## 4 Discussion  
Meta-industry, as a term introduced for the first time in this paper, concerns the sustainable waste valorization amenable for the simultaneous pollutant load reduction and the subsequent added-value metabolites production. Dairy and cheese-making industries have always been pillars of the economy, in both international and national (Greek) level. However, cheese-making practice leads to the production of enormous cheese whey quantities, a by-product with high organic load. In the present study, sustainable and eco-friendly conversions of second CW, a pollutant wastewater with destructive effects when released into the environment, but with novel products when properly utilized [4; 23; 32], were performed and important microbial metabolites (yeast biomass, lipids, and polysaccharides) were produced. In our research, the capability of seven yeast strains to catabolize lactose and accumulate lipids was evaluated. Out of the seven strains, five of them were capable to assimilate lactose without previously hydrolyzing it at the extra-cellular level (in accordance with [PERSON] et al. [26]) and grow on this medium. Subsequently, these microorganisms were cultivated in pretreated second CW, without any addition of extra elements. CW was primarily used for whey cheese \"Mizithra\" manufacture, and the second CW, which was left over after this process, was treated through centrifugation and filtration, to remove protein substances. From the obtained results, it was revealed that _C. curvatus_ ATCC 20509 demonstrated the highest potential concerning SCO production, and, for this reason, it was further studied in trials with different C/N and initial lactose concentrations, but also in fed-batch trials with second CW and pulsed lactose additions into the medium. In the current culture condi  
Figure 5: (**a**) Separation of neutral lipids fractions, extracted by the five strains dry biomass, cultivated in second CW through thin layer chromatography. a: triacylglycerlevel standard; b: free fatty acids standard; c: cholesterol standard; d: C. _curatus_ ATCC 20509/74 h; e: _C. curvatus_ NRRL Y-1511/218 h; f: _P. laurentii_ NRRL Y-2536/119 h; g: _P. laurentii_ NRRL Y-3594/101 h; h: _D. hansenii_ ACA-DC 5079/170 h of cultivation. (**b**) Separation of neutral lipids fractions, extracted at different time spots during the fed-batch cultivation of _C. curvatus_ ATCC 20509, through thin layer chromatography. a: cholesterol standard; b: free fatty acids standard; c: triacylglycerlevel standard; d: 70 h; e: 154 h; f: 178 h; g: 257 h; h: 312 h of cultivation. tions, it has been indicated that the initial C/N ratio \(\approx\)56 moles/moles, although, according to the literature being considered as quite high and amenable to promote considerable lipid accumulation [27; 33; 54; 55; 57], it was revealed as inappropriate to induce noticeable storage of lipid inside the yeast structures. On the other hand, C/N ratios >100 moles/moles seemed capable to enhance intra-cellular lipid storage, especially for C. _curvatus_ ATCC 20509. Specifically, increased biomass (=44 g/L), containing 36% \(w/w\) intracellular lipids, was recorded for this strain at initial C/N ratio adjusted to c. 124 moles/moles and initial lactose concentration \(\approx\)118 g/L (Table 3). Moreover, fed-batch culture, initiated with treated second CW and accomplished with pulsed lactose quantities, resulted in high lipid accumulation, 57% \(w/w\) of lipids content (21.7 g/L) in 38.1 g/L of total dry biomass. Various treatment techniques have been applied in second CW to reduce protein content and achieve high lipid accumulation. C/N ratio increase (from 25 to 70 mole/mole) in _Apiotrichum curcumin_ ATCC 20509 cultures, led to higher lipid content (from 18% to 58% \(w/w\)) [37]. Cultivation of _Candida curvata_ D in ultra-filtrated cheese whey permeate led to 13.8 g/L dry biomass with 55% intra-cellular lipids [36]. In their research, [PERSON] et al.