Epilogue

Epilogue

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Many people avoid WINE CHEMISTRY, because it is considered too difficult and not necessary. Indeed, it needs some efforts, but it is absolutely doable. The French say it beautifully, << Qui veut peut >>. It couldn't be simpler and clearer. When I started my own research on WINE CHEMISTRY, my chemistry knowledge was limited to O₂, CO₂ and H₂O. My interest on WINE CHEMISTRY started when I learned that  6CO₂ + 6 H₂O  à C₆H₁₂O₆ + 6O₂  and  C₆H₁₂O₆ à

2 C₂H₅OH + 2CO₂.

There are many ‘questions’ and ‘mysteries’ in the study of wine. Why do yeast convert sugar into alcohol and bacteria turn malic acid into lactic acid ?  What are glycerol, fatty acids, high alcohols, esters,  diacetyl, and wine diamonds?  And where do they come from ?  Why is oxidation sometimes good, and sometimes bad in wine?

It is not easy to get answers for those questions. Wine is a bio-chemical product, so I think the answers must be found in Wine Chemistry. There are some WINE CHEMISTRY books, but they are more for professional wine makers. For a chemistry layman, like me,  they are difficult to follow. So I decided to ‘google’ for it. Step by step, piece by piece, learning by doing, I put the puzzles together. I cannot assure you that my findings are the accurate. It is my attempt to get an answer to those questions I had in the study of wine.

The beauty of WINE CHEMISTRY is that it, with its metabolic pathways, provides a clarifying explanation to the phenomena in wine. It also, with its molecular structures, gives a 'face' to the wine components so that we can see  their differences. WINE CHEMISTRY is  desirable if you want to have a good picture of the wine components. WINE CHEMISTRY is certainly needed if you want to give a judgement on what you are told. I see WINE CHEMISTRY as grammar. You can learn to speak a language fluently wthout learning its grammar, but it is unlikely that you can truly understand what is right or wrong. You can study wine without WINE CHEMISTRY, but then you have to take everything you are told for granted.

Here, I would like to express my gratitude to Google. All my findings are 'dug' out from Google. Without Google, there is no way I could have gotten access to all this information and knowledge. Without Google, it would have been impossible for me to share my findings with you. THANK YOU VERY MUCH GOOGLE! YOU ARE GREAT!

Finally, I would like to thank you for reading. If you find my blog informative, please share it with your wine friends. Share and discuss knowledge result to increase and improve knowledge. A joy shared is a joy doubled! 

                                                    

Part 13 - Sulfur dioxide & Sulfite

Part 13 – Sulfur dioxide & Sulfite

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v  Sulfur (or Sulphur) - S

Sulfur is commonly known as the substance used in matches and gunpowder. Over 2000 years ago the Romans discovered by accident that wines stored in vessels treated with burning sulfur candles did not develop a vinegar smell. Due to combustion, sulfur combines with oxygen and becomes sulfur dioxide (S + O₂ à SO₂). SO₂ is a colorless gas with a sharp characteristic smell. It is toxic. It kills the micro-organisms and thus disinfects the barrels. Today, most wine makers still use SO₂ or sulfite to stop the growth of harmful micro-organisms.

v Atom, molecule and ion

Sulfur (S) is a non-metallic atom with 16 protons and 16 electrons. Protons are positively charged and electrons are negatively charged. An atom with an equal number of protons and electrons is electrically neutral.

Sulfur dioxide (SO₂) is a molecule. A group of two or more atoms held together by chemical bonds is called a molecule.  A molecule is electrically neutral.

Ion is an atom or molecule which is not electrically neutral. In other words, when an atom or molecule has become electrically charged, it is called an ion. A positively charged atom or molecule is called a cation, a negatively charged atom or molecule is called an anion. Sulfite (SO₃^-2)  e.g.  is an anion.

v Sulfur dioxide (SO₂)  in cylinders

SO₂ is supplied in cylinders. The gas is colorless with a pungent odor and toxic. It works strongly irritating to mucous membranes and eyes. Major winemakers use this to determine the precise dosage and only pure SO₂ is added. However, SO₂ (gas) dissolves in water and becomes sulphurous acid (H₂SO₃).  Acid gives off H-protons  and release bisulfite anion (HSO₃^-) and sulfite anion (SO₃^-2). These reactions are reversible, they go right and left, back-and-forth.

