An novel immobilization method of Saccharomyces cerevisiae to sorghum bagasse for ethanol production

Abstract

Natural sorghum bagasse without any treatment was used to immobilize Saccharomyces cerevisiae at 0.6 ± 0.2 g dry cell weight (DCW)/g dry sorghum bagasse weight (DSW) through solid-state or semi-solid state incubation. The scanning electron microscopy (SEM) of the carriers revealed the friendship between yeast cells and sorghum bagasse are adsorption and embedding. The ethanol productivity of the immobilized cells was 2.24 times higher than the free cells. In repeated batch fermentation with an initial sugar concentration of 200 g/L, nearly 100% total sugar was consumed after 16 h. The ethanol yield and productivity were 4.9 g/g consumed sugar on average and 5.72 g/(L h), respectively. The immobilized cell reactor was operated over a period of 20 days without breakage of the carriers, while the free cell concentration in the effluent remained less than 5 g/L thoughout the fermentation. The maximum ethanol productivity of 16.68 g/(L h) appeared at the dilution rate of 0.3 h⁻¹.

Introduction

Immobilized cell technology has been suggested as an effective means for improving ethanol fermentation. The immobilization of cells leads to higher cell densities with consequent increases in reaction rates and productivity. As a result, shorter residence time and smaller reactor size can be employed.

Ethanol production by immobilized yeast cells has been extensively investigated during the last few decades. The most widely used immobilization methods are based on cell entrapment in gels, such as carrageen and Ca alginate (Luong, 1985, Godia et al., 1987, Hamamchi and Ryu, 1987). Other methods are based on passive adhesion to the surfaces, such as glass beads, stainless steel wire spheres (Van Haecht et al., 1985, Vega et al., 1987, Sho et al., 2001). For the former, the main drawback of these systems is the instability of Ca-alginate against phosphates and the disruption of gel particles due to CO₂ evolution during fermentation. For the second one, as the cells attach to the carriers’ surface by electrostatic interactions or covalent binding of the cells, the two major drawbacks are the limitation of the biomass loading by the carriers surface, and the effect of various factors that can cause cell desorption thereby limiting the operational stability.

Cells have been immobilized on a variety of natural and synthetic supports. One of the extensively used classes of natural support is lignocellulosic materials. Some lignocellulosic supports investigated include sawdust, wood chips/shavings, rice husk, and straw (Shukla et al., 1988, Das et al., 1993, Maryse and Zdravko, 1996). However, there are a few drawbacks, for example, some of these supports have non-uniform structures which are often present in a particulate, powder or chip form. The preparation of stable beds with such materials is difficult and pressure drops or floatation is often observed especially when viscose and high density substrates are employed. In these literatures, the immobilization methods are all based on the recirculation of the concentrated yeast suspension through the reactor.

Sorghum is a natural economical lignocellulosic material which grows well in different parts of the world and is produced in large quantities in most Asian countries, including China where it is used extensively in ethanol industry (Woods, 2000, FAO, 2002). Sorghum bagasse is highly fibrous with a good water retention capacity. The advantages of the Sorghum bagasse as yeast cells carrier are: (1) the surface of the Sorghum bagasse as represented in Fig. 1(a) is much more porous than the straw and wood chips are, which facilitates the transmission of substrates and products between carriers and medium. (2) As we can see from Fig. 1(b), there are lots of pores among the sorghum cells which make the sorghum bagasse integrative. This makes the transmission of substrates and products among the bagasse cells more easily. Both of them can solve the problem of mass transfer which occurs in the systems of calcium alginate and straw. The present paper describes the possibility of using sorghum bagasse as a support; delineates a novel and simple technique for the rapid and strong immobilization of cells in sorghum bagasse and also discusses the application of sorghum bagasse containing the yeast cells for the ethanol fermentation.