The Chemistry of Metabisulfite and Sulfur Dioxide in Wine.

TRIGGER WARNING:  I assume that the reader knows a couple of things about chemistry.  If you are little rusty in your chemistry knowledge, here are some quick tips:

• Sulfur, oxygen, and hydrogen atoms are given the symbols S, O, and H.
• Compounds are groups of atoms that are bonded together.  The formula of a compound gives the type and number of all the atoms it consists of.  For example, the formula "H₂O" means that the compound has two hydrogen atoms and one oxygen atom.  Charges of ions are given as superscripts at the end of a formula  (e.g. SO₃^2-, which means one sulfur and three oxygen atoms with a negative two charge).
• In molecular structures, we show bonds between atoms with lines.  One line means a "single bond", two lines mean a "double bond", etc...
• To write a chemical reaction, we write the formulas of reactants, draw an arrow, and then write the formulas of the products.

OK, with that out of the way....

Potassium metabisulfite is ubiquitous in wine making.  It is the active component in campden tablets and can also be purchased in crystalline form at your local wine making supply store.  It is added to wine in order to produce sulfur dioxide, which is very effective at killing off unwanted microbes that can spoil the wine.  So, how does it work?

Metabisulfite is a complex anion (negatively charged ion) that consists of two sulfur and five oxygen atoms.  It has a -2 charge.  Generally, metbisulfite is sold as a potassium salt.  In potassium metabisulfite, the negative charge is balanced with two potassium ions (+1 charge each).  Sodium can also balance the negative charge.  However, sodium metabisulfite is not as common as the potassium form.  In terms of generating sulfur dioxide, it does not matter whether you use sodium metabisulfite or potassium metabisulfite - they both react the same way.

To see how metabisulfite is transformed chemically into sulfur dioxide, let's start by looking at the molecular structures of three compounds:

1.  Sulfur dioxide, SO₂.  The structure is relatively simple.  A sulfur atom is bonded to two oxygen atoms in a bent structure.

2.  Sulfite (SO₃^2-) and hydrogen sulfite (HSO₃^-).  These two chemical species are the principal forms of sulfite that exist whenever a sulfite compound is dissolved in water.  They are simply acid-base forms that interconvert with the addition or subtraction of hydrogen ions (H⁺).  That means that the relative amounts of the two depend on the hydrogen ion concentration, which is commonly measured as the pH.  At the pH of wine, which is, say, between 3 and 4, sulfite (SO₃^2-) only exists in minute amounts.  The most abundant form is hydrogen sulfite (HSO₃^-).

3.  Metabisulfite (S₂O₅^2-).  In the structure, it kind of looks like there is an SO₂ molecule attached to an SO₃^2- ion, with a bond between the two sulfur atoms.

When metabisulfite is added to water, it reacts immediately with water to generate hydrogen sulfite:

From a chemical point of view, you can think of the oxygen atom in H₂O "attacking" one of the S atoms (the one bonded to two oxygen atoms) to form a new sulfur-oxygen bond.  This breaks the bond between the two sulfur atoms and results in two hydrogen sulfite ions.  This may not be the exact mechanism (I have not looked it up), but it's a nice way to think about it.

Let's say you are making wine with a kit and the secondary fermentation is complete.  You just added the packet of potassium metabisulfite to your wine and you start stirring.  The reaction above just took place, and you now have a lot of HSO₃^- dissolved in your wine.

HSO₃^- reacts with H⁺ ions in the wine (remember, your wine is somewhat acidic) to form dissolved, "molecular" SO₂, which is written as SO₂^.nH₂O.  This means that SO₂ is "hydrated", or associated with water molecules, in solution.

This reaction is written as a chemical equilibrium, which means that at any instant, some of the compounds on the left are reacting to make compounds on the right, and vice versa.  This is why there are two reaction arrows.  After some time, the reaction comes to equilibrium, which means that the concentration of each compound stops changing.  At equilibrium, the relative concentrations of reactants and products are defined by an equilibrium constant, K, that is unique to the reaction.  For this reaction, K is written as:

Concentrations are indicated by the square bracket around each compound.  If you were to measure the equilibrium concentrations of SO₂^.nH₂O, HSO₃^-, and H⁺, entered the values in the expression above, and did the calculation, the answer would be 62.5.

If any of the concentrations change, then the concentrations of the other compounds must change over time so that the system comes back to equilibrium (and the expression again equals 62.5).  So, let's say you add an acid blend to your wine to change the flavour.  By adding acid, you have increased [H⁺], and the expression will no longer equal 62.5.  In order to get back to 62.5, the equilibrium must shift:  [SO₂^.nH₂O] will increase slightly; and [HSO₃^-] will decrease.  This is an important practical result.  The more acidic the wine, the more SO₂ will be produced from HSO₃^-.  Wines that are more acidic require less added metabisulfite.

