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In this episode of “Wine 101,” VinePair tastings director Keith Beavers discusses how sulfites affect wine. Sulfites are arguably the most important element of the wine fermentation process, besides grapes. Sulfur dioxide, otherwise known as SO2, has been used to preserve wine and food storage for centuries — with mentions of the element showing up in Sanskrit writings and even the Bible.

In addition, listeners will learn about the two types of SO2 — bound and free — and how they affect wine differently. Finally, Beavers explain why sulfur dioxide can create a rotten scent.

Tune in to learn more about sulfites.

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Keith Beavers: My name is Keith Beavers, and aren’t we all, deep down inside, no matter how old we are, a Steely Dan fan?

What’s going on, wine lovers? Welcome to Episode 23 of VinePair’s “Wine 101” podcast, Season 2. My name is Keith Beavers. I am the tastings director of VinePair. And yeah, how are you doing?

This is sulfites, wine lovers. This is sulfites. We got to clear the air with sulfites. Are you ready? Sit down. We got this. Let’s get some science going on.

When you have a glass of wine in your hand and you haven’t even smelled it yet, you haven’t even tasted it yet, you are anticipating it. Oh my gosh, I’m about to enjoy a glass of wine. When you put your nose in that glass, you’re going to get a bunch of information that you will interpret as certain aromas that you may have experienced in your life. If you haven’t or you can’t find it, someone will tell you something. However, what you’re expecting is a journey through this glass or through this bottle.

There are certain grapes and blends that have certain characteristics in the aroma profiles that are standard. Cabernet Sauvignon has a certain standard profile. Nebbiolo has a certain standard profile. When you’re smelling a Cabernet Sauvignon, or even a Cabernet Sauvignon blended with Merlot, Barbera, or a Chardonnay, you’re anticipating the aromas that are often associated with that wine. Then, you’re anticipating the additional layers of aromas that are brought about by the winemaking process. The initial aromas that you smell usually are called precursors, which are just the inherent aromas that are inside the grape that come out through the fermentation process and aging. Then, there are the aromas that come from oak or from stainless steel, which is not really a thing. Also, concrete, which is another weird thing. Yet, there are other aging aromas that will develop.

A wine will get more delicate, or you have a young wine that’s not meant to age. Still with that, you are anticipating something. You know that a red blend is going to smell like all the berries and vanilla. It’s going to be soft but it’s going to be pleasant. You know you’re about to experience that pleasant stuff. This experience with wine is only possible because of nature’s ability to combat elements in nature that can compromise that experience. In the next episode, we’re going to talk about some of those things that can compromise your wine experience.

Today, I want to talk about the thing that can protect the wine from some of these things that nature wants to throw at it. That thing is called sulfur, a natural element that takes up 0.5 percent of the weight of the Earth’s crust. You can go on Amazon and just type elemental sulfur. You can buy sulfur from Amazon. It’s a yellowish, pale yellow, brittle, solid substance. When it’s burned in the air, it creates a gas called sulfur dioxide, otherwise known as SO2. It’s in this form as a chemical compound that it’s most widely used by winemakers.

It is one of the most important elements of winemaking besides the actual grapes in the fermentation process. Sulfur dioxide has been used since antiquity to preserve wine and food storage. From ancient Sanskrit speakers, it was called “sulvere.” In the Bible and the Book of Genesis, they call sulfur “brimstone.” In the 15th century, where German winemakers would burn wood shavings, they would use sulfur and herbs inside barrels to prepare them to put the wine in. Finally, in the 18th century, some of the most prestigious Bordeaux chateaux learned from the Dutch to do the same thing to their barrels. Today, SO2 (sulfur dioxide) is used in pretty much all winemaking.

