2.1. Material. Orange peel wastes were collected from
Br¨amhults Juice AB (Bor˚as, Sweden). The wastes were from
orange juice process and contained 21.3% total solid (TS).
It was then chopped or homogenized prior to pretreatment
process. Inoculum was collected from a thermophilic biogas
plant (Bor˚as Energy and Environment AB, Bor˚as, Sweden).
The inoculumwas kept at 55∘C for 3 days before the digestion
process. Chemicals including hexane, diethyl ether, and
sodium sulfate were purchased from Sigma-Aldrich.