Rapid screening of toxicity to thermotolerant yeasts: inhibition of growth and fermentation by ionic liquids and zwitterions†

For extremely efficient bioethanol production, simultaneous pretreatment, saccharification, and fermentation in the same reaction pot (called a one-pot process) is necessary. Thermotolerant yeast Kluyveromyces marxianus can ferment at around 50 °C and is thus suitable for this process. We have developed cellulose-dissolving zwitterionic liquids, which are suitable pretreatment solvents to enable a one-pot process. On the other hand, there are no studies of the toxicity to yeasts including Kluyveromyces marxianus. We here studied the toxicity after establishing the screening methods applicable to high temperature. The zwitterion was confirmed to be low-toxic in most cases, compared to the most famous cellulose-dissolving ionic liquid. We further subjected two natural zwitterions, trimethylglycine and L-carnitine, to the same screening. Trimethylglycine, especially, was low-toxic, while it does not dissolve cellulose. The inhibition of growth and fermentation depended on the ion species, concentration, microorganism species, and temperature.