Coconut palm (Cocos nucifera) is an important commercial crop in many tropical countries, but its industry generates large amounts of residue. One way to address this problem is to use this residue, coconut husk, to produce second-generation (2G) ethanol. The aim of this review is to describe the methods that have been used to produce bioethanol from coconut husk and to suggest ways to improve different steps of the process. The analysis performed in this review determined that alkaline pretreatment is the best choice for its delignification potential. It was also observed that although most reported studies use enzymes to perform hydrolysis, acid hydrolysis is a good alternative. Finally, ethanol production using different microorganisms and fermentation strategies is discussed and the possibility of obtaining other added-value products from coconut husk components by using a biorefinery scheme is addressed.

The use of high hydrostatic pressure (HHP) is an interesting approach to optimize the production of both first-and second-generation ethanol. It may be applied on Saccharomyces cerevisiae cells to enhance the fermentation pathway and on the lignocellulosic biomass to increase sugar release. HHP has a wide effect on many biological processes, such as growth, division and cellular viability. Actually, conformation, stability, polymerization and depolymerization of proteins are affected by HHP as well as lipid packaging. Moreover, transcriptional profile analysis indicates an activation of the general stress response. In yeast, HHP higher than 100 MPa leads to significant morphological and physiological alteration, and loss of cellular viability occurs over 200 MPa. A yield rate increase in ethanol production occurs at pressures of 10-50 MPa, but over 87 MPa alcoholic fermentation is interrupted.