Pilot Scale Production of Single Cell Oil by _Apiotrichum Brassica_ and _Pichia kudriavzevii_ from Acetic Acid and Propionic Acid  
[PERSON]  
1 Department of Agrobiotechnology, Institute of Environmental Biotechnology, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Str. 20, 3430 Tullah, Austria; [EMAIL_ADDRESS]  
[PERSON]  
2 TECNALIA, Basque Research and Technology Alliance (BRTA), Parque Tecnologico de Alava, C/Leonardo Da Vinci 11, 01510 Minano, Spain; [EMAIL_ADDRESS] (L.O. ); [EMAIL_ADDRESS] (T.D.) [PERSON]  
2 TECNALIA, Basque Research and Technology Alliance (BRTA), Parque Tecnologico de Alava, C/[PERSON] Da Vinci 11, 01510 Minano, Spain; [EMAIL_ADDRESS] (L.O. ); [EMAIL_ADDRESS] (T.D.) [PERSON]  
1 Department of Agrobiotechnology, Institute of Environmental Biotechnology, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Str. 20, 3430 Tullah, Austria; [EMAIL_ADDRESS]  
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dilemma can be partly overcome by the efficient reuse of waste biomass [5]. The range of substrates investigated in this context includes industrial by-products such as crude glycerol [6; 7] and many waste derived materials, such as biomass hydrolyses [8; 9; 10], food wastes [4], and volatile fatty acids (VFAs) [11; 12; 13; 14]. In particular, the use of VFAs offers several advantages. By employing a shortened anaerobic digestion (AD) process, VFAs can be produced from a range of heterogenous waste streams at low cost [15]. Such waste streams, like biowaste or food waste, are at the moment mainly composted or treated with AD to generate methane [16], but they contain various carbohydrates, proteins, and lipids that can be converted to VFAs during the hydrolysis and acidification stage of AD [13]. The uncomplicated pretreatment of the biomass and low associated costs, as well as a wide range of materials available for AD-based VFA production, make VFAs a promising carbon source for heterotrophic lipid production [2], and calculations suggest that VFA-based lipid production offers a high theoretical lipid yield [17]. VFAs at high concentrations or low pH-values, however, have a cytotoxic effect on the yeasts, since the undissociated, uncharged acids can pass the cell membrane at these conditions and change the intracellular pH [12]. The most common approach to preventing this effect in batch fermentations is using low VFA concentrations [10; 18; 19; 20]. Ultimately, this approach severely limits the volumetric lipid productivity. A way to overcome this hurdle is to implement a fed-batch process with an appropriate feed strategy. In this way, the amount of metabolized VFA can be increased while keeping the VFA concentrations in the reactor at a low level [6]. The present study describes how the use of an acidic VFA feed for pH control (pH-stat fed-batch) can be applied to produce lipids in the two yeast strains _Apiotrichum brassicae_ V134 and _Pichia kudriuzvevii_ V194. Both strains were recently identified as promising lipid producers. A screening of natural isolates showed that both strains are able to accumulate lipid contents above 55 g/100 g DCW using acetic acid (HAc) as a carbon source [21]. It was also demonstrated that both strains are not only able to utilize HAc, but also other VFAs such as propionic acid (HPr), butyric acid, valeric acid, and caproic acid [22]. These characteristics make both strains promising candidates for the purpose of the present study. Furthermore, the use of the species _Apiotrichum brassicae_ and _Pichia kudriuzvevii_ for the present study allows us to gain further insight into the lipid production potential of these oleaginous yeasts, since the literature on both strains is rather scarce in this context. Apart from the mentioned publications [21; 22] and our previous study [23], _Apiotrichum brassicae_, basionym _Trichosporon brassicae_ [PERSON] [24], is to the best of our knowledge only once described as a lipid producer [25].