If I've ground a batch of coffee, I know I should use it as soon as possible, but if I store it (in an opaque, air-tight, slightly-below-room-temp container), how long have I got to use it? Are there any key points at which the quality will degrade (e.g. after a day, after a week)?
Whole Bean Coffee Degradation Over Time
Whole bean coffee will go stale within 14 days - on average. The graph below shows the quick 1.5 standard deviation drop in quality within the first 14 days (marked in red). *See note below for details
Staling in the context of coffee has a commonly agreed on definition - the loss of volatile aromatic compounds and the oxidation of surface oils on the roasted coffee beans. The loss of aromatics affects the flavor profile because: 1) they are both part of the same degradation process and 2) the majority of what you taste in coffee is flavors from volatile aromatics through retronasal olfactory sensation.
This graph was created using data from Analytical Flavor Systems where we build quality control and flavor profiling tools for craft beverage producers. Perceived Quality is a non-hedonic assessment of a product's quality. This time series is taken from a degradation study.
Ground coffee, on the other hand, will stale within minutes. 70 cc of ambient air is enough to render one pound of coffee stale. On average, this process takes seven minutes.
Interestingly, the degradation curve looks about the same!
Data Analysis Minutia (notes)
This time series model was segmented using daily Perceived Quality means from a random selection of 15,000 coffee reviews. All coffees included are whole bean, freshly ground, third wave coffee brewed in a Chemex with a bleached filter.
Certain other brewing methods, particularly methods optimized for older coffee such a Nel, may show a different degradation curve.
The staling point was selected by a parametric statistical change point analysis, which searches for shifts in the mean and variance of a time series. This time series was modeled for change point analysis using a Poisson distribution as we're searching for the average number of coffees that go stale within a specific time-point, and the model was set to find at most one change.
The flavor constituents of roasted coffee are the result of high roasting temperatures. After roasting, they continue to be affected by environmental factors, their own natural instability, and interaction with other compounds. The most important of these processes are:
- Dissipation into other media. Aromatics evaporate from the surface of the coffee into the atmosphere or are dissolved into solvents, where they often interact with other chemicals.
- Non-enzymatic browning reactions. These involve carbohydrates, usually sugars, in carmelization and Maillard reactions. Carmelization occurs when a sugar gives up water and carbon dioxide, changing the structure of the sugar and its taste. The Maillard reaction is the result of an interaction between amino acids and carbohydrates in which an aromatically perceived substance is formed. When the Maillard reaction takes place at a high temperature (as in coffee roasting), the result is usually desirable roasted flavors and aromas, but when it takes place at a lower temperature, the result is flat, gluey, and cardboard-like flavors.
- Oxidation. Oxidation is any reaction in which one or more electrons are moved from one chemical to another, producing two different compounds. In coffee, the most common process is that an oxygen molecule donates two electrons to a compound, forming a new (differently perceived) compound and bonding with hydrogen to form water.
The engine that drives all of these processes forward is thermal energy (heat). This energy can be in the immediate environment, a result of other chemical reactions, or already present in the product.
According to a high-rep legit subreddit AskScience user,
Unlike all of the other posters in this thread, I have done this experiment. A cup of brewed coffee left at room temperature for 24 hours had about 60% of its caffeine remaining. There's been lots of talk of caffeine's chemical stability, but it's a carbon and nitrogen source for bacteria. Edit: also, coffee left at 4 degrees celsius for 2 weeks had less than 5% of its original caffeine content. The method of assaying caffeine was capillary electrophoresis, a pretty reliable method.
Generally, there is the Rule of 15.
Green coffee lasts fresh for 15 months, roasted (stored in an opaque, air-tight, slightly-below-room-temp container) 15 days and ground 15 minutes.
This applies mainly to specialty coffee since it has the most aromas and tastes. Even if you open a bag of beans and do not grind them, once the air reaches the beans you can notice that taste degrades in few days. You would have to get most of the air of the container so that the coffee lasts 15 days mentioned above.