The effect of increased yeast alcohol acetyltransferase and esterase activity on the flavour profiles of wine and distillates

Transcription

1 Yeast Yeast 2006; 23: Published online in Wiley InterScience ( Research Article The effect of increased yeast alcohol acetyltransferase and esterase activity on the flavour profiles of wine and distillates Mariska Lilly 1, Florian F. Bauer 1, Marius G. Lambrechts 2, Jan H. Swiegers 3, Daniel Cozzolino 3 and Isak S. Pretorius 3 * 1 Institute for Wine Biotechnology, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7600, South Africa 2 Distell, PO Box 184, Stellenbosch, 7599, South Africa 3 The Australian Wine Research Institute, PO Box 197, Glen Osmond, Adelaide SA 5064, Australia *Correspondence to: Isak S. Pretorius, The Australian Wine Research Institute, PO Box 197, Glen Osmond, Adelaide SA 5064, Australia. Sakkie.Pretorius@awri.com.au Received: 21 March 2006 Accepted: 13 May 2006 Abstract The fruity odours of wine are largely derived from the synthesis of esters and higher alcohols during yeast fermentation. The ATF1- and ATF2-encoded alcohol acetyltransferases of S. cerevisiae are responsible for the synthesis of ethyl acetate and isoamyl acetate esters, while the EHT1-encoded ethanol hexanoyl transferase is responsible for synthesizing ethyl caproate. However, esters such as these might be degraded by the IAH1-encoded esterase. The objectives of this study were: (a) to overexpress the genes encoding ester-synthesizing and ester-degrading enzymes in wine yeast; (b) to prepare Colombard table wines and base wines for distillation using these modified strains; and (c) to analyse and compare the ester concentrations and aroma profiles of these wines and distillates. The overexpression of ATF1 significantly increased the concentrations of ethyl acetate, isoamyl acetate, 2-phenylethyl acetate and ethyl caproate, while the overexpression of ATF2 affected the concentrations of ethyl acetate and isoamyl acetate to a lesser degree. The overexpression of IAH1 resulted in a significant decrease in ethyl acetate, isoamyl acetate, hexyl acetate and 2-phenylethyl acetate. The overexpression of EHT1 resulted in a marked increase in ethyl caproate, ethyl caprylate and ethyl caprate. The flavour profile of the wines and distillates prepared using the modified strains were also significantly altered as indicated by formal sensory analysis. This study offers prospects for the development of wine yeast starter strains with optimized ester-producing capability that could assist winemakers in their effort to consistently produce wine and distillates such as brandy to definable flavour specifications and styles. Copyright 2006 John Wiley &Sons,Ltd. Keywords: alcohol acetyltransferases; esterases; esters; wine yeast; wine aroma Introduction During the primary (alcoholic) fermentation of grape sugars, the wine yeast, Saccharomyces cerevisiae, produces ethanol, carbon dioxide and a number of byproducts, including esters, of which alcohol acetate and C 4 C 10 fatty acid ethyl esters are found in the highest concentrations in wine and brandy (Lambrechts and Pretorius, 2000; Nykänen and Suomalainen, 1983; Schreier, 1979; Swiegers and Pretorius, 2005; Swiegers et al., 2005). Volatile esters are only present in small amounts in fermented beverages, but are extremely important for the flavour and aroma profiles of these products. The characteristic fruity odours of wine, brandy and other grape-derived alcoholic beverages are largely due to a mixture of ethyl acetate, hexyl acetate, ethyl caproate (apple-like aroma), isoamyl acetate (banana-like aroma), ethyl caprylate (apple-like aroma) and 2-phenylethyl acetate (fruity, flowery Copyright 2006 John Wiley & Sons, Ltd.

