[stpug]

There has been some discussion on the effectiveness of metabisulfite in beer brewing as an antioxidant due to the pH of wort and/or beer.  Metabisulfite powder is composed of 57.6% sulfur dioxide, and when sulfur dioxide dissolves in a solution it creates three compounds known as 'sulfites' (molecular SO2, bisulfite ion, and sulfite ion) depending on the pH.  The graph below illustrates the compounds that sulfur dioxide takes in solutions of varying pH.  One of these compounds has antimicrobial properties (molecular SO2), and the other two are primarily antioxidant properties (bisulfite and sulfite ions).

In beer brewing we are only concerned with the latter two, i.e. the antioxidant properties.  In fact, we do not desire antimicrobial control since boiling will control microbes in beer brewing and don't want to potentially affect yeast performance/health.  "Because of sulfur dioxide’s ability to bind with the precursors and the products of oxidation, it can be used as both a preventative and a treatment." [Practical Winery & Vineyard Journal Jan/Feb 2009:2]  For beer brewing of standard pH beer, we are generally working in the 4-6 pH range, and by looking at the graph you can see that the compounds are primarily bisulfite ions, and these ions provide indirect antioxidant properties by attaching to oxidation precursors rendering them useless.  As the pH starts to move higher from ~5 pH we increase the sulfite ion quantities which can directly interact with (scavenge) oxygen in the solution.  Minimizing oxidative reactions and oxygen in the mash, in turn, minimized oxidative reactions down stream provided they are used in an appropriate quantity for the amount of oxygen ingress of the brewing system.  Keeping the system's oxygen intake as close to zero as possible (and maintaining that low threshold) will allow us to use a minimal dose of metabisulfite to aid in any oxygen ingress throughout the brewing process until the next oxidative inhibitory mechanism is introduced (i.e. active yeast). In a perfect world, we would use a brewing system that prevents oxygen ingress through mechanics and give ourselves control over how much oxygen we allow into the system depending on brewing desires.



Source: http://www.moundtop.com/pdf/Winemaking-SO2.pdf
Source: https://www.practicalwinery.com/janfeb09/page2.htm
Source: https://www.practicalwinery.com/janfeb09/page3.htm
Source: https://www.winebusiness.com/tools/?go=winemaking.calc&cid=15
Source: https://www.extension.iastate.edu/wine/sites/www.extension.iastate.edu/files/wine/SulfurDioxideAdditions.pdf

Thanks to bayareabrewer for providing a succinct presentation of the usefulness of metabisulfite in brewing:
https://www.homebrewersassociation.org/forum/index.php?topic=27965.msg368881#msg368881

[Big Monk]

Just finished up dinner and transitioning into movie night with my wife and kids but want to share my thoughts on this when I'm done.

I'm planning a blog post along these lines that will include some theory, empirical observations and the email I got back from Dr. Bamforth.


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[lupulus]

Sorry.  Cannot see the presentation of the usefulness of  smb in brewing in the link provided.  Can you be more specific?

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[Big Monk]

Ok so here was what I sent to Dr. Bamforth:

Dr. Bamforth,

I've read many of your papers concerning the role of Oxygen in the brewing process and I am very interested to know your position on the use of chemical antioxidants, Sodium and Potassium Metabisulfite in particular, in the process of making beer. What is your opinion on the utility of these substances and do you see any negative affects?

Our current SOP is the deoxygenation of strike water through pre-boiling whereupon we add between 20-50 ppm of NaMeta or 20-30 ppm KMeta (depending on the brewer and system). By the time we exit the boil and aerate/oxygenate, sulfite testing strips show low/no residual sulfites.

Concerns on the popular forums and message boards have been raised about the possible connection between metabisulfite and sulfury/"eggy" flavors in finished beer. We have not experienced this and are receiving increased collective experience that echos our own findings.

If you can spare the time to give me your opinion it would be greatly appreciated.

Best Regards,

Derek Scott


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[Big Monk]

This is the initial response I received back:

Derek,

Coincidence or what? I had another email about this yesterday.


My response to that person was:

"We did experiments in the pilot brewery in Burton putting sulfur dioxide into the mash. Without permission, one of the brewers in at our Belfast location tried the same thing. It bit him hard because the yeast made hydrogen sulfide. The same thing happened in the pilot brewery. However there was a big difference: in Burton we had a piece of copper piping, which leached enough copper to bind the sulfide. There was zero copper in the Belfast brewery.

