 µmolTE/g), ABTS (146.95 µmolTE/g), and ORAC (293.5 µmolTE/g). The guarana husk, despite its low Folin value (2.01 mgEAG/g), exhibited the highest DPPH response (126.51 µmolTE/g) and an ABTS value of 121.21 µmolTE/g, demonstrating that the redox capacity did not correlate linearly with total phenolic content. Coffee husk exhibited 12.09 mgEAG/g and an ORAC value of 226.0 µmolTE/g, while cocoa husks exhibited 5.61 mgEAG/g and responses of 37.40, 87.13, 55.08, and 77.98 µmolTE/g in FRAP, ABTS, DPPH, and ORAC, respectively. LC–MS/MS revealed complementary chemical signatures: signals consistent with caffeine (nominal m/z 195→138) in coffee and guarana; a theobromine-type methylxanthine feature (m/z 181, with products around 163/138/137) shared by cocoa, coffee, and guarana; and an oligomeric feature consistent with proanthocyanidins in peanut skins (nominal m/z 865, with products near 577 and 287). These tissues do not constitute a homogeneous class of residues: their food utilization can combine the full use of the fibrous fraction with the selective recovery of bioactive compounds, depending on the chemical and technological profile of each matrix. It is concluded that each fraction can address nutritional deficiencies and/or enhance the bioactive profile.

Acknowledgments: CNPq (PIBIC / PIBITI – 2026 and 400122/2025-1), FAPESP (2025/11374-6 and 2024/14698-4) and PIDA/PRPI/USP 160 2025/01