Evaluating the impact of boiling and roasting on the properties of cashew nutshells (Anacardium occidentale L.) for biomass valorization

Abstract

Abstract Cashew nutshells (CNS), a lignocellulosic byproduct from the industrial crop Anacardium occidentale L., are a renewable material that constitutes 75 wt.% of the nut yet remains underused due to limited understanding of its processing. Before shelling—and the generation of this biowaste—thermal pretreatments such as boiling and roasting are commonly applied, but their effects on CNS remain underexplored. This study assesses their impact on the physical, chemical, thermal, and spectral properties of CNS. Boiling increased cashew nutshell liquid (CNSL) retention, influencing physical properties such as color, moisture content, density, and porosity. Chemical analysis revealed a high extractive fraction (46 wt.%), along with high lignin content and moderate cellulose and hemicellulose levels. Thermal degradation profiles differed notably: boiled CNS showed peaks linked to CNSL decarboxylation, while roasted and defatted samples exhibited typical lignocellulosic behavior. Boiled CNS exhibited reduced crystallinity in X-ray diffraction analysis; however, after defatting, the crystallinity of both pretreated samples became comparable. These results demonstrate that industrial thermal pretreatments significantly affect CNS composition, particularly through changes in residual CNSL. The combined compositional and thermal characterization provides valuable insight to guide future applications of CNS in biomass valorization pathways, supporting its integration into circular bioeconomy strategies.