Wood chips were directly pretreated by dilute acid (DA) and
Sulfite Pretreatment to Overcome Recalcitrance of Lignocelluloses
(SPORL) [21,22]. These two processes were applied because DA is
the most widely studied and SPORL has demonstrated robust performance
for woody biomass conversion [19] even when applied to
softwood species [23,24]. The pretreated wood chips were disk
milled after the separation of solids from the pretreatment hydrolysate
(spent liquor) as shown in Fig. 1. All pretreatments were
conducted using 150 g wood chips in oven-dried (od) weight in a
1-L reactor with a liquor to wood ratio of 3:1 at 170 C for
20 min. This set of mild conditions was used to avoid over pretreatment
so that the differences in the recalcitrance among the poplars
could be shown. The pretreatment liquor was made of sulfuric acid
and sodium bisulfite. Acid concentration was 0.2% (v/v) or 1.1% on
od wood for DA pretreatment. Sodium bisulfite of 2% on od wood
was applied in addition to the application of the same amount of
sulfuric acid in SPORL pretreatment. Three, 1-L vessels were
mounted inside of a 23-L wood pulping digester (pressure vessel)
in an autoclave configuration as described elsewhere [21]. Therefore,
DA or SPORL pretreatments of the three poplar samples were
conducted in the same run. The 1-L pressure vessels were heated
externally using steam while the wood pulping digester was rotating
at the speed of 2 rpm for mixing. The temperature increased
to170 C in approximately 7 min. Aspen pretreatment was conducted
separately, with aspen substrates produced in a previous
study using two sodium bisulfite dosages of 1.5% and 3.0% on wood
for SPORL pretreatment [20].