Re-useable Vacuum Sealer Bags

[Hildy]

I did my experiment as described: one way valve bag in a zip lock bag, sucked to a minimal vacuum with a straw, then closed. I don't know how much the zip lock bags leak in terms of air. There is a small amount of loss of "vacuum" in the zip lock now, without the one way bag being visibly very puffed - but I can squeeze a small amount of gas out of it.

[MrJack]

To 'escape' the gas inside the bag must overcome the ambient pressure outside the bag. The gas does NOT escape or the bag would not puff up at all - the CO2 is contained until the internal pressure passes equivalence to the outside air - type of molecule doesn't matter, nor does partial pressure. If the CO2 is less than the air pressure outside it will not leave through the valve.

The inherent assumption in print.

I think you might be putting too much faith in the "one way" valve functionality being defined by its name.

I think this will be well and truly setting off Yelta's "Minutiae Alarm" by now, but here goes...

The gas in the bag will be at atmospheric pressure until such time as the volume of gas inside begins to stretch the bag (i.e it is completely "puffed up" ). Once the bag starts resisting deformation the pressure will rise above atmospheric (and may leak).

If you were to carefully cut the bag before this point, CO2 would escape, and O2 and N2 would mix with the gas in the bag. This is despite the lack of a pressure gradient (and even in the absence of external air movement). Why? Diffusion - a process which is only observed where there is a concentration gradient; it's nothing magical, just a statistical phenomenon.

It is entirely possible that both oxygen and carbon dioxide molecules could pass through microscopic holes/gaps in the valve, even if the bag is not puffed up. If the outgassing of the beans is rapid, the bag will expand. If on the other hand the outgassing is slow enough (say, if most of it has already evacuated, and you then deflate the bag), then the volume of gas inside the bag may remain relatively static.

Fill a foil balloon with CO2, and watch it deflate over time.

The fact that volatile coffee oils are detectable outside of the bag supports this (or perhaps indicates low rates of bulk gas flow occur).

[Journeyman]

To 'escape' the gas inside the bag must overcome the ambient pressure outside the bag. The gas does NOT escape or the bag would not puff up at all - the CO2 is contained until the internal pressure passes equivalence to the outside air - type of molecule doesn't matter, nor does partial pressure. If the CO2 is less than the air pressure outside it will not leave through the valve.

The admixture of O2 or other gas is, presumably, prevented by the 'one-wayness' of the valve. If the bag is reduced by pressing out the CO2 the O2 cannot get in because it would have to somehow have a greater pressure than the outside air. The only way I could see O2, N2 or other gas entering is almost a quantum level of exchange as CO2 is actively exiting the bag. i.e. the valve is being held open by the escaping CO2. But the problem there is the CO2 will be at a higher pressure than the outside air (or it would not operate the valve) and so we aren't really talking more than trace amounts.

I don't see any way around that.

[MrJack]

Ok, so we are on the same page. I would say that the one way valve would prevent bulk flow but that it would not stop effusion/diffusion. Oxygen will still enter, just slowly (after all, it doesn't extend shelf life indefinitely).

The concentration gradient from near pure CO2 to ~0% CO2 is greater than the concentration gradient between ~21% O2 and 0% O2. Granted, oxygen is slightly smaller than carbon dioxide.

[Hildy]

if it doesn't completely seal, then it is no longer one way, because oxygen from the outside can diffuse/effuse into the bag - isn't the point of a one way bag to stop oxygen from getting in?

[MrJack]

For this to come into play, you have to violate the second law of thermodynamics (because you'd need a Maxwell's daemon or similar to enforce the one way nature of the valve. A one way valve like what we use in coffee requires a pressure gradient.

How so? This is not an isolated system (nor a one way valve, from an effusion point of view).

In the no-one-way-valve situation, a concentration gradient _is_ a pressure gradient.

If you consider a single component gas, that is true.
But in a gas mixtures, where the pressure either side on the opening is equal (like we could assume in this case) it is the component partial pressure (i.e. the combination of concentration and pressure) which matters. The valve is only "one way" because external pressure "closes" the opening, preventing bulk flow. However, I suspect it would not completely seal.

Like I said earlier, I'm not saying it does happen, only that it could (and thus should be considered).

[Hildy]

This net movement through small orifices does not require an absolute pressure gradient to occur; it will occur under a concentration gradient, in a phenomenon known as effusion*.
While you might not witnesses bulk flow, if the process of effusion and evolution of CO2 are occurring at similar rates, gas could be leaving the bag without it "puffing up".
Not saying that IS happening, just that it's easy to draw false conclusions (and design poor experiments) if you don't consider all of the factors at play.
*Effusion in a nutshell (as I tend to think about it), is when a gas molecule randomly happens to pass through the hole in the process of "bouncing around" in whatever container it's in.
If you have lots of small molecules bouncing around quickly (I.e. low molecular weight, high temperature/pressure/concentration) the number of molecules which happen to pass through the hole is greater than if you have a few great big molecules moving slowly.

For this to come into play, you have to violate the second law of thermodynamics (because you'd need a Maxwell's daemon or similar to enforce the one way nature of the valve. A one way valve like what we use in coffee requires a pressure gradient.

In the no-one-way-valve situation, a concentration gradient _is_ a pressure gradient.

