1. Introduction
Biofuels, which are currently the only direct substitutes for fossil fuel in the transport industry, can be produced on a large scale [1]. However, first-generation biofuels from grains are recently in a downturn, whereas second-generation biofuels are not competitive [2]. Sweet sorghum has attracted attention as a non-food feedstock for biofuel. Sweet sorghum is known for its strong tolerance because it can grow under harsh environmental conditions, such as an arid climate and alkaline soil; it also possesses the combined advantages of corn, sugar cane, and switchgrass [3] and [4]. Sweet sorghum instead of starch-based grain is therefore used in bioethanol production. The ethanol from sweet sorghum is regarded as a 1.5 generation biofuel. This excellent energy crop has not been widely applied in commercial ethanol production because no cost-effective technology to process it is available. Traditionally, ethanol is produced from sugar-based feedstock, such as sugar cane, sugar beet, or sweet sorghum stems, via liquid-state fermentation (LSF). However, LSF requires much energy to obtain juice from crops. Additional energy is required to concentrate the juice by boiling. Serious issues in wastewater treatment also exist in LSF [4].