Abstract
Malolactic fermentation (MLF) is an integral step in red winemaking that results in a decrease in acidity and can also influence additional wine quality parameters. This study examined the impact of MLF on red wine color and the ability of Oenococcus oeni to degrade compounds important to the development of polymeric pigments. Pinot noir and Merlot wines were produced where a portion of the wines underwent a simultaneous alcoholic and malolactic fermentations with O. oeni strain VFO. Post-alcoholic MLFs were conducted using VFO and two additional O. oeni strains in sterile filtered wines. A control wine that did not undergo MLF was pH adjusted to the same pH as wines that had undergone MLF. Wines that underwent MLF (MLF+) contained lower concentrations of acetaldehyde and pyruvic acid than wines that did not undergo MLF (MLF-). Wines that underwent MLF with O. oeni VFO also had higher concentrations of caffeic and trans-p-coumaric acids. MLF+ wines had significantly lower color and polymeric pigment content than MLF-wines and contained significantly higher monomeric anthocyanins. Vitisin B concentrations were also significantly lower in wines that underwent MLF. These differences remained throughout nine months of storage, demonstrating that MLF can affect red wine color independent of pH change. While O. oeni influenced the concentration of phenolic and nonphenolic compounds involved in red wine color development there were no strain specific differences in color and polymeric pigment content.
The color of red wine is an important sensory attribute that originates primarily from anthocyanins present in the grape skins (Fulcrand et al. 2006, Vivar-Quintana et al. 2002). However, once anthocyanins have been extracted into the wine, they can rapidly form copigmentation complexes (Boulton 2001) and undergo numerous other reactions resulting in a number of new pigmented compounds (Fulcrand et al. 1998, Bakker and Timberlake 1997). These new color compounds are often much more stable and produce greater color than would be expected from their concentrations in the wine. For example, copigmentation between compounds such as p-coumaric acid, caffeic acid, catechin, quercetin, and anthocyanins may result in a hyperchromic shift at 520 nm as well as a bathochromic shift toward the blue end of the visible spectrum (Schwarz et al. 2005, Boulton 2001, Brouillard et al. 1989). Pigmented compounds such as the pyranoanthocyanins vitisin A and vitisin B demonstrate more resistance to SO2 bleaching and oxidation (Fulcrand et al. 1998, Bakker and Timberlake 1997) while polymeric pigments formed through reactions between anthocyanins and tannins are generally accepted to account for the majority of observed color in older red wines (Salas et al 2003, Fulcrand et al. 1998, Bakker and Timberlake 1997).
Given their importance in red wine color, copigmentation pigments, polymeric pigments, and factors that impact their formation have been investigated. Anthocyanin concentration is primarily determined in the vineyard, but other compounds involved in red wine color development can be impacted by winemaking practices such as increasing fermentation temperature (Reynolds et al. 2001, Girard et al. 1997), extended maceration (Zimman et al. 2002), and saignée (Harbertson et al. 2009). Some recent studies have investigated the impact of wine microorganisms on red wine color. For example, Cabernet Sauvignon wines produced by two different yeast strains, a Saccharomyces cerevisiae and a Saccharomyces bayanus, resulted in significantly different color properties (Hayasaka et al. 2007). The authors attributed these differences to the higher production of acetaldehyde by the S. bayanus strain, leading to greater vitisin B formation. Morata et al. (2003) also demonstrated a strong correlation between pyruvate and acetaldehyde production by yeast and vitisin A and B formation. Furthermore, Medina et al. (2005) reported that yeast may influence red wine color through adsorption of anthocyanins to their cell walls. While these and other studies have reported the action of yeast on red wine color, the development of wine color is an ongoing process that continues long after alcoholic fermentation is complete. In particular, other microorganisms may play a role in red wine color development, as the concentration of compounds such as pyruvic acid and acetaldehyde do not remain stable and can be affected by winemaking practices such as malolactic fermentation (MLF).
