16.3.1.1 Milling

Milling is a mechanical process of pretreatment that breaks down the structure of lig — nocellulosic materials and reduces the crystallinity of the cellulose [14]. During ball milling, biomass is grounded with the contact of the balls inside a cycle machine to get the uniform particles size [25]. This method can be considered environment-friendly due the absence of added chemicals in this process that produce toxic substances

[14] . A disadvantage of milling is the high power required by the machines and consequent high energy costs [14]. Buaban et al. [26] reported that the increase of the time of milling increased the amounts of the sugars (glucose, 89.2 ± 0.7 % and xylose, 77.2 ± 0.9 %) after 4 h of milling.

16.3.1.2 Irradiation

Gamma-rays-mediated pretreatment is irradiation pretreatment which allows the breakdown of beta-1, 4glycosidic linkages, thus enhancing the surface of area and crystallinity of cellulose [16]. It is a physical pretreatment which increase the surface area, consequently reducing the crystallinity. This method is expensive for large-scale operations with considerable environmental and safety concerns [16].

16.3.1.3 Microwave Irradiation

The use of high-energy radiation such as microwave causes one or more changes in the characteristics of cellulosic biomass, such as an increase in surface area, reduction in the degrees of polymerization, and crystallinity of cellulose and hemicelluloses, and the partial depolymerization of lignin [27]. However, irradiation pretreatment have the disadvantage of high energy consumption, and the methods are slow and expensive. Microwave irradiation process acts under the structural change in cellulose with the occurrence of the lignin and hemicellulose degradation, thus increasing the enzymatic accessibility [14]. To further improve the sugar yield after pretreatment, microwave radiation process was combined with chemicals. Binod et al. [28] tested different microwave pretreatment conditions for SB and reported highest reducing sugar yield (0.83 g/g dry biomass) in the microwave-alkali pretreatment followed by acid pretreatment.