This is another lengthy "rave on" by me. I have covered much of this recently in responding to posts. However, there has been a bit of discussion and questions raised lately by newer members of HBT (welcome, by the way) in a variety of posts regarding carbonation and bottle pressure. So, I figured that maybe a bit of general understanding about carbonation is warranted. After all, the only way for us to know how many volumes of CO2 are in a sealed bottle is to monitor the pressure inside the bottle.
For those of us not blessed with a kegging setup, bottle priming is the process of choice for carbonation, so we need to know the relationship between carbonation and bottle pressure in order to stop fermentation (i.e. pasteurise) at the right time to avoid volcanoes and bottle bombs. Of course, opening bottles periodically and "tasting and wasting" is another way to go, but you might not have many bottles left at the end of the process.
For years when bottling a batch of cider destined for carbonation, I have bottled a sample in a Grolsch type bottle fitted with a pressure gauge to monitor CO2 development in the sealed bottle. This was initially inspired by a post from Maylar way back on 10 Dec 2015. Unfortunately, the fittings listed in that post weren't readily available here in Oz at the time, so I went down the path of making something similar using Grolsch type bottles. For anyone interested, attached below is a "How to do it".
There have also been concerns about using the "soft drink bottle squeeze test" to monitor carbonation because resulting carbonation has been lower than expected. I haven't had any issues with it, but most of the time I use my Grolsch bottles. I am currently part way into a trial to monitor carbonation with both a Grolsch bottle/pressure gauge and a squeeze bottle to see if there is some subjective correlation between measured pressure and perceived squeeze pressure... So, "watch this space", although as it is still in progress and I am unlikely to post anything for a few weeks yet.
As we are on the subject, it is also worth a brief review of carbonation. The most common measure is volumes of CO2. However there seems to be various opinions about how many volumes of CO2 are needed. Basically, a freshly fermented beverage (cider) will already have about a volume of residual dissolved CO2 which is in balance with the ambient air pressure of typically 1 bar or 15psi at bottling time. This volume doesn't contribute to the "fizz". To get "fizz" more CO2 needs to be forced into solution (the cider) under pressure from a gas cylinder or generated from priming sugar. When the bottle is opened and the gas pressure under the cap suddenly decreases, the concentration of CO2 in solution has to be in equilibrium with the ambient air pressure and so the gas escapes from solution, creating effervescence.
For the priming sugar approach which many of us use for "craft' cider, sugar is added to create the extra CO2 needed for "fizz" on top of the residual volume that is already there (the sugar ferments and 47% of its mass is converted into CO2). However, we don't really know exactly what volume of residual CO2 is already there in the cider. This depends on factors like temperature, age of the beverage, atmospheric pressure, etc, and according to various opinions it can be anything between 0.5 and 1 volume with 0.8 - 0.9 being a popular "best guess" when calculating priming sugar. So, it can be a somewhat inexact science.
There are several ways to prime for carbonation, with 2.5 volumes being a common recommendation... on-line calculators take the residual volume and calculate the amount of sugar needed to create the target volume of CO2 (i.e. in simple terms 1 volume of residual + 1.5 volumes from the sugar = 2.5 volumes of CO2). These calculators are generally based on multiple regression analysis work by Dr Michael Hall and published in Zymurgy Magazine, Summer 1995, and are quite accurate. Other approaches are more "rule of thumb" and calculate the amount of sugar needed to create CO2 for "fizz", i.e. Andrew Lea's 10g per litre (a teaspoon per pint) increases SG by 5 gravity points, two gravity points of fermentation results in one volume of C02, etc.
Then comes the interesting part and the reason for assessing bottle pressure. Once a primed bottle is sealed, the only way to know how many volumes of CO2 have been created is by measuring the pressure inside the bottle air space. i.e. using "the soft drink bottle squeeze test", or a pressure gauge. But... to convert pressure to volumes of CO2, the temperature must be known because absorption of CO2 into solution is temperature dependent. e.g. according to Andrew Lea's Carbonation Table (which is based on Henry's Law which relates dissolved gas in a liquid to pressure) with one volume of residual CO2 already in the cider, at a typical bottling or fermenting room temperature of 20C (68F) 2.5 volumes of CO2 will have a gauge pressure of 1.9 bar (27psi), whereas at 15C (16F) the gauge pressure will only be 1.5 bar (21psi) because at this temperature more CO2 will be taken into solution so airspace pressure is less.
Typically, I will prime to SG1.010 or bottle at that SG and will hot waterbath pasteurise when the test bottle gauge pressure is 1.9 bar (27psi) at 20C which indicates that about 1.5 volumes of CO2 have been generated for "fizz" (i.e. the SG from the unfermented sugar should then be around 1.005 for 5g/L residual sugar). So, at a typical lower drinking temperature I still have 2.5 volumes of carbonation to effervesce even though the pressure is lower. The best part of the pressure gauge approach is that you can "taste test" before pasteurising to check if the carbonation seems to be about right.
