## 1 Introduction
Here, brewer's spent grain (BSG) as a by-product of the brewing industry was used as the feedstock to improve the overall renewability of the process. BSG is found mainly in livestock feed applications, however, it has significant biotechnological potential in the production of value-added products due to its lignocellulosic composition and chemical characteristics [21]. BSG has an estimated annual production of \(\sim\)3.4 million tons in the EU [22], therefore developing processes that take advantage of these large amounts are needed [23]. Apart from bioenergy, volatile fatty acids (VFAs) are produced as intermediates during the acidogenesis stage of AD [24]. VFAs are short-chain fatty acids (C2-C5), with the most common ones being acetic acid (C2), propionic acid (C3), and butyric acid (C4), and have been proven viable substrates for yeast cultivation [25]. Depending on the substrate, the microbial population, and the digestion conditions, a mixture of different types of VFAs in different ratios can be produced [26]. This VFA mixture can be further valorized for the production of SCO by different oleaginous microorganisms [3; 19; 27]. Apart from biodiesel production, SCO produced by oleaginous yeasts can contain important amounts of odd-chain fatty acids (OCFAs) when yeasts are grown in propionic acid. Although OCFAs, such as C15, C17, or less commonly C25 fatty acids, are found in small proportions in natural sources, their importance is very significant. The most common ones, C15:0 and C17:0, are found in low levels in whole fat milk (up to 1% of the total fatty acids) [28]. Certain types of fish are also sources of OCFAs, as well as some plants and bacteria, while it has also been reported to be present in ruminant fat [29; 30]. OCFAs are also present in human plasma, red blood cells, and liver [31; 32]. Additionally, C25:0 has been detected in glycosphingolipids located in the brain tissue [33]. Lately, OCFAs have attracted significant interest due to their association with several health benefits such as reducing the risk of metabolic disorders [34]. Additionally, C17:1 has been noted to exhibit good anti-inflammatory effects and can also regulate allergies, psoriasis, and autoimmune diseases [35]. OCFAs can also be used as biomarkers for the detection of potential coronary heart disease or diabetes type II mellitus since they are known to be inversely correlated to these medical conditions [33; 36]. However, the direct association of OCFAs in health-related problems needs to be studied extensively with more in vitro studies. Aside from their nutritional and biological applications, they can be used for the production of certain industrial chemicals, such as pesticides, flavor and fragrance compounds, hydraulic fluids plasticizers, and coatings [37]. Several microorganisms have been reported to produce OCFAs including thrust-tochytrids [38; 39; 40], yeasts, and bacteria [41] by using propionic acid, or 1-propanol as carbon sources [42]. These substrates can be converted to propionyl-CoA, the main biosynthetic precursor of OCFAs, by the action of propionyl-CoA synthetase [42]. However, microbial production of OCFAs results in very low yields, since the cell growth on the aforementioned substrates can be inhibited above a certain concentration. To solve this problem, genetic engineering has been applied to improve OCFAs production [37; 41]. There is still, however, a need to investigate new promising strains with the ability to tolerate increased concentrations of propionic acid while producing OCFAs. Similarly, the cultivation of those microorganisms needs to be further optimized in relation to the OCFAs yield. In this study, we investigated the potential of two oleaginous yeast strains, _Rhodotorula toruloides_ NCYC 1576 and _Cryptococcus curvatus_ DSM 101032, as natural producers of OCFAs and SCOs suitable for biodiesel. Initially, their tolerance and preference towards certain VFAs was assessed on synthetic VFAs.