## 1 Introduction
Various ways of reducing the pollutant load, such as anaerobic digestion [10; 11; 12; 13], dark fermentation [14; 15], usage as animal feed [16], filtration to separate whey protein concentrates, or directly its dehydration (through mostly lyophilization process) and its subsequent use as food additives or supplements in athletes' diet [17; 18], have been proposed. On the other hand, cultivation of microorganisms into lactose-based substrates, to synthesize microbial compounds including biomass (and also mycelial mass of pharmaceutical mushrooms) [19; 20; 21; 22; 23], single-cell oil (microbial oil) [19; 24; 25; 26; 27], organic acids, such as lactate [28; 29], ethanol [30; 31], microbial biosurfactants [32], etc., have been reported in the literature. Second CW is the deproteinized by-product resulting from the manufacture of whey cheese (Mizithra, Anthotyro, Manouri), which is produced by thermal aggregation of whey proteins (at c. 90 \({}^{\circ}\)C), or from the deproteinization through filtration and is described as a yellow-green liquid, rich in lactose and salts [27]. Single cell oils (SCOs), lipids produced by single celled entities (viz. yeasts, molds, algae, and bacteria, which accumulate lipids to more than 20% \(w/w\) of total biomass), present a great industrial interest as alternative source of animal and vegetable oil and as starting materials for the synthesis of the so-called \"second generation\" biodiesel and various types of olechemicals [3; 33]. Oleaginous microorganisms, cultivated on sugars or related substrates, accumulate lipids through the de novo lipid production pathway when a nutrient source, mainly nitrogen, is exhausted. Through an anabolic biochemical process, by virtue of quasi-inverted \(\beta\)-oxidation reactions, acetyl-CoA generates fatty acids, which are then esterified with glycerol, generating structural and storage lipid compounds [34; 35]. The selection of microorganisms that could grow on a by-product like second CW, which contains mostly lactose as microbial carbon source, presupposes their capability to hydrolyze the disaccharide into glucose and galactose and subsequently to catalyze the liberated monosaccharides. Several studies have indicated the utilization of pretreated or non-pretreated CW, potentially blended with other co-substrates, for the cultivation of oleaginous yeasts such as _Candida curata_ D [36], _Apiotrichum curatum_ (_Cryptococcus curata_) ATCC 20509 [37; 38], various _C. curatatus_ stains [24], _C. curatatus_ KCTC 27583 [39], _C. laurentti_ 11 [25], _Yarrowin lipolytica_ B9 [40], _Debaryomyces etchellsii_ BM1 [41], _Wickerhamomyces anomalus_ [42], _C. curatus_ NRRL Y-1511 [26; 27], _C. laurentti_ UCD 68-201 [27], and _Cystobisidium oligophagum_ JRC1 [43]. Lipids that are produced through CW valorization from the mentioned yeast strains, given their abundance in monounsaturated fatty acids, could be used as non-conventional fatty materials amenable to be used in the synthesis of second generation biodiesel [33; 34]. On the other hand, and given that in the last years there has been a constant search for the identification of \"new\" and \"non-conventional\" proteins amenable to be used as feed or food supplements [34; 35], (lipid-rich) yeast cells produced through CW valorization could be also used for this purpose, since most of the mentioned yeast species (i.e., _D. hansenii, Y. lipolytica, C. curatatus_, etc.) as also many other oleaginous fungi have not been reported to produce toxic compounds or substances with \"anti-nutritional\" properties (in contrast, these microorganisms produce lipids that contain valuable cellular fatty acids) [34; 35]. Some of these such as _Y. lipolytica_ have already been classified as GRAS-type microorganisms [34]; therefore, another viable option would be the valorization of CW-based substrates by yeast fermentations in order to produce (lipid-rich) yeast biomass, amenable to be used as food or feed supplement. The aim of this study was the valorization of the second CW, through bioprocesses.