SW are generally considered as being much more refractory than (HW) or agricultural residues. This is due to the fact that SW have a more rigid structure and contain more lignin. However, various conditions for SWDAPs have been investigated (Table 8.4), which were performed using H2SO4 or SO2.

The effect of the pretreatment is usually evaluated by the cellulose conversion yield during subsequent enzymatic hydrolysis process. Cellulose conversion yield is defined as the ratio of sugars liberated in the enzymatic hydrolysis to the theoretical value based on the sugars available in the raw material [77]. Some recent results on cellulose conversion yields from softwood are also shown in Table 8.4. It can be seen that the cellulose conversion yields of DAP treated SW are less than 65 %, which are generally lower than hardwood species [5]. This is, in part, due to the fact that SW have a more rigid structure and contains more lignin [12]. It should be noted that the addition of surfactant (i. e., Tween 80) in the post-DAP-treated substrate could enhance the cellulose conversion by 30 % [76]. During the hydrolysis, the surfactant

was added simultaneously with the enzyme. The surfactant concentration ranged 1-3 g/L [78]. This was attributed to the fact that the surfactant could change the nature of the substrate by increasing the available cellulose surface or by removing inhibitory lignin [79]. The surfactant could also increase the stability of the enzymes and reduce enzyme denaturation during the hydrolysis [80, 81]. Moreover, the surfactant could facilitate desorption of enzymes from substrate [82]. It should be noted, as indicated in Table 8.4, the cellulose conversion yields (52-63 %) is still low even with the addition of Tween 80. However, this research at least afforded a way to enhance the cellulose conversion yield through the addition of surfactant. Further study might be needed in the selection of effective surfactant.