A Sulfur Dioxide application to table grapes, which are those destined for the fresh market, is common practice, and has been since the 1920s. The application of Sulfur Dioxide on grapes is also required to be included on the labeling. Have you ever noticed the labeling on fresh table grapes that alludes to the grapes being treated with Sulfur Dioxide?
The pictures below are examples of the labeling I found at grocery stores this past week.
Why are Grapes Treated with Sulfur Dioxide?
Grapes are treated for two main reasons:
- To control gray mold rot (Botrytis cinerea) – This is one of the main postharvest issues that grapes encounter since the Botrytis cinerea pathogen that causes the disease can grow at low storage temperatures, which is how the grapes are stored in order to prolong their shelf-life. The disease originally starts in the field and continues during the postharvest life.
- To reduce the darkening of the rachis – As the grapes continue respiring postharvest, their rachis darken. (The rachis is what holds all of the grapes together in the cluster.) It is a minute detail that us grape lovers do not pay attention to when slurping up grapes, but it is important from a marketing perspective.
This leads to a longer shelf-life for the grapes, including longer storage potential and greater transport distance potential.
How are Grapes Treated with Sulfur Dioxide?
Harvested grapes are fumigated with Sulfur Dioxide. There is a fumigation method called the Total Utilization System that results in no excess Sulfur Dioxide fumigant at the end of the fumigation process, thereby reducing air pollution and residues on the grapes.
Grapes are commonly fumigated more than once. The first fumigation takes place before or after packing, and then the grapes continue to be fumigated on a weekly basis until they are shipped. This needs to be a strategic treatment because the impact of Sulfur Dioxide on grapes is most effective based on the fumigation time and the Sulfur Dioxide concentration.
Another method used, most commonly in addition to the fumigation method, is the use of Sulfur Dioxide generating pads with a box plastic liner that are placed inside the grape boxes during transport. Under proper temperature conditions, these pads slowly release Sulfur Dioxide throughout transport, and absorb moisture generated by the grapes.
The Dangers of Sulfur Dioxide
Some people are allergic to sulfites. As a result, the EPA put a limit of 10 ppm for sulfite residues on table grapes, in order for them to be sold in the United States.
How to Tell if Grapes Have Been Overtreated with Sulfur Dioxide
You can tell if the grapes have been over fumigated by looking at them. If you see any bleaching spots, a brown shade on the grape where it is connected to the rachis, or any sunken areas on the grapes, those are signs that the grapes have been overtreated.
Are Organic Grapes Treated with Sulfur Dioxide?
Organic growers are restricted from using the Sulfur Dioxide on grapes as a fumigation method. In recent years, multiple alternatives have been researched, with the most successful being ozone and calcium chloride treatments.
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You might also be interested in reading:
- Why Are Mangoes Hot Water Treated?
- How is Your Produce “Washed”?
- What Do All Of These Food Labels Mean?
Resources:
Chervin, C., Aked, J., Crisosto, C.H. “Grapes.” Crop Post-Harvest: Science and Technology, First Edition. 2012. 187-211.
“Grapes, Fumigation with Sulfur Dioxide (SO2).” (2008). WFLO Commodity Storage Manual.
Lindsey, P.J., Briggs, S.S., Moulton, K., Kader, A.A. “Sulfites on Grapes: Issues and Alternatives.” Chemical Use in Food Processing and Postharvest Handling: Issues and Alternatives, Agricultural Issues Center. University of California, Davis, 1989. 5-19. 1989. Online.
Dear Megan
Your info is predominantly related to the US market. California and Chile are the biggest SO2 fumigators of table grapes. The problem however is that SO2 gas react with moisture to form a VERY corrosive by product. That, and the concentrations in gas chambers are lethal to operators. Due to this the FDA is looking at overhauling legislation aimed at what is called “Bystander Exposure” – apart from this, you article is fairly spot on. In the rest of the world however SO2 generating pads are used. There are a dozen of them on the market ranging from very good to fairly acceptable. SO2 is produced as a by product during the reaction between sodium metabisulphite and H2O. Sodium Metabisulphite again is a very, very commonly used food additive in everything from baby food to sausages as a preservative. As to the SO2 damage you mentioned: remember SO2 is a preservative. where bleaching has occurred the green parts of a berry will be washed out. usually over a crack in the berry. if you see this the gas has done its job. Otherwise an infection would has ensued.
SO2 has been very effective for almost a 100 years. chances are we will be seeing it around for some time still 😉
Megan, Your post on SO2 fumigation answered all my questions. Thank You.
Very useful information, your reference to how organic grapes are treated is helpful.
I am searching this topic to find out how to best wash them. Or if that’s a waiste of time.
Check this out: https://www.theproducenerd.com/2019/05/how-to-wash-fruits-and-vegetables/