Carbon dioxide (CO2) plays an integral role in the quality and freshness of coffee. Built up during roasting, it can affect everything from acidity to extraction to aroma. For specialty roasters, understanding how different levels of CO2 influence the characteristics of roasted beans is crucial to ensuring their coffee is consumed at its best.
However, the level of CO2 in roasted beans can differ from coffee to coffee. Factors such as variety, processing method, and roast profile can all change the amount of gases that are built up during roasting. As such, the rate of degassing will not be the same across all coffees.
It’s important, therefore, for specialty roasters to learn how to measure the CO2 content of their coffee beans. Doing so will maximise the quality of their coffee, help them plan roasting dates, and improve consistency across batches.
To learn more about measuring the CO2 levels of roasted coffee, I spoke with three-time Australia Barista Champion and Head of Training at ONA Coffee, Hugh Kelly.
See also: What’s a degassing valve?
How does CO2 affect the freshness of coffee?
When green coffee beans are roasted, thermally driven chemical reactions cause the formation of volatile gases in their porous structures. This buildup increases the volume of the beans by up to 80%, most of which is made up of carbon dioxide (CO2). Although some of the CO2 is released during roasting, the majority remains in the beans and is gradually released during storage, or more quickly during grinding and extraction.
CO2 is linked to important characteristics of coffee and is a key indicator for freshness. It also plays an important role in shelf life and can affect the sensory profile in the cup. Hugh highlights the vital role of CO2 in roasted coffee by describing it as a “preserving layer” that staves off oxidation.
Oxidation is when oxygen replaces the CO2 lost through degassing. It causes compounds in the coffee to change, typically resulting in a loss of aroma and the development of unpleasant, rancid flavours. Research suggests that for every 1% increase in oxygen levels, there is a 10% increase in the rate of degradation.
“CO2 is extremely important,” Hugh says. “Oxygen can get to the surface and start oxidising coffee, so it’s essential to grind the beans while there’s still CO2 present. There’s a notable correlation between an increase in oxygen and a change in characteristics. The coffee tends to become drier and take on an oily taste.”
However, more CO2 doesn’t necessarily mean a better tasting cup. Consuming coffee immediately after roasting will usually result in an uneven extraction due to the formation of a barrier of released gases between the coffee and the water. This can impart the coffee with an astringent flavour, masking its distinct characteristics.
Instead, each coffee should be left to “rest” (or degas) for a period of time after roasting before it’s brewed. The length of time can vary from a few days to a few weeks, depending on a number of factors, including bean density, roast profile, and processing method.
How to measure the CO2 levels of roasted coffee
For specialty roasters, measuring the CO2 levels of their coffee is important for ensuring maximum quality when consumed. If roasters sell coffee that has degassed beyond its peak flavour and aroma, customers may find it stale and look for a brand offering fresher coffee.
One of the most effective ways of measuring CO2 levels in roasted coffee is using scales. The Coffee Excellence Center in Zürich carried out research using this method in 2016, when they placed samples in containers with fitted capillaries to allow gases to escape. The time-resolved release of gases was then measured via the weight loss of the containers.
They found that while origin, varietal, and processing method all had a significant impact on the rate of degassing, roast profile was by far the most important variable.
Darker roasts, they discovered, emit CO2 much quicker than lighter roasts: 8.5 milligrams of CO2 per gram of coffee diffused over at least 400 hours, compared to 2.7mg/g for light roasts. Roasting speed also had an impact. For a medium roast on high speed, 6.6mg/g of CO2 was released, whereas a slower speed resulted in 4.7mg/g.
Based on his experiences during the 2017 Australia Barista Championships, Hugh advises playing around with different types of coffee and seeing how each variable affects the rate of degassing.
“Test different roast profiles and different processing methods,” he says. “Try six or seven bags of coffee from one roast profile and test that over a number of days. I had five different bags of coffee that I tested around 75% CO2 and slightly lower oxygen levels around 10% led to less dryness and more texture in the coffee.
“But a different bag with exactly the same amount of coffee that was packed on the same day could be below 70% CO2 with oxygen content above 10%, even when it sat in the same bag under the same conditions.”
How do different packaging materials affect degassing rates?
In recent years, sustainable packaging materials such as kraft paper, polylactic acid (PLA), and low-density polyethylene (LDPE) have become increasingly popular. But how do they affect the rate of degassing?
Hugh explains that traditionally, recyclable and compostable materials by themselves aren’t the most reliable in preventing exposure to external factors such as oxygen, leading to a “steeper curve of oxidation”.
However, when reinforced with laminates or extra layers, their barrier properties improve significantly. According to Hugh, the addition of a degassing valve on packaging can also help.
“Degassing valves are important because we don’t want to be suffocating coffee beans,” he says. “CO2 is heavier than oxygen. There’s a layer of coffee beans and the CO2 sits around them like a cloud, while the oxygen gets pushed up to the top and, ideally, pushed out of the valve. Without a degassing valve, you will get heavy cup profiles that can obscure the natural flavour profile of the coffee.
At MTPak Coffee, we understand the importance of preserving freshness and quality. All of our sustainable packaging options, including compostable and recyclable bags, include two or more layers of material to protect and preserve your coffee.
Our BPA-free degassing valves help to prolong shelf life and improve storage, while our range of resealable features offer a convenient packaging option for consumers.
For more information on our sustainable coffee packaging, contact our team here.
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