What is Positive Pressure & why is it Important to Consider?

If you have witnessed me speak about fermentation lengths, you have most definitely heard me use the phrase, “positive pressure”. Some of you may even think I am a little obsessed with it, and you would not be entirely wrong. I promise there’s a handful of very good reasons for this and that is what we will be discussing today!

What is “positive pressure”, anyway? The dictionary defines the term as: “Air or gas pressure greater than that of the atmosphere, as used e.g., in the artificial ventilation of the lungs.” This is a very broad and basic definition, so let us talk about its application in brewing. When we use this term as it relates to your beer, sitting in the fermenter, we are typically referring to the presence of active fermentation resulting in the production of C02 gas, which in turn, pushes other atmospheric gases like Oxygen, away. You can visualize this positive pressure as a sort of protective “force field”.

So, what exactly creates this “positive pressure”? Well, as mentioned above, fermentation, but more specifically it is initiated by the yeast consuming and metabolizing wort sugars; one of the bi-products of that feasting is C02, along with roughly 500 other fermentation compounds! In the most active stages of fermentation, the yeast is releasing large quantities of this gas over a short period of time, so your fermenting beer is generally well protected in your LBK or other appropriate fermenters, as this C02 release pushes oxygen out and away from the beer. As you can probably imagine, as the C02 production slows with the activity of the yeast, that positive pressure drops, until it is no longer present at all. As a result of this, unwanted oxygen can come into contact with your beer, and create a whole host of undesirable issues.

You might be wondering, what are some of the possible consequences for the loss of positive pressure? Let us talk about a few of the “big” ones, but spoiler alert, most (all) of them have to do with the resulting oxygen exposure… Color: When pressure is no longer protecting a fully fermented beer, oxygen enters and intensifies a phenomenon called the “Millard effect”, which can cause your beer to darken, significantly. Clarity: Oxygen exposure due to pressure loss, contributes to a clarity issue known as “chill haze”, causing the molecular chains of certain compounds to collect oxygen and become so large that they create a “haze” when cold, that is visible to the naked eye. Flavor: The absence of pressure causes air to change the flavor of certain fermentation components and can give a “wet cardboard” character, or even intensify a strong apple cider flavor, in beer. You also lose desirable flavor and aroma compounds without positive pressure. Infection: One of the most significant vectors for infection within homebrewing (and arguably, brewing in general), is oxygen. Wild spores and microbes can travel right into your fermenter and take root in your beer! Condition: The success you have with carbonating the beer in your bottles is also indirectly affected by positive pressure, as yeast that has sat hungry for a week, with no active fermentation, will not be as healthy or vital within the bottle. Having yeast that is unhappy in the bottle, can cause your beer to condition very slowly, or not at all. Believe it or not, these are not even all the consequences of losing your positive pressure, but they are typically the most common!

If you have been brewing with Mr. Beer for the last couple of years or more, you may have noticed our general direction for fermentation and conditioning times, has changed a bit. The primary reason for this change came from our testing and observation on many batches. What we found, was that on average, except for a high gravity wort or certain types of yeasts, when the correct fermentation parameters are maintained, most beers will fully ferment in 10-14 days, inside the LBK. Adding an extra 7 days beyond this time means the beer is no longer under the protective veil of “positive pressure”. You might be thinking, “but Ash, what about letting the yeast clean up?”, you’re not wrong… but how much sense does it make to “mop a floor, in dirty boots?”, Not a lot, unfortunately. Once the correct FG has been achieved, it is best to bottle that beer and allow it to mature and clean up in a stable, and airtight environment! The rapid release of positive pressure is also why we discourage you from opening the fermenter to “look”. Your next thought might be, “well what about adding the additions to recipes?”. Not to worry, when a quick addition is made, such as hops or fruit, the yeast is usually going to continue to produce some C02 after the fermenter is closed back up again.

It is worth mentioning that many brewers opt to apply outside means of pressure, as well. This is not needed for the average homebrewer but can help brewers that do use it, keep very tight control over the fermentation conditions of a batch. Fermenting under “added” pressure, is a subject all on its own… that we will save for another day!

I hope this helped you to better understand the role and importance of positive pressure in the beer-making process. As always, if you have any questions about this or any other brewing topic, feel free to get in touch with Zach and myself! We are always happy to help you better understand the “why” behind what we do!

Pints up! Cheers!