Resting is the length of time between roasting and using coffee, during which its flavor improves. It is also known as de-gassing. Once the flavor starts to deteriorate the coffee is no longer resting.
What happens during resting?
Roasting produces around a thousand different flavour chemicals. Their creation is a symphony of different chemical reactions, and the balance of those chemicals changes constantly both during and after the roast. This is because while heat triggers the inital reactions, time also comes into play as the reactions, plus evaporation, run their course. There is a point where the chemical balance is optimal for flavour, and resting the coffee is a matter of waiting for that optimal point.
This is made clear by studying figure 6.8 in Illy's comprehensive work on the subject. This graph shows the disapearance of compunds responsible for "the fine and pleasant smell arising from freshly roasted beans". The ratios of the different compounds vary dramatically over the first ten days and this explains the significant change in flavour over that time.
What Illy's graph doesn't show is that the ratios are also changing during roasting. So the journey to peak flavour starts during the roast, and ends when it ends, depending on when the ratios are optimal. This means that there is no fixed point in time when roasted coffee will reach its peak, it could conceivably happen during or after the roast.
The literature on resting coffee
Although most coffee authorities have some recommendation to make about resting, they tend to contradict each other. Rao recommends consuming freshly roasted coffee within 2-3 days of roasting unless it is stored in a valve bag where he recommends consume within 2 weeks (assuming the bag is not being opened every day) (p 70). In other words he does not recommend resting at all. Davids, on the other hand, recommends resting anywhere from 4 hours to one day (p 149). Lamason recommends using roasted coffee between 3 and 10 days after roasting if making espresso, on the basis that in the first three days the shot may be too bubbly and will taste a bit tangy (p 28). This range of advice from the experts seems contradictory and confusing, but it reflects the reality that different roast profiles give different times of peak flavour.
The three-day hypothesis
Despite the variation in advice given by the experts, there is a generally accepted notion that coffee should be rested for at least 3 days before consuming. Our hypothesis is that this is not because there is anything definitive about the 3-day period in coffee, but that commercial roasters choose profiles that require 3 days of resting. This is because 3 days is a good compromise between risk of wastage which would arise if coffee was at its best straight after roasting (think of a bakery making fresh bread which cannot be programmed to peak at three days old, and consequently there is always some wastage at the end of the day) and the need for storage space (the longer you have to keep the coffee before consumption the greater the amount in storage at any one time). So it is commercial practice, and not the coffee itself, that has given rise to the concept of 3 days resting.
Programmed peak flavour
Given that that the canonical 3 days of resting is a choice, not an intrinsic property of coffee, would it be possible to alter a roast profile so that peak flavour occurred on-demand at the desired time? In principle, yes, but it requires fine control of the roast profile.
The Kaffelogic roast control system is so precise that it is possible to influence the rate of the different chemical reactions during the roast, and thereby alter the timing of peak flavour. This means coffee can be roasted ready to drink, or, by designing the appropriate profile, roasted ready to rest for several days before drinking.
The most significant thing that comes from the abilty to progam peak flavour is the ability to create ready to drink profiles. Wayne Burrows is the pioneer of this technique. He published ready to drink profiles as part of the Kaffelogic Nano 7 Core Profile Set in July 2020. This is the first time ready to drink roast profiles have been published.
The importance of ready to drink profiles
With a nano batch roaster (100g) RTD profiles are tremendously useful both in the home and commercially. In the home roast on demand for immediate consumption is the simplest way to manage coffee supplies. It is in fact the only way for home coffee roasting to be more convenient than buying ready roasted coffee. In the commercial environment, the ability to taste the results of a sample roast immediately speeds up both the coffee selection and profile development cycles.
There are other factors that affect resting. Coffee ages faster in warmer temperatures, and slower at altitude. Darker roasts age faster than lighter roasts, but conversely they may benefit from more resting . Oxygen and moisture contribute to ageing. Using a bag with a valve keeps oxygen away from the beans. This will delay ageing and allow you to rest the coffee for longer and then enjoy it fresh for longer. The coffee will age faster if you keep opening the pouch to make coffee because this lets oxygen in. Also, some chemical changes happen independently of the supply of oxygen. Don't expect coffee kept in a valve bag for a week to taste exactly the same as coffee kept in a paper bag for a couple of days.
On top of all this, different coffees respond differently to the various profiles. Some fussy coffees will not respond well to RTD profiles. You will always need to taste your coffee, taking note of how long since it was roasted.
Wayne Burrows - Head Roaster and Customer Services Manager, Kaffelogic Ltd
Chris Hilder - CEO and Director Product Development, Kaffelogic Ltd
Coffee ages faster in warmer temperatures, perhaps 1.5x faster for each 10 deg C warmer (Illy et al, p 238). Darker roasts age faster than lighter roasts (Rao, p 70). Changes in chemical makeup of coffee start immediately after roasting with the first 5 days being the most dramatic, leveling off after about 20 days (Illy et al, fig 6.8, p 242).
Espresso Coffee: The Science of Quality
The Coffee Roaster's Companion
Home Coffee Roasting: Romance and Revival
People's Coffee Barista Handbook
How To Brew Coffee Like a Barista