The expansion of urban green areas has boosted the accumulation of gardening lignocellulosic residues. Due to the composition of these residues, rich in lignocellulosic sugars, they can be interesting raw materials to produce platform chemicals like lactic acid.
When using lignocellulosic substrates, pretreatment is the first stage to favor the release of sugars in any production process. During pretreatment two fractions are generated: i) a solid fraction rich in cellulose (i.e. glucose) and ii) a liquid fraction rich in hemicellulosic sugars (mainly xylose). Taking into account that the conversion of glucose from cellulose has been widely addressed, this work studies the fermentation of xylose to produce lactic acid. It should be highlighted that xylose, which is a five-carbon sugar, is difficult to ferment by most wild-type microorganisms.
In this work*, different steam explosion pretreatment conditions were applied and the resulting liquid fractions were analyzed. The enzymatic hydrolysis conditions necessary to favor the release of monomeric sugars in the liquid fraction were determined. Furthermore, lactic acid production from hemicellulosic sugars was performed with Lactobacillus pentosus CECT4023T. This heterofermentative bacterium is capable of metabolizing glucose and xylose even in the presence of inhibitory compounds generated in the pretreatment. However, by metabolizing xylose, it produces acetic acid concomitantly with lactic acid.
Weak acids produced during pretreatment (i.e., acetic acid, formic acid) and fermentation (lactic acid, acetic acid) can significantly affect sugar consumption due to the pH drop when they accumulate in the medium. For this reason, different methods of pH control were compared and studied to improve xylose consumption. When comparing the production of lactic acid in shake flask with CaCO3 and in bioreactor with pH control by NaOH addition, the highest lactic acid yields were obtained in the second case.
The results obtained in this research provide new insights for the valorisation of emerging lignocellulosic materials like gardening residues into high added-value products, such as lactic acid.
(*) Cubas-Cano, E., González-Fernández, C., Ballesteros, I., Tomás-Pejó, E. Efficient utilization of hydrolysates from steam-exploded gardening residues for lactic acid production by optimization of enzyme addition and pH control (2020) Waste Management, 107, pp. 235-243. DOI: 10.1016/j.wasman.2020.04.003
More information: Elia Tomás Pejó, Senior Assistant Researcher, Biotechnological Processes Unit. firstname.lastname@example.org