question about sulfites & stabilizing

[jonalexdeval]

I have a few questions about sulfites that have not been cleared up by some of the books I've been reading (Schramm, Vargas).

As I understand, when stabilizing you usually add both sulfite and sorbate (at rates of 1 tablet per gallon and 1/2 tsp per gallon respectively). Vargas' book (Making Wild Wines and Meads) mentions that any of the recipes can be sulfited at must preparation as a precaution against wild yeast (by dissolving campden tablets in must and allowing to sit 24 hrs prior to pitching, p. 21). This seems somewhat strange as I think sulfites are usually added in the same manner when racking onto fruit at secondary (as a precaution against wild yeast in the fruit but also to inhibit further fermentation??)

Also optional of course is sulfites at stabilizing. Vargas says that the effects of the initial sulfiting at must preparation dissipate after 24 hours (p. 31), almost as if suggesting that the initial sulfites somehow disappear completely.

Schramm briefly addresses sulfiting but does not go into the mechanism by which they work in detail. (I'm somewhat amazed at Schramm's vagueness on certain topics...)

My question is essentially the following: If the effects of sulfites dissipate after 24 hours when used at must preparation (in order that the proper yeast may thrive), then why does sulfiting at a later stage not also have dissipating effects and hence fail to preserve the mead/wine?

Or, to put it another way, if sulfites kill wild yeast and bacteria (Vargas, 31) then presumably they kill off the wine/mead yeast at stabilizing. The sulfites then hang around to preserve the mead for a long while. But then why does sulfiting at must preparation not also inhibit the yeast when it is eventually pitched?

I'm not really interested in sulfiting at must preparation. It's just out of my own curiosity. I want to use sulfites when racking onto fruit at secondary and also for stabilizing though and would like to know more about the chemistry of how they work.

 

[wayneb]

Well, as you have learned from your reading to date, sulfites provide multiple benefits to wine or mead, depending on when and how they are used. Prior to pitching yeast into a must, added sulfites can be used to control wild yeasts (and other potential spoilage organisms) since free SO2 levels at or above 50ppm usually are sufficient to kill off those wild organisms. And since the concentration of sulfite decreases over time (both as free sulfite chemically binds with other substances in the must, and as the SO2 dissipates into the air), it doesn't continue to have this antimicrobial effect indefinitely. That is why it is generally adivised that you add sulfite to a must at least 24 hrs before you pitch your commercial yeast. A combination of the dissipation in net free sulfite over that time, coupled with the fact that most commercial yeast strains can tolerate larger concentrations of sulfite and still survive, allow you to manage which yeast actually accomplish the fermentation in your brew.

When sulfites are added to wine or mead post-fermentation, they again provide an immediate antimicrobial effect that can be beneficial in suppressing other spoilage organisms such as acetobacter, and additionally sulfite acts as an anti-oxidant by suppressing the activity of certain oxidases in the wine itself. Oxidases are enzymes that promote the oxidation of other organic molecules in the wine, and that oxidation is generally not a good thing.

The sulfite that you introduce post-fermentation does also dissipate with time, for the same reasons that it does in the pre-pitched must. So, for sulfite to be effective as an antimicrobial and an antioxidant in the long run, you must add it a short while prior to bottling, and you must take care to use scrupulous sanitation practice during bottling.

The reason that most stabilization protocols call for both sulfites and potassium sorbate to be used post-fermentation, is that you are never 100% sure that all of your commercial yeast have been dispatched by the later sulfite addition, and they may "wake up" some time down the road to re-start the fermentation, if additional fementables are present. Sorbate keeps those hardy yeast stragglers in check by preventing them from reproducing - and in doing so keeps those few remaining viable cells capable of doing little to that remaining sugar until they naturally die off.

 

[jonalexdeval]

Ah, ok. Thanks wayneb for that reply! I'm compiling a binder on mead and winemaking, so I'm trying to organize everything I learn. This is good stuff... why aren't the books this clear?

