Brettanomyces/Dekkera is considered as one of the main causes of microbial spoilage and sensory deviations of red wines. This work compares the sensitivity and effectiveness of conventional microbiological culture and real-time polymerase chain reaction (Q-PCR) methods for Brettanomyces/Dekkera detection and quantification and demonstrates a positive correlation between both methods. Moreover, an improved DNA extraction protocol enabled quantification of Brettanomyces/Dekkera cells by Q-PCR down to 20 cells/mL in turbid wines in a total of 324 red wine samples. The conventional culture analysis is time-consuming but has lower cost than Q-PCR, and it is simple and efficient in quantifying viable Brettanomyces/Dekkera cells.
Brettanomyces is a genus of yeast implicated in wine spoilage and responsible for severe quality problems in the wine industry. Brettanomyces refers to the nonsporulated anamorphic yeast form and Dekkera to the sporulated asco-sporogenous state, and they are described in the literature as part of the microbiota of many fermented products such as wine, beer, cider, and kefyr (Suárez et al. 2007). Although these yeasts have been rarely reported in grapes (Renouf and Lonvaud-Funel 2007) or in fermenting musts (Renouf et al. 2006), they are commonly detected in red wine during aging in oak barrels, and in vats, pumps, and equipment that are difficult to sterilize (Fugelsang 1998). Wines infected by this yeast develop off-favors, the so-called Brett character, which is described as animal, stable, horse sweat, mousy, burnt plastic, and wet wool (Licker et al. 1998), a problem often detected in association with other wine faults, including oxidation and volatile acidity (Grbin and Henschke 2000). The mousy taint is the result of pyridine synthesis by Brettanomyces/Dekkera from lysine and ethanol, while the stable odors are caused by volatile phenols, mainly 4-ethylphenol and 4-ethylguaiacol, secondary metabolites produced by Brettanomyces/Dekkera from phenolic acids naturally present in wine (Coulon et al. 2010, Woolfit et al. 2007). Off-favors start to be detected when active cells are present in wine at concentrations >103 cells/mL and, consequently, when volatile phenols concentrations reach 425 μg/L (Chatonnet et al. 1995). Control of these microorganisms is difficult due to the number of adaptations they have to survive in the physiologically stressing environmental conditions of a finished wine, such as high alcohol content (up to 150 mL/L; EU Regulation 110/2008), residual or no sugar availability, anaerobiosis, and CO2 pressure. Slight amounts of oxygen have a positive effect on the growth of Brettanomyces/Dekkera in a reducing environment, which may explain why microoxygenation favors its growth, and the naturally mild oxidizing conditions of aging in oak barrels can activate metabolically viable residual Brettanomyces/Dekkera cells (Suárez et al. 2007). Identification and quantification of this yeast are extremely important for appropriate actions of winemakers to avoid sensory deviations.
Traditional methods for identification and enumeration of Brettanomyces/Dekkera in wine are based on culturing in selective media, and typically it takes one or two weeks to perform. In this case, identification and selectivity are based on one or several of the following characteristics: production of the Brett character due to the presence of hydroxycinnamic acids in the culture medium; production of acetic acid, which is detected by color change due to the presence of a pH indicator in the selective medium or to calcium carbonate; presence of antibiotics that prevent the growth of numerically superior yeasts (especially Saccharomyces spp.) and bacteria; and presence of ethanol in the growth medium as a selective carbon source for Brettanomyces (Snowdon et al. 2006). Nevertheless, none of these culture media is totally selective, and false-positive and false-negative results can occur. Molecular biology methods for rapid detection and identification of Brettanomyces/Dekkera have been developed, and many are based on polymerase chain reaction (PCR) analysis of the DNA (Cocolin et al. 2004, Ibeas et al. 1996). They offer the advantage of high specificity, rapid results, and reliability; nevertheless, quantification is not achieved by these classical PCR methods. Real-time or quantitative PCR (Q-PCR) assays not only offer the possibility of detecting and identifying with high specificity and reliability but also permit quantification of target DNA and, consequently, of yeast population in wine (Delaherche et al. 2004, Hierro et al. 2006, Martorell et al. 2005, Phister and Mills 2003, Phister et al. 2007, Salinas et al. 2009, Tessonnière et al. 2009, Zott et al. 2010). Nevertheless, Q-PCR has the handicap of requiring the absence of inhibitors of the DNA polymerase in the analyzed sample, among which wine polyphenols are notably included (Delaherche et al. 2004, Tessonnière et al. 2009).
