Bart Vanderhaegen

Expression Levels of the Yeast Alcohol Acetyltransferase Genes ATF1, Lg-ATF1, and ATF2 Control the Formation of a Broad Range of Volatile Esters

ABSTRACT Volatile aroma-active esters are responsible for the fruity character of fermented alcoholic beverages such as beer and wine. Esters are produced by fermenting yeast cells in an enzyme-catalyzed intracellular reaction. In order to investigate and compare the roles of the known Saccharomyces cerevisiae alcohol acetyltransferases, Atf1p, Atf2p and Lg-Atf1p, in volatile ester production, the respective genes were either deleted or overexpressed in a laboratory strain and a commercial brewing strain. Subsequently, the ester formation of the transformants was monitored by headspace gas chromatography and gas chromatography combined with mass spectroscopy (GC-MS). Analysis of the fermentation products confirmed that the expression levels of ATF1 and ATF2 greatly affect the production of ethyl acetate and isoamyl acetate. GC-MS analysis revealed that Atf1p and Atf2p are also responsible for the formation of a broad range of less volatile esters, such as propyl acetate, isobutyl acetate, pentyl acetate, hexyl acetate, heptyl acetate, octyl acetate, and phenyl ethyl acetate. With respect to the esters analyzed in this study, Atf2p seemed to play only a minor role compared to Atf1p. The atf1Δ atf2Δ double deletion strain did not form any isoamyl acetate, showing that together, Atf1p and Atf2p are responsible for the total cellular isoamyl alcohol acetyltransferase activity. However, the double deletion strain still produced considerable amounts of certain other esters, such as ethyl acetate (50% of the wild-type strain), propyl acetate (50%), and isobutyl acetate (40%), which provides evidence for the existence of additional, as-yet-unknown ester synthases in the yeast proteome. Interestingly, overexpression of different alleles of ATF1 and ATF2 led to different ester production rates, indicating that differences in the aroma profiles of yeast strains may be partially due to mutations in their ATF genes.

Bioflavoring and beer refermentation

Various techniques are used to adjust the flavors of foods and beverages to new market demands. Although synthetic flavoring chemicals are still widely used, flavors produced by biological methods (bioflavors) are now more and more requested by consumers, increasingly concerned with health and environmental problems caused by synthetic chemicals. Bioflavors can be extracted from plants or produced with plant cell cultures, microorganisms or isolated enzymes. This Mini-Review paper gives an overview of different systems for the microbial production of natural flavors, either de novo, or starting with selected flavor precursor molecules. Emphasis is put on the bioflavoring of beer and the possibilities offered by beer refermentation processes. The use of flavor precursors in combination with non-conventional or genetically modified yeasts for the production of new products is discussed.

Applicability and trends of anaerobic pre-treatment of municipal wastewater

Abstract The most important motive for direct anaerobic treatment of municipal wastewater is the low sludge production. Indeed, treatment and disposal of sewage sludge is technically cumbersome and economically a heavy burden. Provided wastewater temperatures are a minimum of 10–15°C, the influent organics are higher than 500 mg/l and contain soluble high-energy COD, and provided levels of sulfate and suspended solids are low, direct anaerobic treatment of municipal wastewater can be performed satisfactorily. Yet, direct anaerobic treatment of municipal wastewater can only be considered as a pretreatment since sulfide is formed and nutrients, such as nitrogen and phosphate, are hardly removed. The high substrate threshold of methane-producing bacteria will always render direct anaerobic treatment of low strength wastewater less performing. It is argued that initial removal and digestion of the particulate organics, in combination with treatment of the soluble organics by means of a trickling filter with thick biofilm, might constitute an optimal way of maximizing anaerobic electron flux, resulting in an overall lower final sludge production and yet still achieving a high quality, low nutrient effluent.

Experimental demonstration of an all-optical 2R regenerator with adjustable decision threshold and "true" regeneration characteristics

Experimental 2R regeneration characteristics of an optical decision circuit based on a Mach-Zehnder interferometer with gain clamped semiconductor optical amplifiers in both arms are reported for the first time. The decision circuit gives a true regeneration, i.e., a digital characteristic on a linear instead of a logarithmic (or dB) scale for the input and/or output power.

Experimental characteristics of an all-optical 2R regenerator with adjustable decision threshold

We present measurements on a fiber-optic Mach-Zehnder interferometer with Gain Clamped SOAs in both arms. We obtained a true (i.e. on a linear scale) decision characteristic with extinction ratios of 10 dB. We also demonstrate that by varying the SOA currents, the decision threshold can be varied.