why let Coffee Rest
Is fresh always the best? The answer is still yes, but with a caveat. The introduction of third wave coffee and the proliferation of local roasters increased the awareness and access to fresh coffee. You now have the option to buy and brew coffee roasted the same day! That is great, however there is a multitude of research being done today that is expanding our understanding of coffee quality over time after it comes out of the roaster. Studies and industry leaders are finding that resting your coffee, that is leaving it in a sealed container for 1-2 weeks after it has been roasted, can improve the quality of your cup! In this article I am going to reference the people who are at the forefront of understanding this phenomenon and outline how you can adapt this practice to your own home brewing.
Resting increases quality due to trapped CO2 having time to escape from roasted coffee. How does gas get in the bean in the first place?
When coffee is dropped into a roasting machine, multiple chemical reactions begin to transform the coffee seed into the aromatic bean we love so much. A by-product of the compositional transformation of coffee beans are gasses. A book published in 1987 called Coffee by Clarke and Mcrae broke down the composition of gasses produced and concluded Co2 is the most prevalent. The carbon dioxide is formed as a result of chemical reactions that occur including Strecker degradation (what is strecker degradation?), pyrolysis of sugars, and the Maillard reaction. While Co2 is not the only gas being produced, because it claims 87% of the gas composition it is safe to refer to the effects of degassing to the escape of Co2 from the beans.
Why does the coffee taste better after resting?
Currently the clearest answer to that question is better extraction during brewing. When the beans are fresh, and there is a greater amount of trapped gas, water rapidly diffuses the bulk of the trapped co2 which disrupts the brewing process. Due to this disruption, a balanced extraction is harder to achieve and as a result decreases overall quality in the cup. In speaking with Perfect Daily Grind, 2017 Roasting Champion Mark Michelson explained his experience at Onyx Coffee Lab letting coffee used for competitions sit for 10 to 21 days “Because it makes it easier to pull the coffee, it is easier for the extraction”. This is true for both filter and espresso options. The experience of Michelson and other industry leaders led to the practice of purposely resting coffee to allow gas to escape before brewing.
There will be a point at which the coffee begins to decrease in quality due to staleness and each coffee will have its own individual tipping point. However it is safe to say for light roasted coffee that point will exist after the two week mark. At this point, you will be able to experience a coffee in the window at which it still retains its volatile aromatics while containing less trapped CO2.
While disruption to brewing can be observed and tested, another process is occurring in the beans during resting that is not fully understood yet. Scott Rao attributes the change to either a weakening of the beans cellulose structure or a chemical reaction taking place over time that affects the coffee flavor to appear more developed. This is supported by a study from Cafe Kreyol which challenged industry standards for resting. In How Long Should Coffee Rest (Post Roast) Before Industry Professionals Assess Quality? The Cafe Kreyol team conducted a study on 21 coffees over 22 days to assess how aging affects coffee quality. They concluded “only a single coffee (out of the 21 coffees studied) had its peak score at 24 hours post roast rest. Over 73% of the studied coffees reached a peak score after eight or more days of resting from the time of roast. One coffee in the study increased by over three points over the span of the 22 days of testing, and continued to improve with the peak score being on the final day.”. In this study a double blind cupping format was used to assess quality, which steeps coffee over the course of 4 minutes to achieve an extraction equilibrium. In doing so, this method eliminates the effect gas interruption has on the brewing process and isolates time as the variable affecting coffee quality. Further research is needed to understand the reason behind the increased quality due to resting post-roast, but this study provides evidence of coffee age improving quality in a short term window. In the meantime you can experience the evolution in flavor at home!
How long you will want to rest your coffee will depend on a few different factors, but from these experiments and experiences we can safely say our coffee is evolving in positive ways after it is roasted. Less disruption during brewing and for some coffee an increased complexity in flavor development are going to benefit the quality of the end cup. Next we will go over what those different factors are and how you can modulate your home brewing procedure to benefit the most from post-roast resting!
Do all coffees degas the same?
No! There are a few variables that have a big impact on the rate at which coffee will degas. You will want to look out for
Roast Degree
Processing Method
The type of roasting machine
Coffee that is roasted dark will degas faster than coffee that is roasted light. According to the Journal of Agricultural and Food Chemistry, a light roast was recorded as degassing at a rate almost three times slower than a dark roast with the same roasting speed. If you have a coffee that is on the darker side, it would be better to drink it closer to the roast date than a coffee that is light! This is because dark roasts are more porous which allows the gas an easier path to escape from whereas a light roast is very dense trapping all the gas inside for longer. If you are unsure of the roast degree of your coffee, talk to your local coffee company or check out their website for a agtron level/roast degree signifier.
The Cafe Kreyol team found some variance in resting and processing method. “Of the coffees studied, we saw strong evidence that washed coffees did not change as significantly from day one through the end of the study, especially in an overall positive direction. Coffees that were naturally processed (the process of leaving the seeds inside of the coffee fruit during the period of drying), and honey processed (the process of de-pulping the seeds from the fruit, and drying the seed with mucilage intact), appeared to have the largest and most consistent changes in coffee quality as the coffee was allowed a longer period of post roast resting time”. If you want to experience an evolution of flavor due to resting, washed coffees might not be your best bet in comparison to a natural or honey. This doesn’t mean you shouldn’t rest your washed coffees because they will still benefit from less gas disruption during brewing!
This last variable is more situational and may be harder to find out for your individual experience. Coffee that is roasted on an air roasting machine appears to benefit from resting more so than coffee roasted on a traditional drum roaster. Scott Rao has experienced significant difference resting coffee specifically roasted on a Loring. In my personal experience roasting coffee on a Giesen (traditional drum roaster) and an IMF (Drum air roaster) I have also preferred coffees roasted on the latter after an extended resting period. I think adjusting your own resting practice using roast degree and processing method will do you just fine but factoring in machine type will allow you to fine tune each coffee to perfection.
Conclusion
I hope you feel comfortable foraying into the world of coffee resting! Just in-case your are not lets go over the key points to help wrap up all this information.
If your coffee is light, rest for longer. If your coffee is dark, drink it close to the roast date
If your coffee is washed, rest for brewing improvements
If your coffee is processed naturally or honey, rest to experience a flavor evolution!
If your coffee is roasted on an air roaster, try resting the coffee longer than if you have a coffee roasted on a traditional drum roaster.
Following these key points will help guide you to your own preference for resting time. I did not want to specify an exact time period length for resting because you get to decide what tastes best for you. Explore how your coffee ages and wherever you like it best is going to be the right amount of time.
Thank you for indulging in the exploration of coffee quality!
- Sean