Abstract
Lipid-based biofuels, such as biodiesel and hydroprocessed esters, are a central part of the global initiative to reduce the environmental impact of the transport sector. The vast majority of production is currently from first-generation feedstocks, such as rapeseed oil, and waste cooking oils. However, the increased exploitation of soybean oil and palm oil has led to vast deforestation, smog emissions and heavily impacted on biodiversity in tropical regions. One promising alternative, potentially capable of meeting future demand sustainably, are oleaginous yeasts. Despite being known about for 143 years, there has been an increasing effort in the last decade to develop a viable industrial system, with currently around 100 research papers published annually. In the academic literature, approximately 160 native yeasts have been reported to produce over 20% of their dry weight in a glyceride-rich oil. The most intensively studied oleaginous yeast have been Cutaneotrichosporon oleaginosus (20% of publications), Rhodotorula toruloides (19%) and Yarrowia lipolytica (19%). Oleaginous yeasts have been primarily grown on single saccharides (60%), hydrolysates (26%) or glycerol (19%), and mainly on the mL scale (66%). Process development and genetic modification (7%) have been applied to alter yeast performance and the lipids, towards the production of biofuels (77%), food/supplements (24%), oleochemicals (19%) or animal feed (3%). Despite over a century of research and the recent application of advanced genetic engineering techniques, the industrial production of an economically viable commodity oil substitute remains elusive. This is mainly due to the estimated high production cost, however, over the course of the twenty-first century where climate change will drastically change global food supply networks and direct governmental action will likely be levied at more destructive crops, yeast lipids offer a flexible platform for localised, sustainable lipid production. Based on data from the large majority of oleaginous yeast academic publications, this review is a guide through the history of oleaginous yeast research, an assessment of the best growth and lipid production achieved to date, the various strategies employed towards industrial production and importantly, a critical discussion about what needs to be built on this huge body of work to make producing a yeast-derived, more sustainable, glyceride oil a commercial reality.
Highlights
All microorganisms are composed of lipids usually comprising around 6 to 8% (w/w) of their dry cell weight [1]
Despite several advantages over plant oils [7, 8], it was not until recently that commercial production has commenced, in the form of a speciality oil [9, 10]. Within this topic this review focuses mainly on the upstream processes and aims to address three key questions: firstly, what central efforts have been undertaken within this field; secondly, what are the most popular and promising feedstocks, organisms, operation conditions and applications for oleaginous yeasts; and thirdly, how can the knowledge of the past performance aid commercialisation of affordable yet sustainable yeast lipid processes? To facilitate addressing those questions, oleaginous yeast performance data from the majority of published research articles concerning oleaginous yeast since 1972 was extracted, analysed and interpreted
Other examples include the expression of multiple plant fatty acid elongase genes in R. toruloides to produce long-chain MUFAs (C22:1 and C24:1) [186]; a flax delta-15 desaturase gene in L. starkeyi to obtain increased amounts of ALA, which in turn was converted into EPA and docosahexaenoic acid (DHA) [201]; a fungal delta-12/omega-3 desaturase and an algal delta-9 elongase gene in C. oleaginosus to enhance ALA production and produce nonnative long-chain polyunsaturated fatty acid (PUFA) (C20:2 and C20:3), respectively [194]
Summary
All microorganisms are composed of lipids usually comprising around 6 to 8% (w/w) of their dry cell weight [1]. Oleaginous yeast species of major scientific interest include Y. lipolytica, R. toruloides, C. oleaginosus, L. starkeyi, and R. glutinis, together accounting for over 50% of oleaginous yeasts cultured, typically because of their high attainable lipid content, substrate suitability, growth performance, or genetic tractability (Table 1).
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