## 4 Conclusions  
This review summarizes and discusses advances and developments in the biotechnological production of commercially important biochemicals using the yeast _Yarrowia lipolytica_. Towards the goal of building and ensuring a sustainable society, the bioconversion of waste materials into value-added compounds should be in the spotlight, and to that end, the biodiesel-derived waste glycerol is the focal point of the present review. By presenting significant accomplishments that integrate strain, medium, and process engineering, this review articulates the necessity to design production processes in a cross-disciplinary manner in order to achieve previously unattainable targets.  
Strain engineering strategies implemented in _Yarrowia_ mainly aimed at: (i) enhancing the biosynthetic pathway of interest (overexpression of _GUT1_, _GPD1_,2_, _SCT1_, _SLC1_,_DGA1,2_, PDAT, _LRO1_, etc.); (ii) increasing the availability of biosynthetic precursors (G3P, FFA, acetyl-CoA, mal-onyl-CoA, 6-PGL, Ribu-5-P, R-5-P, S-7-P, E-4-P, mevalonate, NPP, FPP, etc.); (iii) increasing the availability of redox and energy equivalents (manipulation of PPP, cofactor switch, overexpression of heterologous enzymes, co-feeding of carbon sources); (iv) introducing novel pathways (overexpression of heterologous enzymes e.g., _LS, FS_); and (v) shutting down consuming/degradation pathways (\(\beta\)-oxidation, deletion of _GUT2_, _TGL_, _EYD_, _EYK_, etc.). The performance of the producer strain was often further enhanced by medium and process optimization strategies. An optimized cultivation medium will guarantee fast microbial growth, increased production metrics (titer, rate, yield; TRY), and decreased accumulation of unwanted by-products to facilitate downstream processing. Just as importantly, it should be cheap to decrease the production costs and easily transferable to an industrial environment for a commercialized production process. Often, medium engineering can alter or tailor-make the profile of the synthesized bio-products (e.g., biomass-lipids-citric acid ratio, profile of lipids or polyols) by rearranging the involved carbon fluxes, balancing the redox supply or by supplying the required precursors. The herein presented medium engineering strategies involved: (i) fine-tuning of medium composition (C/N ratio, concentration of nutrients); (ii) supplementation with critical nutrients or elements (e.g., fatty acids, amino acids, minerals, vitamins); (iii) selection of the most favorable carbon source (glycerol in the vast majority of cases) or combination of different ones (co-feeding or \"doping\"); (iv) selection of the nitrogen source (inorganic or organic); and (v) type of medium buffering. The optimization of the bioprocess will then secure an ideal environment for the strain to acquire and utilize all the necessary nutrients in order to support cell growth and proliferation, and as well, the biosynthetic pathways leading to the target compounds. Process engineering strategies involved the optimization of critical process parameters (inoculum density, pH regulation or no regulation, oxygen supply, agitation) and the operational feeding mode (batch, fed-batch, continuous or hybrid process) in order for a high-yielding production process to be established.  
The presented examples clearly demonstrated not only the huge potential of glycerol as a favorable sustainable feedstock for _Yarrowia lipolytica_ bioprocesses and the outstanding biosynthetic routes leading to a wide spectrum of valuable compounds, but also how a bioprocess design based on integrated strategies can maximize the performance of a producer strain.