Acid hydrolysis is an old process still applied in some ethanol industries. Sulfuric acid is the most commonly applied acid in this process, where starch is converted to low-molecular-weight dextrins and glucose [8]. Main advantages of this process are rapid hydrolysis and less cost for catalyst, compared to the enzymatic hydrolysis. However, the acid processes possess drawbacks including (a) high capital cost for an acid — resistant hydrolysis reactor, (b) destruction of sensitive nutrients such as vitamins present in raw materials, and (c) further degradation of sugar to hydroxymethylfurfural (HMF), levulinic acid, and formic acid, which lowers the ethanol yield and inhibits the fermentation process [9].

The acid hydrolysis process can be performed either in batch or in con­tinuous systems. Dilute-acid hydrolysis can also be used as a pretreat­ment for enzymatic hydrolysis. It is common to soak the starch or starchy materials in the dilute acid prior to enzymatic hydrolysis, then to con­tinuously pass it through a steam-jet heater into a cooking tube (called a jet cooker or mash cooker) with a plug flow residence time for a couple of minutes, and then subject it to enzymatic hydrolysis.