2 642 M. Lilly et al. flavour with a honey note) (Lambrechts and Pretorius, 2000; Swiegers and Pretorius, 2005; Swiegers et al., 2005; Fujii et al., 1994). These esters are synthesized in the yeast cells by alcohol acetyltransferases (AATases), using higher alcohols and acetyl-coa as substrates. Higher alcohols, such as isobutanol, isoamyl alcohol and 2-phenylethyl alcohol, can also have an impact on the aroma of wine and are formed as part of the branched-chain amino acid metabolism (Lilly et al., 2006). The role of ester production in yeast metabolism is unclear, but several hypotheses have been proposed. One school of thought suggests that esters might be formed to remove toxic fatty acids from the yeast cell (Nordström, 1962, 1964), whereas another proposes that esters could simply be overspill products from the yeast s sugar metabolism during fermentation and might be of no advantage to the yeast cell (Peddie, 1990). However, it has been shown that fatty acids with a chain length of C 8 C 14 are toxic to the yeast and exhibit strong antimicrobial activity and that the effect is intensified if these fatty acids are unsaturated (Bardi et al., 1998). Esters of shorter chain fatty acids (C 2 C 6 ) would be produced through the same detoxification process. Another reason for ester formation could be to reduce the acetyl charge, as it is essential for the yeast cell to maintain a balance between acetyl-coa and CoA- SH (Thurston et al., 1982). Yeast might synthesize esters to redress any imbalance in the CoA- SH : acetyl-coa ratio caused by the cessation of the lipid synthesis pathway through fermentation (Thurston et al., 1981, 1982). This is supported by the fact that the genetic regulation of the ATF1 gene is repressed by oxygen and unsaturated fatty acids (Nordström, 1962). Furthermore, it has been suggested that the ATF2 gene is involved in a detoxification mechanism to reduce the inhibitory action of steroids on the growth of yeast cells (Thurston, 1982). Several enzymes are involved in the formation of esters, of which the ATF1-encoded alcohol acetyltransferase (known as AATase I or Atf1p) is the best studied and has the most activity in S. cerevisiae (Schreier, 1979; Thurston et al. 1981; Cauet, 1999; Fujii, 1996, 1997). It was also demonstrated that the manipulation of the expression of ATF1 alone could alter the ester production significantly during wine fermentation, thereby adjusting the aroma profiles of wine and distillates considerably (Malcorps and Dufour, 1987). A second alcohol acetyltransferase (known as AATase II or Atf2p) encoded by the ATF2 gene has also been characterized (Malcorps et al., 1991). Atf1p and Atf2p are responsible for the production of ethyl acetate and isoamyl acetate during fermentation (Thurston et al., 1981; Malcorps et al., 1991). Another enzyme involved in ethyl acetate and isoamyl acetate production is Lg-Atf1p, an AATase found in lager yeast that is homologous to Atf1p (Lilly et al., 2000). A third ester-synthesizing enzyme, designated EHT1, has been suggested to encode an ethanol hexanoyl transferase, which generates ethyl hexanoate from ethanol and hexanoyl- CoA (Nagasawa, 1998). Recently, it has been shown that EHT1 belongs to a three-member gene family also containing the newly identified EEB1 (Saerens et al., 2006). In this work it was shown that Eht1p and Eeb1p have medium chain fatty acid ethyl ester (including ethyl hexanoate)-synthesizing and -degrading activity. It has also been shown that the balance between ester-synthesizing enzymes and esterases, such as those encoded by IAH1, which hydrolyses isoamyl acetate, is important for the net rate of ester accumulation (Yoshimoto et al., 1998; Mason and Dufour, 2000). Despite the complexity of wine aroma and our limited understanding of the formation of flavouractive esters during winemaking, steady progress is being made to unravel the basis of the large degree of variation regarding different commercial wine yeasts ability produce these esters at optimal concentrations under different fermentation conditions (Nykänen, 1983). It is hoped that this research would eventually assist winemakers in predetermining the outcome of their product by inoculating grape juice with flavour-active yeast, which have been tailored and selected according to specific flavour profiles and market specifications (Fukuda et al., 1998; Horsted, 1998; Pretorius, 2000, 2003, 2004, 2006; Pretorius and Bauer, 2002; Howell et al., 2004, 2005; Swiegers and Pretorius, 2005; Swiegers et al., 2005; Pretorius et al., 2006). In today s competitive, market-driven wine industry it is increasingly important to consistently produce wine and grape-based distillates to definable specifications and styles (Pretorius and Bauer, 2002; Pretorius, 2006; Swiegers et al., 2006). The objectives of this study were: (a) to overexpress genes encoding ester-synthesizing (ATF1,

3 Yeast-derived flavour-active esters in wine 643 ATF2 and EHT1 ) and an ester-degrading (IAH1 ) enzyme in a widely used industrial wine yeast strain (); (b) to prepare Colombard table wine and base wines for distillation using the transformed and untransformed strains; and (c) to analyse and compare the ester concentrations and aroma profiles of wines and distillates prepared with these host and transformed strains. In the future, these strains could be developed further as aids to enable winemakers and distillers to adjust the flavour profiles of their products in order to satisfy the different sensory preferences of consumers. Materials and methods Microbial strains, media and culture conditions All yeast and bacterial strains used in this study and their relevant genotypes are listed in Table 1. Escherichia coli cells were grown in Luria Bertani (LB) broth (BioLab, Midrand, South Africa) at 37 C (Sambrook et al., 1998). S. cerevisiae cells were grown at 30 C in either a synthetic medium, SCDSM [containing 0.67% yeast nitrogen base without amino acids (Difco, Detroit, MI, USA), 0.13% amino acid stock solution (Ausubel et al., 1994) lacking valine and isoleucine and supplemented with 0.5% glucose and 400 µg/ml sulphometuron methyl (SMM; Dupont, Wilmington, DE, USA) dissolved in N -N -dimethylformamide], or in a rich medium, YPD (containing 1% yeast extract, 2% peptone and 2% glucose). Solid media contained 2% agar (Difco). The bacteria were grown at 37 C and the yeast at 30 C. Recombinant DNA methods and plasmid construction Standard procedures for the isolation and manipulation of DNA were used throughout this study (Sambrook et al., 1998). Restriction enzymes, T4 DNA-ligase and Expand Hi-Fidelity DNA polymerase (Roche, Mannheim, Germany) were used in the enzymatic manipulation of DNA, according to the specifications of the supplier. Gene constructs are shown in Figure 1. Primers (listed in Table 2) were synthesized to amplify the coding regions of the different genes by means of the polymerase chain reaction (PCR) technique. To identify possible cloning artifacts, all inserts were sequenced. Genomic DNA from the commercial wine yeast strain,, was used as template to amplify the coding sequences of the respective genes. A multicopy, episomal S. cerevisiae E. coli shuttle vector phvxii (Volschenk et al., 1997), containing the promoter (PGK1 P ) and terminator (PGK1 T ) sequences of the yeast phosphoglycerate kinase I gene (PGK1 ), was used for subcloning the respective full-length open reading frames (ORFs). PCR-generated 1608 bp ATF2, 1356 bp EHT1 Table 1. Microbial strains and plasmids used in this study Strain or plasmid Genotype or construct Reference or source Escherichia coli DH5α F enda1 hsdr17 (r k m + k ) supe44 thi-1 reca1 GIBCO-BRL/Life Technologies gyra (Nal r ) rela1 (laclzya-argf)u169 deor [F80dlac DE(lacZ)M15] Saccharomyces cerevisiae Industrial strain Commercial wine yeast strain Anchor Yeast, Cape Town, South Africa Transformants (patf1-s) SMR1-140 PGK1 P -ATF1-PGK1 T Lilly et al. (2000) (patf2-s) SMR1-140 PGK1 P -ATF2-PGK1 T This study (peht1-s) SMR1-140 PGK1 P -EHT1-PGK1 T This study (piah1-s) SMR1-140 PGK1 P -IAH1-PGK1 T This study Plasmids phvxii bla LEU2 PGK1 P -PGK1 T Volschenk et al. (1997) pdlg42 bla SMR1-140 La Grange thesis (Stellenbosch University) patf1-s bla SMR1-140 PGK1 P -ATF1-PGK1 T Lilly et al. (2000) patf2-s bla SMR1-140 PGK1 P -ATF2-PGK1 T This study peht1-s bla SMR1-140 PGK1 P -EHT1-PGK1 T This study piah1-s bla SMR1-140 PGK1 P -IAH1-PGK1 T This study

