Steam explosion after sulphur dioxide impregnation of wood chips is an effective method for improving the enzymatic digestibility of cellulose in the softwood Pinus radiata. Digestibility of pretreated fiber was progressively increased by altering the conditions of steam explosion. With increasing digestibility, there was an observed increase in fiber porosity as measured by the solute exclusion technique. Accessible pore volume and accessible surface area to a 5-nm dextran probe positively correlated with both 2- and 24-h digestion yields from pretreated fiber. The increase in accessibility was probably the result of hemicellulose extraction and lignin redistribution. A subsequent loss in accessibility, brought about by structural collapse or further lignin redistribution, resulted in a corresponding loss in digestibility. It appears that steam explosion increases cellulose digestibility in P. radiata by increasing fiber porosity.

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The discovery of xylose-fermenting yeasts has considerably enhanced the prospect of more efficient conversion of biomass resources to ethanol. In previous work, we have demonstrated the ability of Pachysolen tannophilus to ferment wood sugars obtained via the acid hydrolysis of softwood and hardwood species.' However, fermentations of wood hydrolysate media with this yeast are very slow, even when relatively high yeast concentrations are used. Many researchers are investigating genetic manipulation of P . tannophilus' in order to improve undesirable properties such as slow fermentation rate and low ethanol yield. However, our approach has been to investigate the possiblity of obtaining more rapid conversion of substrate by the use of high concentrations of flocculated yeast in continuous tower fermentation. Flocculence has not previously been demonstrated in P . tannophilus, but tower fermentation has recently been shown to be a suitable means of inducing flocculation in yeasts and bacteria that were considered nonflo~culent.~*~ This article reports the induction of flocculation with P . tannophilus by the use of controlled aeration in a tower fermentor. The observed environmental and physiological conditions for flocculent growth are described. Although most studies with P . tannophilus have used xylose as substrate, a synthetic glucose medium was chosen for this study as the faster growth rate of the organism on glucose was considered to favor more rapid selection of a flocculent strain. The fermentation of hexose sugars such as glucose, by P . tannophilus, is also of interest in its own right since any industrial use of this organism in New Zealand will involve the fermentation of softwood hydrolysates, which contain not only the pentose sugar xylose but also the hexoses glucose, mannose, and galactose. For example, hemicellulose hydrolysates derived by dilute acid hydrolysis of New Zealand's major commercial species, Pinus radiata, have the composition arabinose 6%, xylose 17%, glucose 25%, mannose 40%, and galactose 12% at a total sugar concentration from 3.5-6% w/v.' MATERIALS AND METHODS Organism and MediumThe yeast used in these experiments was Pachysolen tannophilus, strain NRRL Y-2461. The culture medium contained glucose, 50 g/L; (NH4)2S04r 2 g/L; KH2P04, 0.75 g/L; MgS04 . 7H20, 0.5 g/L; yeast extract (Difco), 2.5 g/L and CaCl,, 0.25 g/L. The medium was sterilized at 121°C for 30 min. The CaCl, was autoclaved separately in a concentrated form and then added to the rest of the medium to avoid precipitation. Glucose concentration was determined after sterilization of each batch of medium and varied from 45-50 g/L. Tower FermentorThe fermentor was based on a design used at the University of Sydney, A~s t r a l i a .~ It was constructed of glass QVF pipeline sections of inside diameter 40 mm, with an upper, internal settling region of 80 mm diameter. The settling region was fitted with two baffles to provide a nonturbulent region to enable cells to settle and eventually recycle to the main section of the ...