In this study, we investigated and evaluated different protocols for preparation of wine samples and DNA extraction and applied the optimized protocol to a total of 324 red wine samples with turbidities ranging from 0.1 to 1000 NTUs that were subsequently analyzed by Q-PCR. Red wines were also submitted to the conventional microbiological culture method on a selective medium to detect, identify, and quantify Brettanomyces/Dekkera. We assessed the quantification limits of both Q-PCR and microbiological culture analyses and compared the obtained results to show a correlation between the methodologies as well as their strengths and limitations.
Materials and Methods
Yeast strains and culture conditions.Brettanomyces bruxellensis BY24 and Saccharomyces cerevisiae LV5 strains of the collection of enological isolates of the Department of Food and Agriculture of the University of La Rioja were used. Yeasts were cultivated at 30°C for 48 hr onto YPD-agar plates [(10 g/L yeast extract; Oxoid, Basingstroke, UK), 20 g/L peptone (Difco, Oxford, UK), 20 g/L glucose (Panreac Química, Barcelona, Spain), 20 g/L agar (Difco)] under aerobic conditions. DBDM medium (Rodrigues et al. 2001), modified with 10% (v/v) ethanol, was used as Brettanomyces/ Dekkera selective and identification medium containing an off-odor indicator and a pH color indicator that detects acetic acid production (named as Brettanomyces-specific medium).
Wine samples and microbiological analysis.Vitis vinifera L. cv. Tempranillo red wine from the Spanish Appellation of Origin Rioja was used for comparison and finally developing an optimized protocol for DNA extraction and Q-PCR analysis. Moreover, 324 samples of red wines of Tempranillo variety from the Spanish Appellations of Origin Rioja and Ribera del Duero, aged in oak barrels and suspected of being spoiled by Brettanomyces under sensorial analysis, were analyzed by microbiological culture on the Brettanomyces-specific medium and by Q-PCR analysis. These red wines presented turbidity values from 0.1 to 1000 NTU. Wine samples were processed for microbiological culture as follows: a 10 mL volume of the homogenized wine sample was placed in a sterile tube and centrifuged at 3,000 g for 10 min. The supernatant was discarded; the cell pellet was resuspended in 1 mL of sterile 0.15 M NaCl; and 100 μL of this cell suspension was spread onto YPD-agar and Brettanomyces-specific medium plates and incubated at 30°C for total yeasts and Brettanomyces counting. Colonies obtained on the Brettanomyces-specific medium were isolated and reconfirmed as positive Brettanomyces by the specific conventional PCR analysis described below.
Wine samples and washing procedures. Two washing procedures were applied before yeast DNA extraction from wine samples in order to perform subsequent PCR and Q-PCR analyses. For washing method A, low turbidity wines (≤50 NTU) were submitted to the following procedure: 1 mL of the homogenized wine sample was placed into an Eppendorf microtube and centrifuged at 13,600 g for 5 min in a Biofuge-Pico microfuge (Heraeus, Hanau, Germany). Supernatants were discarded and pellets resuspended in 1 mL of washing buffer (0.05 M Tris.Cl, 0.15M NaCl, 100 mL/L ethanol). After briefly vortexing, samples were centrifuged again and supernatants discarded. This washing and centrifugation process was repeated once more, supernatants were discarded, and cell pellets were conserved for DNA extraction.
