Liquid Hot Water Pretreatment as a Sustainable Strategy for Cardoon Stalks Valorization

Cardoon (Cynara cardunculus L.) is a perennial lignocellulosic crop with high potential as a biorefinery feedstock due to its adaptability to marginal lands and Mediterranean climates. In this study, liquid hot water (LHW) pretreatment was investigated as a sustainable, chemical-free strategy to enhance the enzymatic saccharification and overall conversion of cardoon stalks. Pretreatment was conducted at 190 °C for 5, 20, and 35 min. The pretreated solids were characterized for structural carbohydrates and lignin, while the pretreatment liquors were analyzed for total carbohydrates, total phenolic content, and total carboxylic acid content. Hemicellulose removal reached 84% (w/w) after 20 min, while cellulose recovery remained close to 100% under all conditions. Enzymatic digestibility of cellulose increased from 10% (w/w) in the raw material to 68–74% (w/w) in the pretreated solids, with the highest overall conversion achieved at 20 min. The pretreatment liquors showed increasing concentrations of carbohydrates, phenolics, and carboxylic acids with residence time. These results demonstrate that LHW pretreatment at 190 °C for 20 min is an effective and environmentally friendly approach for cardoon valorization, achieving high cellulose accessibility without the need for chemical additives.