Lactic acid production by lactobacillus pentosus from wood extract hydrolysates

Hot-water extracted hemicelluloses from sugar maple wood chips were hydrolyzed by dilute acid at an elevated temperature and concentrated using a nano-filtration membrane process to obtain a fermentable sugar stream containing arabinose, galactose, glucose, mannose, rhamnose, and xylose. Lactobacillus pentosus was directly adapted to using the concentrated wood extract hydrolysate to produce lactic acid. The effect of initial sugar loading was investigated by diluting the concentrated wood hydrolysate to obtain four sugar concentrations: 54.08 g/L, 61.47 g/L, 94.10 g/L, and 129.50 g/L; the media are labelled as med1, med2, med3, and med4 respectively. After 55 hours of fermentation in a 1-L bioreactor at 37°C and pH 6.0, medium med1 had the highest lactic acid yield (Y_p/s) of 0.83 g-lactic acid/g-sugar, representing approximately 97.3% of theoretical yield. Acetic acid was produced after glucose was depleted as the main by-product at up to 49% of the obtained lactic acid concentration. Adaptation of an L. pentosus strain to concentrated wood-extract hydrolysate led to a 10-h reduction in fermentation time and a 15.5% increase in lactic acid production. L. pentosus simultaneously utilized both six-carbon and fivecarbon sugars; arabinose, galactose, glucose, and rhamnose were preferably utilized, whereas mannose and xylose were slowly utilized. Acetic acid was also produced after glucose had been completely consumed.