Steam Explosion of the Softwood Pinus Radiata with Sulphur Dioxide Addition. II. Process Characterisation

Abstract

The enzymatic digestibility of the softwood Pinus radiata is substantially enhanced by SO₂-catalysed steam explosion. The factors controlling the effectiveness of this pretreatment have been studied. Under conditions of constant time (3 minutes) and temperature (215°c) the effects of SO₂-impregnation levels from 0–11.8% were examined. Up to about 3% SO₂ there is a pronounced effect on the enzymatic digestibility of the pretreated substrate and a rapid increase in the yield of water soluble sugars. At 2.55% SO₂, a total sugar yield of 57.4 g/100 g oven-dry wood was obtained. SO₂ levels above 3% had no beneficial effects in terms of increased sugar yields. Equivalent total sugar yields could be obtained by shortening the treatment time to 1 minute and increasing the temperature to 235°c (still at 2.55% SO₂). Percentage solubilisation of wood glucan correlates strongly with the enzymatic digestibility of the pretreated substrate and also with the surface area accessible to a 51Å molecule. This suggests that carbohydrate solubilisation and cell wall accessibility are closely related. The major, beneficial effect of SO₂ during steam explosion of P. radiata is to promote hemicellulose and cellulose hydrolysis while limiting excessive carbohydrate degradation. However, sugar reversion products were observed under near-optimal treatment conditions, this being an undesirable consequence of SO₂ catalysis. Evidence is presented suggesting a very low level of lignin sulphonation and consequently very little soluble lignin is produced. The almost total retention of lignin in the pretreated substrate appears to present little hindrance to the enzymatic digestion of its cellulose component.