A few weeks back, I unintentionally breached the Serious Eats code of taste-testing with my post about whether or not to freeze coffee beans. In retrospect I realized I didn't test a broad enough series of variables to really get down to the nitty-gritty facts.
Mea culpa, Eaters. I hereby pledge never to take your love of absolute food truths for granted again!
So with the help of Kenji I concocted a new series of samples (eight in total) and hosted a blind tasting at Serious Eats Headquarters in an attempt to finally get to the bottom of the beans.
The table was littered with tiny paper cups numbered one through eight, each representing a different method for storing coffee beans:
- 1. Whole beans stored at room temperature in a Ziploc bag (Ziploc bags are not hermetically sealed—air can still escape and enter the bag)
- 2. Whole beans stored at room temperature in a one-way valve bag (from which CO2 can escape but stale-making air can't get in)
- 3 and 4. The same beans stored in the freezer
- 4, 5, 6, and 7. Ground coffee stored in the same 4 manners
The grinds and whole beans all came from the same batch. The coffee was stored for two weeks before we cracked it out, to get the full effect. (It was Ecco Caffe's lovely El Tambor from Guatemala, roasted on September 17, measured and stored on September 19, and tasted on October 5.)
Seven Serious Eaters put themselves at risk of caffeine overdose for the experiment, and I prepared all the samples the same way: Each batch of beans weighed 24 grams and was brewed using 12 ounces of 30-seconds-off-boil water poured in two 6-ounce increments through Bonmac coffee drippers.
Each taster was given a form on which to rank the coffees' flavor, acidity (or fruitlike brightness), body, and aftertaste from one to ten, leaving comments about what stood out from each cup. At first, we compared samples in pairs while they were hot, revisiting the collection as a whole once the cups had all become lukewarm.
The results were reassuring, since they closely mirrored what we had found in the first test, and demonstrated remarkably clear trends.
As you can see from the graph, the overriding factor in coffee quality is storing at room temperature versus freezing, with room temp beans scoring a good point or two higher on average than frozen beans. The valve bags, with their hermetic seal also produced better tasting cups than the Ziploc bags, which allow air to breathe in and out (thin plastics are air-permeable). The difference between whole beans and preground beans stored at room temperature was actually minimal.
Valve-sealed bag produced the best-tasting cups of both whole bean ("bright and balanced") and preground coffee ("smooth—no weird aftertaste").
The poorest performance was from frozen whole beans in a Ziploc bag: "Bitter, like nail-polish remover," one taster commented. Not really what something you'd want to wake up to.
Fear the Freeze
So now we have the benefit of impartial opinions from several Serious coffee drinkers, but the question remains: Why is freezing so bad for your cup quality? As I said in my original post, freezers and refrigerators freeze moisture in order to prevent decay and mold, but the beans' internal moisture content is pivotal to the way they retain and translate their flavor. Temperature fluctuations (caused, for instance, by opening and closing the ice box) can cause ice crystals to constantly form and defrost, throwing that yin and yang out of balance.
But the most detrimental effect is likely the sweating caused by the thaw after being removed from the freezer. Brewed coffee has two ingredients: coffee and water. When coffee beans come in contact with water, the liquid begins to absorb some of the solubles that normally wind up in your finished morning cup. Messing with the dynamic interaction of water and coffee (in this case caused by the condensation as they thaw) is one of the easiest ways to end up with a subpar, bitter mug.
In the end, I think it's safe to repeat the earlier recommendation for ideal storage: Buy beans fresh in small batches, store them whole or freshly ground at room temperature in a sealed valve-bag, and drink them quickly for the most delicious results.