Author: Cade Jobe

Oxidation of beer produces stale off-flavors and can lead to shelf-stability issues in finished beer, hence the reason brewers of quality beer go to great lengths to limit cold-side exposure to oxygen. Most methods for reducing this risk involve the use of CO2 to displace oxygen at various points in the process, for example, by using it to push the beer from the fermenter to a keg that’s already been purged of oxygen. However, there are some chemical options as well.

Potassium metabisulfite (PMB) is a crystalline compound that’s commonly used in the wine and cider industries to rid must of wild yeast and bacteria prior to fermentation, as it converts to sulfur dioxide, a disinfectant, once added to the must. Perhaps the most common usage of PMB among beer brewers involves adding a small amount to the brewing liquor to eliminate cholormines, though recently, some have been relying on it as a chemical means of reducing the risk of cold-side oxidation. Like sodium metabisulfite (SMB), once PMB dissolves into beer, a redox reaction occurs wherein sulfite ions bind with oxygen in solution, thus reducing the risk of oxidation. It should be noted both PMB and SMB are sold as Campden tablets.

Given the significant results of our two xBmts on the impact of adding SMB at packaging, I began to wonder if using PMB in the same way would have a similar effect. In response to those prior xBmts, some questioned whether the perceptible difference was caused by the sodium in SMB rather than the redox reaction, so I decided to test it out for myself.

| PURPOSE |

To evaluate the differences between a beer dosed with potassium metabisulfite at packaging and a beer without the chemical.

| METHODS |

My goal being to accentuate any impact of the variable, I went with a simple pale lager for this xBmt.

Safer In Space

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.2 gal 60 min 8.2 IBUs 3.1 SRM 1.046 1.011 4.7 % Actuals 1.046 1.005 5.4 % Fermentables Name Amount % Pale Malt (2 Row) US 7.75 lbs 79.49 Corn, Flaked 1.75 lbs 17.95 Corn Sugar (Dextrose) 4 oz 2.56 Hops Name Amount Time Use Form Alpha % Tettnang 14 g 60 min Boil Pellet 4.5 Yeast Name Lab Attenuation Temperature Harvest (L17) Imperial Yeast 72% 50°F - 60°F Notes Water Profile: Ca 194 | Mg 17 | Na 18 | SO4 389 | Cl 50

I started off my brew day by collecting the proper volume of water for a 10 gallon/38 liter batch, adjusting it to my desired profile, then lighting the flame under the kettle.

While waiting on the water to heat up, I weighed out and milled the grain.

When the water was ready, I stirred in the grains then checked the mash temperature.

During the mash rest, I collected out the kettle hop addition.

When the 60 minute mash was complete, I sparged to collect the target pre-boil volume.

The wort was boiled for 60 minutes then quickly chilled with my IC.

A hydrometer measurement showed I’d hit my target OG.

I racked the wort to a sanitized 14 gallon Brew Bucket and put it in my fermentation chamber controlled to 68°F/20°C before pitching two pouches of Imperial Yeast L17 Harvest.

After 11 days, signs of fermentation activity were absent so I took a hydrometer measurement indicating fermentation was complete.

At this point, I sanitized two kegs and added 0.3 grams of PMB to one of them in order to achieve approximately 10 ppm in the beer.

After purging both kegs with CO2, I proceeded with splitting the batch equally between the kegs.

The filled kegs were placed in my keezer, burst carbonated, then left to lager for 4 weeks before I began serving them to participants.

| RESULTS |

A total of 16 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer dosed with PMB at packaging and 2 samples of the non-dosed beer in different colored opaque cups then asked to identify the unique sample. While 9 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 6 did (p=0.45), indicating participants in this xBmt were unable to reliably distinguish a pale lager dosed with PMB at packaging from one that was not dosed with PMB.

My Impressions: Out of the 4 semi-blind triangle tests I attempted, I happened to guess the odd-beer-out one time. These beers were exactly the same to my senses. Both tasted and smelled like the delicious fizzy yellow lager I intended!

| DISCUSSION |

In addition to methods that use CO2 to displace oxygen at packaging, some rely on chemical additives like potassium metabisulfite (PMB) and sodium metabisulfite (SMB) to reduce the risk of oxidation on the cold-side. Countering the results of a previous xBmt exploring the impact of SMB at packaging, tasters in this xBmt were unable to reliably tell apart a light lager dosed with PMB from one without the compound when the kegs were purged with CO2 prior to being filled.

One impetus for this xBmt was to test whether the sodium in SMB may have been responsible for the perceivable difference in the aforementioned xBmt. The fact tasters generally failed to notice a difference between a PMB dosed beer and a non-dosed version suggests there may be some merit to this claim. It’s important to note that the beers in both this and a prior SMB xBmt were pressure-transferred to CO2 purged kegs, so their exposure to oxygen was already quite limited.

I began packaging with SMB as a direct result of the two previously mentioned xBmt results because I like the additional insurance against cold-side oxidation. While these results seem to suggest PMB may have a different impact than SMB when used at packaging, both undeniably lead to a redox reaction when blended with beer. As such, I look forward to repeating this xBmt in the future with the difference being that neither keg is purged with CO2 before packaging. Until then, I’ll continue using either PMB or SMB when racking my beer to kegs.

If you have any thoughts about this xBmt, please do not hesitate to share in the comments section below!

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