Physical pretreatment methods such as ultrasonic radiation
have also been applied to improve the efficiency of alkaline washing.
The frequency at which the radiation is emitted produces a
cavitation phenomenon that increases the temperature and pressure
of aqueous reaction systems. For instance, the cavitation
phenomenon produces free radicals from water molecules that are
capable of cleaving bonds between lignin and hemicelluloses. Subhedar
and Gogate [20] showed that the use of ultrasound reduces
the alkali requirement and the processing time for delignification,
whereas for Velmurugan and Muthukumar [21], the most
attractive features of the sono-assisted alkaline pretreatment were
the substantial reductions in pretreatment time and temperature,
although the substrate accessibility to enzymatic hydrolysis was
also improved considerably.