So sulfite doesn't actually kill the wine yeast, it just inhibits it or makes it go dormant? (because Vargas clearly states that sulfites kill live yeasts... or is it, as you say, that the wine yeasts are more resistant to sulfites and so merely go dormant instead of dying like the wild yeasts?)

So the action of the sulfite is really the same in both pre- and post-fermentation additions... in both cases the sulfite works temporarily so to speak. So it must really be the sorbate that makes stabilizing work... because otherwise the yeast would theoretically just restart... unless perhaps they weaken over time as their food supplies run out (or the alcohol concentration helps to suppress them).

So it's just those additional factors after fermentation (alcohol, decreasing food supplies) combined with sulfite and sorbate that give the yeast an extra kick to knock them down... whereas at pitching they have ideal conditions except for the sulfite, which is already dissipating anyway.

Interesting, because presumably that might mean one should not rely entirely on stabilizing to get to the target gravity... yeast alcohol tolerance is also key. Not that anyone in their right mind would want to be taking readings every few hours to know when to stop the fermentation, but the last mead I did took 4 weeks to reach expected target gravity and moved very slowly in the last 2 weeks...

Makes sense... random question: is it then common to sulfite just after fermentation and then again just prior to bottling?

PS - is there a good mead/wine making book that addresses these kinds of topics in more detail than schramm and vargas?

 

[wayneb]

Sulfites do kill yeast cells; the different commercial strains are just more resistant to sulfites than their wild cousins are. The commercial strains typically can carry on active fermentation - not even being forced to go dormant - at sulfite levels at or above 50 ppm. Additionally, sulfites tend to be more effective at killing dormant cells than those that are actively fermenting. That is why you can't rely on a sulfite addition to stop an ongoing fermentation -- in general it will take more sulfite than you would want to add.

And although most of us try to add as few chemicals as possible, for those meads that will undergo extended time aging in bulk (in barrels or large carboys), sulfites are sometimes added at the transfer to the bulk vessel, and again just prior to bottling.

And if you REALLY want to get technical about this, and other winemaking topics, pick up a copy of Wine Science by Jackson.

 

[akueck]

Sulfites seem to do different things to different bugs, and different things at different concentrations. With a high enough concentration I think that the yeast will die. "High enough" varies by yeast strain, with wild yeasts the least hardy. Some yeasts can survive (and ferment sugar) at sulfite levels above 250ppm! In general, at "normal winemaking levels" of 50-100ppm the sulfites are going to slow down (commercial strains) or slow down to zero/kill (wild strains) the yeast in the must/mead.

Used pre-fermentation, the difference between commercial and wild strains is enough to let your preferred yeast thrive essentially unchallenged. Post-fermentation, the hope is to knock out the survivors. They might not die, but dormant yeast tend to settle out so you can leave them behind when you rack. As you say, the sorbate is what is really "stabilizing" since it keeps whatever population of bugs (yeast and/or bacteria) constant over time by preventing reproduction. The 1-2 punch reduces the cell count and then disallows later growth. Dormant yeast can wake up (for whatever reason), but if you have lowered the cell count using sulfites the few survivors shouldn't affect much before they naturally run out of steam and if you added sorbate they can't make copies of themselves. It's also important to have a low cell count before using sorbate because large populations can actually start to break down the sorbate, rendering it ineffective and apparently producing a lovely geranium smell.

I've had some weird oxidation-after-bottling problems of late, so yes I have done the sulfite/sorbate thing and then added a tiny bit of extra sulfite a few months later during bottling. I added the sulfite to the bottling bucket and racked onto it, then bottled. So far, no oxidation. ;D

 

[Medsen Fey]

This Thread in the Patron's area links in some articles with very detailed explanation of sulfites and their chemistry. In fact, you'll find some of the most comprehensive and up to date info on fermentation management discussed in some of the Patron's threads. For $25, you won't find better information in any book.