First, a quick refresher on how
gas effects beer during dispense. These three drawings
represent dispense with 100% CO2.
Illustration "A" represents CO2 pressure
adjusted properly for the storage temperature and carbonation
level of the beer. There is an equal amount of CO2
dissolving into and breaking out of the beer.
Illustration "B" represents too little
CO2 pressure. Gas breaks out of the beer into the head
space of the keg and in the lines. Pockets of gas will
accumulate in the lines creating pouring problems as well as flat
tasting beer.
Illustration "C" represents too much CO2
pressure, more CO2 is dissolving into the beer than is breaking
out. The carbonation level of the beer is increasing.
Pouring problems and "buzzy" tasting beer will result
when the keg is low and dispenses the over carbonated beer.
Over carbonated is often perceived as tasting extra bitter.
This keg has the CO2 pressure in balance with the beer's
carbonation level, but is using a blend of co2 and Nitrogen to
increase the total pressure. Note there is and equal amount
of CO2 in the beer and in the head space. The carbonation
level of the beer does not change during dispense.
This animation represents what takes place in a keg when
using a blended gas which contains too little CO2. The
combined pressure of the CO2 and nitrogen in the keg is high
enough to keep gas from breaking out of the beer in the lines,
thus preventing foam. The problem is that there is not
enough CO2 in the blend to maintain the carbonation of the beer
at the keg. The gas comes into direct contact with the beer
inside the keg, therefore using the proper percentage of CO2 is
essential in order to dispense beer that tastes the way the
brewer intended. The animation above illustrates CO2
concentration of the gas in the head space lower than the CO2
concentration of the beer. CO2 leaves the beer until there
is an equal amount of CO2 in both the beer and the head
space. The beer is loosing carbonation or going flat.
I have known that low CO2 blends cause beer to
go flat for some time, based on my experiences tasting beer in
the market while cleaning taps and fixing "off taste"
complaints for retailers. I wanted to prove that this takes
place beyond everyone just taking my word for it. I set up
a keg to dispense with a blend of N2 and CO2. I then took
carbonation level readings and weighed the keg as I dispensed the
beer from it. The results follow.
75th Street Saxy Golden Ale
stored at 38 to 42 degrees F
Tapped Wed night with beer gas at 25% CO2 / 75% N2
at 18psig at 1040 ft above sea level
Full 160 lbs 2.43 vol, wed night
120 lb 2.43 vol, thurs night
90 lb 2.43 vol, fri night
75 lb 2.14 vol, mon night
65 lb 2.05 vol, mon night
40 lb 2.05 vol, mon night
36 lb 1.96 vol, mon night
34 lb 1.89 vol, mon night
I tested one more time before the keg blew but it
was so low it was off the chart (the chart I was using cut off at
1.8 vol). The loss in carbonation was obviously very
severe. I finished off the test monday night because I was
looking to document what takes place in a keg over a reasonable
amount of time. I thought five days was a reasonable amount
of time to represent what a retailer takes to blow an ordinary
keg of micro brew.
In order to simplify, the
following diagram represents the severity of the loss in
carbonation in the test keg. The red section represents beer that tastes very
flat. This is about 5% of the keg or 6 of the 124 pints in
a 1/2 bbl keg. The
yellow section represents beer that is noticeably flat.
This is about 25% of the keg or 30 of the 124 pints in a 1/2 bbl
keg. The green
section represents the remaining 70% of the beer that tested at
or near the proper carbonation level. My two cents worth, 30% off-taste beer
is NOT acceptable.
Illustration "A" represents CO2 pressure
adjusted properly for the storage temperature and carbonation
level of the beer. There is an equal amount of CO2
dissolving into and breaking out of the beer. 
This keg has the CO2 pressure in balance with the beer's
carbonation level, but is using a blend of co2 and Nitrogen to
increase the total pressure. Note there is and equal amount
of CO2 in the beer and in the head space. The carbonation
level of the beer does not change during dispense. 
This animation represents what takes place in a keg when
using a blended gas which contains too little CO2. The
combined pressure of the CO2 and nitrogen in the keg is high
enough to keep gas from breaking out of the beer in the lines,
thus preventing foam. The problem is that there is not
enough CO2 in the blend to maintain the carbonation of the beer
at the keg. The gas comes into direct contact with the beer
inside the keg, therefore using the proper percentage of CO2 is
essential in order to dispense beer that tastes the way the
brewer intended. The animation above illustrates CO2
concentration of the gas in the head space lower than the CO2
concentration of the beer. CO2 leaves the beer until there
is an equal amount of CO2 in both the beer and the head
space. The beer is loosing carbonation or going flat.