4 644 M. Lilly et al. Table 2. PCR primers used in this study Primer name Sequence Enzyme ATF1 F 5 -GATCCTCGAGATGAATGAAATCGATGAGAA-3 XhoI ATF1 R 5 -GATCCTCGAGGTAAGGGCCTAAAAGGAGAG-3 XhoI ATF2 F 5 -GATCAGATCTATGGAAGATATAGAAGGATA-3 BglII ATF2 R 5 GATCAGATCTTTAAAGCGACGCAAATTCGC-3 BglII EHT1 F 5 -TCGACTCGAGATGTCAGAAGTTTCCAAATGGCC-3 XhoI EHT1 R 5 -TCGACTCGAGTCATACGACTAATTCATCA-3 XhoI IAH1 F 5 -AATTGAATTCATGGATTACGAGAAGTTTCT-3 EcoRI IAH1 R 5 -GATCAGATCTATTCAAGACATTATGTTATA-3 BglII The enzyme sites are indicated in bold and the region homologous to the corresponding genes is underlined. et al., 1988), a mutant allele of an endogenous gene (ILV2 )of S. cerevisiae conferring resistance to the herbicide sulphometuron methyl (Sm R ). Plasmids patf2-s, peht1-s and piah1-s were linearized with ApaI in the SMR1-410 terminator region for integration into the genomic copy of the ILV2 gene of the wine yeast strain. Figure 1. Maps of the different gene constructs and plasmid pdlg42. The open reading frames of the ATF1 (alcohol acetyltransferase I), ATF2 (alcohol acetyltransferase II), EHT1 (ethanol hexanoyl transferase) and IAH1 (esterase) genes of Saccharomyces were placed under the control of the promoter (PGK1 P )andterminator(pgk1 T ) sequences of the yeast phosphoglycerate kinase I gene (PGK1). These gene cassettes were cloned into the HindIII site of the pdlg42 yeast integrating plasmid, which contains the dominant selectable SMR1-410 marker gene. SMR1-410 is a mutant allele of ILV2 that confers resistance to the herbicide sulphometuron methyl (Sm R ) and 717 bp IAH1 fragments were digested with Bgl II, XhoI and EcoRI, and Bgl II, respectively, and subcloned into phvxii, thereby generating plasmids patf2-m, peht1-m and piah1-m. The HindIII HindIII fragments containing the PGK1 P, the ORFs of interest and the PGK1 T were obtained from the respective multicopy plasmids (patf2- m, peht1-m and piah1-m) and inserted into the unique HindIII site of plasmid pdlg42 (Figure 1), generating single-copy integrating S. cerevisiae E. coli shuttle plasmids patf2-s, peht1-s, and piah1-s. Plasmid pdlg42 contained the dominant selectable SMR1-410 marker gene (Casey Transformation All bacterial transformations and the isolation of DNA were carried out according to standard protocols (Sambrook et al., 1989). was transformed by means of electroporation (Ausubel et al., 1994). YPD (10 ml) was inoculated with yeast cells, and the cells were incubated at 30 C until the stationary phase. A 500 ml volume of YPD was then inoculated with 10 ml of the preculture and incubated until the mid-logarithmic growth phase was reached [absorbance at 600 nm (A 600 ) of ]. The cells were harvested, washed with 80 ml sterile water, resuspended in 10 ml TEbuffer (0.1 M Tris HCl, 0.01 M EDTA, ph 7.5) and 10 ml 1 M lithium acetate stock solution, and incubated at 30 C while shaking gently (Ausubel, 1994). After 45 min, 2.5 ml 1 M 1,4-dithio-threithol (DTT) solution was added and the mixture incubated for another 15 min. The solution was then diluted to 500 ml with water and centrifuged. The cells were first washed with 250 ml ice-cold water and 30 ml ice-cold 1 M sorbitol, and then suspended in 0.5 ml 1 M sorbitol. In a sterile, icecold 1.5 ml tube, 40 µl concentrated yeast cells were added to 5 15 µg DNA and transferred to an ice-cold 0.4 cm gap electroporation cuvette. The EasyjecT V Twin pulse (EquiBio, Ashford, UK) apparatus was used for electroporation. The

