By Rachel Beebe
At Steampunk we roast coffee every day, package it up and put it on our shelves, so you might be able to walk in and buy a coffee that was roasted as little as 24 hours previously. But does that mean you should go home and brew it straightaway? And on the flip side, how old is too old for a roast to taste good? Like most specialty roasters we put the roast date on our coffee, but what should you do with that information as a customer and coffee lover? These are some of the questions we set out to answer when we started our Coffee Resting Project.
The thinking around coffee freshness has shifted in the 20 years since the start of the third wave. People used to think the fresher the better, hence the advent of putting roast dates on bags. Back in 2010, just having a roast date on the packaging set specialty coffee apart from non-specialty. This is no longer the case, and if you do a quick search about coffee freshness you’ll come across a ton of information, mostly backed up by anecdotal evidence, about how long to rest your coffee before brewing.
There is a general consensus (and scientific evidence) that coffee needs to de-gas for at least a few days after roasting. This is because when coffee is roasted it generates CO2, some of which gets trapped inside the cellular structure of the bean. This CO2 then slowly dissipates over time. Roasted coffee is porous, so the trapped CO2 gas seeps out slowly into the air.
Trapped CO2 is the reason fresh coffee blooms when you brew it. If you’re a regular filter coffee drinker, you will have noticed that when you initially pour water on your coffee grounds, they puff up into a dome-like shape. The fresher the coffee, the more trapped CO2 gets released, the puffier the bloom.
A general rule of thumb is that lighter roasts de-gas more slowly than darker roasts, because their cellular structure is less porous, so it takes longer for the CO2 to dissipate. But you should also consider how you’re planning to brew the coffee. If you’re brewing espresso you’ll want to rest your coffee for longer before brewing. All the flavours in espresso are more intense than in other brewing methods, including the metallic tang of very fresh coffee.
Indeed, very fresh coffee brewed in any method can taste metallic or astringent and some of the complex fruitiness or florals, if they’re present, might be muted. The best explanation we’ve found for why fresh coffee tastes this way is on a blog post by Harmony Roasters, which says that the CO2 in fresh coffee binds with the H2O during brewing and creates H2CO3, or carbonic acid, which tastes astringent.
Another thing we know for sure is that when coffee starts to stale the oils begin to oxidize and the vibrancy of the fruitiness begins to flatten. Sometimes an unpleasant cereal note creeps in. Sometimes it can taste papery or cardboardy or even woody. The volatile aroma compounds that give coffee its nice flavours are degraded and what’s left is oxidized oil, sugars and cellulose…not very tasty.
But, between the two extremes of very fresh and definitely old, the picture gets a little more vague. As a rule, Steampunk sells coffee until two weeks past its roast date. But if our coffee peaks in flavour four to six weeks post-roast, should we change this policy? If our coffee tastes too fresh the week it’s roasted should we even be selling it that early? So, we set out to design an experiment that would help us understand when our coffee tastes best and when it starts tasting old.
This is easier said than done though and, although we learned a lot, (spoiler alert) we failed to generate any conclusive data. We’re coffee roasters not scientists so the truth is we struggled right from the start. We wanted to design an experiment that would measure the perceived quality of coffee as it aged. So we set aside some coffee (actually two roasts of six different coffees) and tasted it week after week, scoring it each time using the Specialty Coffee Association’s Affective Form, which generates a quality score for coffee. We used coffee representing a range of processing methods (washed, natural, anaerobic natural and decaf). The coffee was already a few weeks old when we started and we cupped them for six weeks in a row.
Except in the case of one coffee (an anaerobic natural), the resulting data points were so scattershot that at week six we took a step back to reassess. We started questioning everything. Were our personal preferences introducing bias into our scores? Were the roast curves consistent enough? Was the mixture of varieties and uneven fermentation in some of the coffees introducing inconsistency? Were our palates good enough to perceive the differences we were looking for? Questioning, it turns out, is the part of the scientific method we’re very good at.
According to the SCA, “The sign of a high-performing affective cupper is consistency with their own scores and preferences over time, not their alignment with other cuppers.” So, to ensure that our palates were performing well enough we decided to cup the same coffees two days in a row. We would expect to score the same coffees the same way because 24 hours isn’t really going to make or break the perceived quality in a cup. Unfortunately, only one out of four of us was 100 percent consistent in our scores across all three coffees that we tasted those two days. This was humbling information given that we cup professionally two to three times a week and one of us is a Q grader.
