In this chapter, we will discuss the use of sulfites. By better understanding what they do, we will better understand where they are likely to be present. We will better understand why some foods, or certain forms of preservatives, are to be avoided. We will have more ease in choosing to consume some foods when we are in unusual situations. We will better detect information that seeks to confuse us rather than help us.
Sulfites are the Swiss Army knife of food industries, cosmetics and pharmaceuticals. We will review their “fantastic” powers.
Sulfites are part of the family of preservatives also called antioxidants.
Sulfites are used to prevent spoilage and discoloration of products during storage and distribution. It is their antioxidant properties that are sought in particular. Sulfites capture the oxygen molecules and, by imprisoning them, prevent them from oxidizing food.
Specifically, sulfur dioxide, SO2, is used to fumigate fruits and vegetables prior to packaging to protect them from decay. It can also be released through the transport packaging specially provided for this purpose.
You know lychees and their color:
But take a look at them being prepared:
You notice their yellow color? And yet they are exactly the same! This color is due to their treatment with sulfur, or with sulfites anyhow now that we know some chemistry principles. Lychees are put in boxes at the base of which is ignited sulfur. It turns into sulfur dioxide and enters the lychees giving them this yellow color. In contact with the water present in the lychees, this sulfur dioxide turns into sulfites. Lychees regain their normal color during transport and do not let anything show once arrived at your supermarket display.
If sold from the display, there will be no mention of the presence of sulfites, whereas if they are sold packaged, the sulfites are indicated on the label. Of course, the lychees are the same and contain as many sulfites. Why this difference in labeling? We will return to that later.
They should be called anti-browning agents instead of bleaching agents. Their function is not to make foods white, but to prevent them losing their natural color.
Many foods change color. It is a phenomenon of browning that occurs during aging or during the implementation of different manufacturing processes or storage. This browning is either desired because it can improve the appearance and taste, or it is fought to keep a fresh appearance.
Think of an apple you have just cut. It will turn brown within minutes.
There are two types of food browning:
- Non-enzymatic browning also popularized by cooks as the “Maillard reaction.” It is the result of a curing reaction of saccharides.
- Enzymatic browning concerns fruits and vegetables and some shellfish. It occurs spontaneously without cooking.
Sulfites have the power to permanently inactivate an enzyme (polyphenol oxidase) required for browning and thus block the natural browning reaction.
For fresh fruits and vegetables:
Sulfites are used as a means of preventing browning whether they are whole or precut. They are treated by dipping, spraying or fumigation. For example, sliced fruit or sliced potatoes are soaked in a potassium bisulfite solution. The maximum permitted dose is 30 mg/kg for fruit and 50 mg/kg for vegetables.
For frozen fruits and vegetables:
The maximum permitted dose is 500 mg/kg for fruit and 50 mg/kg for vegetables. In general, the more the products will be handled and are light in color, the more they are at risk of containing sulfites.
Dried fruits and vegetables:
Dried fruits are among the foods treated with the largest amounts of sulfites. The maximum dose allowed is 1000 mg/kg.
And 2000 mg/kg for apricots. Detected exceedances are frequent. This is what dried apricots treated with sulfites look like:
And those that are not treated:
You now understand the benefit of sulfites for whetting our appetites, but you also understand that a good food is no longer judged by its appearance.
Dried vegetables and legumes are also concerned, particularly if their natural color is crystal clear, like it is for white beans or chickpeas. The maximum permitted dose is 500 mg/kg.
These products are very susceptible to enzymatic browning, named “Black Spot” by professionals. Shrimp and other crustaceans are generally soaked immediately after fishing in a sodium bisulfite solution. Sulfites are also added as tablets in water which is used to make ice for transport and for presentation at your fishmonger’s stall. You will note in passing that frozen shrimp indicate the presence of sulfites on the label, but not shrimp sold without their heads. It is essentially the head of the shrimp that begins to turn green and brown, and gives the shrimp a not very appetizing aspect. Shrimp that are sold headless are not always sulfite-treated.
Overall, the stabilizing role of sulfites is used in fermented foods.
Stabilization consists in fixing the fermentation process to maintain the food in the desired state.
Sulfites allow you to select yeasts and promote those sought for a good fermentation. They also help block the fermentation where it has reached the desired threshold.
This concerns both wine and beer, but also bread and pastries, because the dough that rises is fermented–and sauerkraut and tea which are also fermented foods.
The Case of Wine
Wine is a textbook case for understanding the multiple uses of sulfites. I discuss in detail sulfites and wine in the section on elimination, but I will give you an overview of their uses:
- Disinfection of barrels: sulfur is burned in the barrel and the sulfur dioxide released penetrates the wood. When filling, it migrates into the wine, meets water and turns into sulfites.