For washing method B, wines with turbidity >50 NTU were submitted to the same procedures of method A, and when necessary one additional washing step was included to recover a colorless supernatant. Pellets were resuspended in 100 μL washing buffer and spread onto a sterile YPD-agar plate. The suspension was carefully distributed onto the plate surface with a sterilized inoculation loop and incubated overnight at 30°C. This step promoted absorption of wine color particles into the agar and prevented the inhibition of DNA polymerase by wine tannins and pigments during PCR analysis (Delaherche et al. 2004, Phister and Mills 2003, Tessonnière et al. 2009, Jara et al. 2008). After incubation, yeast cells grown on the surface of the whole plating area were recovered with an inoculation loop repeatedly and resuspended in 1 mL washing buffer. Samples were centrifuged again in the microfuge at 13,600 g for 5 min, and pellets were submitted to DNA extraction.
Control wine samples were prepared to evaluate the different protocols for DNA extraction and subsequent Q-PCR analysis. Serial dilutions of B. bruxellensis BY24 cells were suspended either in an amicrobically filtered red wine (0.20 μm; Corning Inc., Munich, Germany) or in 0.15 M sterile NaCl solution. The cell concentration range studied was 4 × 107 to 4 cells/mL, determined by optical density at 600 nm with an Ultrospec-2000 spectrophotometer (Amersham Pharmacia Biotech, Little Chalfont, UK), where 1 OD600 corresponds to ~2 × 107 cells/mL (Ausubel et al. 1992), and by colony counting on YPD-agar plates. These control samples contained a fixed concentration of 2 × 105 cells/mL of S. cerevisiae LV5 strain to resemble enological conditions and to quench cell losses during DNA extraction procedures.
DNA extraction methods. Six different DNA extraction methods were tested. In method 1, cell pellets were resuspended in 200 μL of InstaGene Matrix (Bio-Rad, Hercules, CA) and manipulated according to manufacturer’s recommendations. In method 2, cell pellets were resuspended in 200 μL of PrepMan Ultra reagent (Applied Biosystems, Foster City, CA), and treated according to manufacturer’s recommendations.
In method 3, cell pellets were resuspended in 200 μL of lysis buffer (500 mM pH 8.0 Tris, 100 mM β-mercaptoethanol). A volume of 25 μL of fresh lyticase (3 mg/mL; Sigma-Aldrich, St. Louis, MO) was added to each sample, and samples were incubated in a water bath at 37°C for 25 min; this step was repeated once more. Samples were vigorously vortexed, boiled in water bath for 15 min, frozen and thawed, and centrifuged at 3,400 g for 1 min in the microfuge. Supernatants were recovered and transferred to DNeasy Plant Mini Spin-Columns (Qiagen, Hilden, Germany), and manipulated according to manufacturer’s recommendations.
In method 4, cell pellets were resuspended in 200 μL of lysis buffer (500 mM pH 8.0 Tris, 100 mM β-mercaptoethanol). A volume of 25 μL of fresh lyticase (3 mg/mL; Sigma-Aldrich) was added to each sample, and samples were incubated in a water bath at 37°C for 25 min; this step was repeated once more to obtain cell lysates, and finally boiled in water bath for 15 min. Samples were vigorously vortexed, frozen and thawed, and centrifuged at 3,400 g for 1 min in the microfuge. After centrifugation, 100 μL of the supernatant was recovered in a sterile tube, and protein precipitation and DNA isolation were carried out according to Sambrook et al. (1989) with the following modifications: 200 μL of solution II (0.2 N NaOH, 0.01 g/mL sodium dodecyl sulfate) was added to the 100 μL sample, gently mixed, and incubated in an ice bath for 15 min. Then 150 μL of solution III (3 M for potassium ion, 5 M for acetate) was added, gently mixed, and incubated in the ice bath for 5 min. The sample was then centrifuged at 13,400 g for 5 min at 4°C (5415R centrifuge; Eppendorf, Hamburg, Germany) and 900 μL ethanol at 4°C was added and gently mixed. The sample was once again centrifuged and the supernatant was gently eliminated with a micropipette. After drying, the pellet was resuspended in 50 μL ultrapure water.