5 Yeast-derived flavour-active esters in wine 645 pulse programme was as follows: voltage, 1500 V; capacity, 25 µf; shunt, 201c. The yeast cells were then immediately plated onto SCDSM agar plates and incubated at 30 C for at least 3 days. Southern blot analysis Genomic DNA was isolated from the control yeast strain,, as well as from the corresponding transformed S. cerevisiae strains [((patf2- s), (peht1-s) and (piah1-s)], using the standard mechanical method (Ausubel et al., 1994). (patf2-s) genomic DNA was digested with EcoRV and StuI, (peht1-s) genomic DNA was digested with StuI and NruI, and the genomic DNA of the remaining strains was digested with EcoRV only. The DNA fragments were separated by agarose gel electrophoresis and transferred to a Hybond-N nylon membrane (Amersham, Little Chalfont, UK), and Southern blot hybridization was performed using the DIG Luminescent Detection kit (Roche). The ATF2, EHT1 and IAH1 ORFs were labelled with the digoxigenin molecule, using PCR, and applied as probes. Production and analysis of table wine The wine yeast strains,, (patf1-s), (patf2-s), (peht1-s) and (piah1-s) were each inoculated ( cells/ml) into 4.5 l Colombard grape juice and fermented at 15 C until dry (<1 g/l residual sugar). The wine was then cold-stabilized, filtered and bottled according to standard practices for white wine production. All fermentations were done in triplicate and wine samples were scanned using a Wine- Scan FT 120 instrument (Foss, Hillerød, Denmark), which employs a Michelson interferometer that was used to generate the FT-IR (Fourier infra-red) spectra (Table 3). The samples (7 ml) were pumped through the CaF 2 -lined cuvette (optical path length 37 µm) that is housed in the heater unit of the instrument. The temperature of the samples was brought to exactly 40 C before analysis. Samples were scanned at cm 1 at 4 cm 1 intervals, which includes a small section of the nearinfrared (NIR). The frequencies of the NIR beam transmitted by a sample were recorded at the detector and used to generate an interferogram. The latter was calculated from a total of 10 scans before being processed by Fourier transformation and corrected for the background absorbance of water to generate a single beam transmittance spectrum. Two transmittance spectra for each sample were generated in order to calculate the absolute repeatability of the spectral measurements. The calculation of the absolute repeatability has been described (WineScan Table 3. Infra-red (FT-IR) analysis of the Colombard table wine after bottling and the Colombard base wine after alcoholic fermentation but before distillation Colombard table wine Colombard base wine Parameters (patf1-s) (patf2-s) (peht1-s) (piah1-s) (patf1-s) (patf2-s) (peht1-s) (piah1-s) ph Volatile acid g/l Total acids g/l Malic acid g/l Lactic acid g/l Glucose g/l Fructose g/l Glycerol g/l Ethanol % v/v

6 646 M. Lilly et al. FT120 Type and Reference Manual, Foss, Denmark, 2001). The transmittance spectra were finally converted into linearized absorbance spectra through a series of mathematical procedures. Production and analysis of base wine and small-scale distillation The wine yeast strains, (patf1- s), (patf2-s), (peht1-s) and (piah1-s) were each inoculated ( cells/ml) into 15 l Colombard grape juice, to which no sulphur dioxide had been added, and fermented at 15 C until less than 0.5 g/l total sugar remained. These fermentations were done in triplicate. Routine WineScan analysis was performed on the base wines just after alcoholic fermentation (Table 3). Three 5 l round-bottomed flasks were each filled with 4.5 l base wine and yeast lees derived from the original 15 l base wine fermentation volume. Two copper plates and 3 g copper sulphate were added to the base wine and heated in heating mantles. The distillation flow rate was maintained at 5 ml/min, and the distillate was collected until 30% (v/v) alcohol was reached. The same procedure was followed with the second distillation, except that the first 40 ml of distillate, collected at a flow rate of 2 ml/min, was discarded. The flow rate was then adjusted to 5 ml/min and the heart was collected until 70% (v/v) alcohol was reached. Gas chromatographic analysis To each wine sample (10 ml Colombard table or base wine), 0.8 ml internal standard (4-methyl-2- pentanol, mg/l, 12% v/v ethanol) and 6.5 ml solvent (diethyl ether) were added. The tube was then mechanically rotated at 60 r.p.m. for 30 min. The top layer of ether was separated and the extracts were analysed. For the 70% distillates, a 5 ml sample was taken and 0.25 ml 4-methyl- 2-pentanol (2 g/l, 70% v/v ethanol) was added. After mixing, 2 µl sample was injected into the gas chromatograph (GC). The extractions were done in triplicate. The analysis of volatile compounds was carried out on a Hewlett-Packard 5890 Series II GC coupled to an HP7673 autosampler and injector and an HP 3396A integrator. The column used was a Lab Alliance organic coated fused silica capillary with dimensions of 60 m 0.32 mm i.d. with a 0.5 µm coating thickness; hydrogen was used as the carrier gas for a flame ionization detector (FID) held at 250 C. The injector temperature was 200 C, the split ratio 20 : 1, the flow rate 15 ml/min, and the injection volume 3 µl. The oven temperature programme was as follows: 35 C (10 min) rising to 230 C(0min)at3 C/min. For the distillate analysis, the conditions were as described above, except for a different oven programme and 2 µl injection volume: 30 C (5 min) rising to 80 Catarateof 2 C/min, and 80 C (0 min) rising to 230 C at 3 C/min. For each of the compounds measured, a specific amount was measured for the standard used to calibrate the machine. The internal standard and the chemicals were sourced from Merck (Cape Town, South Africa). Table wine extractions were done after bottling. Extractions from the base wine were made after alcoholic fermentation but before distillation. Samples from the distillates were taken after the second distillation. Sensory evaluation The table wines and distillates were sensorially evaluated for different fruity aromas, as well as for flowery and solvent or chemical intensity, by a panel of six experienced judges. The wines and distillates were evaluated on a percentage scale of 0 100, where 0% represented the absence of a specific flavour and 100% represented a very high intensity of the flavour. Statistical analysis The statistical differences between the GC results for the wines and 70% distillates produced by the modified and the control yeast strains were determined using the Bonferroni test, by which the p value is determined where the difference is significant if p Principal component analysis (PCA) models of sensory descriptive analysis (SDA) and GC data were constructed to describe compositional changes that occur in the samples due to the different yeast strains. The mean values of either the SDA or the GC data for wines and distillates from each yeast were processed for chemometric analysis, using The Unscrambler software (version 9.2, CAMO ASA, Norway). Before performing PCA, both SDA and GC data were preprocessed using 1/SDev. This preprocess is called

