Repeated-Batch Ethanol Production from Sweet Sorghum Juice by Saccharomyces cerevisiae Immobilized on Sweet Sorghum Stalks

Pongthep Ariyajaroenwong,Lakkana Laopaiboon,Pattana Laopaiboon,Prasit Jaisil

doi:10.3390/en5041215

Abstract

Sweet sorghum stalks were used as a low cost carrier for immobilization of Saccharomyces cerevisiae NP 01 to produce ethanol from sweet sorghum juice. The effects on ethanol production of carrier size (6 × 6 × 6 to 20 × 20 × 20 mm3) and initial cell concentrations (5 × 107 to 2 × 108 cells mL−1) for cell immobilization were investigated. The ethanol production medium was the juice containing 230 g L−1 of total sugar without nutrient supplementation. The fermentations were carried out under static conditions in 500-mL air-locked Erlenmeyer flasks at 30 °C. The results showed that the optimum size of sorghum stalk pieces for repeated-batch ethanol production was 6 × 6 × 6 mm3, while the optimum initial cell concentration for the immobilization was 1.0 × 108 cells mL−1. The immobilized yeast under these conditions could be used for at least eight successive batches without any losses of ethanol production efficiencies. The average ethanol concentration, productivity and yield of the eight successive batches were 99.28 ± 3.53 g L−1, 1.36 ± 0.05 g L−1 h−1 and 0.47 ± 0.03 g g−1, respectively.

Highlights

Bioethanol has emerged as one of the most viable options in the area of non-conventional sources of energy [1]
The aims of this research were to study the ability of sweet sorghum stalk pieces as the low-cost carrier for the immobilization of S. cerevisiae cells for ethanol fermentation from sweet sorghum juice and to investigate their stability in repeated-batch ethanol fermentation from the juice
The cell numbers in the 20 × 20 × 20 and 12 × 12 × 12 mm3 sorghums were 0.25 and 0.17 log scale higher than that of the 6 × 6 × 6 mm3 sorghum. This finding was supported by the results of Yu et al [18] who reported that if the total mass of the carrier was equal, the large pieces had more intact stalk cells than the small ones, and more yeast cells could be immobilized on every unit

Summary

Introduction

Bioethanol has emerged as one of the most viable options in the area of non-conventional sources of energy [1]. Apart from the raw materials i.e., sugarcane, molasses, corn and cassava that are widely used for industrial ethanol production, one of the prime sources of raw material being investigated for ethanol is sweet sorghum. The juice from its stalks contains a large amount of fermentable sugar (sucrose, glucose and fructose) and many essential trace elements suitable for microbial growth and ethanol production [3,4]. It can be cultivated at most temperatures and tropical climate areas with the growing period of 120–150 days [5]. In Thailand, the growing period of sweet sorghum cv

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