Obviously we needed to change tack if we were going to salvage anything useful from this endeavour. So, we decided to use triangulations. Triangulations are effective because they don’t rely on the translation of sensory perceptions into numbers or words. A triangulation consists of three cups of coffee: two the same, one different. The cupper’s job is to pick out the odd one.
Once again, this is much harder than it sounds. But because we cup so much and do triangulations pretty frequently with better than average success, we knew we’d be able to draw some conclusions from the exercise. At the very least we’d be able to say whether differences in age between two roasts are perceptible.
We did triangulations for another month. We used two roasts each of four different coffees that were roasted one to two months apart, and at least one month before the date of the cupping. The results?
We couldn’t reliably perceive the difference.
To make it more explicit, because this really shocked us and it should shock you, too: we were tasting this coffee at the end of April; one had been roasted in mid-January and the other in mid-March. And we couldn’t tell the difference between them. That’s three months off-roast and six weeks off-roast. And not only could we not tell which was which, we couldn’t tell the difference between the two.
So, what, if anything, can we conclude? When we started this project we thought we’d see the coffee peak in flavour and then start fading over time. The data we collected didn’t support that hypothesis. Instead of a peak, maybe coffee has more of a freshness plateau. Once it de-gases, it tastes pretty good for a long time.
Also, differences in roast level and other factors like variety, processing method and origin are more perceptible than differences in age. As long as you’re buying reasonably fresh coffee (within three months of roasting) the roast date is probably not what is causing most of the perceptible attributes in the coffee you’re drinking.
And here’s our hot take: we hypothesise that the age of the green coffee is more responsible for flatness and negative flavour notes we associate with age than the amount of time past the roast date. In other words, in most cases if you start with fresh green coffee (what ‘fresh’ means will vary depending on the processing method, but generally it means from the most recent harvest at that origin) it’ll taste good for at least three months past roasting. But if your coffee was past crop when it was roasted, it’ll taste flat and woody.
When we realised how important green coffee freshness is to flavour we started putting the UK arrival date on our packaging. And we make a big effort to start roasting our coffee as soon as it lands in the UK. Of course, landing dates don’t tell you everything. The quickest African coffees can get to us in the UK is two months after leaving the dry mill, which is often two months past picking, while some South American coffees could be in our roaster within a few weeks of dry milling. Coffee from Asia can be on the water for even longer. Also the rate at which green coffee ages is massively dependent on the processing method and how skillfully it was fermented and dried. Washed coffees tend to age more slowly while in our experience any kind of longer fermentation naturally processed coffees age more quickly. This could be because fermentation can break down the cellular structure and kill the endosperm in the bean. But honestly, we don't know enough about coffee processing to say for sure.
How would we test our green coffee hypothesis? Ideally we’d get the same coffee from the same farm two years in a row. We’d roast the coffee on the same day and cup it in a series of triangulations for several weeks. If we can reliably pick out the odd cup and say why it’s different, we’d prove our hypothesis. In practice this would be very tricky to do. As green coffee ages it dries out, so it would be almost impossible to roast the two exactly the same. You could measure the roast level with a refractometer to be sure they match, but even then the two roasts might taste different right from the start.
And what if we wanted to get some better data on how a particular roast ages? We’d roast it the same way every other week and set some aside to keep tasting. We’d start cupping when the oldest roast is eight weeks old (we’d have four roasts). Then we’d continue cupping every week and adding a new roast every two weeks. Each week this would give us a table of coffees ranging from two weeks off-roast to many weeks off-roast. We would rank the roasts from best to worst tasting and see if we could find correlations in the data after many weeks of tasting. Ideally we’d get as many people as possible to taste and rank the coffee to weed out personal bias. If our plateau hypothesis is true then we’d expect to have mixed rankings in the middle-aged coffee, and then more consistency in the rankings of the older coffee.
We’re coming into our busy time of year now, so it’s unlikely that we’ll launch Coffee Rest Project 2.0. But we would challenge you to hold onto one of our coffees for 3+ months and then buy a fresh roast and see if you can tell the difference. If nothing else it’ll challenge some of your preconceptions and get you to focus more on what you’re tasting in your coffee, which will lead to more enjoyment. Win win.