7 Yeast-derived flavour-active esters in wine 647 standardization and is used to give all variables the same variance (i.e. 1). This gives all variables the same chance to influence the estimation of the components, and is often used if the variables are measured with different units, or have different ranges, or are of different types. Sensory data, which are already measured in the same units, are nevertheless sometimes standardized if the scales are used differently for different attributes. PCA was used for reducing the dimensionality of data, detecting the number of components and visualizing the outliers (Naes et al., 2002). It is a mathematical procedure for resolving sets of data into orthogonal components whose linear combinations approximate the original data to a desired degree of accuracy. PCA was used to derive the first few principal components from the data and arrange samples into groups. In empirical modelling, as in this case, it is essential to determine the correct complexity of the model (Naes et al., 2002). With numerous and correlated SDA and GC data, there is a substantial risk of overfitting, where an apparently well-fitting model has little or no predictive power (Naes et al., 2002). Hence, a strict test of the predictive significance of each PCA component is necessary. Full cross-validation (leave-one-out) was used to develop the models. Sample grouping was based on the scores derived from the PCA analysis and the loadings were used to evaluate the most important variables that explain the grouping. Results Constitutive expression of ATF1, ATF2, EHT1 and IAH1 in commercial wine yeast With the aim of assessing the effects of acetyltransferases and esterases throughout wine fermentation, the ATF2, EHT1 and IAH1 genes were cloned from the commercial wine yeast strain, and placed under the constitutive control of the PGK1 regulatory sequences to generate plasmids patf2- s, peht1-s and piah1-s (Figure 1). Previously, we constructed the PGK 1 P -ATF1-PGK1 T gene cassette in a similar way (Lilly et al., 2000). To allow for stable maintenance of the gene constructs in non-selective grape juice medium, was transformed with linearized plasmids to facilitate direct integration into the genomic copy of the ILV2 gene. Integration of the respective plasmids into the Figure 2. Southern blot hybridizations indicating the integration of the ATF2 and EHT1 cassettes into the genomic copy of the ILV2 gene of the transformants. Lanes were loaded with BstEII-digested λdna (lane 1) or digested genomic DNA of the yeast strains (control strain), (patf2-s) (A), (peht1-s) (B) and (piah1-s) (C) genomes of the Sm R transformants (patf2- s), (peht1-s) and (piah1-s) were confirmed by Southern blot analysis (Figure 2). Two hybridization bands of 1854 and 2656 bp were obtained for the StuI-digested ATF2 gene of the control host yeast strain,, whereas four hybridization bands of 1854, 2656, 7762 and 8397 bp, corresponding to the StuI-digested wild-type ATF2 gene and the integrated PGK 1 P - ATF2-PGK1 T gene cassette, were obtained with the recombinant wine yeast strain (patf2-s) (Figure 2A). Two hybridization bands of 2744 and 2910 bp were obtained for the NruI-digested wildtype EHT1 gene in, and three hybridization bands of 2744, 2910 and 6267 bp were detected in

8 648 M. Lilly et al. (peht1-s), corresponding to the 2744 and 2910 bp fragments of the wild-type EHT1, and the 6267 bp fragment of the overexpression cassette (Figure 2B). A single hybridization band of 4419 bp, corresponding to the wild-type genomic copy of IAH1, was obtained in the host strain of, and an additional band of bp was obtained in (piah1-s) corresponding to the integration cassettes (Figures 2C). Therefore, all transformed strains carried the original genomic copies of ATF1, ATF2, EHT1 and IAH, of which the transcription was directed under its authentic promoter and terminator sequences, and another copy that was expressed under the direction of the constitutive PGK1 promoter and terminator sequences. The host (control) strain,, and the transformed strains, (patf1-s), (patf2-s), (peht1-s) and (piah1-s), were used to produce Colombard table wine and rebate (base) wine for distilling purposes. Chemical composition of experimental wines and distillates The concentrations of certain esters, higher alcohols and acids were determined for the wines and distillates (Tables 4, 5). The concentration of most of the esters increased in the table wine prepared with (patf1-s) when compared with the control wines (Figure 3). The concentrations of ethyl acetate, isoamyl acetate, 2- phenylethyl acetate and hexyl acetate, as well as of ethyl caproate, ethyl caprylate and ethyl caprate, increased as described in Lilly et al. (2000). The concentration of isoamyl acetate and 2-phenylethyl acetate in the table wines produced with (patf2-s) increased fold in comparison with the wines fermented with Table 4. Yeast strain effect on the concentration of major volatiles in Colombard white wines Concentration (mg/l) Colombard table wine after bottling Compound (patf1-s) (patf2-s) (peht1-s) (piah1-s) Acetic acid Decanoic acid Hexanoic acid i-butyric acid n-butyric acid i-valeric acid n-valeric acid Octanoic aicd Propionic acid Acetate Phenylethyl acetate Diethyl succinate Ethyl acetate Ethyl butyrate Ethyl caproate C Ethyl caprylate C Ethyl caprate C Ethyl lactate Hexyl acetate Isoamyl acetate Acetoin Methanol Phenylethyl alcohol Propanol i-butanol n-butanol i-amyl alcohol Hexanol