The MLF involves the decarboxylation of malic acid to lactic acid by wine lactic acid bacteria, typically Oenococcus oeni, resulting in a decrease in acidity and possibly changes in wine aroma, flavor, and texture (Rodriguez et al. 1990, McDaniel et al. 1987, Giannakopoulos et al. 1984). Anecdotal reports from winemakers suggest that MLF can cause a decrease in red wine color, an observation noted in some studies investigating MLF (Husnik et al. 2007, Rankine et al. 1970) and attributed to a pH increase post-MLF. Other studies have demonstrated that O. oeni can impact the concentration of compounds involved in red wine color, such as acetaldehyde (Osborne et al. 2000, 2006) and pyruvic acid (Asenstorfer et al. 2003, Wells and Osborne 2012), while other lactic acid bacteria may impact the copigments p-coumaric acid and caffeic acid (Hernandez et al. 2006, 2007, Cavin et al. 1993). However, few studies have focused on the role MLF may have in the color development of red wines. One study investigated the capability of the genetically engineered malolactic yeast ML01 to perform MLF (Husnik et al. 2007); Cabernet Sauvignon wines produced by ML01 (MLF performed by the yeast) and yeast strain S92 had darker color than wine produced by S92 with a bacterial MLF. The pH of the wines produced was very similar, indicating that the loss of color was not due to the change in pH caused by the MLF. The authors speculated the color change may have been due to the metabolic activity of O. oeni impacting negatively on anthocyanins in the wine, although the impact of O. oeni on the concentration of compounds important to color development was not measured. More recently, Abrahamse and Bartowsky (2012) reported an overall slight decrease in wine color density following MLF but noted that the influence of MLF on polymerization and copigmentation of anthocyanins was not clear. Costello et al. (2012) also reported changes in Shiraz wine color densities that were influenced by pH and bacterial strain. Malolactic fermentation clearly has the potential to impact red wine color intensity and stability, but this phenomenon has not been well characterized. Therefore, the purpose of this study was to investigate the influence of the MLF on the color and color stability of red wine and the ability of O. oeni to impact compounds involved in copigmentation and polymeric pigment formation.
Materials and Methods
Microorganisms.
An active dry form of Saccharomyces cerevisiae strain VQ-15 (Lallemand, Montréal, Canada) was obtained and rehydrated according to manufacturer specifications prior to inoculation at a rate of 0.25 g dried yeast/L must (~1 × 106 cfu/mL). Three commercially available strains of Oenococcus oeni were used: Viniflora Oenos (VFO) (Chr. Hansen, Hørsholm, Denmark), VP41 (Lallemand), and Enoferm Alpha (Lallemand). After isolation from a single colony, bacteria were maintained in de Man, Rogosa, and Sharpe (MRS) stabs (pH 4.5) and stored at 4°C. When required, bacteria were transferred from stabs to MRS broth (pH 4.5) and grown at 25°C for five days. Cells were harvested by centrifugation (4,000 × g for 20 min) and resuspended in 0.2 M phosphate buffer (pH 7.0) before inoculation at ~1 × 106 cfu/mL.
Grapes.
Pinot noir and Merlot grapes sourced from Oregon State University’s Woodhall Vineyard (Alpine, OR) were harvested by hand in October 2009. After harvest, grapes were stored at 4°C overnight before being hand-sorted and destemmed. Pinot noir grapes were not crushed while Merlot grapes were crushed. The grapes were then pooled and divided into 100 L stainless-steel tanks, each containing ~60 L grape must. At this point samples were taken for pH, sugar content (Brix), titratable acid (TA), and yeast assimilable nitrogen (YAN). Basic juice parameters of the Pinot noir must were pH 3.35, 24.2 Brix, 0.683 g/100 mL TA (grams tartaric acid), and 208.5 mg/L YAN and of the Merlot were pH 3.55, 23.9 Brix, 0.510 g/100 mL TA, and 147.6 mg/L YAN. Each tank received an addition of 50 mg/L SO2 (in the form of potassium metabisulfite) and the yeast nutrient Fermaid K (Lallemand) was added at a rate of 0.125 g/L.
Fermentation.
All grape must were inoculated with S. cerevisiae VQ-15. A set of three tanks (for Pinot noir and Merlot grapes) were also inoculated ~24 hr later with O. oeni VFO. After inoculation, tanks were placed in a temperature-controlled room held at 27°C. Cap punch downs were performed uniformly twice daily. Temperature and Brix were measured with an Anton-Paar DMA 35N Density Meter (Graz, Austria). Completion of alcoholic fermentation (reducing sugar concentration <0.2 g/100 mL) was confirmed by testing with Bayer Clinitest tablets (Morristown, NJ). Malic acid levels were measure by enzymatic assay (R-Biopharm, Darmstadt, Germany) and completion of MLF was confirmed when malic acid concentration was <0.050 g/L.
