Coffee storage and freshness are influenced by a confusing mix of variables that greatly affect how long your beans stay tasty. We’re a big fan of vacuums as a means of extending your coffee’s shelf life (if you couldn’t tell from our new Atmos Vacuum Canister). Here’s a handy guide on coffee freshness that explains why vacuum canisters are so effective at keeping your coffee fresher, longer.
Why is oxygen so bad for coffee beans?
Oxygen is bad for coffee, snacks, or pretty much any perishable item because it makes things stale through a process called oxidation. To set the stage, we define “staleness” as the loss of desirable flavors and aroma, and/or the creation of undesirable flavors and aroma. When oxygen comes in contact with a molecule, it removes an electron from the molecule. This is called oxidation. The molecule that loses an electron becomes unstable and reactive, which then results in the loss of flavor or creation of an undesirable flavor.
For coffee beans, oxygen negatively affects VOCs (volatile organic compounds). VOCs are chemical byproducts of roasting that evaporate quickly. They are generally considered good because they contribute to aroma.³ When oxygen comes in contact with VOCs, they become unstable and result in coffee losing its aroma.
Oxygen also affects lipids, which are the coffee oils in the bean. Oxidation turns lipids into peroxide which contributes to rancid taste notes.7
What’s a vacuum container and how does it help with oxidation?
First off, let’s define what a vacuum is. A vacuum is a closed system in which pressure is less than atmospheric. In order for this to happen, you need a constant amount of space (i.e. a rigid container) where air particles are removed.
There’s a lot of airtight containers/bags out there that get rid of some air, but these aren’t necessarily a true vacuum. An easy way to tell if you don’t have a vacuum container is if your container shrinks (i.e. is it a bag or does it have a lid that collapses into the container). If your container shrinks, it is not a vacuum and will not decrease the pressure below atmospheric.
But why is pressure so important?
Oxidation speed (the rate of oxidation) depends on pressure.¹A reaction occurs when two molecules collide. If there are more molecules in a smaller area, the chances of collision increases and the reaction happens faster.
Air is a mixture of nitrogen, CO2, and oxygen. For the oxidation reaction, we care only about the pressure of oxygen – or the partial pressure. The partial pressure of oxygen is the fraction of oxygen in the air, times the total pressure of the air. The partial pressure of oxygen can be changed by changing the fraction of oxygen in air (i.e. nitrogen flushing) or changing the total pressure of the air (i.e. vacuum chamber).
Fraction of Oxygen in Air x Total Pressure Air = Partial Pressure of Oxygen
Here’s an example of this equation for calculating the partial pressure of oxygen:
What is the partial pressure of oxygen in a) atmosphere and b) a half vacuum?
Fraction of O2 in air = .2095
Total pressure of atmospheric air = 101.325kPa
Total pressure in half vacuum air = 50.66 kPa
Fraction O2 x Total Pressure = O2 partial pressure
.2095 O2 x 101.325 kPa = 21.23 kPa
.2095 O2 x 50.66 kPa = 10.61 kPa
Therefore, O2 partial pressure a) 21.23 kPa and b) 10.61kPa
From calculating the partial pressure of oxygen in a half vacuum, we can conclude the oxidation rate at full pressure is twice as fast as the oxidation rate at half pressure, which is the environment Atmos creates. Although oxidation and freshness have a very strong correlation, because there are other factors at play like degassing, the relationship is not linear. If it was, we could say Atmos slows the oxidation rate by 2x, and therefore extends coffee’s shelf life by 100%.
We landed on the conclusion that Atmos extends coffee’s shelf life up to 50% due to the many variables that go into how fast a specific bean goes stale (origin, roast, how many times you open your canister, degassing, etc.)
What happens to the degassing rate of coffee in a vacuum?
If vacuum chambers affect gas on the molecular level, then it’s important to consider how vacuums affect coffee degassing. When coffee is roasted, gases form inside the bean. After roasting, gases (mostly carbon dioxide) start seeping out. When coffee is a few days old and very fresh, a bulk of the carbon dioxide formed leaves your beans. Because vacuums create a pressure gradient, CO2 degassing will occur quicker than at full atmospheric pressure. When we first started designing a vacuum canister, we wanted to make sure this didn’t harm the flavor profile of coffee or its ability to stay fresher longer.
Our conclusion was twofold. First, 40% of CO2 in coffee escapes within the first 24 hours. Most of the degassing happens very quickly in the first few days after roasting.² By the time you get your coffee, it has probably already done a bulk of its degassing so vacuum canister effects on CO2 are minimal.
If you do get your coffee extremely fresh (1-3 days off roast), a vacuum canister will degas coffee quicker, which in theory, is a negative outcome because degassing too quickly can also lead to the loss of volatile organic compounds (VOCs) that create aroma. However, as we talked about earlier, it turns out those same VOCs, along with lipids, are the molecules most affected by oxidation. Oxidation is the primary reason for coffee staling.5 When assessing the impact of fast degassing versus oxidation, it is clear oxidation has such an overwhelming impact on coffee’s flavor that protecting your beans from oxidation is significantly more impactful than preventing a small increase in the degassing rate.
Since this is a sealed environment, will this mean there is a buildup of CO2?
We’ve received a few questions about CO2 buildup in Atmos compared to in a bag with a standard one-way valve. Commercial coffee bean bags with CO2 degassing valves are designed to relieve CO2 pressure buildup and prevent bag rupture, not to necessarily keep coffee fresher for longer. In a vacuum chamber, the pressure is low enough that CO2 buildup is not a concern. CO2 in the air around the beans is non-reactive and will not affect the flavor of the coffee. CO2 does, however, affect your coffee’s flavor once you start brewing, so make sure to bloom your bed of grounds before brewing!4
Can I put Atmos in the freezer to store coffee?
You can freeze Atmos but it’s not the best use of the product. Since coffee beans should be frozen in individual servings as to not refreeze and freeze beans continuously, Atmos doesn’t make the most sense. Atmos is intended to hold multiple servings, so it would be a waste of space in your freezer to put only one serving of beans inside. Placing Atmos in the freezer will also eventually reduce the life-span of the vacuum seal.
We hope this answers your questions on vacuums, coffee freshness, and Atmos Vacuum Canister! As always, we will continue posting our research as we receive questions from customers and the coffee curious. If you have additional questions or comments reach out to firstname.lastname@example.org, so we can continue serving up the latest and greatest coffee information!
1.) Cengel, Y. A., & Boles, M. A. (2012). Thermodynamics An Engineering Approach. McGraw-Hill Science Engineering.
2.) Illy, A., & Viani, R. (2005). Espresso coffee: The science of quality. Amsterdam: Elsevier Academic Press.
3.) Rao, S. (2014). The coffee roasters companion. Canada: Scott Rao.
4.) Raper, A. Is Your Coffee Too Fresh? Retrieved from https://clivecoffee.com/blogs/learn/is-your-coffee-too-fresh.
5.) Sage, E. (2017). What is the Shelf Life of Roasted Coffee? A Literature Review on Coffee Staling. Retrieved from http://www.scanews.coffee/2012/02/15/what-is-the-shelf-life-of-roasted-coffee-a-literature-review-on-coffee-staling.
6.) Schenker, S. (2000). Investigations on the hot air roasting of coffee beans. Zürich: ETH Zürich.
7.) Smith JP, Daifas DP, El-Khoury W, Koukoutsis J & El-Khoury A. (2004). Shelf Life and Safety Concerns of Bakery Products: A Review. Critical Reviews in Food Science and Nutrition 44(1):19-55.