9 Yeast-derived flavour-active esters in wine 649 Figure 3. Comparison of the fold difference in ester production between the host strain and the strains containing the ATF1, ATF2, EHT1 and IAH1 gene cassettes the untransformed host strain. The ethyl acetate concentration in the table wine produced with (patf2-s) increased 1.3-fold compared to the control. The ethyl caproate concentration showed a slight increase in the table wines produced with (patf2-s) in comparison with that in the wines prepared with. In comparison with the control wines, the table wine fermented with (peht1-s) showed a fold increase in the concentrations of ethyl acetate, ethyl caproate, ethyl caprylate, ethyl caprate and hexyl acetate. The table wines produced with (piah1-s) showed a significant decrease in ester concentrations when compared to the control wines. The concentration of isoamyl acetate decreased fold and hexyl acetate was completely hydrolysed in the wines fermented with (piah1-s). Ethyl acetate and 2-phenylethyl acetate concentrations decreased by fold and fold, respectively, in the wines fermented with (piah1-s). The table wines produced with (patf1- s) showed a decrease in all of the corresponding higher alcohols of which the ester concentrations increased, but the decrease in higher alcohol concentrations did not directly correlate with the quantity of esters produced (Figures 4 and 5). In all of the experimental wines produced with the transformed strains, the n-butanol concentration decreased two-fold or more. The concentration of 2-phenylethyl alcohol decreased 1.2-fold in the table wines fermented with (patf2-s), (peht1-s) and (piah1-s). The propanol concentration decreased only slightly in the wines produced with (patf2-s) when compared to the wines fermented with, but decreased 1.4- fold in the wines fermented with (piah1-s). The isoamyl alcohol concentration remained the same in the wines produced with (patf2- s) and (piah1-s), but the concentration decreased 1.4-fold in the wines fermented with

10 650 M. Lilly et al. Table 5. Yeast strain effect on the concentration of major volatiles in Colombard base wines and the respective 70% distillates Concentration (mg/l) Colombard base wine after alcoholic fermentation 70% Distillate after second distillation Component (patf1-s) (patf2-s) (peht1-s) (piah1-s) (patf1-s) (patf2-s) (peht1-s) (piah1-s) Acetic acid Decanoic acid Hexanoic acid i-butyric acid n-butyric acid i-valeric acid n-valeric acid Octanoic acid Propionic acid Acetate Phenylethyl acetate Diethyl succinate Ethyl acetate Ethyl butyrate Ethyl caproate C Ethyl caprylate C Ethyl caprate C Ethyl lactate Hexyl acetate Isoamyl acetate Acetoin Methanol Phenylethyl alcohol Propanol i-butanol n-butanol i-amyl alcohol Hexanol

11 Yeast-derived flavour-active esters in wine 651 Figure 4. Comparison of the fold difference in higher alcohol production between the host strain and the strains containing the ATF1, ATF2, EHT1 and IAH1 gene cassettes Figure 5. Fold difference in the aroma compound concentrations of the 70% distillates after the second distillation compared to the concentrations in the Colombard rebate wines from which the distillates were produced (peht1-s). The hexanol concentration increased 1.5-fold in the wines fermented with (piah1-s). The decrease or increase in alcohol concentrations corresponded to the increase or decrease in the ester concentrations. The acetic acid concentrations in the wines produced by the transformed strains were drastically decreased in comparison to those fermented with the control strains. Figures 6 and 7 show the PCA biplot (loadings and scores) of yeast strains in table wines and 70% distillates, based on volatile compositional data, respectively. In table wines, the first PC explains 43% of the variation among yeast strains, while PC2 explains 27% of the total variation. Differences were observed between (patf1-s) and (piah1-s) along PC1. The volatile compounds responsible for the separation (loadings) were hexanol and n-butanol, mainly associated with (piah1-s), while esters were associated with (patf1-s). The second PC showed differences between the wild-type strain

12 652 M. Lilly et al. Figure 6. PCA biplot (loadings and scores) illustrating the classification in the Colombard wine set using the volatile compounds (A). PCA score plot illustrating the classification in the Colombard wine set using the sensory data (B) and (peht1-s). The volatile compounds responsible for the differences between and (peht1-s) were associated with acetoin, octanoic acid and hexanoic acid, mainly related with (peht1-s). For 70% distillates, the first PC explains 40% of the variation among the yeast strains, while PC2 explains 30% of the total variation. As was the case with the table wines, differences in 70% distillates derived from base wines fermented with (patf2-s)

13 Yeast-derived flavour-active esters in wine 653 Figure 7. PCA biplot (loadings and scores) illustrating the classification in 70% ethanol set using the volatile compounds (A). PCA score plot illustrating the classification in 70% ethanol set using the sensory data (B) and (piahi-s) were observed along PC1; the concentrations of acetoin, acetic acid and acetate separated the yeast strain tested. Distillates derived from base wines fermented with (piahi-s) were high in acetoin, acetic acid and acetate, while distillates derived from base wines fermented with (patf2-s) were high in decanoic acid and ethyl lactate. Differences