This all sounds a bit heavy but is quite straightforward once you understand what is going on. I hope it helps.
Cheers!
For those of us not blessed with a kegging setup, bottle priming is the process of choice for carbonation, so we need to know the relationship between carbonation and bottle pressure in order to stop fermentation (i.e. pasteurise) at the right time to avoid volcanoes and bottle bombs. Of course, opening bottles periodically and "tasting and wasting" is another way to go, but you might not have many bottles left at the end of the process.
For years when bottling a batch of cider destined for carbonation, I have bottled a sample in a Grolsch type bottle fitted with a pressure gauge to monitor CO2 development in the sealed bottle. This was initially inspired by a post from Maylar way back on 10 Dec 2015. Unfortunately, the fittings listed in that post weren't readily available here in Oz at the time, so I went down the path of making something similar using Grolsch type bottles. For anyone interested, attached below is a "How to do it".
There have also been concerns about using the "soft drink bottle squeeze test" to monitor carbonation because resulting carbonation has been lower than expected. I haven't had any issues with it, but most of the time I use my Grolsch bottles. I am currently part way into a trial to monitor carbonation with both a Grolsch bottle/pressure gauge and a squeeze bottle to see if there is some subjective correlation between measured pressure and perceived squeeze pressure... So, "watch this space", although as it is still in progress and I am unlikely to post anything for a few weeks yet.
As we are on the subject, it is also worth a brief review of carbonation. The most common measure is volumes of CO2. However there seems to be various opinions about how many volumes of CO2 are needed. Basically, a freshly fermented beverage (cider) will already have about a volume of residual dissolved CO2 which is in balance with the ambient air pressure of typically 1 bar or 15psi at bottling time. This volume doesn't contribute to the "fizz". To get "fizz" more CO2 needs to be forced into solution (the cider) under pressure from a gas cylinder or generated from priming sugar. When the bottle is opened and the gas pressure under the cap suddenly decreases, the concentration of CO2 in solution has to be in equilibrium with the ambient air pressure and so the gas escapes from solution, creating effervescence.
For the priming sugar approach which many of us use for "craft' cider, sugar is added to create the extra CO2 needed for "fizz" on top of the residual volume that is already there (the sugar ferments and 47% of its mass is converted into CO2). However, we don't really know exactly what volume of residual CO2 is already there in the cider. This depends on factors like temperature, age of the beverage, atmospheric pressure, etc, and according to various opinions it can be anything between 0.5 and 1 volume with 0.8 - 0.9 being a popular "best guess" when calculating priming sugar. So, it can be a somewhat inexact science.
There are several ways to prime for carbonation, with 2.5 volumes being a common recommendation... on-line calculators take the residual volume and calculate the amount of sugar needed to create the target volume of CO2 (i.e. in simple terms 1 volume of residual + 1.5 volumes from the sugar = 2.5 volumes of CO2). These calculators are generally based on multiple regression analysis work by Dr Michael Hall and published in Zymurgy Magazine, Summer 1995, and are quite accurate. Other approaches are more "rule of thumb" and calculate the amount of sugar needed to create CO2 for "fizz", i.e. Andrew Lea's 10g per litre (a teaspoon per pint) increases SG by 5 gravity points, two gravity points of fermentation results in one volume of C02, etc.
Then comes the interesting part and the reason for assessing bottle pressure. Once a primed bottle is sealed, the only way to know how many volumes of CO2 have been created is by measuring the pressure inside the bottle air space. i.e. using "the soft drink bottle squeeze test", or a pressure gauge. But... to convert pressure to volumes of CO2, the temperature must be known because absorption of CO2 into solution is temperature dependent. e.g. according to Andrew Lea's Carbonation Table (which is based on Henry's Law which relates dissolved gas in a liquid to pressure) with one volume of residual CO2 already in the cider, at a typical bottling or fermenting room temperature of 20C (68F) 2.5 volumes of CO2 will have a gauge pressure of 1.9 bar (27psi), whereas at 15C (16F) the gauge pressure will only be 1.5 bar (21psi) because at this temperature more CO2 will be taken into solution so airspace pressure is less.
Typically, I will prime to SG1.010 or bottle at that SG and will hot waterbath pasteurise when the test bottle gauge pressure is 1.9 bar (27psi) at 20C which indicates that about 1.5 volumes of CO2 have been generated for "fizz" (i.e. the SG from the unfermented sugar should then be around 1.005 for 5g/L residual sugar). So, at a typical lower drinking temperature I still have 2.5 volumes of carbonation to effervesce even though the pressure is lower. The best part of the pressure gauge approach is that you can "taste test" before pasteurising to check if the carbonation seems to be about right.
This all sounds a bit heavy but is quite straightforward once you understand what is going on. I hope it helps.
Cheers!
