There is quite a lot of research going on out there exploring different alternatives to using SO2 during winemaking as a result of the negative connotations associated with excess SO2.¬† While SO2 (a.k.a. sulfur dioxide) is naturally present in grapes and many other fruits and other foods, the addition of extra SO2 can be problematic for aroma and flavor of wine as well as for human health, if levels are too high.¬† In most winemaking regions, the levels of added SO2 are restricted and closely monitored, so as to avoid any of the problems that can arise from too high of SO2 levels.¬† However, even though these policies have been effective at minimizing any large scale health problems in the populations, the stigma behind what could happen in addition to the organic movement has researchers and winemakers
seeking¬†more natural alternatives to SO2 for protecting the finished wine against bacterial infection as well as improving ageability.
Laboratory studies have shown that plant polyphenol extracts can be effective in slowly or stopping the growth of lactic acid bacteria in wine, though to date no study has been done on a larger, real world scale.¬† In the lab, using natural plant polyphenols have been shown to be as effective as SO2 in protecting wine against bacterial infection, and has been shown to be an even better performer when used in concert with lower levels of SO2.¬† These laboratory studies indicate that using natural plant polyphenols either alone or in combination with lower levels of SO2 may be appropriate alternatives to the conventional use of added SO2 in winemaking, though this would need to be tested at a larger scale to see if the laboratory findings coincide with real world scenarios.
The study presented to you today aimed to do just that: examine this idea of using natural plant extracts as a preservative in a commercially-made white wine at the small winery scale, and examined whether using plant polyphenols in addition to lower levels of SO2 changed the organoleptic characteristics in the finished wine in any way that might suggest the alternative is more or less than ideal.
Verdejo wine (white wine) from the 2010 vintage was used for this experiment, and was made by Bodegas Jos√© Pariente S.L. from Valladolid, Spain.¬† Alcoholic fermentation took place in stainless steel tanks, and once that was finished, the wine was divided up into several 225L oak barrels for different treatments.
The treatments for this experiment were as follows:
1)¬†¬†¬†¬†¬† 160mg/L SO2 (typical for white winemaking) (CONTROL)
2)¬†¬†¬†¬†¬† 80mg/L SO2 + 100mg/L eucalyptus leaves extract
3)¬†¬†¬†¬†¬† 80mg/L SO2 + 100mg/L almond skins extract
4)¬†¬†¬†¬†¬† 160mg/L SO2 kept in the stainless steel tank (NOT transferred to an oak barrel)
(Note: All extracts are commercially available, and for this study were purchased from Biosearch Life S.A. in Grenada, Spain)
Two barrels were used for each treatment, with the exception of the control treatment and the stainless steel treatment which used one barrel/tank each.
Wines were sampled and analyzed after 2 and 6 months of aging in the barrels/tanks.
The following were measured and analyzed for each wine sample: total acidity, volatile acidity, pH, alcohol content, lactic acid bacteria, volatile compounds, and phenolic compounds.
Sensory analysis employed the use of 10 judges (3 men and 7 women between the ages of 28 and 65) who were all previously training in detection and recognition of odors and flavors in white wines.¬† Three samples of wine were randomly presented to each judge at a time, and they were asked to identify which of the three was a different wine.¬† The only information that was given to the judges was that two of the three wines were identical, while one of the three wines was different from the other two.
Judges rested between samples, rinsing with water and eating breadsticks when needed.
- Total acidity, volatile acidity, pH, and alcohol content were similar for all wine samples, regardless of the treatment.
- After 6 months of storage, lactic acid bacteria in all wines decreased to less than 106CFU/mL, indicating microbial stability at this point in time.
- Lactic acid bacteria populations were significantly reduced in the SO2+eucalyptus extract compared with SO2 alone.
- Lactic acid bacteria populations were significantly reduced in the SO2+almond skin extract compared with SO2 alone in only one of the two oak barrels of study.
- Volatile Compounds:
- Ethyl butyrate and ethyl hexanoate levels were significantly higher in wines treated with eucalyptus and almond skin extracts compared with wines treated with SO2 alone.
- Hexyl acetate levels were significantly higher in wines treated with eucalyptus and almond skin extracts compared with wines treated with SO2 alone for one out of the two barrels only.
- Concentrations of diethyl succinate and ethyl octanoate varied depending upon treatment and barrel, with some barrels seeing increases and other seeing decreases, even within one treatment type.
- Ethyl lactate did not differ between treatments.
- No differences in alcohols were found between any of the treatments.
- Linalool levels were reduced by about 50% in wines treated with eucalyptus and almond skin extracts.
- ő≤-damascenone levels were higher in wines wines treated with eucalyptus and almond skin extracts (except not in the second barrel of the almond extract treatment).
- Control wines (SO2 only + oak barrel aging) had higher levels of 2,6-dimethoxyphenol and eugenol compared with wines aged in stainless steel tanks.