14 654 M. Lilly et al. between (patf1-s) and (peht1-s) were observed along PC2; loadings were observed in hexyl acetate and some esters (patf1-s). Effect of distillation on the ester content of the rebate wines After the base wine had undergone double distillation, the content changed because only the volatile compounds present in the heart fraction were collected, while other components, such as some acids, were discarded in the heads and tails fractions. Compared to the -derived distillate, the distillate produced from the (patf1-s)- fermented base wine contained a higher content of all the acetate esters, as well as of ethyl caproate, ethyl caprylate and ethyl caprate. The distillate produced from (patf2-s)-fermented base wine showed a 1.6-fold increase in 2-phenylethyl acetate concentration, as well as a 1.2-fold increase in ethyl caproate, ethyl caprylate and ethyl caprate concentrations and only a slight increase in ethyl acetate and isoamyl acetate concentrations. The concentration of 2-phenylethyl acetate decreased 1.5- and 4.5-fold, respectively, in the distillates produced from the (peht1-s)- and (piah1-s)- derived base wines. The distillate produced from the base wine fermented with (piah1-s) showed a 1.5-fold decrease in ethyl acetate concentration, a 15-fold decrease in isoamyl acetate concentration and contained no hexyl acetate. A 1.7-fold decrease in isoamyl acetate concentration in the distillate produced from the (peht1- s)-fermented base wine was observed and this finding is consistent with the decrease observed in the base wines. With the exception of isobutanol, the concentration of all the higher alcohols in the distillate produced from the (patf1-s)-fermented base wine decreased correspondingly to the increased ester concentrations. The concentration of n-butanol decreased by 1.7-fold and more in the distillate produced from base wines prepared with the transformed strains. The concentration of 2-phenylethyl alcohol, propanol and isoamyl alcohol in the distillate produced from the (peht1-s)- fermented base wine decreased 1.7-, 1.5- and 1.4- fold, respectively, but the hexanol concentration of the distillate produced with the (piah1-s)- fermented base wine increased 1.6-fold in correlation with the drastic decrease in hexyl acetate concentration. The distillation process played an important role in the differences in the aroma compound concentrations between the rebate wine and the 70% distillate. Most of the aroma compounds were concentrated during the first distillation, but some were also partly lost during the second distillation (Table 5). All of the ester concentrations increased when the rebate wine was double-distilled. The concentration of ethyl caprate increased the most of all the esters, with a 25-fold increase in concentration, and the ethyl caprylate concentration increased 7.5-fold in the distillate compared to that in the rebate wine. Ethyl acetate was less concentrated and only a 2.8-fold increase was obtained in the distillate. All of the other ester concentrations also increased by fold when the rebate wine was double-distilled. A seven-fold increase in the concentration of isobutanol and propanol was observed in the distillates, while n-butanol, isoamyl alcohol and hexanol were concentrated six-fold. 2- Phenylethyl alcohol was the only higher alcohol of which the concentration decreased during the distillation process. The concentrations of the fatty acid esters were increased and that of acetic acid decreased 14-fold. Decanoic acid was concentrated seven-fold, hexanoic acid 4.9-fold, octanoic acid four-fold and i-butyric acid 1.7-fold. Sensory attributes of the experimental wines All of the sensory results presented here are statistically significant. The fruity aroma, which is usually associated with the combination of many esters, was detected in a higher intensity in the table wines fermented with (patf1-s) and (piah1-s). The intensity of the fruity aroma for the rest of the wines fermented with the transformed strains remained the same as that of the control wine. The peachy aroma was stronger in all of the wines produced with the transformants than in the control, except for the wine fermented with (patf2-s), in which no peachy flavour was detected. The peachy aroma was significantly more intense in the wines fermented with (peht1-s). The apricot aroma was only detected in the wines fermented with (peht1-s) and (piah1-s). An apple aroma was detected in all of the wines except that fermented with (patf1-s). The apple aroma was much more intense in the wines fermented with (peht1-s) than in the control wine. A

15 Yeast-derived flavour-active esters in wine 655 banana aroma was absent in the wines fermented with and (piah1-s), but was quite prominent in the (patf2-s)-fermented wine. The estery/synthetic fruit flavour was overpowering in the wines fermented with (patf1- s), but much more subtle in the (patf2- s)-fermented wines and completely absent in the (piah1-s)-fermented wine. The intensity of the guava aroma in the wines fermented with (piah1-s) was more prominent than in the control wine and the citrus aroma was only detected in the wine fermented with (peht1-s). The floral aroma was also only detected in the wines fermented with. Figure 7 shows the PCA biplot (loadings and scores) for table wines and 70% distillates based on the sensory attributes. In table wine, PC1 and PC2 explain 51% and 22% of the variation, respectively. Sensory differences between (patf1-s) and the other yeast strains (as observed along PC1) mainly related to the strong estery attributes derived from (patf1-s). It was observed that apple, citrus and fruity attributes were high in (peht1- s)-derived wines, while peach and guava characters were observed in wines produced with (piah1-s). Wines produced with the wildtype strain were characterized by floral notes. In most cases, similar changes were observed in the distillates. However, some interesting differences were also observed. The fruity aroma, associated with the combination of all the esters, was detected in a higher intensity in all of the distillates prepared from base wines that were produced with the transformed strains, but was the most intense in the distillates derived from base wines that were produced with (patf1-s), (piah1- s) and (patf2-s). The peach and apricot aromas were stronger in the distillates of (patf2-s)- and (piah1-s)-fermented wines and the apple aroma was more prominent in the distillates produced with (patf2- s)- and (patf1-s)-fermented base wines. As previously described by Lilly et al. (2000), the estery/synthetic fruit aroma was overpowering in the distillate produced from (patf1- s) but much more subtle in the distillates of (patf2-s) wines. In the 70% distillates, PC1 explains 61% of the variation and PC2 20%, respectively (Figure 7). (peht1-s) yeast strains produced wines with strong herbaceous and floral characters, while (patf2-s) tended to produce wines with peach and apricot characters. Discussion In this work, three known ester-synthesizing (the ATF1- and ATF2-encoded alcohol acetyltransferases and the EHT1-encoded ethanol hexanoyl transferase) and an ester-degrading enzyme (the IAH1-encoded esterases) were investigated by overexpressing the ATF1, ATF2, EHT1 and IAH1 genes in an industrial wine strain,. Most of our findings regarding the experimental wines and distillates prepared with the transformed strains were in agreement with observations made with regard to beer and saké (Mason and Dufour, 2000; Verstrepen et al., 2003a, 2003b, 2003c, 2003d, 2004). We confirmed that Atf1p significantly contributes to the production of ethyl acetate, isoamyl acetate and 2-phenylethyl acetate when overexpressed (Figure 3). However, here we show that the abundance of Atf1p also has an effect on ethyl caproate, ethyl caprylate and hexyl acetate concentrations. This could be due to the broad specificity of the enzyme or because increased levels of esters might influence the expression of Eht1p, which has been implicated in the synthesis of hexyl acetate. These effects would have to be investigated further by using strains deleted for the different alcohol acetyltransferase genes in combination with their overexpression equivalents. We confirmed that Atf2p plays a role in ethyl acetate, isoamyl acetate and 2- phenylethyl acetate ester production, but it appears that the effect of Atf2p on the final concentrations of these compounds are substantially less than that of Atf1p. However, Atf2p also slightly increased the concentrations of ethyl caproate, ethyl caprate and hexyl acetate, but it had no effect on ethyl caprylate production, as in the case of Atf1p. Interestingly, overexpression of the EHT1 gene slightly increased the concentration of all of the esters, with the biggest increases in the concentrations of ethyl caprate, ethyl caproate and ethyl caprylate. The effect of increased concentrations of ethyl caprate, ethyl caproate (apple aroma) and ethyl caprylate was evident in the sensory analysis, where EHT1 overexpression resulted in a significantly enhanced apple aroma of the

