Home Roasting and Freshness
This month’s newsletter topic was inspired by a very mediocre cup of coffee. It was an Auction Lot Kenya, and it was originally a very good coffee (the name isn’t important but it is one we sold in mid-2000). Yet this cup was dull, totally lacking the distinct fruity and flowery aromatics of a good Kenya. What was wrong?
Freshness! I looked back at my sample roast notes and saw that this coffee was about 9 days old, and if I remember correctly I left it sitting out in a sample tray on the cupping table for 4 days (for no good reason I can think of).
Home roasters can be sticklers for the details and that includes freshness. After all, freshness is the main benefit to wrestling the "means of production" out of the paws of that questionable coffee source you previously bought from, and adopting the DIY —do it yourself- strategy.
The conventional wisdom for home roasters is to store coffee in small, airtight glass jars after roasting. Jelly-sized canning jars will do, and the favorite is the 8 ounce Ball Wide-mouth type because it’s so much easier to get the coffee into it, and easier to clean.
And the basic home roaster’s dilemma isn’t figuring ways to keep a pound of coffee fresh for as long as possible. The home roaster is usually fighting off the temptation to allow coffee some time to rest and de-gas after roasting before consuming the preciously small batch.
For this reason, the home roaster doesn’t really need to consider seriously the many ways that coffee flavor degrades over time. But it seems useful to illustrate some of the basics of freshness, if for no other reason than to show the fragile nature of coffee flavor, and gain a greater respect for its complexity.
Chemical Basis of Freshness: The enemies of coffee flavor are open air (oxidation and dissipation of aromas), heat (coffee stored in warm environment), and moisture (high humidity especially). My dull and lifeless cup of Kenya is the perfect starting point, because East African coffees have an abundance of Aldehydes that account for their strongly attractive aromatics and fruit flavors. Aldehydes combine with acids during roasting to form aromatic Esters. Esters and aldehydes are the most volatile aromatic compounds, broken down by heat and moisture or oxidation/dissipation. Certain aromatics are diminished by 50% after ground coffee is left 15 minutes in open air!
The basic set of compounds responsible for perceived coffee freshness are a particular set of Sulpherous Mercapatans, and in particular Methyl Mercapatan. This chemical is lost rapidly through dissipation and oxidation, especially when ground coffee is left in open air in a humid environment. Lab results show noticeable loss in 24 hours, and up to 70% loss within 3 days.
Many compounds in coffee are perceived as positive in moderation, then as negative in higher concentrations. Furfuryl Mercapatan is one. At the very slight concentration of .01 to .5 parts per billion it communicates freshness to the senses, and any higher it is perceived as staleness! Such is the topsy-turvey world of coffee chemistry.
Phenols are sensed as spicy notes in darker roasted coffees, and especially noticeable in dark roasted Ethiopians or East Africans. Also in specific roast tastes you find Pyradines (smoky, ashy), Pyrazines (earthy, musty) and Pyrroles (smoky, roasty). Phenols evaporate easily. Pyrroles are more stable but are a negative taste sensation in greater concentrations. Pyrazines are highly volatile and dissipate easily. Pyrroles occur in the coffee oils and are subject to oxidation. The above compounds can be dramatically reduced or converted to negative flavors within 8-10 days of roasting, and much sooner under the influence of oxygen, heat and moisture.
In general, staling occurs as fats and oils are oxidized. As coffee takes on oxygen, Lipids are converted. The main undesirable compounds produced are Peroxides, and the more peroxides present the more will be produced…a sort of exponential regression into staleness. Also, fats are subect to aromatic taints, so keep coffee (both green and roasted) away from your onions.
De-gassing coffee is not immune to the effects of oxygen. It simply has less oxygen surrounding the coffee due to the presence of CO-2 gasses eminating from the fresh coffee. But it takes very little oxygen to stale coffee over time, so the key is to minimize exposure to oxygen, and minimize the time until the coffee is consumed.
Degradation of coffee flavor is inevitable, and pre-roasted coffee, whether it is packaged in a valve-bag or not, has inevitably lost flavor. So it makes sense that the simplest way to combat all the forces that ruin coffee quality over time is to reduce the time between roasting and consumption.
In addition, home roasters should store coffee in a re-sealable valve bag (because you can squeeze out air from the bag as you seal it) or a small airtight jar that approximately fits the size of the roast —not a large 1 Quart mason jar!
Keep coffee away from heat. This includes transferring the coffee out of the roast chamber when the roasters stops, then transfer it into a container when it is room temperature.
And keep coffee way from moisture. In humid environments, this may be difficult. But just remember that your small homeroast batch only has a short time from roaster to cup. Just think of the humilation a 2 lb bag of Starbucks suffers during a Southern summer, from purchase at a Costco or Sam’s Club (already old) to final consumption.
Home roasters are storing most of their coffee in green form. Green coffee is chemically stable and physically immune to the host of degrading forces. Green coffee is, by its nature, made to be stored, awaiting the moment in the roaster when its chemical compounds are converted by heat into the fresh coffee flavors we so value!
For more information on the chemistry of coffee: Espresso Coffee: the Chemistry of Quality by A. Illy, Etc.
Coffee Technology and Coffee Quality: Michael Sivetz
Much of the chemical references in this article from
A Question of Freshness: Paul Songer, SCAA Chronicle
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Indian Monsooned Robusta AA $5.20 $9.88 $22.62 $80.08
Mexican Washed Robusta $3.02 $5.74 $13.14 $46.51
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