Single cell oil production in solid-state fermentation by Microsphaeropsis sp. from steam-exploded wheat straw mixed with wheat bran

Abstract

Microsphaeropsis sp. was used to produce SCO in solid-state fermentation (SSF) from a substrate consisting of steam-exploded wheat straw (SEWS) and wheat bran (WB). The yield of SCO was 42 mg/g dry substrate (gds) without adding cellulase. To achieve a higher SCO yield, cellulase was added to the solid-state medium, resulting in an increase of SCO from 42 to 74 mg/gds with a cellulase loading of 10 FPU/gds. Other SSF parameters such as ratio of SEWS to WB of the dry substrate, initial moisture content, and incubation temperature were optimized under the condition of cellulase loading of 10 FPU/gds. So optimized, the SCO yield was 80 mg/gds, and the SCO content of the dry fermented mass was 10.2%. This research explored a novel method to produce SCO from the abundant and cheap agricultural residues – wheat straw and wheat bran.

Introduction

Microbial oils, called single cell oils (SCO), have potential commercial applications as nutraceuticals, pharmaceuticals, and feed ingredients for aquaculture (Lewis et al., 2000) Recently, biodiesel, produced from oils (triglycerides) and alcohols by transesterification, has become more attractive because it is renewable and environmentally beneficial. SCO can, as recently reported (Miao and Wu, 2006, Xue et al., 2006), also be used as the feedstock for the production of biodiesel although vegetable oils are used as the main feedstock now, for in general their structure and composition are similar to those of common vegetable oils. This extends the industrial prospect of SCO. At present the high production cost of SCO is a major barrier to its commercialization (Ratledge, 2004). One of the main cost contributive parameters of SCO is that of the raw material. For the reduction of production cost, cheap raw materials have to be used.

During the last decade, solid-state fermentation (SSF), that is, cultivation of microorganisms on solid substrates in the absence or near absence of free water (Pérez-Correa and Agosin, 1999), has experienced particular interest due to many advantages of this bioprocess in comparison to submerged fermentation (SmF), e.g., smaller bioreactor volume, reduced downstream processing cost, higher productivity, simpler technique, reduced energy requirement, low wastewater output (Burke and Cairney, 1997, Kim et al., 1985). Fungi play a key role in SSF, for their hyphal development allows them to effectively colonize and penetrate the solid substrate (Pandey et al., 2000). Furthermore, they can utilize the bound water of the substrate and thus grow in the absence of free water (Tengerdy, 1985).

The main raw materials for the production of SCO are glucose (Papanikolaou et al., 2004), soluble starch (Chen and Liu, 1997), glycerol (Papanikolaou and Aggelis, 2002), and cereals (Conti et al., 2001), and the principal method is submerged fermentation (SmF) (Papanikolaou et al., 2004, Chen and Liu, 1997, Papanikolaou and Aggelis, 2002). Lignocellulose which composes more than 60% of plant biomass is a vast potential resource for biofuels, biofertilizers, animal feed and chemical feedstocks (Tengerdy and Szakacs, 2003). Straw is the most abundant agricultural residue in China with an annual production rate of about 700 million tons which can be microbiologically converted to industrial products, though not much reported to SCO. In the present research we used the abundant and cheap wheat straw as the main substrate to produce SCO by endophytic fungus Microsphaeropsis sp. in SSF, which may decrease production cost and increase production scale to satisfy the huge market demand of SCO.