Chirality of the gamma-lactones produced by Sporidiobolus salmonicolor grown in two different media

Biotransformation of ricinoleic acid to γ-decalactone using novel yeast strains and process optimization by applying Taguchi model

Flavour molecules are generated now-a-days through microbial fermentation on a commercial scale. γ-Decalactone (GDL) is an important molecule due to its long-lasting flavouring impact as buttery, coconut and peach-type. In the current study, 33 microorganisms were isolated from different fruit sources, and their screening for target GDL production was performed. Using DNA sequencing, two potential strains yielding good amounts of GDL were identified from pineapple and strawberry fruits. The identified strains were Metschnikowia vanudenii (OP954735) and Candida parapsilosis (OP954733), and further optimized by Taguchi method. The effectiveness of lactone production is influenced by the rate of microbial growth under various operating conditions. The factors such as substrate concentration, pH, temperature, cell density and rotation (rpm) with 3 levels were applied for the GDL production using M. vanudenii (OP954735) and C. parapsilosis (OP954733) strains. The results revealed that the highest molar conversion of GDL was 24.69% (115.7 mg/g quantitative yield) and 52.69% (272.0 mg/g quantitative yield) at the optimal conditions using SB-62 and PA-19 strains, respectively. The two novel strains are reported for the first time for production of γ-decalactone and overall, this study opens up the possibility of using Taguchi design for large scale up process development for producing food flavours utilising environmentally friendly natural strains.

Microbial Biosynthesis of Lactones: Gaps and Opportunities towards Sustainable Production

Lactones are volatile organic compounds widely present in foods. These chemicals are applied as flavors and fragrances in the food, cosmetics and pharmaceutical industries. Recently, the potential of lactones as green solvents and fuel precursors reinforced their role as platform compounds of future bio-based economies. However, their current mode of production needs to change. Lactones are mainly obtained through chemical synthesis or microbial biotransformation of hydroxy fatty acids. The latter approach is preferred but still needs to use more sustainable substrates. Hydroxy fatty acids are non-abundant and non-sustainable substrates from environmental, health and economic points of view. Therefore, it is urgent to identify and engineer microorganisms with the rare ability to biosynthesize lactones from carbohydrates or renewable lipids. Here, we firstly address the variety and importance of lactones. Then, the current understanding of the biosynthetic pathways involved in lactone biosynthesis is presented, making use of the knowledge acquired in microorganisms and fruits. From there, we present and make the distinction between biotransformation processes and de novo biosynthesis of lactones. Finally, the opportunities and challenges towards more sustainable production in addition to the relevance of two well-known industrial microbes, the filamentous fungus Ashbya gossypii and the yeast Yarrowia lipolytica, are discussed.

Metabolic engineering of Ashbya gossypii for deciphering the de novo biosynthesis of γ-lactones

Background

Lactones are highly valuable cyclic esters of hydroxy fatty acids that find application as pure fragrances or as building blocks of speciality chemicals. While chemical synthesis often leads to undesired racemic mixtures, microbial production allows obtaining optically pure lactones. The production of a specific lactone by biotransformation depends on the supply of the corresponding hydroxy fatty acid, which has economic and industrial value similar to γ-lactones. Hence, the identification and exploration of microorganisms with the rare natural ability for de novo biosynthesis of lactones will contribute to the long-term sustainability of microbial production. In this study, the innate ability of Ashbya gossypii for de novo production of γ-lactones from glucose was evaluated and improved.

Results

Characterization of the volatile organic compounds produced by nine strains of this industrial filamentous fungus in glucose-based medium revealed the noteworthy presence of seven chemically different γ-lactones. To decipher and understand the de novo biosynthesis of γ-lactones from glucose, we developed metabolic engineering strategies focused on the fatty acid biosynthesis and the β-oxidation pathways. Overexpression of AgDES589, encoding a desaturase for the conversion of oleic acid (C18:1) into linoleic acid (C18:2), and deletion of AgELO624, which encodes an elongase that catalyses the formation of C20:0 and C22:0 fatty acids, greatly increased the production of γ-lactones (up to 6.4-fold; (7.6 ± 0.8) × 10³ µg/g_{Cell Dry Weight}). Further substitution of AgPOX1, encoding the exclusive acyl-CoA oxidase in A. gossypii, by a codon-optimized POX2 gene from Yarrowia lipolytica, which encodes a specific long chain acyl-CoA oxidase, fine-tuned the biosynthesis of γ-decalactone to a relative production of more than 99%.