After all alcoholic fermentations (and MLF in the case of the simultaneous fermentations) were complete, wines were pressed and placed in a cold room at 4°C for 48 hr to cold settle. Following cold settling, wines were racked and then filtered through a plate and frame filter fitted with Beco K-1 3.0 μm nominal filter sheets (20 cm × 20 cm; Langenlonsheim, Germany). Wine was then filtered through a 1.0 μm nylon cartridge and a 0.45 μm sterile polyethersulfone cartridge (G.W. Kent, Ypsilanti, MI) in succession. Filtered wine was then dispensed into sterile 4 L glass carboys inoculated with either O. oeni VFO, Alpha, or VP41 or not inoculated (control). All treatments were conducted in triplicate. Wines were held at 21°C and MLF progress was monitored by enzymatic assay (Roche Pharmaceuticals, Basel, Switzerland). After completion of MLF (malic acid <50 mg/L), wines were cold settled at 4°C for 48 hr before all treatments received an addition of 35 mg/L SO2. A portion of the wine that did not undergo MLF (MLF-) was pH adjusted (by addition of 2 N NaOH) to match the final pH of wines that had undergone MLF (MLF+). All wines were then filtered through a sterile 0.45 μm PES cartridge (G.W. Kent) and bottled in 350 mL brown glass beer bottles (sparged with nitrogen) and sealed with crown caps. Bottled wines were stored at 13°C until needed for analysis.
Color, pigmented polymer, and phenolic analysis.
Wines were analyzed every 90 days for a variety of parameters. For purposes of analysis, the day of bottling was considered the day zero time point. Color was determined by spectrophotometric analysis (Shimadzu UV-3101PC, Kyoto, Japan) at 520 nm in a 1 mm path-length quartz cuvette after pH adjustment to 3.60 by addition of 2 N NaOH or 25% H3PO4. Polymeric pigment and copigmentation were measured by spectrophotometric analysis (Thermo Scientific Genesys, Madison, WI) according to Levengood and Boulton (2004).
Reversed-phase high-performance liquid chromatography (HPLC) was performed using a Hewlett-Packard/Agilent Series 1100 (Palo Alto, CA) equipped with HP ChemStation software and photodiode-array detector (DAD). The HPLC was fitted with a LiChroSpher reversed-phase C18 column (4 × 250 mm, 5 mm particle size) (Merck, Darmstadt, Germany) held at 30°C. HPLC grade 98% formic acid and 99.8% methanol were obtained from EMD Chemicals (Darmstadt, Germany) while malvidin-3-glucoside, caffeic acid, and trans-p-coumaric acid were obtained from Sigma Aldrich (St. Louis, MO). Gradients of solvent A (water/formic acid, 90:10, v/v) and solvent B (methanol) were applied as follows: 5 to 35% B linear (1.0 mL/min) from 0 to 15 min, static at 35% B (1.0 mL/min) from 15 to 20 min, 35 to 80% B linear (1.0 mL/min) from 20 to 25 min, and 5% B (1.0 mL/min) from 25 to 32 min to re-equilibrate the column to initial conditions. Prior to HPLC analysis, wine samples were centrifuged using an Allegra X-22 instrument (Beckman Coulter, Brea, CA) at 12,000 rpm for 10 min. Wines were sampled in 20 μL aliquots and anthocyanins and hydroxycinnamic acids were detected by scanning from 190 to 700 nm. Identification and quantification of anthocyanins (including vitisin A and vitisin B) were determined from a standard curve for malvidin-3-glucoside at 520 nm and expressed as malvidin-3-glucoside equivalents. Identification and quantification of hydroxycinnamic acids were performed at 320 nm based on caffeic and trans-p-coumaric acid standard curves, with caftaric acid expressed as caffeic acid equivalents.
Acetaldehyde and pyruvic acid concentrations were measured by enzymatic assay (R-Biopharm).
Statistical analysis.
A univariate analysis of variance (ANOVA) was used to determine differences among treatments. The ANOVA was performed by Minitab 16.1.1 (State College, PA) and Tukey’s HSD multiple comparison was performed to test least squares means of treatment effects at the 0.05% significance level.
Results
Alcoholic fermentations for both the Pinot noir and Merlot wines proceeded similarly, with Pinot noir fermentations completed in nine days (<0.5 g/L reducing sugars) and Merlot fermentations completed in six days. Pinot noir and Merlot wines undergoing simultaneous alcoholic and malolactic fermentations completed alcoholic fermentation in nine and six days, respectively, with the MLF in the Pinot noir wines completed in nine days (<0.05 g/L malic acid). However, for the Merlot simultaneous fermentations, the MLF proceeded slower and were completed after 13 days. Wines that were inoculated for MLF post-alcoholic fermentation completed MLF in <21 days. There was no significant difference in the time taken to complete MLF among the three O. oeni strains. Pinot noir wines that underwent MLF had an average pH of 3.67 ± 0.06 and the control had an average of 3.55 ± 0.03. Merlot wines that underwent MLF had an average pH of 3.62 ± 0.07 and the control had an average of 3.44 ± 0.02.