On the basis that it is NOT a good idea to have copper in the beer, because it can seriously jeopardize freshness, then it is not a good idea to be having TOO MUCH (emphasis added here by me) sulfur dioxide in the brew upstream. Once the yeast is removed then sulfite will of course benefit freshness, though commercial beers in this country must label it if total sulfite is over 10 ppm.

There is of course a proviso in all this: and that is of course that famed Burton cask ales DO have an eggy note. That is because the water in Burton contains a huge amount of sulfate and the yeast reduces that to hydrogen sulfide."

Kind regards,

Charlie

[stpug]

Sorry.  Cannot see the presentation of the usefulness of  smb in brewing in the link provided.  Can you be more specific?

SMB is sodium metabisulfite, and metabisulfite is ~56% sulfur dioxide.  The usefulness of sulfur dioxide in beer brewing is as an antioxidant.  Given the pH range we work within, we're primarily preventing oxidation via indirect method of binding the precursors (bisulfite ions), but to a smaller extent the direct binding of oxygen (sulfite ions).

Hopefully I've understood your question correctly.  I'm not a chemist, or engineer, just a plain old homebrewer working with some information that another member here kindly provided.  If you feel I've misunderstood and misrepresented some information, please let me know and I'll look into it more.  My novice understanding of the chemical interactions of sulfur dioxide in solutions of varying pH is just that, novice.  I also respect your input and would gladly accept criticism from you.

[lupulus]

Sorry.  Cannot see the presentation of the usefulness of  smb in brewing in the link provided.  Can you be more specific?

SMB is sodium metabisulfite, and metabisulfite is ~56% sulfur dioxide.  The usefulness of sulfur dioxide in beer brewing is as an antioxidant.  Given the pH range we work within, we're primarily preventing oxidation via indirect method of binding the precursors (bisulfite ions), but to a smaller extent the direct binding of oxygen (sulfite ions).

Hopefully I've understood your question correctly.  I'm not a chemist, or engineer, just a plain old homebrewer working with some information that another member here kindly provided.  If you feel I've misunderstood and misrepresented some information, please let me know and I'll look into it more.  My novice understanding of the chemical interactions of sulfur dioxide in solutions of varying pH is just that, novice.  I also respect your input and would gladly accept criticism from you.

I have no criticism. What you described above appears to be correct, and I just say appears because I have not read them :-); they are all wine related, right? I am sure there is something to be learned from the wine references, but given the pH differences, I am not sure the same mechanisms apply to beer. Maybe they do. My request was because when I click on the link provided there was nothing as to the usefulness of sulfites in beer brewing. If you need some references, Guido 2016, Ilett 1995 and Kaneda 1991 are good references. All the best :-)

[lupulus]

Ok so here was what I sent to Dr. Bamforth:

Dr. Bamforth,

I've read many of your papers concerning the role of Oxygen in the brewing process and I am very interested to know your position on the use of chemical antioxidants, Sodium and Potassium Metabisulfite in particular, in the process of making beer. What is your opinion on the utility of these substances and do you see any negative affects?

Our current SOP is the deoxygenation of strike water through pre-boiling whereupon we add between 20-50 ppm of NaMeta or 20-30 ppm KMeta (depending on the brewer and system). By the time we exit the boil and aerate/oxygenate, sulfite testing strips show low/no residual sulfites.

Concerns on the popular forums and message boards have been raised about the possible connection between metabisulfite and sulfury/"eggy" flavors in finished beer. We have not experienced this and are receiving increased collective experience that echos our own findings.

If you can spare the time to give me your opinion it would be greatly appreciated.

Best Regards,

Derek Scott


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Hi Monk,
Thanks for sharing.
Bamforth has told this story at least once in one of the podcasts, either BeerSmith or BrewStrong. These podcasts are nuggets of wisdom :-)

[Big Monk]

Bryan and I were talking about this yesterday and today and it seems, at least initially, that the mechanism driving the issues with sulfur people are having is ultimately yeast choice+Sulfate content. That is backed up at least partially by what Dr. Bamforth said.

Some trends to look for would be the following:

1.) What is the Sulfate content in the water for beers people are having sulfur issues with? Are you adding additional Sulfate on top of source water content and the amount estimated to come from scavenging power of metabisulfite?

2.) What is the metabisulfite dose used in beers where people are having sulfur issues? You need to find the line where you have adequate margin but do not introduce additional Sulfate into the fermenting beer.

3.) Was a yeast with Low to moderate sulfur characteristics used? If so, was that exacerbated by a high dose of metabisulfite or the use of additional Sulfate in the form of gypsum?

