Sub- and supercritical water hydrolysis of agricultural and food industry residues for the production of fermentable sugars: A review

• Agrifood industry wastes are investigated as source of fermentable sugars.
• Sub- and supercritical water as hydrolysis medium is reviewed.
• Process parameters that influence the process and equipment design are discussed.
• Relevant results to the field are presented and discussed.

Bioethanol has been researched as a potential alternative to substitute liquid fossil fuels due to its eco-friendly characteristics and relatively low production cost when compared to other bio-based fuels. First generation bioethanol is produced from raw materials rich in simple sugars or starch, such as sugarcane and corn, which are food sources. To avoid the fuel versus food dilemma, second generation bioethanol aims at using non-edible raw materials, as lignocellulosic agricultural residues, as source of fermentable sugars. Hydrolysis with sub/supercritical water has demonstrated great potential to decompose the lignocellulosic complex into simple sugars with several advantages over conventional processes. This review provides an overview of the state of the art on hydrolysis with sub- and supercritical water in the context of the reuse of agricultural residues to produce suitable fermentation substrates for the production of second generation bioethanol. Recent applications and advances are put into context together, providing an insight into future research trends.