All wines that underwent MLF (MLF+) had significantly reduced color compared to the control. Pinot noir wines had ~18% reduction in color at 520 nm at the completion of MLF (day 0), regardless of which O. oeni strain was used or the timing of the MLF (simultaneous or consecutive inoculation) (Figure 1A). Over time this reduction in color remained, as after 270 days of storage, MLF+ wines still had ~18% less color at 520 nm than the control wines. No significant difference in color at 520 nm between the pH-adjusted control wine and the control was observed. An even greater loss in color was observed for Merlot wines that had undergone MLF (Figure 1B). At the completion of the MLF (day 0) wines had ~22% less color at 520 nm than the control wine and this difference remained during storage. Again, there was no significant difference in color among the O. oeni strains and also no difference between the pH-adjusted control and the control wine. There was, however, a significant difference in the color of Merlot wines that had undergone a simultaneous alcoholic and MLF and Merlot wines that underwent MLF at the end of the alcoholic fermentation (Figure 1B), with the simultaneous wines having the lowest color at 520 nm at every time point except day 0. Furthermore, Merlot wines that underwent MLF simultaneously with VFO contained significantly less color than wines that underwent MLF with VFO at the end of alcoholic fermentation. Copigmentation content of the wines was also measured. However, very low levels were observed and no significant differences at any time point were noted for Pinot noir or Merlot wines that had or had not undergone MLF (data not shown).
Polymeric pigment content was also significantly lower in MLF+ wines. For Pinot noir, MLF+ wines contained ~17% less polymeric pigment content at the completion of MLF (day 0) than the control wine (Figure 2A). This difference remained over time, at 20% after 180 days of storage and 18% after 270 days. The differences were greater for Merlot: MLF+ wines contained ~23% less polymeric pigment at the completion of MLF and close to 40% less after 180 days of storage (Figure 2B). The difference between the control wines and the MLF wines lessened after 270 days of storage, although there was still a 20% reduction in polymeric pigment content. As with color at 520nm, there was no significant difference in polymeric pigment content among O. oeni strains or between the pH adjusted control and the control wine. However, Merlot wine produced by a simultaneous fermentation consistently contained the lowest level of polymeric pigment (Figure 2B) and contained significantly less polymeric pigment than wines that underwent MLF with VFO at the end of alcoholic fermentation rather than simultaneously. For all Pinot noir and Merlot wines, whether MLF- or MLF+, polymeric pigment content was highest at day 180 and had declined by day 270.
As with color at 520 nm and polymeric content, there were also significant differences in monomeric anthocyanin concentrations between the control wines and MLF+ for both Pinot noir and Merlot (Figure 3). Overall, monomeric anthocyanin concentrations were highest at day 0 and decreased over time. MLF+ wines contained significantly higher concentrations than the control wines at every sampling point during storage. However, at day 0 the control pH Pinot noir wine contained statistically the same amount of monomeric anthocyanins as all the MLF+ wines except for the simultaneous fermentation (Figure 3A). For the remaining sampling points, however, the control pH Pinot noir wine contained a lower concentration of monomeric anthocyanins. As with color and polymeric pigment, no differences were noted in monomeric anthocyanin concentrations in wines inoculated with different O. oeni strains.
Vitisin A and B were measured by HPLC and significant differences between treatments were noted (Table 1). At the completion of MLF, vitisin A content of Pinot noir and Merlot wines that had or had not undergone MLF were statistically the same except for Merlot wines that underwent a simultaneous fermentation. In this case, vitisin A content was higher than in the control as well as in other wines that underwent MLF. In contrast, vitisin B concentrations were significantly lower in both Pinot noir and Merlot wines that underwent MLF compared to the control. The control Pinot noir contained 3.3 mg/L vitisin B (expressed as malvidin-3-glucoside equivalents) and the Pinot noir that had undergone MLF contained between 0.7 and 2.0 mg/L.