In method 5, cell pellets were resuspended in 200 μL of the lysis buffer and submitted to the cell lysis with fresh lyticase as described above. After incubations with lyticase, samples were vigorously vortexed, frozen, thawed, and centrifuged at 3,400 g for 1 min in the microfuge. The supernatants obtained after centrifugation were carefully transferred to Amicon flters (0.5 mL, 100 K membrane; Millipore, Billerica, MA) and manipulated according to manufacturer’s recommendations.
In method 6, cell pellets were resuspended in 200 μL of the lysis buffer and submitted to the cell lysis with fresh lyticase described in method 4. After incubations with lyticase, samples were vigorously vortexed, frozen, thawed, and centrifuged at 3,400 g for 1 min in the microfuge. After centrifugation, 25 μL polyvinylpolypyrrolidone (PVPP) (PolyClar, Laffort, Bordeaux, France) were added to the cell lysate and the sample was kept under gentle agitation at 4°C for 60 min. It was then centrifuged at 13,400 g for 1 min, 4°C (5415R Centrifuge; Eppendorf), and 100 μL of the supernatant recovered in a sterile tube. Protein precipitation and DNA isolation were carried out as previously described in method 4. Two concentrations of PVPP were assayed, 10 and 20 mg/mL.
PCR analysis. To confirm Brettanomyces/Dekkera identification of wine isolates recovered by growing on the selective medium, the specific amplification of Brettanomyces by PCR previously published by Cocolin et al. (2004) was used, with some modifications. PCR reactions were carried out in 50 μL final volumes containing 10 μL DNA template, 0.5 μM of the respective primers (DB90F and DB394R), 5 μL of 10 × NH4 reaction buffer (Bioline, London, UK), 3 μL of 50 mM MgCl2 solution (Bioline), 0.05 mM of each deoxynucleotide (Deoxynucleotide Mix; Sigma-Aldrich), and 1.5 U Taq DNA polymerase (Bioline). PCR conditions were: initial denaturing at 95°C for 5 min, followed by 40 cycles of denaturing at 95°C for 1 min, annealing at 67°C for 45 sec, and extension at 72°C for 7 sec. PCR reaction based on the variable D1/D2 domain of 26S ribosomal subunit of ascomycetous yeasts (Kurtzman and Robnett 1998) was subsequently used to check DNA quality and to detect possible failures on DNA extraction protocol.
All reactions were performed with T3000 thermocycler (Biometra, Göttingen, Germany). PCR products were analyzed by electrophoresis on 15 g/L agarose gel (D1 low EEO; Pronadisa, Madrid, Spain) in Tris-borate-EDTA buffer (90 mM Tris-borate, 2 mM EDTA; Panreac,), stained with ethidium bromide (0.3 mg/L; Bio-Rad), and visualized under a Chemi Genius UV-lighter (Syngene, Cambridge, UK) with GeneSnap software (ver. 6.01; Syngene).
PCR (Q-PCR) analysis. The specific Q-PCR method to detect and quantify Brettanomyces/Dekkera previously described by Phister and Mills (2003) was used. Reactions were performed in triplicates using an iCycler-iQ thermocycler with iCycler-optical module (Bio-Rad). Each 25 μL reaction mix contained 12.5 μL iQ-SYBR Green Supermix (Bio-Rad), 7.2 μL ultrapure water, 0.225 μL DBRUX-F primer (900 nM), 0.075 μL DBRUX-R primer (300 nM), and 5 μL DNA sample. Q-PCR results were analyzed with iCycler-iQ software (ver. 3.1.7050; Bio-Rad).
