Welcome to the eighth part of the series Terroir & vinens kemi. We have followed the wine from soil and climate through acids, phenols and aroma compounds to fermentation and barrel ageing. Now we are nearing the finish line, and it is time to look at what keeps the wine alive in the bottle: stabilisation.
Stabilisation is about preventing the chemical and physical changes that make a wine less acceptable over time. Sulphur dioxide plays a leading role here, but it is far from the whole story. We dive into the difference between free and bound SO2, into the chemistry of oxidation and into the physico-chemical techniques that ensure a clear and durable wine.
Hvad du lærer
- What functions sulphur dioxide has in wine, and how it works as an antioxidant
- The difference between free and bound SO2, and why it is decisive for protection
- The chemistry of oxidation, from enzymatic browning to perfectly timed protection
- How wine is stabilised both chemically and physically against precipitates and haze
Hvorfor vin skal stabiliseres
Wine instability covers undesirable chemical or physical changes that arise during production and storage, and which affect both quality and durability. Some of them you see directly: a wine that ought to be clear becomes hazy. In fact, the passage of light through the wine can be used as a simple indicator. Clear light through the glass points to a clear, stable wine, while dimmed light reveals haze.
Instability can have many sources. Metal ions are one of them. Copper above 0.2 mg/L and iron above 5 mg/L can trigger haze and precipitates. Proteins are another, and so are tartrate crystals and oxidation. The point of stabilisation is to step in before these changes have time to ruin the wine's expression in the bottle.
An important keynote is that wine is microbiologically more stable than must. During the alcoholic fermentation, Saccharomyces cerevisiae produces ethanol, fatty acids and other metabolites that increase toxicity towards other microorganisms. The rising ethanol concentration does the rest of the work. But chemical and physical stability do not come of their own accord, and this is where the winemaker must make choices.
Svovldioxid: fri og bunden form
Sulphur dioxide (SO2) is the winemaker's most versatile tool. It works both antimicrobially and antioxidatively, and it is precisely this dual function that makes it hard to do without. Addition before bottling, after fermentation has ended, provides simultaneous protection against both microbes and oxidation.
Antioxidant gennem kemien
As an antioxidant, SO2 removes hydrogen peroxide, an aggressive intermediate in the wine's oxidative reactions. The reaction proceeds like this:
H2O2 + SO2 + H2O → H2SO4 + H2O
By neutralising the hydrogen peroxide, SO2 slows the chain of oxidation reactions that would otherwise mature the wine too quickly. In addition, SO2 delays the oxidation of the easily oxidisable phenols and can thereby reduce early browning in white wine.
Fri kontra bunden SO2
This is where it gets interesting. When you add SO2, about half quickly binds to the wine's other components, while the other half remains as free SO2. It is the free part that provides the antioxidative protection. The bound part is still present, but not active in the same way.
That explains why you cannot simply measure the total SO2 and count on protection. It is the distribution between free and bound form that determines how well the wine is actually guarded. The addition is therefore timed carefully: typically 30 to 50 mg/L at pressing and 30 mg/L before bottling to prevent oxidation.
SO2 also has a role for acid and colour. Low pH increases SO2's activity, and that is one of the reasons why a fresh, low-pH wine is easier to protect. We went through the importance of pH in Vinens syrer og pH, and it is worth keeping in mind here: the lower the pH, the more you get out of the same amount of sulphur.
Oxidation og dens kemi
Oxidation begins already with the grape. The oxygen content in the must can even be calculated from temperature and sugar content, which underlines how early oxygen comes into play.
Enzymatisk brunfarvning
In the grape itself, the enzymes polyphenol oxidase and laccase catalyse oxidative browning reactions. The precondition is that all three elements are present: oxygen, enzyme and substrate (the phenols). If one of them is missing, the reaction stops.
An elegant natural stop sets in during the alcoholic fermentation. The alcohol the yeast produces denatures the oxidative enzymes, and thereby the enzymatic browning ceases. So fermentation is not only the conversion of sugar into alcohol, but also a chemical shift that closes the door to a particular type of oxidation.
