Tamas' Guide to Coffee Freshness, a post roast conundrum...

A very common question I have received over the years is "When is the best time to drink coffee after roast?". I have received innumerable inquires regarding roast dates, coffee freshness, when to brew after roast and how to store coffee both short term and long term. This is very interesting subject and of paramount importance when considering how to maximally enjoy your coffee. We want to make sure our customers are getting the most from their investment in our coffees and thus feel it deserves an entire article. 

To start with I would like to review some of the science associated with coffee aging and the staling process. However, first - by definition roasted coffee is a shelf stable product. This means that it will remain safely consumable indefinitely at normal temperatures and pressures (ie most anywhere on the surface of our planet). Anyone who has consumed several months to several years old coffee (most of us) knows that it is a drastically different experience than drinking coffee that is less than 1 month from roast and even more so 2 weeks or less from roast. While it may technically still be consumable at these well matured ages - it has lost most of, if not all its flavor and aromatic qualities -- which for most of us is the compelling reason we drink this exquisite dark elixir (well that and the caffeine of course!).   

Alright, let's get to it and explore a dilemma as old as roasted coffee itself; the consumer management of freshness and off gassing following the roasting process. Simply put - what is the best time to brew your coffee after roast and what is freshness in terms of coffee? These factors are critical in influencing the flavor, consistency and enjoyment of our favorite beverage. The mysterious science of what is called “degassing kinetics” is surprisingly not well understood nor extensively studied. The interface between properly aged coffee and stale coffee is not a black and white issue - in fact it has many shades of gray. Factors ranging from coffee species, cultivar, post harvest processing, roast level and time all impact gas kinetics (the movement of CO2 gas from the coffee). Here, we are going to endeavor to examine the gas phenomena in our favorite morning brew and it's significant impact on quality.  

Soon, it will be obvious that we are barely scratching the surface of an extremely complex and dynamic system containing enumerable variables and this will feel not unlike a flashback to Linear Algebra. Fear not, no math is required and there will be no test at the end. So, to narrow our focus a bit we are going to look at oxidation (staling) and Carbon Dioxide (CO2 Gas) degassing. 

During the roasting process two primary byproducts of the maillard and pyrolysis reactions are formed (the aforementioned reactions are basically sugar browning); water (H2O) & Carbon Dioxide (CO2). Much of the water formed escapes in the vapor phase and is released into the atmosphere. However, the CO2 is less fortunate and is trapped in the porous cellular structure of the coffee. Bound to the lipids (fats), remaining water and other porous structures, this CO2 is destined for a slow gradual release over the course of up to ninety days post roast. This loss represents up to 1.7% of the coffees total roasted weight and depending on the coffee and roast, can release up to 10 Liters of CO2 per hour! This unique combination of factors create a dynamic and changing chemical system as the coffee ages.  

In a business where consistency is key, this progressive and somewhat chaotic set of variables produces a real post roast conundrum during extraction (brewing) and beyond. All brew methods are affected, immersion and filter based to espresso and siphon. No brew is free from the complexities and intricacies of degassing kinetics. To further convolute this already ever-changing matrix of variables is the relationship between CO2 and water. This relationship has its own set of unique interactions that directly affect flavor, sensory perception and extraction yield.   

CO2 & water react to form Carbonic Acid, which increases the actual acidity (decreased pH) of the solution (as opposed to perceived acidity or “brightness”). This acidic upset results in an increase of sourness and changes the mouth feel of the cup. When we brew coffee just off roast, enjoying the pinnacle of freshness, no doubt on our way to experience an almost ethereal brew transcending space and time - we can observe CO2’s presence in both the bloom of the grounds, and also taste a distinct sharp metallic sourness in the cup. Ultimately, resulting in an overall negative sensory addition to what was to sure be the most incredible cup ever. This interaction of CO2 can also change the way your taste buds perceive the coffee.   

In espresso the presence of CO2 is critical, as it is responsible for the formation of the delicious tawny brown foamy emulsive layer we all know and love – “Crema”. However, too much CO2 produces aggressively frothy bubbles as the espresso is extracted, resulting in uneven hydraulic resistance across the puck or grounds bed – leading to changes in the extraction and pressure curve, ultimately resulting in a poorly extracted shot of espresso, which typically results in increased sourness and astringency. 

Studies have found that the primary precursor to the formation of CO2 in coffee roasting is Chlorogenic Acid. The higher the Chlorogenic acid content in the green (unroasted) coffee, the higher the CO2 gas formation during roasting. Chlorogenic acid content ranges amongst coffee species, with Robusta having significantly higher total quantities than Arabica. Interestingly, the significantly higher chlorogenic acid in Robusta is likely contributing factor to the significant increase of Crema produced during an espresso extraction of Robusta as compared to Arabica. However, when selecting coffees where CO2 may be a delicate factor (ie espresso) total chlorogenic acid should be considered. 

