I roast coffee, and currently package it in plastic bags with one way 'degassing' valves.

As soon as coffee is roasted, it starts giving off $\ce{CO2},$ a process which continues for days, and sometimes weeks. The degassing valve allows the $\ce{CO2}$ to escape the bag, while it prevents oxygen, which is the staling enemy of coffee beans, from entering the bag.

I fold the top of each bag over with three folds and then tape it with a piece of tape.

Due to composting concerns (the valves are not compostable) I would like to stop using the valves and package the coffee in plain bags with the three folds and piece of tape. My theory is that the pressure of the off-gassed $\ce{CO2}$ will allow it to escape out of the three folds at the top of the bag, and that the oxygen not being under as much pressure from the outside, will not find it's way into the bag for a long time (months, hopefully), thereby maintaining a $\ce{CO2}$ flush of the beans in the bag, which prevents them from staling.

Does this seem like a plausible, likely scenario to keep the beans fresh, by keeping the oxygen away from the beans, allowing the $\ce{CO2}$ to escape under the initial pressure, and then maintaining the $\ce{CO2}$ environment in the bag?

Your informed answers are appreciated.

  • $\begingroup$ I think the easiest way to answer this would be via an experiment. What you're hoping for doesn't seem implausible. Even if oxygen does seep in, maybe it's not enough to matter, etc... $\endgroup$ – Tunk Oct 1 '19 at 4:29
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    $\begingroup$ Hmm,but if your concern are non compostable valves, then it looks like the bags are compostable. If the bags are compostable, they must be made from paper or organic material. If they are from paper or similar, my concern would be oxygen diffusion through bags..... $\endgroup$ – Poutnik Oct 1 '19 at 7:10
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    $\begingroup$ Indeed, where the valves really necessary? Likely nothing would change, just the valve did nothing in the past (if the bag was already compostable). I agree with the Poutnik comment as well as with the suggestion to experiment. $\endgroup$ – Alchimista Oct 1 '19 at 9:04
  • $\begingroup$ Another thought: Maybe the buildup of CO2 (higher pressure) will inhibit O2 from reaching the beans? $\endgroup$ – Tunk Oct 1 '19 at 21:49
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    $\begingroup$ @Tunk But gas diffusion follows partial pressure gradient, not total pressure gradient. Oxygen would diffuse through bag in spite of higher pressure inside. $\endgroup$ – Poutnik Oct 1 '19 at 22:45

If we consider this from a diffusion perspective, your concept might not be as effective as you might think. Graham's Law of diffusion and effusion of gases states that the rate of diffusion for a gas is inversely proportional to its molar mass ($M$): $$rate_\text{gas A} \propto \frac{1}{\sqrt{M_\text{gas A}}}$$

So, if we want to compare the rate of diffusion for $\ce{O2}$ ($rate_\ce{O2}$) against that of $\ce{CO2}$ ($rate_\ce{CO2}$), it would become:

$$\frac{rate_\ce{O2}}{rate_\ce{CO2}} = \frac{\sqrt{M_\ce{CO2}}}{\sqrt{M_\ce{O2}}}$$

Since $\ce{CO2}$ has molar mass $\pu{12 g/mol}$ more than $\ce{O2}$, we can see that $rate_\ce{O2}$ is going to be greater than $rate_\ce{CO2}$, and indeed, when we plug in the molar mass values ($\pu{32 g/mol}$ for $\ce{O2}$ and $\pu{44 g/mol}$ for $\ce{CO2}$), this ratio $\frac{rate_\ce{O2}}{rate_\ce{CO2}} = 1.173$ (In this simplified understanding of diffusion and gas interactions). This means $\ce{O2}$ diffuses $17\%$ faster than $\ce{CO2}$.

You might be able to use this information to determine if the amount of $\ce{CO2}$ being produced is significant enough to overcome the inward diffusion of $\ce{O2}$ into the coffee bags without the valves.

For reference: https://openstax.org/books/chemistry-2e/pages/9-4-effusion-and-diffusion-of-gases

As a possible suggestion, you could easily set up an off-gassing box with pure $\ce{CO2}$ by have a supply of dry ice (solid $\ce{CO2}$) in the bottom of the chamber and a vent at the top. As the dry ice sublimates its colder and denser vapor will displace the oxygen from the local environment of the box and allow the coffee bags to off-gas without oxygen present. Dry ice can be available at certain markets.


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