[MrJack]

My evidence, as in, in my own cupboard, indicates the outgassing happens at the start mainly and after a few days, it doesn't puff the bag out any more. The bags that did get puffed out prior to being opened, did not seem to achieve the pressure required to operate the one-way valve - I deduce this because it took a bit of extra pressure to start the gas through the valve. (as in, merely picking up the bag wasn't enough, as it should have been if the pressure was right at the operational level of the valve)

Have you considered that perhaps the inherent assumption (that gas can only escape if the bag is 'puffed up') might be invalid?

Gases are surprisingly mobile things, and as I recall, can potentially pass through near enough any 'hole' larger than the gas molecule. This includes going through materials we typically think of as 'solid' (a latex balloon for example). This is the reason for the metallic film on the bag; it is less pervious to oxygen than the plastic bag itself.


This net movement through small orifices does not require an absolute pressure gradient to occur; it will occur under a concentration gradient, in a phenomenon known as effusion*.

While you might not witnesses bulk flow, if the process of effusion and evolution of CO2 are occurring at similar rates, gas could be leaving the bag without it "puffing up".

Not saying that IS happening, just that it's easy to draw false conclusions (and design poor experiments) if you don't consider all of the factors at play.


*Effusion in a nutshell (as I tend to think about it), is when a gas molecule randomly happens to pass through the hole in the process of "bouncing around" in whatever container it's in.

If you have lots of small molecules bouncing around quickly (I.e. low molecular weight, high temperature/pressure/concentration) the number of molecules which happen to pass through the hole is greater than if you have a few great big molecules moving slowly.

[GrahamK]

I lay absolutely no claims to having scientific credentials QLD or otherwise. With that in mind, I would suggest there could be a number of factors that could reflect/affect the anecdotal outcomes being different for various people:

1. Not all 250g bags types have the same dimensions. I have used a variety over the past 5 years and they are not all equal in terms of how much coffee can be fitted into them. Therefore each would surely house differing levels of emission gas before the valve gets involved.
2. Not all bags have the same valve types. Which would lead me to believe some valves may release emissions at lower pressure
3. Beans - I would suspect type, age, processing style etc would lead to different emission outcomes
4. Roast profile/depth - again I'm sure this would also have an impact on gas emissions

But I also think, (and in this case guilty as charged), it's probably really not worth sweating the small stuff. I'm sure there is credible scientific info already published to back up any theories.

GrahamK

[Journeyman]

Time stamp is from when loaded to the site, not when the pics were taken. You'd need to check the metadata on the original picture files for that.

Yes, I know. I was simply making the point that, from the evidence given, we do not have Science going on, so taking potshots at someone else's Science is probably not warranted.

And I am quite happy to take the word of those with experience... where it doesn't contradict other data. Because of what has been said here (CS) I do not keep my beans in the fridge, nor do I vacuum-seal them. And, thanks to such advice, my beans stay very tasty right till the last of the bag.

My evidence, as in, in my own cupboard, indicates the outgassing happens at the start mainly and after a few days, it doesn't puff the bag out any more. The bags that did get puffed out prior to being opened, did not seem to achieve the pressure required to operate the one-way valve - I deduce this because it took a bit of extra pressure to start the gas through the valve. (as in, merely picking up the bag wasn't enough, as it should have been if the pressure was right at the operational level of the valve)

[fatboy_1999]

Both your pics have the same time stamp. Now I am not saying you took pics of 2 different bags to make a point and so abrogated any claim to scientific method, I am just saying your evidence does not show what you claim it to show.

Time stamp is from when loaded to the site, not when the pics were taken. You'd need to check the metadata on the original picture files for that.

[smokey]

Leaving the CO2 in the bag may be a protective factor against staling.

"It should be noted that in order to be able to vacuum pack coffee, industrial coffee producers actually let the coffee sit for a while before it is packed. As soon as coffee is roasted it starts to release CO2, in a process called outgassing. This can actually help to protect the bean from going stale. Unfortunately for the people vacuum packing coffee or putting coffee in tins, this also will inflate the bags. This outgassing is the reason that you may very well see one-way valves on coffee bags. These valves allow the CO2 to escape, while keeping oxygen from entering the bag."
Coffee: Quality Is Important

[GrahamK]

The easiest way may be to accept that the experienced people know what they are talking about and have no agenda to push other than to provide an informed opinion?

I roast & bag immediately after roasting and concur on the aromas emanating from a batch of sealed valve bags.

Grahamk

[Hildy]

Isn't the easiest way to do this to place a bag with a valve inside another bag with the residual air sucked out? so that if it does not off-gas, the outer bag has minimal volume, whereas if it does off-gas, the outer bag will puff up.

I'll try this when I get back to my coffee roaster.

(and yes, I am a professional, with a science degree, in a science-related profession.)

[chokkidog]

I open my cupboard at least once a day and the bags of coffee in there do not make my room smell like fresh coffee unless I press the gas out of them.

Man, my whole house smells like coffee!! Caramel sweet, vanilla, roast nuts all sorts going on and I don't go around squeezing bags.
Admittedly there is normally at least 30kgs of roasted hanging around..... ;-D but unless gremlins are squeezing the bags the aromas are doing it themselves, with a little help from
the positive pressure and a one-way valve!! ;-D

JM..........You should get some fresh beans. ;-D