# Microbial Biosynthesis of Lactones: Gaps and Opportunities towards Sustainable Production
## 4 Microbial Production
### Biotransformation
#### 4.1.2 Other Fungi  
The microbial production of lactones is dominated by the biotransformation processes employing _Y. lipolytica_. Nonetheless, there are other microorganisms that deserve attention. _Trichoderma harzianum_ was used for the production of \(\gamma\)-decalactone from castor oil. However, the titers obtained were modest, 260 mg/L after 7 days in a 14 L stirred bioreactor that involved several process difficulties related to appropriate mixing of oil, water and mycelia [68]. _S. salmonicolor_, the model used to study the conversion of lactones, was also exploited in bioreactor biotransformation of castor oil for the production of \(\gamma\)-decalactone [69]. The titers obtained in this work were also low for a traditional biotransformation process, only achieving a maximum of 208 mg/L \(\gamma\)-decalactone after 7 days and using a stepwise fed-batch strategy with an aeration rate between 1.0 or 1.5 vvm. Other strains such as _S. ruintenii_ achieved superior success in similar biotransformation processes, being able to produce 5.5 g/L \(\gamma\)-decalactone after \(\sim\)40 days in a 7 L bioreactor, in which the medium was renewed every 10 days. These differences between the different species could be justified by a selective production of the open form of \(\gamma\)-decalactone, 4-hydroxydecanoic acid, which is less toxic than the lactone by _S. ruintenii_. In this case, the lactone was probably formed outside the cell by spontaneous lactonization [25]. Inthese yeasts, high concentrations of methyl ricinoleate led to modifications at the cellular membrane level and consequently to cell death [70], indicating a diminished tolerance to these substrates in comparison with _Y. lipolytica_. Biotransformation of ricinoleic acid in \(\gamma\)-dodeclactone was also perfomed in _Pichia guilliermodii_, reinforcing the pathways proposed in the works with _S. salmonicolor_[71]. In this yeast, the production media did not influence the enantiomeric ratios of the lactones produced. Given the potential impact of different enantiomers on the sensory perception and aroma intensity of lactones, some works sought to decipher the production patterns of different fungi. In this context, _Y. lipolytica_ and _Kodamaea (Pichia) olmeri_ were studied for their biotransformation capacity of different hydroxy fatty acids into lactones through the \(\beta\)-oxidation pathway [72]. These yeasts were able to convert hydroxy fatty acids derived from palmitic acid (C16:1), oleic acid (C18:1) and linoleic acid (C18:2) into \(\gamma\)-declactone, \(\gamma\)-dodeclactone and cis-6-\(\gamma\)-dodecenolactone. The lactones were produced in the range of 50-150 mg/L from 1 g/L of pure or mixed hydroxy acids after 48 h. Interestingly, some differences were observed between these two yeasts. _Y. lipolytica_ was able to produce \(\gamma\)-declactone from the C16:1-derived hydroxy acid (monounsaturated) but not from the other two, C18:1-derived hydroxy acid (monounsaturated) and C18:2-derived hydroxy acid (polyunsaturated). This was a clear contrast with the well-known ability of this yeast to produce \(\gamma\)-declactone from ricinoleic acid (C18 monounsaturated hydroxy acid). _K. olmeri_ was able to produce \(\gamma\)-declactone from all the precursors studied. Regarding \(\gamma\)-dodeclactone and cis-6-\(\gamma\)-dodecenolactone, both yeasts had a similar behavior, producing these lactones from all the precursors with the exception of the C16:1-derived hydroxy acid (monounsaturated). The enantiomers of the lactones formed by these yeasts were also opposites: for the saturated \(\gamma\)-declactone and \(\gamma\)-dodeclactone, _Y. lipolytica_ predominantly produced the (S)-enantiomer and _K. olmeri_ the (R)-enantiomer, while for the unsaturated cis-6-\(\gamma\)-dodecenolactone, _Y. lipolytica_ predominantly produced the (R)-enantiomer and _K. olmeri_ the (S)-enantiomer [72].