Alternatives to Sulfur Dioxide in Winemaking

By Becca | January 05, 2012

source: http://winobrothers.files.wordpress.com/2011/10/contains-sulfites1.jpg?w=640

It is well known that microorganisms (i.e. yeasts, etc) play an important role in the winemaking process.  Unfortunately, other microorganisms such as certain species of yeast and bacteria, negatively affect the winemaking process by causing spoilage which causes a decrease in wine quality and acceptability.  Specific spoilage examples that are caused by certain species of yeasts and bacteria include haziness in the wine, increased levels of acetic acid (vinegary taste), increased ethanol concentrations, and increased in sulfur and wine viscosity.  Because of these risks, wine must frequently be monitored during the winemaking process, and certain steps need be taken should the presence of certain species of yeasts and bacteria arise.

Traditionally, sulfur dioxide (SO2) has been used by winemakers to stabilize the microbial population in wine.  As a result of increasing evidence of possible health risks associated with SO2 in wine, there has been increased interest in finding an alternative approach.  Filtration and fining techniques have also been used in reducing the microbial population in wine; however, there are sometimes negative sensory effects on the final wine that makes the procedure less desirable.

Some new techniques have very recently been examined as alternatives to SO2, which may or may not prove to be a superior method.  Recently, scientists have shown that using ultraviolet (UV-C) radiation in conjunction with pulsed electric fields and high hydrostatic pressure systems in reducing microbial populations in food, without causing any major changes to the sensory characteristics of the items.  As a result of these studies, scientists have begun to wonder if UV-C radiation could perform similar tasks in winemaking, thus negating the need for SO2.

Another new technique that has recently been investigated as an alternative to SO2 in winemaking is using a colloidal silver complex.  Silver has been used since ancient times and is widely known for its antimicrobial properties.  At this point in time, silver is most often used for medicinal purposes, air purification, cosmetics, clothing, household products, and food treatment.  Due to its widespread use in an array of every day products, scientists have begun to examine if silver would be an acceptable alternative to SO2 in antimicrobial defense.

SO2

The use of SO2 in winemaking is very common, and is an antiseptic/antimicrobial compound that is effective against various yeasts at differing levels of concentration.  SO2 also functions as an antioxidant, which helps reduce the effects of dissolved oxygen in wine, which also inhibiting enzymes such as polyphenoloxidase, tyrosinase, and peroxidase, all of which are known to cause fungal infections.  Other benefits of SO2 in wine are that it allows greater release of phenolic compounds into the wine, which are known to harbor many positive health benefits; and also helps stabilize color.

As indicated earlier, the use of SO2 in wine can have negative effects, aside from the positive antimicrobial properties.  SO2 can sometimes produce negative sensory effects in the wine, resulting in a taste and smell of hydrogen sulfide and mercaptans.  In regards to human health, SO2 in food came under fire years ago as a result of allergic reactions such as hives and bronchoconstriction.  As a result of these allergic reactions to SO2 in food, the World Health Organization has since recommended limits on SO2 in food items, which then prompted the International Organization of Vine and Wine to establish SO2 limits in wine.  It is because of these negative sensory and health effects that scientists and winemakers have been searching for alternatives.

Filtration and Fining

Filtration and fining, while often used in winemaking to clear up haziness in wine due to the presence of certain proteins, can also be used as an antimicrobial mechanism for reducing the presence of certain yeasts or bacteria.  There are many fining agents available on the market, many of which are specific to certain types of proteins and compounds. For example, bentonites (clays) are of a laminar structure with an aluminum silicate base, which is often used to remove unstable proteins that cause haze in wine.  It has been shown; however, that bentonite is sometimes associated with a decrease in sensory quality of the wine, as well as a loss of wine structure.

Other fining agent options include; non-swelling clays, macroporous ion-exchange resins, and other protein adsorbers such as silica gel and alumina, the results of which are more favorable than bentonite, and does not seem to negatively affect the sensory characteristics of the wine.  However, all of these agents are designed primarily for protein removal, thereby making their effectiveness against microorganisms questionable.

New Alternatives to the Status Quo

Two new possible alternatives to using SO2 as a microbial agent in winemaking are UV-C radiation and colloidal silver complex.  Research for both of these alternatives is still in its infancy; therefore one should wait and see what comes next before deciding to use either of these methods in one’s own winemaking adventures.

UV-C Radiation

Research has shown that using UV-C radiation in liquid food products is effective in reducing the microbial population without causing any major sensory problems in the finished product.  Basically, the mechanism behind this treatment is that the UV-C radiation causes interruptions and rearrangements in the nucleic acid (DNA/RNA) of the microorganism, thereby rendering it incapable of reproducing.  The ability for the UV-C rays to penetrate into the food/beverage is dependent upon several things, including the color, absorbance, density, and concentrations of dissolved and suspended solids in the food/beverage.  One can presume then, that it’d be more difficult for the UV-C rays to penetrate a darker beverage than a clear one.  This approach has been documented to be successful in many beverages, including apple cider, orange juice, apple juice, strawberry nectar, and mango nectar.

To date, only one study has examined UV-C radiation as an antimicrobial mechanism in wine the results of which were found to be intriguing.  This study, published recently in 2011, found that UV-C radiation was effective against many microorganisms in wine, including Brettanomyces, Saccharomyces, Acetobacter, Lactobacillus, Pediococcus, and Oenococcus.  When comparing a red wine (Pinotage) and a white wine (Chardonnay), the study found that UV-C radiation was more effective at reducing microbial populations in the white wine than the red (which may be explained by the color penetration theory described earlier).

Even though the microbial efficiency of the UV-C radiation is sufficient enough to be a suitable replacement for SO2 in winemaking, the authors stressed that more research need first be done to examine any long term and short term effects of the radiation itself, in regards to sensory and chemical composition of the wine, and also possible negative health effects in humans.

Colloidal Silver Complex

Silver has been used since ancient times and is widely known for its antimicrobial properties.  Since it’s already used in many facets of everyday life, one recently published study opted to examine if it could be used as an alternative to SO2 in antimicrobial defense in wine.  This study found that in younger red wines treated with colloidal silver complex, they showed very similar sensory and chemical characteristics as though wines treated with SO2.  However, they did not poor phenolic content in these younger red wines treated with colloidal silver complex, so more work may need to be done to see if different aging methods may reverse that effect.

In white wines treated with colloidal silver complex, the study again found similar sensory and chemical characteristics to white wines treated with SO2.  However, it was noted that these colloidal silver complex treated white wines exhibited much higher oxidation than those treated with SO2, thus prompting the need for more research on possible antioxidants to be added along with the colloidal silver complex to reverse that effect.

Conclusions

Currently, there are several mechanisms employed to reduce the microbial populations in wine, with SO2 as the more traditional approach.  The two new alternative approaches: UV-C radiation and colloidal silver complex; show great promise to functioning as alternatives for SO2 in winemaking, however due to some unwanted negative results and some uncertain long term results, more research need be done to confirm their validity and safety.  It is possible that some combination of methods, in conjunction with reduced levels of SO2 may be superior to higher levels of SO2 alone, though more sensory, chemical, and health research will be required.

About The Author
Rebecca Yeamans

Becca Yeamans is the owner/writer of the science-based wine blog, The Academic Wino, as well as the Tasting Room Manager at a vineyard in central Virginia. With a Bachelor of Science in Biology and a Master of Science in Environmental Science, her solid background in science and research allows her to approach wine blogging with a unique style that is both informative to those in the industry, and entertaining for those simply wishing to learn.

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