Wines were analyzed for compounds involved in polymeric pigment formation and copigmentation reactions. For Pinot noir and Merlot MLF+ wines, there was a significant reduction in acetaldehyde and pyruvic acid concentrations compared to the control regardless of which O. oeni strain was used (Table 2). However, O. oeni VFO demonstrated the greatest reduction in both acetaldehyde and pyruvic acid in the Pinot noir wines. VFO was also used in the simultaneous fermentation treatment, which showed a similar reduction of both compounds. In the control wines, higher concentrations of pyruvic acid were noted in Merlot and higher concentrations of acetaldehyde were noted in Pinot noir.
Hydroxycinnamic acids were also quantified at the completion of malolactic fermentation because of their involvement in copigmentation reactions. There was no statistically significant difference in concentrations of caftaric, caffeic, and trans-p-coumaric acids between MLF+ and control wines, except for both Pinot noir and Merlot wines that had undergone MLF with O. oeni VFO (Table 3). In the case of VFO MLF+ and simultaneous (VFO) wines, there was a significant reduction in caftaric acid concentration and a corresponding increase in caffeic acid concentration. In addition, although trans-p-coumaric acid concentrations were low in Pinot noir and Merlot wines, they were significantly higher in wines that had undergone MLF with O. oeni VFO.
Discussion
The effects of malolactic fermentation on the color of Pinot noir and Merlot wine as well as compounds involved in polymeric pigment formation and copigmentation were investigated. Malolactic fermentation caused a significant decrease in color in both Pinot noir and Merlot wines, a reduction that remained even after nine months. While changes in red wine color due to MLF have recently been reported in Shiraz (Abrahamse and Bartowsky 2012) and Cabernet Sauvignon (Costello et al. 2012), this study reports on the long-term color loss in Pinot noir and Merlot as well as possible causes. Although the pH change resulting from the conversion of malic acid to lactic acid was previously suggested as an explanation for color loss due to MLF (Pilone and Kunkee 1972), this factor was accounted for in the present study and was shown not to be the cause of color loss. Furthermore, the change in pH due to MLF did not impact color during aging, as after 270 days of aging there were no significant differences in color between control wines and control wines adjusted to the pH of MLF+ wines. Although wines of higher pH are more susceptible to oxidation and loss of color (Singleton 1987), that was not observed in this study.
The color loss due to MLF corresponded with decreased polymeric pigment content in MLF+ wines compared to control wines. Differences in the polymeric content of the wines were observed immediately at the completion of the MLF and these differences remained or increased during aging. The lower polymeric pigment content also corresponded with a significantly higher concentration of monomeric anthocyanins in the wines. For example, Merlot wine in which O. oeni Alpha had completed MLF contained 173 mg/L malvidin-3-glucoside, whereas the control wine that had not undergone MLF contained 107 mg/L. This result is in contrast to suggestions that loss of color by MLF is probably due to bacterial metabolism of monomeric anthocyanins or results in no change to the anthocyanin profile (Mangani et al. 2011). However, the findings of the present study are in agreement with others (Abrahamse and Bartowsky 2012) where higher anthocyanin content was reported in Shiraz wine that underwent MLF compared to a control wine that did not undergo MLF. Costello et al. (2012) reported lower anthocyanin concentrations in some wines that underwent MLF and higher concentrations in others and suggested that these differences were likely due to differences in the length of time required to complete MLF and/or differences in the wine matrix (pH, ethanol, source of grapes). In the present study, the time required to complete MLF was relatively short and consistent between strains, and monomeric anthocyanin concentrations did not differ in MLF+ wines where different ML strains were used. The higher monomeric anthocyanin in MLF+ wines in the present study was likely due to lower incorporation of monomeric anthocyanins into polymeric pigment compounds (Morata et al. 2007), as indicated by their lower polymeric content compared to MLF-wines.
The differences in polymeric pigment content of wines that did or did not undergo MLF may have been due to degradation of pyruvic acid and acetaldehyde by O. oeni. As noted in previous research (Osborne et al. 2000, Wells and Osborne 2012), all strains of O. oeni degraded acetaldehyde and pyruvic acid. Acetaldehyde plays an important role in stable pigment formation by providing an ethyl bridge to link flavanols to anthocyanins or anthocyanins to other anthocyanins (Cheynier et al. 2006), reactions favored at wine pH (Dallas et al. 2003). Loss of acetaldehyde due to bacterial metabolism may have reduced the formation of these ethyl-linked compounds compared to the control wines. Acetaldehyde and pyruvic acid can also be incorporated into vitisin A and B, compounds with increased absorbance at 520 nm and resistance to bleaching by SO2 (Schwarz et al. 2003). While vitisin A concentrations were often not significantly different between MLF- and MLF+ wines, there were significant differences in vitisin B concentrations. Wines that did not undergo MLF had significantly higher vitisin B than those that underwent MLF. Given the relative importance of vitisins to stable red wine color, even at low concentrations (Rentzsch et al. 2010), the differences found here may have contributed to the color differences between MLF- and MLF+ wines. Furthermore, because acetaldehyde is involved in both the formation of ethyl-linked pigments and vitisin B, it was likely more important than pyruvic acid to red wine color development in the present study.