At room temperature, SO₂ exists as a gas (not a liquid or solid).  Consequently, some of the SO₂ will be released as a gas.  This is one reason why, after adding metabisulfite to wine, you might observe bubbles coming out of the wine as you stir it.  Stirring can also release dissolved CO₂ that was produced during fermentation.

What happens to the dissolved, molecular SO₂ in the wine?  Several things:

• some of the SO₂ will become chemically bound to various wine components, like complex sugars.
• some escapes as a gas
• some stays free and dissolved, in equilibrium with HSO₃^-.

The goal is to have enough dissolved, molecular SO₂ in the wine to keep the microbes at bay.  Typically, 0.5 ppm SO₂ is needed for red wines, and 0.8 ppm is needed for white wines.  Commercial winemakers use analytical chemistry techniques to measure the SO₂ content in their wines.  This lets them make sure they meet regulations for the SO₂ content of wines that are sold on the market.  My wife, who is also a chemist, once had a summer job at a winery near Melbourne where she performed SO₂ assays, among other tasks.

The average home winemaker does not need to worry about measuring the SO₂ in their wines.  We can use a simple rule of thumb:  one campden tablet per gallon of wine will give you approximately the right SO₂ concentration.

For all the expert and amateur winemakers out there, I would love to know your thoughts on how much SO₂ is really needed in wines.  For example, in the book The Way to Make Wine, the author suggests adding campden tablets every time you rack your wine.  This strikes me as overkill.  On the other hand, some winemakers use none at all.  What is your experience?

Let me know if you find this useful, or, even better, if you have questions.  I will do my best to answer.

Merry Christmas!

Bibliography

Chemistry of the Elements, Greenwood, N.N. and Earnshaw, A., Pergamon Press, New York, 1984.

The Way to Make Wine, Warrick, S., University of California Press, Berkeley, 2010.

Fermenting Port Alberni grapes

Today I racked 4 batches of wine:
a) the juice from 57 lbs of green grapes
b) the juice from 45 lbs of purple grapes
c) the skins from the green grapes plus 5 lbs of blackberries
d) the skins from the purple grapes plus 5 lbs of blackberries

Each batch had the following ingredients added:
2 Tsp acid blend
2 Tsp yeast nutrient
1 Tsp pectic enzyme
2 tsp metabisulfite 
1/2 tsp grape tannin

9 to 10 lbs of sugar and water to bring the SG up to 1.085

The mixtures were allowed to ferment for 4 days in a primary container and then racked into the glass secondary containers … great aromas and rapid evolution of carbon dioxide. In the above picture at the back we have the purple grape juice, the green grape juice, the green grape skins plus blackberries and in the front the purple grape skins and blackberries.

Apple Wine Racking & Tasting

It was just under three weeks ago that I racked the apple wine into a carboy where it could ferment to dryness.  At that time, I added three cinnamon sticks to add some flavour.  The bubble rate died off after a week, but there has been a low level of activity since then, with tiny bubbles forming on the surface.  This could be outgassing or maybe even the result of malo-lactic fermentation (MLF).  There is a lot of malic acid in apples, so MLF would not be surprising.  Today, I decided it was high time to rack it and perform a taste test.

I siphoned the wine into a clean carboy and added two crushed Campden tablets.  During the siphoning, I dispensed a small amount into a glass.  After stirring the wine to remove sulfur dioxide, measuring the specific gravity, and installing an air lock, I sat down with my notebook, pen, and glass of apple wine.

This is the best wine I have made to date.  The bouquet was of obviously of apple, but there was a hint of pear.  The taste was simply delicious and mild.  I was surprised at how much body it had, given that the ingredients were so simple.  There was hint of butter, which suggests that there may have been some MLF.  The cinnamon flavour was very mild, and in the background.  I only really noticed it at the end.  It confers a subtle amount of spicy 'heat' to the wine.  The acidity was just right.  I tasted it at room temperature, and I suspect it would be even nicer when chilled.

Tasting and the taking of notes.

For me, as a scientist and chemist, it is a joy to sit down with a new wine I have never tasted, and taste it carefully and slowly, with the intent to observe as much as I can.   A certain oenophile colleague of mine once pointed out how satisfying it is to try to describe what you see, smell, and taste.  Wine tasting is a lot more fun when you really slow down and pay attention to the complex mix of aromas and flavours that you experience in your nose and mouth.  A wine tasting wheel is a great help for this.

(As the wine isn't quite finished yet, this may be premature, but thanks to +Bob Perkins and +Pete Bottiglier for some of the tips you shared that I followed with this apple wine.  To your health!)
  

