# Microbial Biosynthesis of Lactones: Gaps and Opportunities towards Sustainable Production
## 4 Microbial Production
### Biotransformation
#### 4.1.2 Other Fungi
In the few works using the baker's yeast, the most successful process was related to the utilization of _Saccharomyces cerevisiae_ to produce \(\delta\)-declactone from massoia lactones (2-decen-5-olide and 2-dodecen-5-olide) naturally present in the massoia bark oil of the Massoia tree (_Cryptocaria massoia_) [36]. Hydrogenation by _S. cerevisiae_ leads to the reduction of these unsaturated lactones into \(\delta\)-decalactone. Therefore, in a 2 L bioreactor several parameters were taken into account in order to optimize this biotransformation process: the presence of glucose as substrate, high oxygenation, 35 \({}^{\circ}\)C and an optimum pH near 6.0 resulted in high hydrogenation activity. The authors speculated that the high performance observed with glucose and high oxygenation was due to the fact that these conditions favored co-factor regeneration, namely of NAD(P)H and FADH2. Another interesting observation was related to the toxicity of both the substrate and the product towards the yeast. Increasing concentrations of both 2-decen-5-olide and \(\delta\)-decalactone resulted in a significant decrease in hydrogenation activities. However, this effect was counteracted by the addition of cyclodextrin to the medium, which formed a complex with lactones, thus reducing its inhibitory effect. In practical terms, this allowed the authors to instead add all the substrate at once by stepwise addition during the fermentation, consequently reducing the time required for the process. Overall, 1.2 g/L of \(\delta\)-decalactone was obtained by biotransformation of 1.7 g/L of 2-decen-5-olide after 8 h [36]. A patent application was filled concerning this subject. Several years later, [PERSON] and colleagues optimized the biotransformation of ricinoleic acid into \(\gamma\)-decalactone by _S. cerevisiae_ in shake-flask fermentation [73]. In this work, the yeast was able to produce \(\sim\)3.48 g/L from 40 g/L of ricinoleic acid in a medium where the addition of L-carnitine had a positive effect on shortening the biotransformation period and improving the lactone production. Moreover, according to the authors, addition of another lactone to the medium (\(\gamma\)-octadecone) prevented the degradation of \(\gamma\)-decalactone by competitively inhibiting a putative lactonase. This lactonase was not identified by the authors [73]. Other studies used _S. cerevisiae_ to convert 11-hydroxypalmitic acid or 3,11-dihydroxyoxynymistic acid into \(\delta\)-decalactone. Other precursors for the production of this lactose could also be the coriolic acid (13-hydroxyoctadeca-9,11-dienoic acid) obtained from the seed oil of _Coriana napalensis_[6]. A surprisingly good result was obtained by a specific strain of _Rlodototula aurantiaca_, isolated in the Antarctic. This specific strain, unlike others isolated in the same region, was able to produce 5.8 g/L and 6.6 g/L of \(\gamma\)-decalactone from 20 g/L of castor oil in a shake flask and a bioreactor, respectively. These titers were achieved after 6/7 days of culture. Similar levels of performance can only be found in patents [74]. Subsequently, gum was added to the process in order to increase titers and to diminished the toxicity of \(\gamma\)-decalactone, but with limited success [75]. As mentioned above, a critical point with biotransformation processes is to obtain hydroxylated fatty acids in abundance and in an economically feasible way in order to produce lactones. Therefore, several biotransformation processes had to include an additional hydroxylation step, which normally was performed by enzymatic peroxidation of polyunsaturated fatty acids [11]. However, some attempts tried to engineer microorganisms with the ability to produce ribonucle acid from oleic acid in order to provide a substrate for biotransformation processes. This was the case of the heterologous expression of the oleate \(\Delta\)12-hydroxylase from _Claviceps purpurea_ in _Schizosaccharomyces pombe_[76].