Lipid production from hemicellulose with Lipomyces starkeyi in a pH regulated fed‐batch cultivation

Abstract

This study investigated lipid production from the hemicellulosic fraction of birch wood by the oleaginous yeast Lipomyces starkeyi. Birch wood chips were thermochemically pretreated by hot water extraction, and the liquid phase, containing 45.1 g/l xylose as the major sugar, 13.1 g/l acetic acid and 4.7 g/l furfural, was used for cultivations of L. starkeyi CBS1807. The hydrolysate strongly inhibited yeast growth; the strain could only grow in medium containing 30% hydrolysate at pH 6. At pH 5, growth stopped already upon the addition of about 10% hydrolysate. In fed-batch cultures fed with hydrolysate or a model xylose-acetic acid mixture, co-consumption of xylose and acetic acid was observed, which resulted in a pH increase. This phenomenon was utilized to establish a pH-stat fed-batch cultivation in which, after an initial feeding, hydrolysate or model mixture was connected to the pH-regulation system of the bioreactor. Under these conditions we obtained growth and lipid production in cultures grown on either xylose or glucose during the batch phase. In cultivations fed with model mixture, a maximum lipid content of 60.5% of the cell dry weight (CDW) was obtained; however, not all xylose was consumed. When feeding hydrolysate, growth was promoted and carbon sources were completely consumed, resulting in higher CDW with maximum lipid content of 51.3%. In both cultures the lipid concentration was 8 g/l and a lipid yield of 0.1 g/g carbon source was obtained. Lipid composition was similar in all cultivations, with C18:1 and C16:0 being the most abundant fatty acids.

Bioethanol and lipid production from the enzymatic hydrolysate of wheat straw after furfural extraction

“…The physiological basis of this difference on the same substrate is not clear; however, it fits well to results on other lignocellulose hydrolysate, where L. starkeyi -strains produced rather low lipid amounts compared to other oleaginous yeasts (Slininger et al 2016). The order of magnitude of the lipid yield of L. starkeyi was similar to a recent study in our group, where we obtained the highest lipid yield from hemicellulose hydrolysate that has been reported so far (Brandenburg et al 2016). Between the two tested species, R. babjevae seems to be the yeast of choice in terms of further research for biofuel production from lignocellulose.…”

“…Accordingly, all three yeasts, including the two more inhibitor sensitive oleaginous species could grow on undiluted hydrolysate. In earlier experiments, hydrolysate had to be diluted by a factor of 3–10, to get non-adapted yeasts to grow (Blomqvist et al 2011, Tiukova et al 2014, Brandenburg et al 2016). In the ethanol fermentation, the theoretical maximum of ethanol production was reached; in fact, slightly more ethanol was produced than it could be expected according to the measured amount of glucose in the medium.…”

“…Yeast cell biomass was quantified by OD measurements or direct in 2-mL samples by drying as described in Brandenburg et al (2016). Correlations were established for OD and dry matter.…”

“…For a rapid testing, glucose concentration of the medium was measured using Medi-Test Glucose strips (Macherey-Nagel, Düren, Germany) as described earlier (Blomqvist et al 2012). Lipids of the oleaginous yeasts were extracted using a modified Bligh & Dyer method (Brandenburg et al 2016, Folch et al 1957). One hundred milligrams of freeze-dried yeast cells were suspended in 1 ml 1 M HCl, soaked for 15 min and incubated for 1 h at 75 °C.…”

“…As lipid extraction is laborious and requires a substantial amount of fermentation medium and because it was not the purpose of this study to follow the kinetics of lipid formation, lipid concentration was only determined at the beginning and end of each fermentation with the oleaginous yeasts. For determination of the fatty acid composition, the fatty acids were methylated by addition of BF 3 according to Appelqvist (1968), and the methyl esters were analysed by gas chromatography (GC) as recently described (Olstorpe et al 2014, Brandenburg et al 2016). Fatty acids were identified according to standard mixture 68A (Nu-Check, Elysian, MN) and retention times.…”