Harnessing and Evaluating Almond Hulls and Shells for Bio-Based Products

Almond hulls and shells, the byproducts of the almond industry, were analyzed to assess their potential valorization pathways. Shells showed a higher content in lignin and polysaccharides, but very low levels of extractives and inorganics. Hull’s high polar extractives fraction showed poor phenolic preponderance and antioxidant activity, but high sugar and mineral contents, and its lipophilic extracts were highly enriched in triterpenes (from 73.5% to 91.3%), while shells presented more fatty acids (27.4% to 34.2%) and sterols (17.4% to 29.1%). Shells exhibited much higher S/G ratio (syringyl to guaiacyl units) in their lignin polymer (1.0 to 1.4), compared to hulls (0.5 to 0.6). After mineral analyses, hulls showed high amounts of potassium (3.7–4.9%). Fixed carbon content was similar for both materials, but shells showed a higher energetic content, ~20 MJ/kg. Finally, both hulls and pellets increased the water holding capacity (WHC) of the soil by 50%, when added in weight percentages of 6.25% (hulls) and 25% (pellets). With these results, the range of possibilities for these waste materials is exciting: shells could be used to obtain hemicellulose oligosaccharides, while hulls could be used in sugar extraction for biotransformation or as a soil amendment.