Beyond green coffee selection, roasting plays a critical role in the volume of CO2 produced and the subsequent retention after roast. We observe varying density and permeability of coffee from species to species and across cultivars, which can play a role in retained CO2. However, roasting ultimately plays the final and key part in the production and retention of CO2.  

Alright this is what I like to call the gas curve or perhaps the 4th wave of coffee (just kidding in case the seething dad joke humor was lost in translation). The retention of CO2 for any given coffee typically peaks at medium roast levels, then sharply drops. While the actual formation of CO2 continues to increase into darker roast levels, as the expansion and fracturing of the fibers and pores increase in the coffee bean itself the CO2 is able to freely move from the roasted coffee into the atmosphere. Whereas, from the start of maillard to the medium roast level CO2 retention is the highest but not at its peak production volume. Meaning that the gas trapped in the coffee will take far longer to naturally off gas at light to medium roast levels, then that of a medium to dark roasted coffee. The pores and fibers of the coffees have not fractured enough to allow for rapid degassing to occur. Additionally, this overall increase in CO2 retention can greatly contribute to carbonic acid formation, and uneven extractions in both espresso, immersion and drip applications. A total mess.  

While we all love the perfectly sweet, lively and florally high toned light roast – ultimately it can produce significantly more ill-fated extraction attempts due to excessive interference of the CO2 immediately following grinding than its classically dark roasted counter parts.  

How do we ensure freshness and peak flavor without the complications of CO2 in a seemingly endless and ongoing cloud of degassing? How do we have consistent gas management without having to Dark roast all coffees? Working with a kinetic system these questions do not have one answer to end all answers. More-over it comes down to a management program, that must be in place in your coffee shop, cafe or at home. A system which ensures your able to consistently deliver the flavor and quality you or your customers expect day in and day out. 

Several critical points to isolate and understand to best ensure consistent extraction each time are; the coffee type / process, roast level, and roast date. When you have a steady supply of coffee from the same species, cultivar, process and roast profile / roaster - your gas management algorithm becomes about time and rotation. If you are constantly rotating roasts, and styles you may spend more time dialing in your coffees than actually extracting them and enjoying them.  

To get the most from your coffees, I recommend being sure to allow any coffee to rest for at least 5 days following roast. It takes this long at the very least for the coffee to begin to develop its true identity. Depending on the process (natural, washed, semi-washed, etc.), roast level and storage method used; these times can range drastically. Light to Medium roast coffees degas significantly slower than dark roasted as we have already discussed. Storage in a nearly airtight container can slow the degassing dramatically, as opposed to a bucket or paper bag. Ensuring the coffee was allowed to rest prior to packaging is also something that should be considered. Packaging styles and resting times are all variables that continue to play into the evolution of this kinetic degassing system. Dizzy yet? Probably all that CO2.

Even with every variable accounted for from roasted coffee, espresso machine / brewing device, grind, barista / home coffee wizard and extraction style we can see anomalies. Humidity, barometric pressure, temperature and micro variances in green coffee all can lead to irregularities in the results of final extractions and brews.  

Ultimately, we are working with an agricultural product which from seed to seed can have slight botanical differences even within the exact same lot of coffee. These slight differences can lead to unexpected issues associated to excessive degassing and hydrodynamic changes.  

The science of degassing kinetics in coffee seems daunting but ultimately it is important for understanding why coffee flavors change and evolve over time. Knowing how to elegantly manage the variables is the key to consistency, which often means developing a strong program or relationship with a roasting company that can help you ensure the critical points are met and maintained to ensure your battle with CO2 does not become a post roast conundrum.  

Some final thoughts;

Fresher does not necessarily mean better. We have seen countless people pursue the quest of ultimate freshness. Endeavoring to drink the coffee before it has even cooled from the roaster. This does not however result in ultimate flavor and enjoyment. Again - I recommend at least 5 days post roast up to 21 days post roast in the case of unique naturals and anaerobic coffees to begin what I call the "Golden 14" the two weeks (or 14 days) that follow with the best flavor and aromatics possible. 

Store your coffee in a cool dark place in as airtight vessel as you can. Preferably with a one-way valve. If you are not going to consume your coffee in the 30 days following roast - freeze it. Only unthaw it once when ready to consume. I will talk about storage in a later article. 

Finally, enjoy the coffee for what it is. Often we look for so much from each coffee we miss what is right in front of us; an exceptional coffee. It took a lot of work to get it to you. Literally the coffee you are drinking is thousands of miles from its point of origin. Passing through hundreds if not thousands of hands along the way, each one doing their best and in the end hoping to survive and feed their family from those efforts. Don't let extreme freshness be a huge driver of your perception of quality. This is only one part of a greater process and only limits the enjoyment into a narrow range which as we discovered is an ever changing complexly layered system. Okay - so what are you waiting for go brew a cup! 

About the Author:

Tamas Christman is the Founder / CEO & Q-Grader of Dragonfly Coffee Roasters. He studied Chemical Engineering & Computer Science at the University of Colorado Boulder.

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