Lignocellulosic biomass is an attractive raw material for the sustainable production of chemicals and materials using microbial cell factories. Most of the existing bioprocesses focus on second-generation ethanol production using genetically modified Saccharomyces cerevisiae, however, this microorganism is naturally unable to consume xylose. Moreover, extensive metabolic engineering has to be carried out to achieve high production levels of industrially relevant building blocks. Hence, the use of non-Saccharomyces species, or non-conventional yeasts, bearing native metabolic routes, allows conversion of a wide range of substrates into different products, and higher tolerance to inhibitors improves the efficiency of biorefineries. In this study, nine non-conventional yeast strains were selected and screened on a diluted hemicellulosic hydrolysate from Birch. Kluyveromyces marxianus CBS 6556, Scheffersomyces stipitis CBS 5773, Lipomyces starkeyi DSM 70295, and Rhodotorula toruloides CCT 7815 were selected for further characterization, where their growth and substrate consumption patterns were analyzed under industrially relevant substrate concentrations and controlled environmental conditions in bioreactors. K. marxianus CBS 6556 performed poorly under higher hydrolysate concentrations, although this yeast was determined among the fastest-growing yeasts on diluted hydrolysate. S. stipitis CBS 5773 demonstrated a low growth and biomass production while consuming glucose, while during the xylose-phase, the specific growth and sugar co-consumption rates were among the highest of this study (0.17 h^–1 and 0.37 g/gdw*h, respectively). L. starkeyi DSM 70295 and R. toruloides CCT 7815 were the fastest to consume the provided sugars at high hydrolysate conditions, finishing them within 54 and 30 h, respectively. R. toruloides CCT 7815 performed the best of all four studied strains and tested conditions, showing the highest specific growth (0.23 h^–1), substrate co-consumption (0.73 ± 0.02 g/gdw*h), and xylose consumption (0.22 g/gdw*h) rates. Furthermore, R. toruloides CCT 7815 was able to produce 10.95 ± 1.37 gL^–1 and 1.72 ± 0.04 mgL^–1 of lipids and carotenoids, respectively, under non-optimized cultivation conditions. The study provides novel information on selecting suitable host strains for biorefinery processes, provides detailed information on substrate consumption patterns, and pinpoints to bottlenecks possible to address using metabolic engineering or adaptive evolution experiments.

Lactic acid and its derivatives are widely used in pharmaceutical, leather, textile and food industries. However, until now there have been few systematic reports on fed-batch fermentation for efficient production and high concentration of l-lactic acid by lactic acid bacteria. This study describes the obtainment of L (+) lactic acid from sucrose using the Lactobacillus casei Ke11 strain through different feeding strategies using an accessible pH neutralizer such as CaCO₃. The exponential feeding strategy can increase lactic acid production and productivity (175.84 g/L and 3.74 g/L/h, respectively) with a 95% yield, avoiding inhibition by high initial substrate concentration and, combined with the selected agent controller, avoids the cellular stress that could be caused by the high osmotic pressure of the culture media. The purification of the acid using charcoal and celite, followed by the use of a cation exchange column proved to be highly efficient, allowing a high yield of lactic acid, high removal of sugars and proteins. The described process shows great potential for the production of lactic acid, as well as the simple, efficient and low-cost purification method. This way, this work is useful to the large-scale fermentation of L. casei Ke11 for production of l-lactic acid.

The aim of the present study was to isolate and characterize bacterial producers of optically pure D(-) and L(+) lactic acid for use in the synthesis of polymers employed in the production of resistive and biodegradable materials. Four hundred microorganisms were isolated. Of these strains, 20 had high capacity for production of only one lactic acid isomer which was selected for characterization. D(-) lactic acid producers were identified as Weissella paramesenteroides (25%), Leuconostoc mesenteroides (6.25%),Leuconostoc lactis (6.25%) and Lactobacillus delbrueckii (6.25%). L(+) lactic acid producers were identified as Lactobacillus casei (12.5%), Enterococcus sp. (18.75%),Lactobacillus paracasei (6.25%), Streptococcus sp. (6.25%) and Lactobacillus sp. (12.5%). The most promising isolate for L(+) lactic acid production was Ke8, in the presence of sucrose. Otherwise, Ke6 and Ke11 were the best producers, by using glucose as the carbon source. The best microorganism for the production of D(-) lactic acid was isolated from fermented milk (L. delbrueckii Y15C). However, the growth of this microorganism is easily inhibited on media containing sugar concentration greater than 60 g/L. Key words: Lactic acid bacteria, identification, characterization, carbon sources, lactic acid, optimization.