Optimization of phosphoric acid catalyzed fractionation and enzymatic digestibility of flax shives

Flax shives are the woody residue left over from processing flax straw into fiber, and are an abundant renewable lignocellulosic material with a potential for the conversion into bioethanol and other value added products. In this study, prior to enzymatic hydrolysis for the liberation of fermentable sugars, such as glucose and xylose, flax shives were treated with concentrated phosphoric acid. In order to optimize the phosphoric acid pretreatment and enzymatic hydrolysis steps, the effects of three process variables on the fractionation of flax shives, and enzymatic digestibility of pretreated flax shives were evaluated. The optimization process employed a central composite design (CCD), where the variables selected were concentration of phosphoric acid (40.8–86.2%), pretreatment time (9.5–110.5 min), and cellulase loading (13.1–71.9 FPU/g cellulose). Using three-variable and five-level CCD, all tested independent variables were identified to have significant effects (P < 0.05) on the digestibility of pretreated flax shives. It was found that the level of phosphoric acid (P < 0.0001) affects the digestibility most significantly when compared with other variables. When the optimization was conducted under a constrain of minimum cellulase loading, the maximum digestibility of 94.8% was predicted when the phosphoric acid concentration, pretreatment time, and cellulase loading were 86.2%, 110.5 min, and 13.1 FPU/g cellulose at 50 °C and 120 h, respectively. Under these conditions, digestibility of pretreated flax shives in the validation study reached a maximum of 93% at 120 h of incubation, showing good agreement with the values from the validation experiment of 93.4%, indicating high accuracy of the CCD procedure. When triticale straw, pine wood, and poplar wood were pretreated and hydrolyzed under optimum conditions obtained from the flax shives experiment, the digestibility reached 98.2, 74.8, and 95.7%, respectively, suggesting that the modest pretreatment process using phosphoric acid is an effective method for perennial plants as well as hard wood.