Valpolicella Racking, Stabilizing, and Degassing

Today, I racked, stabilized, and degassed the Valpolicella wine.  The kit instructions are to do this at day 14, but this is day 19.  I let the wine sit an extra five days because the fermentation was not quite complete at day 14.


After the usual sterilization of equipment, I transferred the wine to a clean carboy via autosiphon, without incident.  During this 'racking' step, I took a sample for a tasting.  Well, well, this is nice stuff.  This wine is dry and gentle.  There are woody and nutty tones with a hint of almond.  The tannins are noticeable at the end.  I have hopes for a very nice table wine that will pair well with pasta dishes.

Tasting.  Note-taking is important when it comes to stuff like this.

The specific gravity was 0.990.  From the starting point, this gives approximately 16% alcohol.  Yes, it's pretty dry.


The next step is the addition of potassium metabisulfite, which is provided with the kit.  Out of curiosity, I used my little portable balance to measure the mass of K₂S₂O₅.  The mass of K₂S₂O₅ was 4.4 g.  For comparison, I weighed a Campden tablet.  It was 0.6 g.  The recommended use of Campden tablets is 1 tablet per gallon of wine.  This is a 23 L kit, which is about 6 gallons.  6 gallons x 0.6 g per gallon = 3.6 g worth of Campden tablets.  So, this amount isn't too far off.

When you add the K₂S₂O₅ to the wine, SO₂ gas is formed (see picture).    The chemistry of this step is quite straightforward, and one of these days I'm going to write a blog post about it.

SO₂ bubbles.  You can get rid of the excess gas by stirring vigorously.

After the K₂S₂O₅, I added potassium sorbate and then a small packet of kieselsohl.  Tomorrow, I complete the addition of clarification agents, and then I wait three weeks until bottling.

It's a Learning Curve

For a number of reasons, I waited until today to rack and stabilize the rhubarb wine.  This was partly due to just being busy, and partly a conscious decision to let this fermentation go to virtually 100% completion.  This afternoon, the bubble rate was about one every four minutes.  If that's any indication, then this fermentation was awfully close to being 100% complete.

The racking went smoothly.  I took a specific gravity reading (0.990), which would correspond to 16.3% alcohol.  Hmmm.  This is pretty strong stuff!  I also took a small amount in a glass for tasting...

Oh dear.  It's rhubarb mouthwash.  The last time I tasted the wine, at the first racking, there was still some sugar present, and it was delicious.  The acidity of the rhubarb and the residual sugars balanced each other and gave some nice flavours.  At this point, all the sugar is gone.  There is no more sweet!  It's just pure acidity.  This is a dry, acidic wine, probably only good for marinating fish or something like that.

So, what would I do differently next time?  In hindsight, I should have stopped the fermentation and stabilized it a few days after the first racking, when there would have been some sugar left to balance all the acid from the rhubarb.

In any case, I went ahead and stablized the wine.  When I added the potassium metabisulfite, I noticed a colour change.  This compound certainly appears to be causing a bleaching of the colour (see below).  It remains to be seen whether this is reversible, like it was the first time:  http://randomfermentations.blogspot.ca/2014/05/rhubarb-wine-beginning.html

(1) Rhubarb wine after racking and addition of a small amount of postassium metabisulfite (left).  The photo on the right was taken about 1 minute after the first one.  Notice the colour change.

This really is a bit of a bummer.  I put a lot of work went into making this rhubarb mouthwash!  On the other hand, I learned a good lesson:  Listen to what your taste buds tell you!  If the alcohol level is close to where it should be, and it tastes good, then it's time to rack and stabilize.

Just like many things in life, there is a learning curve to this.  It's important to continue and try to get it right the next time.  My rhubarb plant has grown back (it's gigantic), so I have the materials to start again.  Next time, I'm going to listen to my taste buds.

Pomegranate/Acai White Merlot

Sometime in mid-March, I visited the Wine Kitz store in St. Albert.  I was overdue to make a new batch of wine and asked if they had any good kits on sale.  The Wine Kitz dude, whose name I cannot remember, pointed out the "Country Mist" series of wine kits.  All of these involve a mixture of various fruit juices with the grape juice, yielding sweet wines that you would enjoy while sitting on a patio on a summer afternoon or evening.  The Wine Kitz dude strongly recommended the Pomegranate/Acai White Merlot.  The price was right and it sounded interesting, so I picked up a kit and got started a few days later.

