Simultaneous saccharification and fermentation (SSF) is a process in which enzymes saccharify cellulosic biomass while microbes simultaneously ferment the released glucose, for example to ethanol. Compared with the traditional two-stage process, SSF can run at lower temperatures, cutting operating costs. Recent research has focused on engineering strains to ferment cellulosic substrates directly. It is key to economical bioethanol production. It makes biofuel production more viable. Efficiency gains matter at scale. One vessel does the work of two. Recent research has focused on engineering strains to ferment cellulosic substrates directly for economical bioethanol production.