Statistical analysis. Comparison of quantification data obtained by Q-PCR and concomitantly by counting on the specific medium was performed using bilateral Pearson’s correlation with a 0.01 significance level. Analyses were performed using IBM-SPSS Statistics software for Windows (ver. 19.0; IBM-SPSS, Chicago, IL). Comparison of results obtained by culture on the specific medium and Q-PCR analysis was expressed as percent agreement between both methods and kappa statistics. The estimated kappa, which excluded the chance-induced agreement, was interpreted as follows: values <0.4 represented poor agreement, 0.4–0.6 represented moderate agreement, 0.6–0.8 represented good agreement, and >0.8 represented excellent agreement.
Results and Discussion
DNA extraction methods and PCR analysis.Brettanomyces-specific PCR analysis was performed with DNA samples obtained from control wines containing 10-fold dilution series of Brettanomyces strain BY24 cells and submitted to the six DNA extraction methods (Table 1). No amplification bands were obtained when methods 1, 2, and 3 were used to obtain DNA from Brettanomyces cells. In these cases, negative results were obtained as well for ascomycetous yeasts PCR (Kurtzman and Robnett 1998) (results not shown), indicating that those methods did not work for DNA extraction in such a complex matrix as wine. With method 4, Brettanomyces-specific bands were detected for every cell concentration from 4 × 107 to 4 × 102 cells/mL. For methods 5 and 6, Brettanomyces-specific bands could be detected for every cell concentration assayed to a limit of 40 cells/mL. Method 6 extraction was performed using 10 and 20 mg/mL PVPP, and band amplifications by PCR were obtained only when 20 mg/mL was used.
DNA extraction methods and Q-PCR analysis. Based on the results obtained for PCR analysis, DNA extraction methods 4, 5, and 6 were tested for subsequent DNA detection and quantification by Q-PCR. As is known, in Q-PCR analysis the increase of the recorded fluorescent signal is proportional to the amount of DNA synthesized during each amplification cycle, and individual reactions are characterized by the cycle fraction at which fluorescence first rises above a defined background fluorescence, or the threshold cycle (Ct) (Bustin 2004). specific melting temperature (Tm) was consistently verified (Figure 1) and the threshold cycles for each triplicate were considered for application on every standard curve. Figure 2 illustrates the amplification curves obtained by Q-PCR analysis of DNA extracted from an amicrobically filtered red wine containing 10-fold dilution series, from 4 × 106 to 4 cells/mL, of Brettanomyces BY24 control cells and following DNA extraction method 4. The Q-PCR results from methods 4, 5, and 6 are shown as threshold cycle values versus Brettanomyces cell concentration in wine (logarithm values), with the linear correlation coefficient resulting from the corresponding analysis (Figure 3). Although methods 4 and 6 worked for DNA extraction from cells in red wine and PCR polymerase inhibitors were apparently eliminated, in both cases the target DNA was in a very low concentration and the data obtained did not return good linear correlation factors. Both methods were not considered valid for detection and quantification of Brettanomyces/Dekkera, above all because wine spoilage is considered critical when the population of this yeast reaches levels of 103 cells/mL (Chatonnet et al. 1995, Lonvaud-Funel and Renouf 2005). Method 5 provided satisfactory results, showing minimal losses of genetic material as well as no interfering inhibition in the Q-PCR analysis. Under these conditions, the linear correlation was maintained in the concentration range from 40 to 4 × 107 cells/mL.
When method 5 was used for analysis of high turbidity wines, results were quite different and inhibition of Q-PCR reaction was frequent. Therefore, washing method B was applied when wines showed turbidity >50 NTU and, subsequently, DNA extraction method 5 was performed. The results from three independent experiments performed with Brettanomyces BY24 control cells suspended in filtered red wine (turbidity-free) following cell washing method A and subsequent DNA extraction method 5 are shown in Figure 4A. The results from experiments performed with Brettanomyces BY24 control cells suspended in the nonfiltered red wine (turbidity = 73.6 NTU) following cell washing method B and subsequent DNA extraction method 5 are shown in Figure 4B. Both methods rendered good linear correlation factors (R2 ≥ 0.96); detection limits were 4 to 4 × 106 cells/mL and quantification of Brettanomyces was possible down to a value of 25 cells/mL for low turbidity wines and 16 cells/mL, for wines with turbidity >50 NTU submitted to cell washing method B, therefore resulting in an average threshold value of 20 cells/mL.