Conclusions

This study demonstrates the potential of A. gossypii as a model and future platform for de novo biosynthesis of γ-lactones. By means of metabolic engineering, key enzymatic steps involved in their production were elucidated. Moreover, the combinatorial metabolic engineering strategies developed resulted in improved de novo biosynthesis of γ-decalactone. In sum, these proof-of-concept data revealed yet unknown metabolic and genetic determinants important for the future exploration of the de novo production of γ-lactones as an alternative to biotransformation processes.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1113-1) contains supplementary material, which is available to authorized users.

New biotransformation process for production of the fragrant compound γ-dodecalactone from 10-hydroxystearate by permeabilized Waltomyces lipofer cells

A new biotransformation process for the production of the flavor lactone was developed by using permeabilized Waltomyces lipofer, which was selected as an efficient γ-dodecalactone-producing yeast among 10 oleaginous yeast strains. The optimal reaction conditions for γ-dodecalactone production by permeabilized W. lipofer cells were pH 6.5, 35°C, 200 rpm, 0.7 M Tris, 60 g/liter of 10-hydroxystearic acid, and 30 g/liter of cells. Under these conditions, nonpermeabilized cells produced 12 g/liter of γ-dodecalactone after 30 h, with a conversion yield of 21% (wt/wt) and a productivity of 0.4 g/liter/h, whereas permeabilized cells obtained after sequential treatments with 50% ethanol and 0.5% Triton X-100 produced 46 g/liter of γ-dodecalactone after 30 h, with a conversion yield of 76% (wt/wt) and a productivity of 1.5 g/liter/h. These values were 3.7- and 3.8-fold higher than those obtained using nonpermeabilized cells. These are the highest reported concentration, conversion yield, and productivity for the production of the bioflavor lactone.

Volatile analysis of ground almonds contaminated with naturally occurring fungi

Aflatoxigenic aspergilli inflict major economic damage to the tree nut industry of California, with the highest negative impact to almonds. Aspergilli and fungi in general are known to emit volatiles in varying quantity and composition dependent upon their growth media. The goal of the study was to determine the volatile emission of whole and blanched almonds that had been picked out and labeled as inedible by processors. The aflatoxin content and number of colony forming units of each sample were also determined. A total of 23 compounds were consistently detected and identified. Several volatiles from the blanched almonds demonstrated significant increases when compared to the emissions of whole almonds. Several of these volatiles are considered fatty acid decomposition products and included hexanal, heptanal, octanal, nonanal, 3-octen-2-one, tetramethylpyrazine, and decanal. The almond samples investigated were characteristic of a typical postharvest environment and illustrative of potential contamination within a stockpile or transport container. Volatiles indicative of fatty acid decomposition were predominant in the samples that underwent some form of blanching. The emission amounts of hexanal, heptanal, octanal, and hexanoic acid increased 3-fold in samples contaminated with aflatoxin; however, due to variability between samples they could not be considered as indicator volatiles for aflatoxin content. The emission profile of volatiles from almond kernels contaminated with naturally occurring aspergilli and associated fungi is heretofore unreported.

Catabolism of hydroxyacids and biotechnological production of lactones by Yarrowia lipolytica

The gamma- and delta-lactones of less than 12 carbons constitute a group of compounds of great interest to the flavour industry. It is possible to produce some of these lactones through biotechnology. For instance, gamma-decalactone can be obtained by biotransformation of methyl ricinoleate. Among the organisms used for this bioproduction, Yarrowia lipolytica is a yeast of choice. It is well adapted to growth on hydrophobic substrates, thanks to its efficient and numerous lipases, cytochrome P450, acyl-CoA oxidases and its ability to produce biosurfactants. Furthermore, genetic tools have been developed for its study. This review deals with the production of lactones by Y. lipolytica with special emphasis on the biotransformation of methyl ricinoleate to gamma-decalactone. When appropriate, information from the lipid metabolism of other yeast species is presented.