There is a whole host of information, both scientific and anecdotal, on how to pair wine with food, which is in a continual state of flux. One example of a wine and food pairing that is often not recommended is red wine with seafood. This pairing is often not recommended as a result of the perception of a fishy aftertaste and metallic odors due to the tannins and other compounds in the wine. It has also been observed that white wine sometimes carries with it this same fishy aftertaste when paired with seafood, and since white wine has very little tannin, the recommendation that red wine not be paired with seafood as a result of the tannin is likely anecdotal. In fact, a recent study found that iron was actually a key cause of the fishy aftertaste in wine, occurring both in red and white wines.
Finding a way to eliminate this fishy aftertaste in wine is very important, especially since statistics show that the world consumption of seafood is on the rise, and one should be able to drink any wine they’d like with their meal without incurring any unwanted flavors. The average concentration of iron found in wines around the world ranges from 2.8 to 16mg/L. Iron gets into the wine by way of the soil, dust, and the winemaking equipment. When there are excess levels of iron present in the wine, certain fining agents or cooling may be used to precipitate the iron and subsequently remove it from the finished wine.
One major problem with this removal method is that only very low levels of iron are needed to elicit the fishy aftertaste in wine after seafood pairing, and current methods used for removing iron would not be able to bring iron levels down low enough without removing other important quality compounds in the wine. Ferrocyanide is known to effectively remove iron, however, its’ disposal is problematic as it is considered toxic waste. Phytic acid is a more trusted and safe compound for removing iron in food products though, like other removal methods, it does not effectively remove enough iron to prevent the fishy aftertaste.
The goal of the study presented today was to determine an effective method for removing iron from wine in order to reduce the fishy aftertaste in wine-seafood pairings by using yeast as an adsorbent.
Wine yeast OC-2 was cultured in pre-prepared media at 25oC for 3 days.
Generic white grape juice concentrate was dissolved and diluted to 20oBrix. The final adjusted concentration of iron in the juice was 4-7mg/L.
Fermentation between the OC-2 yeast and prepared grape juice was performed at 20oC for 30 days in a 50mL test tube while in the presence of oxygen. Samples were taken throughout the fermentation process to measure sugar, yeast concentration, and yeast viability.
Wines made for sensory analysis started off with the same grape juice base as described previously. Active dry yeast DV10 was used for fermentation, which was performed at 20oC for 7 days in a 15L jar fermentor. After fermentation, wine was centrifuged and fined with chitosan and bentonite. Other wines (both red and white) were purchased from the domestic market in Japan.
Titratable acidity, sulfur dioxide, and pH of all wine samples were measured. Yeast cell viability was also measured.
To prepare yeast cell with alcohol (in order to function as the adsorbant), yeast cells were cultured for 3 days then alcohol was added to a final concentration of 20-70% (v/v). Wet alcohol-treated yeast cells were then dried.
Once alcohol-treated yeasts were created, they were added to the wines and grape juices at varying concentrations and shaken. Wines were treated by the addition of 1, 2, and 3g/L of alcohol-treated yeasts. Yeasts were left to sit in the wines for one day, then were centrifuged and removed.
A Phytic acid treatment was also included in this study. Phytic acid was added to the wines and grape juices, left for one day, then centrifuged and removed.
Efficacy of iron removal from wine by using alcohol-treated yeast cells was determined by measuring the iron left over in the wines after treatment and comparing these values with the iron levels measured before the treatment began.
A sensory analysis was also performed on the wines to determine the intensity of fishy aftertaste. The panel was made up of 11 staff members (7 men and 4 women) ranging from age 26 to 59, all with wine tasting experience.
A triangle test was performed by comparing an untreated control wine to wine treated with alcohol-treated yeast cells, followed by a 30 minute incubation and then clarification using fining agents. Panelists were given three glasses of wine and were told one differed from the other two. 20 sets of wine in two sessions were evaluated. Alcohol-treated yeast wines were always compared to a no treatment control.
Panelists were first trained to the fishy aftertaste by way of a reference solution that contained 3mM FeSO4.
Perceived intensity of the fishy aftertaste was measured as: 1) barely detectable; 2) weak; 3) moderate; 4) strong; and 5) strongest imaginable.
Commercial dried scallops were used for the wine-seafood pairing, since scallops are known to elicit a very strong fishy aftertaste in wine. The protocol was as follows: 1) chewing and swallowing scallops; then 2) taking a sip of wine; and then 3) evaluating the intensity of the fishy aftertaste. Drinking water was provided in between each food-seafood pairing.