Previous works successfully applied Q-PCR for detection of Brettanomyces/Dekkera in red wines. Notwithstanding, the lowest limits of quantification that were reported were ~104 cells/mL, and authors mainly attributed such an absence of sensitiveness to the presence of inhibitors or also to interactions between phenolic compounds and cell components (Delaherche et al. 2004). Phister and Mills (2003) could quantify down to 10 cells/mL when the obtained DNA sample was diluted 10-fold before Q-PCR reaction, and moreover, assays were performed in previously filtered red wine. Tessonnière et al. (2009) also proposed a DNA preparation method using PVPP for direct analysis by Q-PCR of naturally spoiled wines, obtaining limits of detection ranging from 26 to 100 cells/mL for red wines, but reported failures on detection of target DNA when wines had some turbidity. The major handicap for all these methods applied to naturally spoiled red wines was the presence of solid particles in suspension, which meant wine turbidity and wine tannins and pigments that inhibit PCR; consequently, the reported threshold detection limits were higher than the quantification limit of 20 cells/ mL that we report in this work for wines with high turbidity (>50 NTU), which allows detecting and quantifying Brettanomyces/Dekkera in all winemaking stages, regardless of the turbidity of the sample. In this case, previous washing steps and intermediated overnight incubation on plates allowed, respectively, removal of color pigments and other particles, thereby eliminating inhibition on PCR reactions.
Q-PCR and microbiological analysis of wine samples. Results for the 324 red wine samples submitted to both analyses—microbiological culture on the Brettanomyces-specific medium and the Q-PCR analysis—were grouped into five classes (Table 2). Class 1 represented those samples that had both specific amplification signals by Q-PCR and growth of acidogenic colonies on the Brettanomyces-specific medium (21.0% of total samples). Analysis of these samples by Q-PCR rendered quantification results either in the same range of magnitude as that cfu/mL obtained by culture analysis or up to 103-fold higher numbers (data not shown). These differences can be explained by the fact that Q-PCR quantifies both viable and nonviable Brettanomyces cells, which include dead cells and remaining DNA from broken Brettanomyces cells in the wine sample, whereas the culture analysis on the specific medium quantifies only viable, and therefore, metabolically active Brettanomyces cells. Q-PCR analysis allowed estimating Brettanomyces populations, including both viable and nonviable cells, down to 7 cells/mL, and the microbiological culture analysis on the specific medium allowed quantifying even 1 cfu/mL of viable Brettanomyces cells. Statistical analysis of these data returned a positive Pearson’s correlation of 0.772 between both methods (p < 0.01).
Class 2 represented samples that were Brettanomyces-free according to both analyses, accounting for 34.6% of the total samples. Among these, 46% had very low non-Brettanomyces yeast populations (from 0 to <10 cfu/mL) due to clarification procedures performed by the winemaker during wine elaboration, and 54% had total yeast populations between 10 and 106 cfu/mL (data not shown). The overall agreement between Q-PCR analysis and the microbiological culture method was 86.8%, with a “good” (0.6 to 0.8 interval) kappa value of 0.69, resulting from the data shown in Table 2. This agreement range was obtained for the analysis of the wide range of red wine samples of this study (turbidities from 0.1 to 1000 NTU) that contained their own indigenous microbial population, which included bacteria and yeast, most probably S. cerevisiae strains remaining from the alcoholic fermentation.