It makes sense because of its protective qualities. To understand this wine protector and what it does for wine, it does two main things. Number one, it’s a minor antimicrobial agent. It sounds pretty cool, right? And number two, it helps to prevent oxidation. It helps to prevent browning agents from browning a wine. Now in the vineyard, remember how I said it takes up some of the Earth’s crust? Well, naturally, there is sulfur in vineyards, and often there’s enough in the soil mix to help prevent whatever is happening out in the vineyard. However, there are certain fungi like powdery mildew, which is a very tough fungus to get rid of. The wineries will sometimes spray sulfur on their vineyards to make sure that powdery mildew does not infect their vines.

Remember in the first couple episodes of the first season, I talked about the challenges that a winemaker has out in the vineyards and how nature is just out there and you have to navigate it? The same thing happens when wine comes into the winery. Once you crush grapes and start the wine process, that grape juice is vulnerable to all kinds of bacteria and oxygen ready to destroy it. Not destroy it, but break it down into something else. It’s nature.

If you were to take an apple, slice it open, and just leave it on the counter, then come back in an hour or so, you’ll notice that oxygen has been soaking into the apple. The apple is browning. That is nature oxidizing and breaking down matter. If you taste that apple, you’ll notice that it’s not as sweet as you expect because the browning agents are starting to take hold and reduce the apple. If you were to shock that apple with SO2, that would protect the apple from browning. If you ever had dried fruit, dried fruit is shot with a ton of sulfur or SO2. A ton, so much more than wine ever gets.

In your typical glass of wine, there are about 0.005 to 0.010 grams of sulfites in your glass. In dried fruit, there’s 10 to 20 times that amount. In wine, we’re just trying to prevent a couple of things from happening, but in food, they’re trying to saturate the whole thing. I find this exceptionally fascinating. I love this stuff. I think science, nature, and chemicals are so amazing. When we understand it and we can harness it, it’s even better.

Now, what happens here is when they shock the must with SO2, two things are going to happen. Number one, during the fermentation process, the SO2 is going to soak into the grape juice. It’s going to bind itself with some of the constituents in the grape juice like sugar and pigment. What that does is it maintains the color and maintains that sugar content. Once the SO2 has saturated as much as it can into the must, there is often still SO2 available that has not binded to the wine. The SO2 that soaked in and did that work is called bound SO2 because it bound itself with constituents in the wine. The SO2 left over from that is called free SO2. That free sulfur dioxide is the sulfur dioxide that does the work of rendering browning agents impotent — not allowing the wine to break down so we can enjoy the things that the wine and the winemakers want us to enjoy.

With all that work being done, there’s going to be some residual sulfites in the resulting wine. There is a very small percentage of the population of humans out there that have a problem metabolizing these sulfites. And because of that, they can have an allergic reaction. It’s very rare, but it happened enough that the TTB, which regulates alcohol in our country, regulated that wine labels need to say, somewhere, “contains sulfites” as a warning. The term “contains sulfites” is the total of the bound and free sulfites together. It’s called the total sulfites. How is your brain doing? Is it a little science-y right now?

It’s a lot, but the thing is, these are things that winemakers have to talk about. What’s really interesting about this whole SO2 addition is it’s going to happen to every wine. It just depends on how much they want to add. It’s really a case-by-case basis. Winemakers are trying to make good wine, and they know that SO2 will protect the wine. They know that if they don’t add enough, something bad is going to happen. If they add too much, something bad is going to happen. If they do that, something may or may not happen on those ends of the spectrum. However, right in the middle, there’s a really good chance that the wine will be sound. It makes sense.

Now that you think about it, if you don’t add enough SO2, more of it is going to bind than be free, or maybe there’s not even enough to bind and be free. Then, you’re letting things in like browning agents and spoilage yeasts. What’s going to happen is that stuff is going to take over the wine. This extreme, no-addition stuff is what people are calling “natural wine” or “low-intervention.” I’m not going to talk about those terms because they have no definition. They’re not really even real. I can’t educate you on things that have no definition. We’ll talk a little bit more about that in the next episode.