- During the harvest: when handling the grapes during harvest and transport, sulfites are used to prevent an uncontrolled start to alcoholic fermentation.
- During alcoholic fermentation: this is monitored by computer, and sulfites can be added to control it if it deviates.
- During the mutage of sweet or dessert wines: sulfites are used to stop the fermentation of the must and conserve residual sugar, which allows you to obtain these very sweet wines.
- At the end of alcoholic fermentation: to stop malolactic fermentation which occurs after alcoholic fermentation, once all sugars have been consumed.
- During air extractions: when changing the container of the wine, which can happen every 3-4 months.
- Just before bottling: to stabilize the wine for transportation and marketing.
This issue should be clarified. Those who use a lot of sulfites willingly talk willingly about natural sulfites as if to clear themselves of those they add. Some maintain, in addition, an intentionally vague aspect between natural sulfur, added sulfur, and sulfites.
So what is the reality of natural sulfites? If some foods contain them, are they as dangerous as added sulfites?
I must say that before researching this question, I already had a preconception. This story about “natural” sulfites was probably like that story about radioactivity when those who were pro-nuclear tried to convince us that there was no danger to the plants since radioactivity was natural. Granted, these are old stories, and they no longer dare talk about it.
I then embarked on some research and tests, and I was able to clarify the following:
I prepared a dish of red onions (with olive oil, salt, pepper, and garden thyme) and just before putting it in the oven, I rubbed a test strip on the white ‘sap’ that came to the surface of one of them. The result was positive at 25 mg/l of SO2.
How could it be that these sulfites had been added? Well, I conducted this test in the United States where the use of sulfites on fruits and vegetables (except potatoes) is prohibited. Looks like these onions therefore contain sulfites naturally.
In attempting to trace the source, that is to say a scientific publication, regulatory or medical, I found a study by our Canadian friends (http://www.hc-sc.gc.ca/fn-an /pubs/label-etiquet/go-ao/index-fra.php).
They studied 411 scientific publications and selected 36 of them to determine whether garlic and onion should be classified as priority allergens. Their goal is to identify allergy problems and not intolerances in order to include them, or not, in the improved labeling list of food allergens and gluten and added sulfites. You will notice that they specifically mention sulfites. It is in this study that I found the details of the chemical compounds in garlic and onions and the reactions that occur. None mentioned sulfites.
Their conclusion is the following: “Although scientific observations give grounds for assuming that some people experience severe reactions following ingestion of garlic and (or) onion, the prevalence of allergies to garlic (or) onion in children and in adults is unknown, and clinical data is insufficient to establish a credible cause-effect relationship for the allergenicity of garlic and (or) of onion taken orally.”
Then I redid a practical test with yellow onions this time:
The result was again positive at 25 mg/kg.
Imagining where these sulfites might come from, I think of the following possibilities:
- The onions are sulfited industrially and illegally because they are white and it improves their conservation.
- The test strips that I use react to other chemical products close to sulfites.
- Fresh, raw onions naturally contain sulfites that the Canadian study did not show.
As is, and before having more tangible evidence, I cut out onions. For garlic, the amount being much lower, I do not preoccupy myself with it any longer.
If wine is, in some ways, a case study, the same elements of confusion are found in other foods.
I do not think that this communication is orchestrated by a particular industry like wine, but is more on the order of rumor. It is also a mixture of baseline information that is rather scientific, but independent of each other, such as:
- Foods that contain sulfur
- White and light foods, which often contain sulfur
- Light colored foods to which sulfites are added, when treated industrially, to preserve precisely this whiteness.
To conclude on the presence of “natural” sulfites, I think that in “normal conditions of temperature and pressure” as scientists say, namely that of the growth of plants and animals that make up our diet, the natural synthesis of sulfites is very occasional and should not, except in a few cases such as onions which we will discuss later, be a concern.
Because sulfites may occasionally be found in nature does not mean that sulfites added industrially to our diets must be acceptable as certain industries sometimes try to make us believe.
They Are Everywhere
We have just seen how active, useful and versatile sulfites are for the food industry. They are all these things just as much in the cosmetic and pharmaceutical industries, and we will return to this.
Agri-food industry professionals note that the concentration can reach 5,000 mg/k without the occurrence of a bad taste, but the main disadvantage of this technique is the total non-elimination of SO2.
The problem we face is vast. That is why we will need a minimum of knowledge, method and organization to cope.
Have you found unexpected Sulfites in your food? What was it?