  

 The molecular sulfur dioxide (SO₂ ) works about 100x as strong as the sulfite anion (SO₃^-2) against micro-organisms. That’s because only the molecular SO₂ can pass through the cell wall, after which it destroys the enzymes and proteins in the cell. However, the availability of the molecular SO₂ is dependent on these anions. The sulfite anion (SO₃^-2)  love to bind with acetaldehyde, anthocyanins and sugar. The binding with acetaldehyde is permanent and so the sulfite is lost forever. The more acetaldehyde to be formed (e.g. by slow fermentation) the more sulfite anions are extracted from the wine. This will be increasingly SO₂  from the left "sucked" to the right, so to speak. In other words, the more acetaldehyde, the greater the need for sulfurizing. As acetaldehydes are binded by sulfites, yeasts will produce glycerol to compensate for the NAD deficit. That explain why addition of sulfite will lead to more glycerol formation (see Part 4).

The bindings with anthocyanins and sugar are temporary. From these bindings, sulfite anion, bisulfite anion  and eventually SO₂  can be free again. These loose bindings act as a buffer stock SO₂.

v  Sulfites ( or sulphites)

Smaller winemakers usually use sulfites : potassium bisulfite KHSO₃  or  potassium disulfite K₂S₂O₅ . Sulfites release sulfur dioxide SO₂ , which is the active component that helps preserve wine and food.

                          

One molecule potassium disulfite can release two molecules SO₂.  1 gram KDS releases roughly  0.57 grams of SO₂. This can be calculated as follows:   

v  pH influence

The SO₂ released from potassium disulfite ( like the SO₂ from cylinders) will dissolve in water and become HSO₃^-  and SO₃^-2 . The lower the pH in wine, the more SO₂ will be free again; the higher the pH in wine, the less.

It is generally accepted that for a good action against micro-organisms in wine, a minimum 0.8 mg/L molecular SO₂ is required. The table below shows that at pH-4, it needs 10x more sulfite in order to have the same amount of working molecular SO₂  as at  pH-3.

v  Wine diamonds

Sulfites release not only SO₂, but also potassium ions (K+), which react with tartaric acid. This causes the precipitation of potassium bitartrate, known as tartaric acid crystals (or wine diamonds), and some (minor) deacidification takes place (tartaric acid disappears from the wine). It crystallizes because it’s a salt.

                                                          

Some winemakers let their wines undergo cold stabilization, a process by which a wine is cooled down before it is bottled. The “crystallized tartaric acid” fall out and can be separated from the wine.

Some winemakers believe that cold stabilization influences the wine’s balance and taste. According to them, the wine is actually ripped apart; as the rapid cooling changes the wine’s colloidal structure. In these wines, wine diamonds may occur.

v  Sulfurization of grapes, must or wine aims to:

1. SO₂ stops growth of good yeast (Saccharomyces cerevisiae) and prevents spontaneous fermentation of grapes.

2. SO₂ kills the oxidation enzymes (laccase and tyrosinase) and prevents premature oxidation (no brown colors). 

3. SO₂ kills wild yeasts (Brettanomyces) and prevents bad aromas  'brett'  in wine.

4. SO₂ kills acetic acid bacteria and prevents excessive acetic acid and ethyl acetate (glue-like smell) in wine.

5. SO₂ binds to oxygen in the wine and thereby limits the oxidation of alcohol to acetaldehyde.

6. SO₂ binds to products such as aldehydes and diacetyl, which suppress the fruity aromas.

v Legal limit sulfite content

mg/l	red	white, rosé	bio-wine	sherry,port	mousser.	sweet		Eiswein
<5 g r/s	150	200	120	150	185(BOB/BGA)	Spätlese	Auslese	BA/TBA
>5 g r/s	200	250	170	200	235	300	350	400

l Total sulfite content = free  SO₂ + HSO₃^-  + SO₃^-2  + bound sulfite (see table above).

l Sugars bind sulfite. Wines with residue sugars have a higher content of bound sulfite. That's one of the reasons

     that the legal limits for total sulfite content for dessert wines is higher than for dry wines.

l Since 25-11-2005 wines containing more than 10 mg/l sulfite must state on the label "contains sulfites" or  

    "contains sulfur dioxide”. That's because sulfite is an allergen, which can be dangerous for asthmatics and people

    with allergies to aspirin.

v Wine completely without sulfite

No, that does not exist. In wine there is always 5-15 mg/L of sulfite produced by the yeast from the sulfur-containing amino acids: cysteine and methionine (see Part 6).  

A wine without  'added' SO₂, can be . The question is: how old can that wine be?

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P.S.

This is my last post. It is a very ‘limited’ blog. Only ‘fermentation’ and ‘wine components’ produced therefrom are discussed. ‘Wine components’ coming from grapes and from oaks, as well as ‘wine-aromas’ and ‘wine-faults’ were left out. Wine Knowledge is a very extensive study. Quite rightly, Clint Eastwood said in the movie ‘Magnum Force’: ‘A man’s got to know his limitations.’