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. 

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