While significant differences in polymeric pigments were noted, color due to copigmentation did not differ between wines that did or did not undergo MLF. This finding was surprising given that wines that underwent MLF with O. oeni VFO had higher levels of the copigmentation cofactors caffeic and trans-p-coumaric compared to MLF- wines and MLF+ wines in which MLF was conducted by O. oeni Alpha or VP41. The increased caffeic and trans-p-coumaric concentrations were likely due to cinnamoyl esterase activity converting caftaric and coutaric acid, respectively. While cinnamoyl esterase activity by O. oeni has been reported (Cabrita et al. 2008), strain variability has not. In addition, Hernandez et al. (2007) reported cinnamoyl esterase activity during a spontaneous MLF as well as a MLF inoculated with Lactobacillus plantarum. However, two O. oeni strains evaluated did not exhibit cinnamoyl esterase activity and the bacteria present during the spontaneous MLF were not identified.
In the present study the copigmentation values for all wines were low compared to those reported in other studies (Gutiérrez et al. 2005). Therefore, changes in the concentrations of cofactors in the present study would likely have had little impact on red wine color. The anthocyanin composition (acylated versus nonacylated forms and percentage of malvidin-3-glucoside) of red wine varieties is varied (Romero-Cascales et al. 2005), and future research should be conducted on a range of red wines to determine if O. oeni cinnamoyl esterase activity is beneficial or detrimental to copigmentation and red wine color development. In particular, research should focus on red wines such as Syrah where the contribution to total red wine color by copigmentation is more significant (Boulton 2001).
Timing of the MLF inoculation did not impact color loss, as wines that underwent MLF at the completion of alcoholic fermentation or concurrently demonstrated the same decrease in color post-MLF. Despite the demonstrated advantageous of performing simultaneous fermentations in white wines (Pan et al. 2011), this practice is not common in red winemaking due to concerns over possible color loss, elevated production of acetic acid, and/or antagonistic interactions between yeast and malolactic bacteria. However, recent studies with Malbec (Massera et al. 2009) and Shiraz (Abrahamse and Bartowsky 2012) have demonstrated that coinoculation for red winemaking could reduce the overall fermentation time while not negatively impacting wine sensory quality. This present study demonstrates that color loss due to MLF can occur in a simultaneous MLF and in a consecutive MLF. However, wines that underwent simultaneous MLF did frequently have the lowest color and polymeric pigment content, particularly Merlot wines, possibly due to the earlier removal of acetaldehyde and pyruvic acid from the wine during the simultaneous fermentation. Still, differences in color and polymeric pigment content between wines that underwent a simultaneous or sequential MLF were minimal compared to wines that did not undergo MLF, suggesting that color loss should not be a determining factor in deciding whether to conduct a simultaneous red wine fermentation.
Conclusions
Malolactic fermentation resulted in a reduction in red wine color and polymeric pigment formation in Pinot noir and Merlot wines and a corresponding higher concentration of monomeric anthocyanins. Color loss was observed regardless of whether the MLF occurred concurrently or consecutively and was not due to the pH change caused by MLF. All O. oeni strains degraded acetaldehyde and pyruvic acid during the MLF, which likely impacted formation of polymeric pigments. Whether the degradation of these compounds is the primary way that MLF impacts red wine color should be further investigated, as it may be possible to mitigate color loss through management of acetaldehyde and/or pyruvic acid concentrations. In addition, the impact of MLF on copigmentation should be studied in wines where copigmentation may play a more significant role, as differences in the ability of ML strains to impact the concentrations of compounds involved in copigmentation were noted in this study. These findings may aid in developing strategies to minimize color loss due to MLF, including use of specific strains of O. oeni.
Acknowledgments
Acknowledgments: The Oregon Wine Board and American Vineyard Foundation provided funding for this project.
- Received January 2013.
- Revision received March 2013.
- Accepted May 2013.
- Published online August 2013
- ©2013 by the American Society for Enology and Viticulture