Part 13 – Sulfur
dioxide & Sulfite
==================================================
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.
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?
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.’
Geen opmerkingen:
Een reactie posten