Class 3 represented samples with false-negative results by Q-PCR and showing positive colonies on the Brettanomyces-specific medium, which were reconfirmed by specific PCR analysis of the isolated strains from the Brettanomyces-specific medium. These samples accounted for 7.7% of the total, with populations from 1 to 104 cfu/mL on the Brettanomyces-specific medium. Most samples (84%) had very low total yeast populations (<100 cfu/mL) (data not shown), which could be why during the washing and/or DNA extraction stages, cells and DNA for Q-PCR were lost and Q-PCR analysis rendered negative results.
Class 4 samples were those with a positive result by Q-PCR and negative results by cell culture on the Brettanomyces-specific medium (31.8% of total samples). These results, similarly to class 1, can be explained by the fact that Q-PCR quantifies both viable and nonviable Brettanomyces cells and DNA present in the wine sample. Nonviable Brettanomyces cells and DNA still persisting in the wine indicate that Brettanomyces cells have existed in that wine at some stage of the elaboration process and can provide useful information for the winemaker, who can thus assess whether a process of sanitation has been successfully applied.
Class 5 samples were those that gave false positive colonies on the Brettanomyces-specific medium (4.9%) and that, after isolation and analysis by specific PCR, were not confirmed as Brettanomyces. False positives on specific or semispecific culture media always occur, and species such as Candida wickerhamii, Candida cantarellii, Debaryomyces hansenii, Debaryomyces anomala, Kluyveromyces lactis, and Pichia guilliermondii have been reported on DBDM medium (Dias et al. 2003). Other researchers have also detected growth of non-Saccharomyces yeasts in Brettanomyces-specific media (Morneau et al. 2011, Renouf and Lonvaud-Funel 2007).
Since the time required for DNA extraction and Q-PCR analysis (~5 hr) is shorter than the microbiological culture analysis on the specific medium, which can take up to two weeks, Q-PCR analysis is of interest for winemakers. Moreover, the cell washing and DNA extraction methods here allow for analysis of red wines with high turbidity (up to 1000 NTU), and therefore at early stages of aging. This rapid method also has high specificity, which prevents false positives due to other yeast cells that can be growing in the same wine. Nevertheless, quantification by Q-PCR includes nonviable and DNA from broken Brettanomyces cells, which renders high numbers for populations that might be inactive. During the preparatory protocol of DNA extraction from wine, some losses could occur (in our study 7.7% of the analyzed samples), and false negative analyses can appear with this Q-PCR analysis; however, this deficiency can be considered of minor importance if we regard as the threshold limit for appearance of the typical unpleasant off-odor 103 cfu/mL of Brettanomyces population (Chatonnet et al. 1995, Lonvaud-Funel and Renouf 2005). Consequently, this method is appealing for the winemaking industry. On the other hand, the conventional microbiological culture analysis on specific media counterbalances its lack of rapidity by its low cost, simplicity, and efficacy in quantifying viable Brettanomyces cells, as it can detect 1 cfu/mL of metabolically active cells that will be the ones ultimately responsible for generating the off-odors that spoil wine.
Brettanomyces is not expected to change wine quality from an unspoiled to a spoiled status in a period of days, but it will take longer, and thus winemakers can choose the best strategy to analyze and control potential Brettanomyces contamination. Strategies include the rapid and efficient Q-PCR method, which can be applied to wine samples with high turbidity that have been submitted to the DNA extraction protocol described in this article, or the conventional microbiological culture method, which has lower cost, requires less equipment, and has the lowest threshold (1 cfu/mL) for detecting viable Brettanomyces spoiling cells in wine.
This research was financially supported by grant CE-NIT-2008/1002 of the Spanish Ministry of Science and Innovation MICINN-CDTI. Cauré Portugal was a contractual researcher supported by grant CENIT-2008/1002.
The authors acknowledge the valuable help of Zenaida Hernández of the Department of Mathematics and Computing of the University of La Rioja to carry out the statistical analysis of data.
Publication costs of this article defrayed in part by page fees.
- Received March 2012.
- Revision received July 2012.
- Accepted September 2012.
- ©2013 by the American Society for Enology and Viticulture