If you add too much SO2, you’re overprotecting the wine. When a wine is opened and you pour it into the glass, it wants to breathe. It needs oxygen to open up. Isn’t it crazy how oxygen is the enemy and the friend of wine? Once the wine is poured into a glass, you want oxygen to start opening it up and do the work of all the aromas and stuff coming out. If you over-shock with SO2, the oxygen can’t do enough work, and it makes the wine a little bit flabby.

I was talking to Scott Kozel, who I interviewed for the oenology episode, and he told me that this is really the only tool in the toolbox winemakers have to stem out oxidation. What’s cool is that it is a natural element. It is not a manufactured thing. A little side note: Scott Kozel wrote his thesis on sulfites when he was in school. And because of that, we had another mind-blowing conversation that confirmed a lot of the research I did for this episode. He said some interesting stuff. One thing was that winemakers are more concerned about free SO2 than they are about bound SO2 because those browning agents are a big deal. They don’t want those to take over.

Then I asked him, what is it about the sulfur in nature or the residual sulfur that is used to spread in the vineyard? Does that make it into the winery? And then how does that work? What’s interesting is he said that the sulfur that comes in from the winery often gets converted into what’s called hydrogen sulfide. (Sulfide with a D, not with a T). This is the form of sulfur that smells like rotten eggs. Actually, elemental sulfur also smells like rotten eggs. Hydrogen sulfide, when fully converted, really has a stank on it. If a winery isn’t practicing good hygiene, then that stink will sometimes make it into the wine. That’s why sometimes when you open a wine, it stinks a little bit. Just let it blow off because it’s residual hydrogen sulfide from the winemaking process, and it was a little dirty.

He also said something that was fascinating. In the early days of canned wine, it showed that SO2 has a reaction to aluminum, which produces hydrogen sulfide. Back in the early days of canned wine, if you were to open a canned wine, it would smell rotten, but it would blow off. That’s basically what was going on. Nowadays, he said that the industry is catching up with that and they’re making more sound wine in cans by limiting the amount of sulfites in the cans so that it doesn’t have enough of a reaction with the aluminum. Interestingly enough, canned wine is low-sulfite wine. Woah. Not only that, but if a winemaker has a lineup of wines, they usually bottle. Then, if they want to add a line of cans to their brand, they really have to make two different wines. That’s crazy.

There you have it: A nice, general rundown of the sulfite thing. I wanted this to be an episode because there is a time in the industry where the idea of sulfites was a hotly debated subject with allergies. Now, sulfites are in a new realm of debate about additions, whether to even add any. I just wanted you guys to know the science behind it so you can make your own decisions based on what people tell you about sulfites. These are the hard scientific facts of what this will do. You may have heard or may be in the middle of a debate about the whole sulfite thing with somebody, but just know that it’s the decisions that people make that define the resulting wine. Science is there. Nature is there. It’s going to do whatever it does.

Winemakers have tools that they use, naturally, that help a wine become something wonderful. SO2 is one of the most important things they have to protect wine, but it comes down to what the winemaker cares about and what they want. Do they want nature to run rampant? Do they want to have control over it? Or do they want to overdo it? It’s a big world out there of wine. All three of those things happen, so this is what sulfites do for wine.

And I know I mentioned some things in this episode that you may not recognize, but just know that in the next episode when we talk about wine flaws, I’ll go into a little more detail about this. I’ll mention sulfites again, but we’re going to talk about what slips through the cracks when SO2 is not there to protect the wine. And things can get iffy. We’ll talk next week.

@VinePairKeith is my Insta. Rate and review this podcast wherever you get your podcast from. It really helps get the word out there. And now for some totally awesome credits.

“Wine 101” was produced, recorded, and edited by yours truly, Keith Beavers, at the VinePair headquarters in New York City. I want to give a big ol’ shout-out to co-founders Adam Teeter and Josh Malin for creating VinePair. And I mean, a big shout-out to Danielle Grinberg, the art director of VinePair, for creating the most awesome logo for this podcast. Also, Darbi Cicci for the theme song. Listen to this. And I want to thank the entire VinePair staff for helping me learn something new every day. See you next week.

Ed. note: This episode has been edited for length and clarity.