Most people who are NOT having sulfur issues are using fairly simple water profiles: Distilled/RO with minimal CaCl and Sulfate driven by metabisulfite dose.

4.) It seems that even in light of the potential for Fenton Reactions and other oxidative precursors, people are having good luck with copper chillers and BTB. That remaining copper in some people's systems may be helping to mitigate the sulfur character as well, with BTB helping to mitigate Fenton Reactions.

Let's also note that people using known sulfur producing yeast that have no copper in their systems are capable of producing beers with only the faintest amount of sulfur, so the issues likely lie in the amount of Metabisulfite used and any additional Sulfate  used.

I was hammering away at a point the other day, the "sulfites will all be gone by aeration/fermentation" line of thinking, and while that's technically correct, if Sulfate is the source of people's issues, the amount of the initial metabisulfite dose will ultimately drive the Sulfate contributed up toward the 101 ppm mark. Thanks to Bryan for talking me through that the yesterday.

[narcout]

Doesn't it seem more likely that sulphur issues are being caused by sulphur dioxide reducing to hydrogen sulfide as opposed to excess sulfate?

[Big Monk]

Doesn't it seem more likely that sulphur issues are being caused by sulphur dioxide reducing to hydrogen sulfide as opposed to excess sulfate?

It seems more likely that given what we know about our use of metabisulfite as a scavenger, and the fact that you can verify, approximately at least, dissolved Oxygen and remaining sulfites, that the pathway would be:

Meta -> Sulfate -> yeast breaks down Sulfate to hydrogen sulfide

I could totally be wrong. I'm just thinking out loud with everyone trying to figure out what people's issues with sulfur are. The fact that some people are having issues with sulfur and others aren't, even when both parties are using the same ingredients, makes me think the yeast choice + Sulfate thing could have some credibility.

[narcout]

The fact that some people are having issues with sulfur and others aren't, even when both parties are using the same ingredients, makes me think the yeast choice + Sulfate thing could have some credibility.

SMB doesn't really add much sulfate though, and I've pushed sulfate over 200 ppm on several beers with no issues (though all of those beers were brewed with US-05, 1056 or 1272).

Do yeast actually break down sulfate to hydrogen sulfide?  Kunze says "sulphate is assimilated from the wort and converted to sulphite, which the yeast cell uses up itself."  After most of the nutrients are used up and yeast growth is inhibited,  the assimilation continues and excess sulphite is excreted from the cell (paraphrasing here).   Though maybe sulphite is intermediate to hydrogen sulfide?  I really don't know.

[Big Monk]

The fact that some people are having issues with sulfur and others aren't, even when both parties are using the same ingredients, makes me think the yeast choice + Sulfate thing could have some credibility.

SMB doesn't really add much sulfate though, and I've pushed sulfate over 200 ppm on several beers with no issues (though all of those beers were brewed with US-05, 1056 or 1272).

Do yeast actually break down sulfate to hydrogen sulfide?  Kunze says "sulphate is assimilated from the wort and converted to sulphite, which the yeast cell uses up itself."  After most of the nutrients are used up and yeast growth is inhibited,  the assimilation continues and excess sulphite is excreted from the cell (paraphrasing here).   Though maybe sulphite is intermediate to hydrogen sulfide?  I really don't know.

From what Dr. Bamforth says, yeast breaks down Sulfate into hydrogen sulphide. I'm not 100% sure of the mechanism.

One thing I'm not sure of is how the highly hopped/high Sulfate dynamic plays out. Obviously people who brew big IPA and even pale ale push the Sulfate up pretty high and don't get egg fart beers. Maybe the oils bind up some of the sulfides.

I'm feeling my way through this stuff in real time so forgive me if any of this seems wonky. We (the royal we at LOB.com and our forum) are not having any major issues with this, so I'm just trying to help others suss it out.

[charles1968]

Another, possibly much bigger, source of sulfur is the protein in barley - barley contains the amino acids cysteine and methionine, both of which contain sulfur. Though most of the protein ends up in the spent grain after mashing, amino acids do get into wort and are assimilated by yeast. Yeast can also manufacture their own cysteine using sulphate in the water.

Hydrogen sulfide (eggy gas) formation is favoured by low pH and reductive (i.e. LODO) environments. Some interesting info on this in winemaking here (though bear in mind wine pH is lower than that of beer):

https://www.practicalwinery.com/novdec05/novdec05p26.htm

[JJeffers09]

Hmmmm

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