Lignocellulose Feedstocks for the Production of Lactic Acid

M. Neureiter, H. Danner, L. Madzingaidzo, H. Miyafuji, C. Thomasser, J. Bvochora, S. Bamusi and R. Braun

Abstract
Conventional substrates based on starch or sucrose are not available in sufficient quantities and at reasonable prices to provide the feedstock for the production of chemical bulk products. Lignocellulosic materials are an attractive alternative since they are abundant and usually low-priced. However, it is comparatively difficult to convert these materials into fermentable sugars. The conversion process usually includes high temperatures and/or the use of chemicals, which leads to the liberation of compounds that are toxic to microorganisms. In order to use the substrate most efficiently, it appears to be necessary that also hemicellulose derived pentoses are utilized in the fermentation process. This paper presents a process for the production of lactic acid from lignocellulosic biomass, which is based on the research conducted at IFA-Tulln in the past years. The use of lignocellulose as fermentation feedstock requires: 1) an effective treatment in order to obtain fermentable sugars, 2) a detoxification procedure, which results in a fermentable sugar solution, and 3) a fermentation process providing high yields and productivities from the respective substrate. The presented data include the evaluation of softwood and straw regarding their suitability for dilute acid pretreatment and the determination of optimal hydrolysis conditions for the recovery of hemicellulose derived sugars. Furthermore, toxic compounds from different hydrolysates have been identified and methods for the detoxification of hydrolysates are discussed. It could be demonstrated that dilute acid hydrolysates from lignocellulosic raw materials can be fermented to lactic acid by thermophilic Bacillus strains. An overall process including continuous fermentation in a membrane bioreactor including product purification by electrodialysis is introduced and evaluated.

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Keywords
lignocellulose, dilute-acid hydrolysis, lactic acid, fermentation, thermophilic strains