Gilles Feron

Prospects for the microbial production of food flavours

The microbial production of natural flavours has been extensively studied during the past decade, and some processes are currently being exploited commercially. However, in the case of flavour compounds such as lactones and some aromatic compounds, the lack of basic information on their metabolism, as well as their high toxicity, are two main barriers to industrial production. The development of novel and cheap production processes, such as solid-state fermentation, may help overcome some of the current limitations of microbial flavour production, as well as widening the spectrum of biotechnologically accessible compounds.

Addition of oxidizing or reducing agents to the reaction medium influences amino acid conversion to aroma compounds by Lactococcus lactis.

Aims:  The aim of this research was to investigate the impact of extracellular redox potential (Eh) on amino acid conversion to aroma compounds by Lactococcus lactis that is commonly used as a starter in the cheese industry.

Influence of cell immobilization on the production of benzaldehyde and benzyl alcohol by the white-rot fungi Bjerkandera adusta, Ischnoderma benzoinum and Dichomitus squalens

Abstract Three white-rot basidiomycetes, Bjerkandera adusta, Ischnoderma benzoinum and Dichomitus squalens, were cultivated on a liquid medium supplemented with l-phenylalanine, a precursor for benzaldehyde (bitter almond aroma) and benzyl alcohol. Remarkable amounts of benzaldehyde (587 mg l−1) were found in cultures of B. adusta. Immobilization of this fungus on polyurethane foam cubes allowed an 8.3-fold increase of the production of benzaldehyde and a 15-fold increase of the productivity as compared with non-immobilized cells. Aryl-alcohol oxidase activity was only detected in B. adusta. This activity was also significantly enhanced in immobilized cells, suggesting that it plays an important role in benzaldehyde biosynthesis. Conversely, consistent amounts of benzyl alcohol (340 mg l−1 for B. adusta and I. benzoinum and 100 mg l−1 for D. squalens) were produced by the three fungi when immobilized. Laccase activity was found only in the strains I. benzoinum and D. squalens. This activity was markedly enhanced in free cells cultures. Immobilization of the fungi did not promote benzyl alcohol production by comparison with free cell cultures (500 mg l−1).

In Situ Detoxification of the Fermentation Medium during γ-Decalactone Production with the Yeast Sporidiobolus salmonicolor

γ‐Decalactone (γ‐C10) is known to be highly toxic for the microorganims used for its production. In this work, three techniques were studied in order to overcome this toxicity during a bioconversion process using ricinoleic acid as precursor of the lactone: in situ trapping in oily phases, in porous hydrophobic sorbents and in β‐cyclodextrins. Oily phases added to the media (olive, Miglyol, tributyrin, and paraffin) had a protective effect on Sp. salmonicolor, and they improved the lactone production. β‐cyclodextrins, which have a hydrophobic cavity that can trap molecules such as γ‐C10, have been used to protect the yeasts. The results showed insufficient preservation of cell viability. Some sorbents (activated carbon and polystyrene‐based sorbents) were successfully tested during bioconversion. In all cases viability exceeded the reference one. Nevertheless the aroma production was 30% lower than the reference. All of these solutions led to some enhancement of the cell viability during bioconversion of methyl ricinoleate to γ‐C10. For improvement of the lactone production, the oil trapping method seemed to be the best with the experimental conditions tested.

Production of γ-decalactone and 4-hydroxy-decanoic acid in the genus Sporidiobolus

Within the genus Sporidiobolus, S. odorus has been widely reported as having the ability to convert castor oil or its derivatives to γ-decalactone, but other species have also shown potential for lactone production. In this work, the bioconversion of ricinoleic acid methyl ester to γ-decalactone was studied with four species of Sporidiobolus: S. salmonicolor, S. ruinenii, S. johnsonii, and S. pararoseus. With 4.1 g/l of ricinoleic acid methyl ester, only S. salmonicolor and S. ruinenii were able to produce γ-decalactone (12 and 40 mg/l respectively). S. johnsonii and S. pararoseus did not produce any lactone in spite of consuming the precursor. The four strains showed different sensitivities to lactone. Between S. salmonicolor and S. ruinenii, the latter was the best biocatalyst. During four successive batch cultivations in a 7-l bioreactor, 5.5 g/l of γ-decalactone was produced with S. ruinenii in each 10-d run. This was essentially due to its ability to produce the open form of γ-decalactone (i.e., 4-hydroxy-decanoic acid), which is far less toxic than the lactone.

Contribution of exofacial thiol groups in the reducing activity of Lactococcus lactis.

Lactococcus lactis can decrease the redox potential at pH 7 (Eh7) from 200 to −200 mV in oxygen free Man–Rogosa–Sharpe media. Neither the consumption of oxidizing compounds or the release of reducing compounds during lactic acid fermentation were involved in the decrease in Eh7 by the bacteria. Thiol groups located on the bacterial cell surface appear to be the main components that are able to establish a greater exchange current between the Pt electrode and the bacteria. After the final Eh7 (−200 mV) was reached, only thiol‐reactive reagents could restore the initial Eh7 value. Inhibition of the proton motive force showed no effect on maintaining the final Eh7 value. These results suggest that maintaining the exofacial thiol (–SH) groups in a reduced state does not depend on an active mechanism. Thiol groups appear to be displayed by membrane proteins or cell wall‐bound proteins and may participate in protecting cells against oxidative stress.

Microbial production of 4-hydroxybenzylidene acetone, the direct precursor of raspberry ketone.

Aims:  To investigate the enzymatic aldol reaction between acetone as a donor and 4‐hydroxybenzaldehyde as a receptor to generate 4‐(4‐hydroxyphenyl)‐but‐3‐ene‐2‐one or 4‐hydroxybenzylidene acetone, the direct precursor of 4‐(4‐hydroxyphenyl)‐butan‐2‐one or raspberry ketone, using different species of filamentous fungi and bacteria.

Gaseous environments modify physiology in the brewing yeast Saccharomyces cerevisiae during batch alcoholic fermentation

Aims:  To investigate the impact of different gaseous atmospheres on different physiological parameters in the brewing yeast Saccharomyces cerevisiae BRAS291 during batch fermentation.

Screening of lactic acid bacteria for reducing power using a tetrazolium salt reduction method on milk agar.

Reducing activity is a physiological property of lactic acid bacteria (LAB) of technological importance. We developed a solid medium with tetrazolium dyes enabling weakly and strongly reducing LAB to be discriminated. It was used to quantify populations in a mixed culture (spreading method) and screen strains (spot method).