On the "Features" section of this website is an article by Ken Fox describing an experiment comparing previously frozen coffee to fresh, never frozen coffee. It concluded that freezing was a reasonable way to preserve coffee intended for espresso use, for a period of at least two months.
There are, however, several potential criticisms: (1) at least some of the tasters were unknown to the online coffee community; (2) doses used for the shots were possibly too large, in the range of 18-20g for a double shot; (3) better espresso grinders might allow more subtle shot differences to appear; and (4), there was no cupping component.
Jim Schulman, although not present for the original experiment, was heavily involved in its design and analysis. He just completed a visit with Ken, in Idaho, and during this time Ken and Jim were able to do a follow up study that addresses these concerns.
This time around, the test coffee used was Sidamo Worka, a delicately fruity and floral coffee that was felt more likely to show any subtle damage inflicted by freezing. The roast profile used was shorter and lighter, completed in 15 minutes to a roast level terminated before the onset of 2nd crack. The frozen test coffees were frozen for a period of 3 3/4 and 1 month . The grinders used were two original Cimbali Max's, of mixed conical/planar design. The tasters were Jim Schulman and Ken Fox, both tasting half the paired shots and with the ability to separate out the results from these two tasters.
Test Design:
One day we compared the coffee frozen for almost 4 months with fresh, and the next day we compared the coffee frozen for only 1 month with fresh. The format was the same both days. First,
we cupped the coffees in a dual triangle test (1 cup fresh/never frozen, placed with two previously frozen and vice versa), and then immediately following we compared identically prepared shots from fresh coffee with those made from previously frozen coffee. All comparisons were single blinded, in that the taster did not know which of the two presented samples was which.
Dual Triangle Cupping
The test coffees that were frozen were packaged immediately after roasting into plastic valve bags (with the valve taped over), then placed into a very cold chest freezer at around -15F, where they reposed until defrosted in these bags couple of days before the comparison tests were conducted. The fresh-never frozen coffee had degassed for 4-5 days before it was used in the test.
Frozen and Fresh Test Coffees
Results:
In triangle cupping, neither Ken nor Jim was able to identify either odd cup. The roasts for this test was suitable for espresso, rather than cupping, so both the fresh and the frozen had aromatics more subdued than a lighter roast's. It could be that frozen coffee is more detectable in roasts suitable for brewing, but that was not the purpose of this test.
In the espresso comparison tasting, we did sixteen side by side comparisons with the older coffee vs. the fresh never frozen coffee, eight by Ken, and eight by Jim, using a factorial design that alternated the pair of machines and grinders between the fresh and frozen coffees in a balanced manner. We preferred the frozen coffee ten times, the fresh four times, with two ties. The results are suggestive, but not statistically significant, even under the severest statistical torture. On the 2nd day, following its own cupping, we did 8 similar comparisons with the coffee frozen for one month vs. the fresh-never frozen coffee. The fresh was preferred three times, the frozen three times, and there were two ties.
Blind Tasting Espresso Shots
There is absolutely nothing in the results that would suggest that the fresh coffee made better espresso.
Discussion
There are two possibilities that explain our slight preference for the frozen coffee, if indeed that preference was meaningful. The first is the more radical one that green coffee stored in a 50F basement degrades more rapidly than roasted then frozen coffee in a chest freezer. The second is that the flavor of frozen coffee, after being taken out of the freezer, peaks at a different rate than fresh coffee out of the roaster. We may have tasted when the frozen coffee was at its peak and the fresh was not.
But none of this affects the main conclusion, which is: Many things affect the taste of coffee. Freezing has an effect in the same order of magnitude as storing green coffee at room temperature, or of coffee resting in the first week after the roast. Since neither of these is regarded as particularly deleterious to the flavor of espresso, freezing should not be either. Freezing remains a viable method for the preservation of coffee roasted for espresso, for a period of at least 4 months.
Ken Fox and Jim Schulman
