Damned if You Do - Screwed if You Don't (15 August)

Regular readers will know that I hate what corks do to wine, and the current level of TCA and oxidised wine corks produce is unacceptable; and whilst I have an open mind about screwcaps, I am yet to be convinced they are a universal panacea, especially for red wine. Therefore anything, be it positive or negative, that provides more meaningful information about the use of  alternative closures and their effect on wine is important. That is why I started my correspondence with Alan. Here is what he said.

"The sulfide issue is very interesting. I wrote an article that covered the topic in full in last years annual tech issue of Aus/NZ Grapegrower/Winemaker but the issue of 'post bottling' sulfides is particularly interesting.

Firstly, we have to make the distinction between wines that have been bottled with obvious sulfide notes, and wines which have been bottled 'clean' ie no obvious sulfide notes.

If the wine has been bottled with obvious sulfide notes, then this really is due to lack of care by the winemaker. But there are many cases of wines being bottled clean, and developing sulfide notes after bottling - especially under screw cap. This is what the AWRI closure trials have consistently shown up. ROTE (s cap) closure show sulfide notes after several months, while those under the cork do not.

Why?

As I explained in my article, the sulfides exist in various forms in the wine. Some can be cleaned out with copper fining (the usual pre-bottling procedure), but others cannot. The ones that can be cleaned out are H2S and thiols (mercaptans). The ones that cannot be cleaned out are the more complex ones, and in particular, disulfides, and thioacetates.

By two different processes, these compounds can both degrade over time to produce thiols.

The thiols are particularly smelly - more so than the parent components. So, a non- smelly (clean) amount of the disulfide, or thioacetate can yield enough thiol to make the wine sulfide smelly. This difference is about 20-40 fold in terms of sensory effect - so it can be significant.

Why not under cork?

This process has gone on since winemaking began - it is an artefact of the fermentation process, and basically impossible to eliminate. So, all wines get bottled with some of these components. What happens to them in terms of thiol accumulation depends on how much oxygen ingress the wine is getting via the closure. Corks allow more than screwcap. The thiols (the smelly bits) are easily oxidised back to disulfides (non-smelly bits). So if the wine is getting enough oxygen the balance of smelly thiol to non-smelly disulfide is in the favour of the disulfides. So, the pool of thiol is small. If the oxygen ingress is cut back further, the balance shifts to more thiol, less disulfide. This is what you see under screwcap. In a completely anaerobic experiment, the AWRI sealed a sample of their trial in a glass ampoule. No oxygen ingress, compared to a slight amount under screwcap. The ampoule produced even more sulfide smell.

Hope this answers your question."

It certainly did, but unfortunately it posed another obvious one. Given that the gating factor to the amount of thiols is the amount of oxygen in the wine, what can the winemaker do to reduce/eliminate the problem. e.g. is there a simple test that can be carried out prior to bottling to see what is likely to happen when bottled under ROTE, or is it pot luck? How do wineries determine how much oxygen and head space is required at bottling time? Obviously the bottle size is predetermined and so is the quantity, so how do they try and minimise the problem. Alan responded with this comment.

"This is a common misconception -- how much oxygen should I give my wine at bottling to prevent this effect. I have seen screwcappers dosing with O2 at bottling, or not inert flushing at filling, or fiddling with headspace etc. The AWRI did some headspace trials with results that were predictable.

It is all about competing rates, post bottling.

Rates of thiol production, from the precursors; the kinetics of which are reasonably understood in the science literature ,and so are the rates of oxygen ingress required to to mitigate.

A couple of slides from a recent presentation I did may help to explain it.

I read in Tyson Stelzers latest book (which has some lousy chemistry which I took him to task over), that the wine must be in the right oxidative state pre bottling. This is nonsense. These things happen post bottling, and the determining factor for every given wine is the rate of O2 ingress thru the closure. Every AWRI trial has shown this effect under ROTE, and nil under cork.

As for how else to mitigate - not so simple , I wrote about a 5000 word follow up article in Aus magazine on this topic.

I don't know how deeply you want to go into this. (TORB's Comment: Stick with it folks, its gets a little technical here but it's not that hard to understand and worth the effort.)

The kinetics of the thiol production are governed by several factors - most of which are basically out of the winemakers hands.
 

They are;
Thioacetate concentration. It is a first order (concentration dependant) relationship to thiol production, via hydrolysis. i.e. twice the amount of thioacetate = twice the rate of thiol production.

Further, the rate of hydrolysis is Ph dependant - higher Ph = faster rate of thiol production, for given amount of thioacetate.

Disulfide concentration, also first order relationship. Twice the disulfide = twice the rate of thiol prod.

We generally know we need an oxidant for oxidation, but we also need a reductant for the disulfide reduction (to thiol). The literature tells us it is SO2. Again a first order relationship - according to the literature. So, double the free SO2 = double the rate of thiol accumulation via disulfide reduction.

So you can see there are many factors at play here in the final result, most of which the winemaker has little choice, or control over. So, take a given wine - say Penfolds Grange 2002. It will have a unique set of the above parameters. Let us assume, it has, high Ph, high disulfide, high thioacetate, and high SO2. The rate of thiol production will be very high, and it will take a considerable rate of O2 ingress to mitigate this.

Now assume the same wine has low Ph, low disulfide, low thioacetate and low SO2. It will require much less O2 ingress to mitigate the effect.

So, the effect is variable, and largely unpredictable from wine to wine. But what the chemistry literature (kinetics) does tell us is that generally, it will peak at about 18 months post bottling . How long it takes to become apparent is dependant on all the above factors, and more.
 

You can screen for the precursors; there is a commercial service for this in the US, but it is not exactly simple. As a result of my article on how we might otherwise possibly mitigate this effect, I have instigated some research work here in NZ on this topic (I chair the research program for the NZ industry). Traditionally the cork with its higher ingress rate has largely spared us from this event. As we move to lower ingress closures, the effect becomes more significant - which you may now be able to see."

So why is all this important? The answer is as simple as it is obvious. Not all wines are best sealed under screwcap and there is no easy way of telling if cork or screwcap will be the best seal for a particular wine.