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

摘要

Thermotolerant yeast Kluyveromyces marxianus performance was evaluated at elevated temperatures (37°C to 45°C) 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 40°C ± 1°C 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 37°C and for the lower carbonate concentrations (pH 4.0 to 4.2) at 40°C. These results were equivalent to the ethanol produced (3.5 wt%, 86% theoretical yield) in SSFs using Saccharomyces ce r evisiae at the same inoculum level after 48 h at 37°C. Ethanol was also produced in SSFs at 42°C and 45°C with K. marxianus , but in significantly lower (18% to 38%) amounts. S. cerevisiae does not perform well above 37°C, 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 35°C are higher than in temperate regions