16 656 M. Lilly et al. Colombard table wine (Figure 6). However, the EHT1-overexpressing strain did not seem to group with ethyl caproate in the PCA analysis (Figure 6). The reason is that the overexpression of ATF1 also resulted in a significant increase in ethyl caproate (more than with the overexpression of EHT1 ) but the excessive concentrations of ethyl acetate probably masked the apple aroma. Again, it is unclear whether Eht1p has a broad specificity or whether the esters it produces have a stimulatory effect on the expression of Atf1p and Atf2p. The individual enzymes of Atf1p, Atf2p and Eht1p would have to be purified and enzyme assays would have to be conducted on various substrates to establish whether or not this is the case. Recently it was reported that purified Eht1p have mediumchain fatty acid ethyl ester synthesis and hydrolysis capacity in vitro (Saerens et al., 2006). However, Saerens et al. (2006) also found that overexpression of EHT1 in a S. cerevisiae laboratory strain did not lead to an increase in ethyl caprate, ethyl caproate and ethyl caprylate concentration, as observed in the study. This apparent discrepancy can be ascribed to the different genetic backgrounds of the host strains; Saerens et al. (2006) used a laboratory strain of S. cerevisiae, whereas we have used an industrial wine yeast strain. It appears that industrial wine strains are more efficient in their ability to produce esters, higher alcohols and other flavour compounds than laboratory strains and are therefore more suitable for studies on these metabolic processes (Howell et al., 2005; Lilly et al., 2006; Saerens et al., 2006). Furthermore, the medium Saerens et al. (2006) used was a synthetic medium and not grape juice and could therefore contribute to the discrepancies. In the case of ester hydrolysis, overexpression of the IAH1 gene, as expected, led to a decrease in ester concentration. However, a decrease in the concentrations of hexyl acetate, isoamyl acetate and 2-phenylethyl acetate was most prominent. It therefore appears that Iah1p has a higher affinity for these substrates, although its effect on the regulation of alcohol acetyltransferases cannot be ruled out. Overexpression of the alcohol acetyltransferase and esterase genes also had an interesting effect on the concentrations of higher alcohols, which act as substrates for ester synthesis. Overexpression of ATF1, ATF2, EHT1 and IAH1 resulted in an overall increase in the concentration of isobutanol and a decrease in the concentration of n- butanol and 2-phenylethyl alcohol. The increase in isobutanol concentrations is interesting, as both isobutanol and isoamyl alcohol are derived from α- keto-isovalerate. When ATF1 was overexpressed, isoamyl alcohol levels dropped in correlation with an increase in the formation of isoamyl acetate. However, this led to an increase in isobutanol concentration, indicating that the regulation of ester formation probably affects the branched-chain amino acid metabolism as well. In this work, we could not find a clear correlation between an increase and decrease in esters and a corresponding decrease and increase in higher alcohols for all the overexpressed genes tested. However, it has to be taken into account that, although we compared the effects of the different enzymes that were overexpressed, we did not measure the actual enzyme activities encoded by the various genes tested in this study, neither did we quantify the mrna levels of these genes to confirm their overexpression. Therefore, it cannot be ruled out that there were differences in the levels of overexpression of the ATF1, ATF2, EHT1 and IAH1 genes. The long-term goal of this work is to develop optimal ester aroma profiles for wines and distillates. The small increase in ethyl acetate concentration when fermenting with (patf2-s) instead of (patf1-s) might be advantageous to the wine and brandy industry, since Lilly et al. (2000) showed that excessively high concentrations of ethyl acetate did not improve the fermentation bouquet and aroma of the wines and distillates. The sensory evaluation of the wines and distillates produced from (patf2-s) showed that lower concentration of ethyl acetate enhances the fruity aromas of wine. Such strains might therefore provide wines with higher complexity. The table wines fermented with (patf2-s) had a much less estery/synthetic fruit character than the wine fermented with (patf1-s), but more intense fruity aromas were observed. On the other hand, distillates produced from (patf2-s) base wine had a more intense peach, apricot and apple aroma than the distillates produced from base wines fermented with or (patf1-s). The perceived intensity of certain fruity aromas is therefore not linked to the total ester concentration, but rather to specific ratios thereof.