I have just taken some time to answer an email from a TV researcher about wine and oxygen, specifically related to shipping and bottling wine. Because it took a fair while to answer the questions, I thought I’d share my responses here. Yes, geeky technical stuff indeed, but I though someone (anyone? anyone?) might find it interesting.
I’ve been told that oxygen is the enemy of wine. What does oxygen do to wine while it’s being transferred and bottled – what processes does it allow to happen?
Once wine has been made it needs to be protected from oxygen, which can cause oxidation of various wine components, such as the fruity esters. The wine is protected from oxygen by sulfur dioxide, which while not an anti-oxidant, binds up the products of the reaction between oxygen and various wine components, and stops further oxidation. For example, it binds with peroxide which is one of the initial products of wine oxidation, thus preventing the peroxide from then going on to oxidise desirable wine compounds. It also binds with acetaldehyde (also known as ethanal), which gives wine an apple-like, sherried character that’s usually undesirable. Acetaldehyde is produced by the interaction of alcohol and oxygen. So as the wine comes in contact with oxygen, the pool of sulfur dioxide that is supposed to protect it (the ‘free’ sulfur dioxide – it exists in both free and bound forms) gets used up. When it falls below a certain level, the wine starts tasting tired and oxidised, with fruity flavours being replaced by flat, nutty flavours.
Are these processes the same ones that cause wine to age, but in the presence of oxygen they just happen more quickly?
Oxygen is involved in wine aging (although wines will still develop in time without any external source of oxygen), but we’re talking tiny, tiny amounts of oxygen over a very long period. In the presence of larger amounts of oxygen the sorts of reactions that take place are less desirable. If you expose the same wine to the same dose of oxygen, but over very different time scales (say, 1 month versus 5 years), the results will be strikingly different. Wine is particularly vulnerable during shipping and bottling, because whenever wine is transferred, there is some pick-up of oxygen. The goal is to keep this to the absolute minimum.
Are there any benefits to wine being in contact with oxygen, or are they all negatives. E.g. do you want some of these reactions to happen?
At some stages during its life, wine does benefit from exposure to oxygen. But when, and how much, depends on the wine style. In short, red wines benefit more from oxygen than whites. And whites such as Chardonnay which are oak fermented and aged benefit more from oxygen than fresh, unoaked whites such as Sauvignon Blanc. All wines seem to benefit from a bit of oxygen during fermentation, as this helps the yeast grow. Let’s call this macrooxygenation. Then, for unoaked whites, that’s it – they are best protected from oxygen from then on. For reds and oaked whites, the wines can benefit from the microoxygenation that takes place during barrel ageing, with tiny amounts of oxygen being introduced to the wine. The wine gets progressively more sensitive to oxygen, so it’s vital that quality isn’t lost during bottling, or where the wine is bulk shipped, during shipping and then bottling, through the introduction of oxygen. There is a tiny bit of oxygen transmission by the closure – call it nano-oxygenation if you will – and this can be of benefit to big reds, and can avoid issues with volatile sulfur compounds developing post bottling. But that’s another, complicated story.
I believe that nitrogen can be added to wine to bring down the dissolved oxygen levels – how does this work at a molecular level? How does the nitrogen lower the oxygen levels?
During wine movements, nitrogen is sometimes added to the wine in small amounts to help prevent oxygen pick-up. I think it’s a sparging effect, where the nitrogen pick up displaces other gases, such as oxygen, from the wine.
Is this standard practice in the wine industry?
I have only seen it in wineries where they are transferring the wines from shipping tanks to storage tanks, before bottling. Remember, many wines are still bottled at the winery, and then shipped. Transporting wine in bulk is getting more popular, but it’s still mainly the cheaper wines that are shipped that way.
Does adding nitrogen change the taste of the wine at all?
It’s neutral. But it can displace dissolved carbon dioxide, which changes the wine’s taste subtly. It can make the wines taste a bit flatter, but then the CO2 level of the wine can be adjusted later.
I believe that bottles are sprayed with nitrogen before the wine is added. How does this work? Is it that spraying in the nitrogen physically forces the air out of the bottle before the wine is added? i.e. a different process to bubbling through the wine to lower the dissolved oxygen content.
The bottles can be filled with nitrogen before they are filled with wine, in an attempt to reduce oxygen pick up during bottling. Some bottling lines also add a tiny drop of liquid nitrogen just before a screwcap is applied, to remove air from the headspace. For an in-neck closure such as a cork, a vaccuum id drawn before the cork is inserted. If no attempt to remove oxygen from the bottle is made before the closure is applied, then the wine can take up quite a bit of oxygen, which is only bad for quality.
Is it liquid nitrogen they use or gaseous? Is this standard practice in the wine industry?
It comes from a cylinder, which because it is under pressure has the gas as a liquid, but which then becomes gaseous when you open the tap. Just like cooking gas cylinders.
What is the purpose of adding sulphur to wine?
I have touched on this already. Sulfur dioxide is added to almost all wines before and during winemaking, and at bottling. Early additions are anti-microbial, later additions are both antimicrobial and also to protect against the effects of oxidation.
Is this standard practice across the industry or only lower-end wines?
It is standard practice.
What are the positives/negatives of adding sulphur?
The positive is that the wine is protected from oxidation and microbial spoilage. Without it, the wines in the supermarket would pretty much all be undrinkable. You can make wine without sulfur dioxide, but you have to be so careful, and then store and transport it with great care. The wine would taste different – and sometimes the no-added sulfite wines can be fabulous, but it’s not wine as we know it. The negatives? For most people, none. There are a very few people who react badly to the relatively low levels allowed in wine, but many people who find they have reactions to wine blame the sulfites simply because the wine says ‘contains sulfites’ (a legal requirement) on the bottle.