Pecan nutshell as a source of bioactive compounds: solvent-extraction optimization, enzyme-inhibitory activity and incorporation into nanoemulsions

Pecan shells [Carya illinoinensis (Wangenh.) K. Koch] are an abundant agro-industrial byproduct in northern Mexico, rich in phenolic compounds with potential applications in the food, pharmaceutical, and cosmetic industries. To analyze possible uses for this byproduct, this work had three objectives: 1) To optimize an ultrasound-assisted extraction method using ethanol as a solvent, for its low toxicity and environmentally friendly properties. 2) To evaluate the effect of the extract on the activity of pancreatic alpha-amylase as a model for studying its antidiabetic potential. 3) To develop double emulsion nanosystems and evaluate their physicochemical properties as a strategy to improve the stability, bioavailability, and controlled release of the extract's bioactive components. To optimize extraction, a full factorial design was used, evaluating three temperatures (20, 30, and 40 °C), two extraction times (15 and 30 min), and two ethanol concentrations (50 and 80% v/v). Yield, total phenol (Folin-Ciocalteu) and condensed tannin (DMAC) contents were assessed as response variables. Pancreatic alpha-amylase activity was evaluated by quantifying reducing sugars with the DNS reagent, and the inhibitory effect of different extract concentrations (2–20 mg/mL) was determined to calculate its half inhibitory concentration (IC50). Double water-in-water (W/O/W) nanoemulsions, were prepared using polycaprolactone (PCL) as the encapsulating polymer and polyvinyl alcohol (PVA) or TWEEN 80 as stabilizers. Emulsification was performed by ultrasonication and evaporation. Particle size, polydispersity index, and zeta potential were determined by dynamic light scattering.

Extraction yields remained relatively constant (18–22%) with no significant effects from temperature, time, or ethanol concentration. In contrast, temperature, ethanol concentration, and their interaction, significantly influenced the total phenol and condensed tannin content. The highest total phenol (543 mg GAE/g of extract) and condensed tannin (121 mg EC/g of extract) contents were obtained using 80% ethanol at 40 °C. Phenolic compounds in the optimized extract were identified by UPLC-QTOF-MS, with trans-resveratrol, 3-phenylacrylic acid, and p-coumaric acid glucoside being the most abundant compounds. The extract showed inhibitory activity against pancreatic alpha-amylase with an IC50 of 6.5 mg/mL. Finally, the nanosystem formulated with PVA exhibited smaller particle sizes (76.4 nm), a lower polydispersity index (0.0504), a lower zeta potential (-14.5 mV), and lower stability than the formulation with TWEEN 80 (344 nm, 0.0801, -24.8 mV), which also displayed a bimodal particle size distribution. In conclusion, it is feasible to produce a walnut shell extract using an 80:20 ethanol:water mixture as a solvent. This extract contains phenolic compounds, exhibits inhibitory activity against an enzyme involved in dietary carbohydrate absorption, and can be encapsulated in PCL-PVA double emulsion nanosystems. However, further optimization of the nanosystems and testing of their biological activity in different models are still needed.

Acknowledgements: This work was funded by Sechiti, México, grant number CF-2023-G-96.