A wine kit comes with a very clear set of instructions.  Generally, the instructions tell you to perform the primary fermentation in a large plastic pail.  I have done this in the past, but this time I used a carboy with an air lock (Pic 1).  Why?  Because in wine making, oxygen should be excluded as much as possible.  The head space in a carboy is much smaller than what you would have in a plastic pail, which means there is less oxygen in the system.  Also, I had the bad habit of peeking into the pail during fermentation, just to see what was going on, and to enjoy the aromas that waft out.  This is another way to introduce oxygen - maybe not much, but it should be avoided.  With the glass carboy, I can watch everything that is happening and not worry about oxygen getting in.

 

(1) All ready to start fermenting...

Once the fermentation gets going, it's fun to just watch all the bubbles rising through the must (Pic 2).

 

 

(2) Primary fermentation, well under way 

Now, I'm a scientist and I love to measure things so I can understand how those things work.  Sometimes, this spills over into my hobbies.  In the first couple of days of the primary fermentation, I noticed that the bubbles in the airlock were spaced at regular intervals of time, and that these intervals seemed to be getting shorter.  I started counting the number of bubbles in a fixed period of time and recorded the number of bubbles per minute.  This is how I obtained the graph shown below.   I only started measuring at the 72 hour mark, which explains the absence of points between 0 and 72, and I don't really know what that part of the curve should look like.  (For now, a straight line will suffice!)

 As shown on the graph, the primary fermentation is basically complete at around 10 days.  The instructions say to let it go until 14 days, when you stabilize the wine.  In Pic 3, you can see about an inch of sediment at the bottom of the wine.  This is all the yeast that settled out.  To separate the wine from all that yeast, you simply siphon the wine into another carboy.  I use an "autosiphon" which has a special plug on the end that helps keep the solids from being siphoned off along with the wine.

  

 (3) Siphoning to a 2nd carboy

Stabilization involves the addition of two chemicals:  potassium metabisulfite and potassium sorbate.  The metabisulfite reacts to form sulfur dioxide, inhibits microbes (e.g. bacteria, yeast) and also reacts with any oxygen that gets into the wine.  Sorbate acts to suppress any yeast that are leftover.  Apparently, sorbate is very important to add to sweet wines, where yeasts might start fermenting the sugar that is still there.  When these two chemicals are added to the wine, a lot of bubbles can be generated (Pic 4).

  

(4) Lots of bubbles after the potassium metabisulfite was added

After the wine was stabilized, I added the pomegranate/acai juice to the wine (Pic 5).  This stuff smelled really good!

 

(5) Stabilized wine in carboy.  Pomegranate/acai juice in the bag.

Shortly after the juice was added, I added the "finings", which consist of two separate solutions.  One is "chitosan", which is a polysaccharide made from crustacean shells.  The other is "kieselsohl", which is colloidal silica.  These two thing work in tandem to remove tiny particles from the wine, clearing it.  Pic 6 shows the wine after the finings were added.

(6) Finings added.  Somewhat cloudy in appearance.

Twelve days after the finings were added, the wine is very clear (Pic 7).  In the photo, you can see the fine sediment at the bottom of the carboy.

  

 (7) Clear wine.

Bottling is a lot of fun.  I carefully siphoned the clear wine into another carboy, making sure to not disturb the sediment.  Then, I siphoned the wine into bottles (Pic 8) and sealed them with corks.  It is so satisfying to have prepared 30 bottles of wine like this.

  

 (8) 30 bottles of wine

 

 (9) Bottles with labels.  (Professional looking, or what?)

Amateur Wine Making

Last summer, my wife announced that she was going to try eating dandelion leaves.  This came a little out of the blue, but I wasn't totally surprised.  In an effort to be more environmentally responsible, we stopped using herbicides on our lawn two years before this.  So, our lawn was certainly more 'natural' and we had no fears about residual herbicides on the vegetation.  My wife is always willing to try new things, and eating dandelion leaves would not be out of character.

During our conversation about this, she mentioned that one could make dandelion wine.  Now, I had always been intrigued by wine making, and this was now a challenge.  Could I make dandelion wine?  I did some research and went to work...

For this post, I'm not going to give a detailed description of my ad hoc recipe, concocted as a fusion of other dandelion wine recipes.  What I will reveal is that I think I added too much sugar.  The wine ended up being almost sickly sweet.  I can't drink it.  (My wife doesn't mind it too much.)

In the end, the failure didn't really matter much.  What I discovered was that for me, making wine was a super amount of fun.  I loved mixing up several liters of liquids, adding ingredients, watching the fermentation bubbles, adding chemicals to stabilize and protect the wine, and adding finings to make the wine crystal clear.  Bottling my own wine was incredibly satisfying.  It's also satisfying to open a bottle of your own wine and know that it tastes great and the cost to make it was a tenth of the price of a bottle of wine at the store.  I was hooked.

Dandelion Wine - 2013 vintage