Simultaneous Saccharification and Fermentation of Orange Processing Waste to Ethanol Using Kluyveromyces marxianus

Abstract

Thermotolerant yeast Kluyveromyces marxianus performance was evaluated at elevated temperatures (37C to 45C) in batch simultaneous saccharification and fermentations (SSFs) using deterpenized high-solids (23.5% dry solids) orange processing waste (CPW) and a mixture of commercial pectinolytic, cellulolytic, and hemicellulolytic enzymes. Effects of inoculum size, temperature, pH, and nutritional supplements on ethanol production were studied. SSFs done at 40C 1C inoculated with K. marxianus cultures containing 0.126 0.027 mg cells g-1 CPW did not perform as well as SSFs with a 10 higher inoculation (1.26 0.27 mg cells g-1 CPW). Using the low inoculum, the highest ethanol production (2.5 wt%) was achieved with the addition of calcium carbonate sufficient to maintain pH above 4.8 and supplemented with ammonium sulfate or yeast extract . SSFs with the higher K. marxianus inoculum produced significantly higher ethanol concentrations of 3.4 to 3.5 wt% (84% to 86% theoretical yield) after 48 h for all treatments at 37C and for the lower carbonate concentrations (pH 4.0 to 4.2) at 40C. These results were equivalent to the ethanol produced (3.5 wt%, 86% theoretical yield) in SSFs using Saccharomyces cerevisiae at the same inoculum level after 48 h at 37C. Ethanol was also produced in SSFs at 42C and 45C with K. marxianus, but in significantly lower (18% to 38%) amounts. S. cerevisiae does not perform well above 37C, and K. marxianus may therefore be suitable for SSF of CPW in tropical and subtropical climates where citrus is processed and costs to cool and maintain temperatures below 35C are higher than in temperate regions.