Pinking og fenoloxidation
A particular phenomenon in white wine is pinking, a salmon-red tint that arises when anthocyanins appear at just 0.3 mg/L. It is a reminder that even small amounts of pigment compounds can give visible, undesirable colour changes if the wine is not correctly protected.
Oxidation is not, however, unambiguously an enemy. In red wine, controlled oxygen uptake during maturation is part of the colour and structure development. The anthocyanins polymerise with other phenols and form, among other things, dimeric compounds such as catechin-anthocyanin linked via acetaldehyde bridges. These bridges form faster at low pH and are slowed by SO2, but they do not stop entirely. It is a fine balance: you want the colour stabilisation, but not the uncontrolled oxidation. You can read more about these compounds in Fenoler: Tannin, farve og struktur.
Fysisk-kemisk stabilisering
In addition to sulphur and oxygen management, there is a range of techniques aimed at specific instabilities: proteins, tartrates and excess phenols.
Proteinstabilitet
Wine proteins range from 10 to 275 mg/L with molecular weights of 11,000 to 28,000 Dalton. Two protein groups account for most of the protein instability: chitinases and thaumatin-like proteins (TLP). Proteins are least soluble at their isoelectric point, where positive and negative charges cancel each other out, and it is here that they risk precipitating and clouding the wine.
Stability can be tested by heating. Heating at 60 °C for 48 hours precipitates 95 to 100 % of the proteins, while 40 °C for 24 hours precipitates around 40 %. Bentonite is the classic agent and most effectively removes protein fractions with a molecular weight below 65,000 Dalton. For white and blush wines, silica sol (a silica colloid) is also used, which reacts with the proteins and gives rapid precipitation, often together with gelatine.
Tartratstabilitet
Potassium hydrogen tartrate (KHT) is readily soluble in must, but when the wine becomes saturated with it after fermentation, it precipitates. This happens especially at low temperatures, and that is why it is well known to find tartrate crystals in a wine that has stood in the cold. Here it is worth remembering a natural ally: mannoproteins, which the yeast produces during the alcoholic fermentation, make up a considerable part of the wine's polysaccharides and act as natural inhibitors of KHT crystallisation.
Fenolisk justering
Finally, you can purposefully bind excess phenols. Polyvinylpolypyrrolidone (PVPP) binds phenolic compounds selectively via hydrogen bonds to ketoamide groups, with a binding preference in the order leucoanthocyanin before catechins before flavonols before phenolic acids. This gives the winemaker the possibility to adjust bitterness and browning tendency without intervening disruptively in the rest of the wine's chemistry.
Kort fortalt
- Stabilisation prevents undesirable chemical and physical changes, from metal-triggered haze to oxidation and precipitates.
- SO2 works both antimicrobially and antioxidatively, among other things by removing hydrogen peroxide. About half binds quickly, the free half provides the protection.
- Oxidation starts enzymatically in the grape via polyphenol oxidase and laccase, but the alcohol from fermentation denatures the enzymes and stops the enzymatic browning.
- Physico-chemical stabilisation targets proteins (bentonite, silica sol), tartrates (cold, mannoproteins) and phenols (PVPP).
- Low pH increases SO2's activity and supports both microbial and chemical stability.
Ofte stillede spørgsmål
Why is the difference between free and bound SO2 so important?
Because it is only the free SO2 that provides the antioxidative protection. When sulphur is added, about half quickly binds to the wine's components and becomes inactive in that respect, while the other half remains free and active. That is why the total SO2 does not tell the whole story about how well the wine is actually protected.
Is all oxidation harmful to wine?
No. Uncontrolled oxidation gives browning and flatness, but a controlled, slow oxygen exposure during the maturation of red wine helps to stabilise the colour. Here anthocyanins polymerise with other phenols, among other things via acetaldehyde bridges. The art is to manage the process, not to avoid it entirely.
Klar til næste skridt?
Now that you know the mechanisms that keep the wine stable, you are equipped for the series' final chapter. In Vinfejl og mikrobiel fordærv we look at what happens when stabilisation fails, and which microorganisms and compounds lie behind the classic wine faults.
And when all the chemistry has been gone through, it is worth remembering the simple truth: the best pairing is the wine you like with the food you like. Do drop by our selection and find a bottle where the craft behind the stability can be tasted as pure, clear pleasure in the glass.