Ambient-temperature sulfuric acid pretreatment to alter structure and improve enzymatic digestibility of alfalfa stems

• Novel pretreatment with acid improved porosity and surface area of alfalfa stems.
• Acid pretreatment increased crystallinity; ensiling and wet storage decreased it.
• Enzymatic hydrolysis was improved by acid pretreatment because of structure changes.
• Ensiling also increased the enzymatic hydrolysis, presumably due to the swelling.

A novel pretreatment with sulfuric acid at ambient temperature had demonstrated positive effects on ethanol production from alfalfa stems. To better understand these effects and help create a process control system, physical parameters (pore dimensions, surface area, and crystallinity index) and the enzymatic digestibility of alfalfa stems were studied in present work. The SEM images showed that more fine particles were present on the surface of alfalfa stems after acid pretreatment. The BET pore size, pore volume, surface area, and crystallinity were increased with increasing acid during pretreatment, might be due to the acid pretreatment hydrolyzed some hemicellulose and amorphous cellulose. Ensiling and wet storage decreased crystallinity, probably due to the swelling increased the pore size of substrates. The enzymatic hydrolysis of alfalfa stems was improved by acid pretreatment because surface damage, improved pore size, pore volume and surface area improved cellulose exposure. Ensiling also improved the enzymatic hydrolysis, likely due to the swelling resulted in increased pore size and broken hydrogen bonding of substrates.