## 9 Metabolic Features and By-Products from _Candida stellata_ Activity
### Alternative Carbon Metabolism: Glycerol Production  
Glycerol can be used as food additive produced from fats and oils, from chemical synthesis, or by microbial fermentation [75]. Glycerol biosynthesis is an important side-reaction of glycolysis pathway produced by reduction of dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate (G3P) and by dephosphorylation of G3P to glycerol (Figure 2). The first step of this conversion is catalyzed by the enzyme NAD-dependent glycerol-3-phosphate dehydrogenase (Gpd) and, subsequently, the glycerol is formed by glycerol-3-phosphatase (Gpp). The enzyme Gpd is encoded as two isoforms by the GPD1 and GPD2 genes [76]. Yeast growth under hyperosmotic stress situation leads to the expression of GPD1 through the so-called HOG (High Osmolarity Glycerol) signaling pathway [77; 78; 79]. On the other hand, GPD2 is believed to help maintain the cell's intracellular redox balance.  
As mentioned previously, _C. stellata_ species exhibits unusual metabolism of sugar; it is usually considered a facultatively fermentative yeast characterized by a very low fermentation rate and high production of secondary metabolites as glycerol, acetaldehyde, acetoin, and succinic acid [67; 80]. In regard to glycerol formation, this behavior of _C. stellata_ is probably owing to low alcohol dehydrogenase activity (4-fold lesser than _S. cerevisiae_) and high glycerol-3-phosphate dehydrogenase activity (40-fold higher than _S. cerevisiae_); thus, this higher Gpd activity causes a strong deviation towards glycerol production [67] (Figure 2, in red).  
In oenology, the glycerol content is appreciated, because it imparts some sensory attributes to the wine. It is an important alcohol with a slightly sweet taste and viscous nature that contributes to the smoothness, consistency, and overall body in wine [82; 83]. Typically, glycerol concentration is higher in red than in white wines ranging from 1 to 15 g/L. The threshold taste level of glycerol is observed to 5.2 g/L in wine, whereas a change of viscosity is only perceived at 25 g/L of glycerol [84]. Also, it is known that its production is raised by the presence of sulfur dioxide, higher incubation temperature, and high-sugar concentration, but it is significantly influenced by yeast strain and species [85]. In particular, _C. stellata_ has typically been described as glycerol producer in wine elaboration [46; 68; 86; 87]. Glycerol concentrations between 9 and 14 g/L have been reported in wines elaborated with _C. stellata_, in contrast with lower amounts produced by _S. cerevisiae_ monoculture [46; 64; 87]. However, glycerol and ethanol content are inversely related; as consequence, the tendency of _C. stellata_ to form glycerol seems to be the reason for its low growth and fermentation rate [67; 81]. Other authors found an ethanol yield produced by _C. stellata_ comparable with that of _S. warrant/bayanus_ strains, although both produced significantly lower ethanol than _S. cerevisiae_[64]. By contrast, [PERSON] et al. [88] reported one _C. stellata_ strain with an ethanol yield (9.09 g/100 mL) and fermentative power (19 g CO\({}_{2}\) evolved) without significant differences from _S. cerevisiae_ (9.05 g/100 mL and 19.2 g CO\({}_{2}\) evolved, respectively).