Wine samples were presented at random. At first, panelists only tasted wines. Afterwards, panelists tasted three wines with dried scallops.
Wine without the alcohol-treated yeast was the positive control.
The following were measured to determine if the treatment affected other compounds in the wine: organic acids, alcohols, esters, aldehydes, and finally, iron.
- Wine yeast cells adsorbed iron rapidly during the early stages of fermentation.
- Gradual release of iron was observed after 10 hours of incubation.
- The OC-2 yeast strain did not take up iron during fermentation as well as the common wine yeast Saccharomyces cerevisiae.
- Approximately 100 hours after fermentation, as the proportion of dead cells increased, the concentration of iron in the wine decreased.
o This suggests dead yeast have a significant ability to adsorb iron in wine.
- Greater than 40% v/v ethanol in treated yeasts was required for effective iron adsorption.
o Less the 30% v/v ethanol is not recommended, as it may not kill all of the yeast and thus risk refermentation or other complications.
- The addition of ethanol-treated yeast cells to wine pretreated with phytic acid was observed to remove all of the residual iron in the wine.
o This synergistic effect was not affected by the drying process of the yeast, as iron removal was the same for both fresh and dried treated yeasts.
- Using 1g/L or 3g/L of ethanol-treated yeasts alone removed all residual iron from the wines.
o Using a mixture of phytic acid plus ethanol-treated yeasts allowed the concentration of yeasts needed to remove all residual iron to drop down to only 0.5g/L.
- The combination of ethanol-treated yeasts plus phytic acid was the most effective treatment in reducing iron levels in wine.
o Other alcohols tested and found to be just as effective as ethanol were: n-propanol and isopropyl alcohol.
- Panelists were not able to tell the difference between control wines and wines treated with 1 and 2g/L of ethanol-treated yeasts.
- Wines treated with isopropyl alcohol-treated yeasts plus phytic acid resulted in the least fishy aftertaste.
- In regards to organic acids, esters, alcohols, and aldehydes, there were no differences in concentrations of these compounds between untreated control wines and wines treated with alcohol-treated yeasts and/or phytic acid.
According to the results of this study, the authors claim that they have successfully developed a novel method for enhancing the iron-adsorbing ability of yeasts (and thus iron removal from wine) by treating the yeasts with greater than or equal to 40% v/v ethanol. The drying process did not seem to have any effect on the iron-adsorbing ability of the yeasts, which is good news for large scale processing and distribution of the product.
Iron removal was found to be very effective with the combination of alcohol-treated yeasts and phytic acid, however, phytic acid alone was not nearly as effective. This study found that phytic acid is more effective in some wines than others, and that using alcohol-treated yeasts in concert with the compound would removal all iron from the wine without question.
The authors suggested one problem with using alcohol-treated yeasts to remove iron in wine and that is that there is a relatively high cost to filtering something with such high a density. When using alcohol-treated yeasts alone, 3 g/L of cells were required to be effective. This could be costly to filter such a relatively dense mass once the iron removal process was complete. By using a combination of alcohol-treated yeasts and phytic acid, the authors found that significantly less alcohol-treated yeasts would be required to effectively remove iron in wine. Specifically, when used in concert with phytic acid, only 0.5 g/L of alcohol-treated yeast cells are needed. By using much less alcohol-treated yeasts, the cost of filtering and removal is markedly reduced.
The results of this study present an effective method for iron removal from wine using alcohol-treated yeast cells followed by a phytic acid treatment. The sensory results were promising in that this treatment resulted in wine that had no hints of fishy aftertaste when consumed after eating scallops; nor were there any negative changes in the desirable sensory characteristics of the wine. I’d be curious to see if the results are repeatable at a commercial level, and also if the treatment is effective in all varietals and blends.
Being that it is the first paper showing the effectiveness of this new iron removal method for wine, much more research needs to be done. I’m curious, what do you all think should be the next step in this line of research? Leave your comments below!
Source: Tsuji, T., Kanai, K., Yokoyama, A., Tamura, T., Hanamure, K., Sasaki, K., Takata, R., and Yoshida, S. 2012. Novel Method to Reduce Fishy Aftertaste in Wine and Seafood Pairing Using Alcohol-Treated Yeast Cells. Journal of Agricultural and Food Chemistry 60: 6197-6203.
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!