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. Methods and Results:  Different classes of micro‐organisms were tested in a medium containing mainly acetone and 4‐hydoxybenzaldehyde. Of the micro‐organisms tested, only bacteria were able to synthesize significant amounts of 4‐hydroxybenzylidene acetone, ranging from 15 to 160 mg l−1 after 21 h of bioconversion, as a function of the bacteria tested. Conclusions:  The biological production of 4‐hydroxybenzylidene acetone has been described with bacteria possessing 2‐deoxyribose‐5‐phosphate aldolase (DERA, EC 4·1·2·4). This result suggests that DERA is involved in the catalytic aldolization of precursors for the production of 4‐hydroxybenzylidene acetone. Significance and Impact of the Study:  Raspberry ketone or frambinone represents a total market value of between €6 million and €10 million. The possibility of producing its direct precursor through a simple process using bacteria is of considerable interest to the flavour market and the food industry as a whole. This paper broadens the spectrum for the use of aldolase to achieve the biological synthesis of compounds of interest.

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The sensitivity of Sporidiobolus ruinenii yeast to the use of reducing agents, reflected in changes in the oxidoreduction potential at pH 7 (Eh7) environment, ricinoleic acid methyl ester catabolism, gamma-decalactone synthesis, cofactor level, beta-oxidation activity, and ultrastructure of the cell, was studied. Three environmental conditions (corresponding to oxidative, neutral, and reducing conditions) were fixed with the use of air or air and reducing agents (hydrogen and dithiothreitol). Lowering Eh7 to neutral conditions (Eh7 = +30 mV and +2.5 mV) favoured the production of lactone more than the more oxidative condition (Eh7 = +350 mV). In contrast, when a reducing condition was used (Eh7 = -130 mV), the production of gamma-decalactone was very low. These results were linked to changes in the cofactor ratio during lactone production, to the beta-oxidation activity involved in decanolide synthesis, and to ultrastructural modification of the cell.

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Aims:  To investigate the impact of different gaseous atmospheres on different physiological parameters in the brewing yeast Saccharomyces cerevisiae BRAS291 during batch fermentation. Methods and Results:  Yeasts were cultivated on a defined medium with a continuous sparging of hydrogen, helium and oxygen or without gas, permitting to obtain three values of external redox. High differences were observed concerning viable cell number, size and metabolites produced during the cultures. The ethanol yields were diminished whereas glycerol, succinate, acetoin, acetate and acetaldehyde yields were enhanced significantly. Moreover, we observed major changes in the intracellular NADH/NAD+ and GSH/GSSG ratio. Conclusions:  The use of gas led to drastic changes in the cell size, primary energy metabolism and internal redox balance and Eh. These changes were different depending on the gas applied throughout the culture. Significance and Impact of the Study:  For the first time, our study describes the influence of various gases on the physiology of the brewing yeast S. cerevisiae. These influences concern mainly yeast growth, cell structure, carbon and redox metabolisms. This work may have important implications in alcohol‐related industries, where different strategies are currently developed to control better the production of metabolites with a particular attention to glycerol and ethanol.