Short CommunicationSingle cell oil production in solid-state fermentation by Microsphaeropsis sp. from steam-exploded wheat straw mixed with 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.
Section snippets
Microorganism
Microsphaeropsis sp. used in the present investigation is an endophytic fungus isolated from fresh stems of Sabina chinensis Linn Ant collected from Handan, Hebei in north China in February 2005. It could accumulate oil and secrete cellulase when cultured on the straw-based solid-state medium (Peng and Chen, 2007). The stock culture was maintained on a potato dextrose agar (PDA) slant at 4 °C.
Substrates
Steam-exploded wheat straw (SEWS) and wheat bran (WB) were used as substrates for the production of SCO.
Yields of oil and cellulase in SSF without adding cellulase
Fig. 1 shows the time course of oil accumulation and cellulase secretion under the SSF parameters as ratio of SEWS to WB 4:1, initial moisture content 75% and temperature 30 °C without adding cellulase. The yield of oil increased progressively with incubation time and reached the maximum 42 mg/gds after 10 d then decreased. The yield of cellulase increased before the fourth day and kept relatively stable from the fourth to the ninth day (the maximum 0.32 FPU/gds) and then decreased sharply. It
Conclusions
This research explored a novel method for the production of SCO in SSF from steam-exploded wheat straw mixed with wheat bran, using the endophytic fungus Microsphaeropsis sp. which is capable of accumulating SCO and of secreting cellulase. The cellulase produced by Microsphaeropsis sp. itself was limited, leading to low SCO yield (42 mg/gds), which could however be increased by adding cellulase to the solid-state medium, leading to a maximal SCO yield of 80 mg/gds.
Acknowledgement
This study is financially supported by National Basic Research Program of China (2004CB719700).
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