The purpose of using mechanical pretreatment finds its success in converting the
larger sizes to extremely fine ones and also reduction in the crystalline property of
cellulose. Milling methods that reused include: pin mill, vibratory mills, knife mills,
extruders, and hammer mills (Cheng and Timilsina 2011; Sun and Cheng 2002).
Among all the mills mentioned above, hammer mills are the ones that have easier
mode of operation, have ability to make a greater range of particle sizes, and also are
cost-effective (Dey et al. 2013). The modes of action by which hammer mills reduce
the sizes of particles are impact and shear. The limitation is that we cannot process
the larger stalk or straws by these mills. For processing such materials they are first
processed by other mills like knife mills to make their sizes compatible with other
mills so that they may be processed without any difficulty as that of choking (Bitra
et al. 2009). Figure 5.6 (Jędrzejczyk et al. 2019) explains the process of milling.
Fig 5.6 Physical methods of pretreatment (Jędrzejczyk et al. 2019)
5 Challenges in Bioethanol Production: Effect of Inhibitory Compounds
125