WE ALL WANT THE BEST POSSIBLE EXPERIENCE FROM OUR COFFEE. OBVIOUSLY,
THIS MEANS IT OUGHT TO BE
FRESH. THAT SOUNDS GOOD, OF COURSE, BUT WHAT EXACTLY DO WE MEAN BY
FRESHNESS? The implication is that at one point in time, coffee is fresh but it loses that
freshness and becomes stale. Ultimately, we’re talking about a taste in the coffee
that changes from good to less good because it changes over time. Each coffee
drinker probably has a different standard for what level of staleness is
unacceptable. That standard is based on their past experience, their level of
sensory acuity, and any number of things that might influence their sense of
freshness.
So, for a well-trained coffee geek, staling may be noticeable a week or
two after roasting, while for a less discriminating consumer, it may be two to ten
months before they notice (or care) about a change in the taste due to staling.
Thus, there is no absolute definition, so we must discuss the issue with some
generalities and wiggle room.
The next step is to consider freshness in light of coffee chemistry. We’ve
established that roasting has an immense impact on coffee but it actually extends
beyond the end of the actual roast. The bean not only passively changes but
chemical reactions continue to occur. Some researchers have attempted to
correlate these chemical changes to sensory response.
While some insight has
been gained, there are so many factors to account for that we only have a
glimmer of the whole picture.
During roasting, many gases, or volatile compounds, are released or generated.
The end of the roasting process doesn’t mean the volatiles are no longer present.
You know this intuitively because anytime you smell coffee, you smell a gas that’s
been released and is no longer in the bean.
In the first twenty-four hours after
roasting, the bulk of gases, composed mostly of carbon
dioxide, are released from
the bean. Over the course of several months, more and more volatiles escape
from the bean structure, which is why coffee smells less intense over time. These
volatiles that you smell are volatiles that you won’t be drinking.
Thus, the loss of
these volatiles is a primary cause of staling. Since the volatiles are trapped in the
bean and must diffuse out, the size of the bean particles play a significant role on
their evolution. Smaller particles, with more surface area relative to their volume,
offer much shorter distances for the volatiles to travel. If coffee is ground just
after roasting, 26 to 59 percent of the carbon dioxide (and undoubtedly other
volatiles) will be released immediately, with the larger value coming from smaller
bean particle sizes that have a larger surface area to volume ratio. The other primary cause of staling is the oxidation of compounds within the bean.
While lipids (fats and oils) have been the main purview of coffee oxidation
research, other molecules react as well and are surmised to play a role.
Independent of the identification of specific oxidation reactions, the data
demonstrate that coffee exposed to oxygen stales quicker than coffee not
exposed to oxygen.
An indirect factor in coffee staling is ambient temperature. Higher temperatures
increase the rate of chemical reactions. Thus, the warmer the room, the faster gas
evolution and oxidation will occur. Also, higher levels of water activity (essentially,
the amount of water available to participate in chemical reactions) hasten staling.
In other words, exposure to humidity will allow coffee to absorb moisture,
permitting bad things to happen.
While many a coffee geek suggests light is
detrimental to coffee freshness, there is no evidence to support this in the
literature. However, as some wavelengths of light contain enough energy to break
chemical bonds (think UV and some plastics), it is reasonable to moot that light
can play a damaging role.
Researchers working on coffee staling chemistry have identified a number of
volatile compounds that either correlate with negative aromas or with negative
aroma experiences.
Unfortunately, there is no agreement on any one compound or even the ratio of
two compounds that guarantees a successful measure of staleness. Part of the
challenge is that the roast profile, roast level, and coffee origin all influence the
volatile composition and thus makes finding definitive staling compound proxies
difficult.
Interestingly, very few experiments that test the taste of coffee freshness
(without any chemistry component) seem to exist. Some use untrained panelists
(i.e, regular consumers) as their assessors while others use trained panelists to
collect more refined data. As there are so few studies from which to draw
conclusions, there isn’t much of a story to tell.
Moreover, each study had a very unique purpose; generating data to help
populate this section of the book was not one of them.
Thus, the next paragraph
is going to be a bit vague.
Average consumers, it seems, have a hard time telling the difference between
coffees that are fresh or just a few weeks old, whether they were stored on the
shelf or in the freezer. In other words, sometimes they can tell a difference and
sometimes they cannot.
This suggests that coffees that are less than a month from the roast date are
probably perfectly acceptable to most consumers.
On the other hand, with coffee
far from the roast date (nine or eighteen months), a trained panel can easily
describe differences between the coffees. Whether those differences are
important (it was descriptive data, not preference data) was not evaluated.
A
trained panel also seems to be able to identify coffees that were stored under
different conditions or are of different ages starting around three weeks from the roast date (there was no statistical analyses in these reports, so it is difficult to be
definitive here).
It is certainly evident that some people can identify the changes in coffee as it
ages.
Unfortunately, there is no one-size-fits-all answer as to what “stale” means in
terms of days after roasting, nor do I think there ever will be one. Since the
change in taste depends on sensory acuity and personal preference, the answer
will always lie with the drinker.