- Wines treated with eucalyptus and almond extracts had lower levels of 2,6-dimethoxyphenol than control wines.
- Wines treated with eucalyptus and almond extracts had higher levels 4-vinylphenol and 2-methoxy-4-vinylphenol than control wines.
- Wines treated with eucalyptus extracts had higher levels of whiskey lactones and lower levels of furanic compounds (in particular, 5-methylfurfural) than all other treatments.
- Wines treated with almond skin extract had lower levels of lactones and furanic compounds compared with control wines.
- Vanillin levels were different depending upon which barrel was used, and not necessarily what antimicrobial treatment was performed.¬† This may be due to differences in toasting levels for each individual oak barrel.
- Many of the volatile compounds in the different wine treatments were above their odor threshold level, indicating that eucalyptus or almond skin extract treatments could potentially change the aromatic profile of the finished wine.
- Phenolic Compounds:
- No significant differences in phenolic compounds were found between the control wine aged in oak barrels and the wine aged in stainless steel tanks, except for cis-coutaric acid and 4-hydroxy-3-methoxy-mandelic acid.
- Benzoic acid levels were lower for both eucalyptus and almond skin extract treatments compared with the control wine.
- Syringic acid levels were higher for both eucalyptus and almond skin extract treatments compared with the control wine, with the almond skin extract treatment having the highest levels.
- P-coumaric acid, cis-coutaric acid, unknown hexose of caffeic acid, and ferulic acid were all lower in wines treated with both extracts, while isoferulic acid levels were found to be higher in these wines.
- Wines treated with eucalyptus extracts and almond skin extracts both had lower levels of flavan-3-ols than the control wine (15%-30% less).
- Procyanidin B5 levels were reduced by greater than 50% in both extract treatment wines.
- There were no significant differences in the levels of 3-(3,4-dihydroxyphenyl)propionic acid, 4-hydroxymandelic acid, tyrosol, and phloroglucinol among any of the treatment or control wines.
- The concentrations of any of the phenolic compounds mentioned above did not reach their odor threshold levels in any of the wines.
- Sensory Analysis:
- Barrel #1 of the eucalyptus extract treatment: Judges identified the different wine correctly 2 out of 10 times.
- Barrel #2 of the eucalyptus extract treatment: Judges identified the different wine correctly 8 out of 10 times.
- Barrel #1 of the almond skin extract treatment: Judges identified the different wine correctly 5 out of 10 times.
- Barrel #2 of the almond skin extract treatment: Judges identified the different wine correctly 7 out of 10 times.
- Analysis of these results showed that the wines treated with natural plant extracts (eucalyptus and almond extracts) were not different enough from the wines made treated with SO2 only for the judges to successfully tell the difference between them.
Overall, the results of this study were fascinating in that it appears as though using plant extracts in addition to reduced levels of SO2 appears to offer similar, if not better, protection against infection and improve ageability than using only higher levels of SO2 alone.¬† According to the results, the judges were not successful in their ability to tell the difference between wines made with plant extracts versus those made with SO2 alone, indicating that using plant extracts may be a viable alternative to using SO2 alone, in that it allows for a reduction in the amount of added SO2 required for winemaking, while at the same time not having enough of an effect on the organoleptic properties for even well-trained judges to tell the difference between the two styles of wine.
The main caveat in this study, for me, was the relatively small sample size.¬† Sure, it‚Äôs great that they were able to have some replication within treatments, however, with only 2 barrels (and in some cases 1, it‚Äôs extremely difficult to determine what exactly is causing any chemical or sensory differences between thetwo.¬† Why is there so much variability within each treatment?¬† Could it be due to differences in the way each barrel was made?¬† I think that‚Äôs highly plausible, and therefore more replicates would be needed in order to help control for this large variability.
Despite the small sample size, I am hopeful in the idea that using plant extracts in lieu of or in combination with lower levels of added SO2 in white winemaking may be doable on the winery scale, and not just in a petri dish in the laboratory.¬† Certainly, more studies using different types of wine and more replicates is needed to add support (or refute) this idea, though again, I am very hopeful that we‚Äôre closer to a more ‚Äúnatural‚ÄĚ or ‚Äúorganic‚ÄĚ approach than what has been done in more conventional winemaking practices.
What do you think of this topic? Please feel free to share any questions, comments, or experiences you may have on this topic!
Source: Gonz√°lez-Rompinelli, E.M., Rodr√≠guez-Bencomo, J.J., Garc√≠a-Ruiz, A., S√°nchez-Pat√°n, F., Mart√≠n-√Ālvarez, P.J., Bartolom√©, B., and Moreno-Arribas, M.V. 2013. A winery-scale trial of the use of antimicrobial plant phenolic extracts as preservatives during wine ageing in barrels. Food Control 33: 440-447.