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Coffee: To Freeze or Not to Freeze
Does freezing preserve coffee used for espresso?
What exactly are we to make of this experiment and these results?
There are several obvious conclusions which I will list below, and
endless speculation which I will leave to the readers of
Two months is safe: Freshly roasted coffee that is immediately
frozen after roasting in a near airtight container in a very cold
freezer, can be kept undisturbed in the freezer for at least 2 months
and be expected to produce espressos that are not obviously
inferior to those made from fresh coffee that has never been frozen.
Freezing does not accelerate staling after defrosting: At
least over a period of time extending to about 8 days after roasting,
using the roasting and freezing procedure used here, there was no
evidence that previously frozen coffee deteriorates more quickly after
defrosting than does coffee that has never been frozen.
This study was primarily designed to evaluate freezing as a method of
coffee preservation available to the average home consumer. As such, it
has demonstrated that freezing, done shortly after roasting in a very
cold freezer delays staling for at least two months and hence extends
shelf life for at least that long.
Two aspects of this study could present difficulties for some readers
and they deserve clarification. First, the coffee was frozen immediately
after roasting, and those who are reliant on parcel delivery services or
who cannot buy just-out-of-the-roaster coffee must compromise on that.
The second is that not all freezers, especially freezer compartments of
refrigerators, can maintain very cold temperatures. Self-defrosting
freezers are especially problematic in that regard. Nonetheless, I chose
to use a "best case scenario," that is, immediate freezing in a very
cold freezer, to study the impact of freezing for coffee preservation.
Had I chosen otherwise and had results been different, we would not have
known whether the compromised results were the result of delay in
freezing or inadequately cold freezer temperatures. The data in this
study can be augmented by reports of readers who have used less rigorous
approaches, and I solicit their findings in the comment thread following
Jim Schulman has previously communicated his results of some
informal coffee "cupping" experiments he has done with frozen coffee.
Cupping is of course a far different process than is espresso making and
some would say that one can have more precision in cupping than one can
have in judging coffee served as espresso. Jim reported that although he
could tell previously frozen from never frozen coffee more often than if
by chance, the differences were subtle and defied characterization along
the lines of anything "systematic" or "easily describable." He felt he
was no better in discriminating between frozen and never frozen than he
would be in detecting subtle differences between different roast batches
of the same coffee, neither of which had ever been frozen. I should add
that Jim does not have a freezer that holds very cold temperatures like
mine does, however he did freeze his coffee immediately after he roasted
Jim's observations imply that the observations in this study can be
extrapolated to include coffee that is frozen and used in preparations
other than espresso. This would require further efforts to prove, should
someone be sufficiently motivated to try to test it.
Conclusions not related to freezing process
The presence of more than one espresso machine and more than one
grinder introduced other potential variation into the study that was
controlled for by proper "balancing" within the experimental design.
Because the study was balanced in this way, it enables us to look at
these other variables independently to learn other things from this
experiment. Below are some conclusions reached as result of further data
analysis that includes these factors.
Old versus new grinder burrs: There is no obvious, tastable
benefit from changing grinder burrs after moderate wear as was done on
one of the two grinders used in this experiment. Some have speculated
that changing planar grinder burrs much more frequently than is the
stated life of those burrs would produce a benefit in espresso.
We did not detect any such difference and therefore believe that very
frequent burr changes, much earlier than the stated lifespan of the
burrs, has no detectable benefit if taste is what is being measured.
Rotary versus vibratory pumps: This study is the third
independent experiment which compares the output of two nearly
identical espresso machines that differ primarily by the type of pump
within them, one with a vibratory pump and the other with a rotary
pump. As in the other two studies,
no consistently tastable difference was
detected. Therefore, all suppositions that rotary pumps produce
superior shots to those made by vibratory pumps must be regarded as
unproven barring further experimentation.
Tasters do have measurable preferences: There was only one
obvious interaction among any of these variables, which was that one
taster, Randy, rated the coffee that had previously been frozen for 4
weeks with lower scores when he tasted it on his second session, 3 days
after the first. In addition, there were several uninterpretable ones
including such things as one taster liking one grinder/machine/coffee
combination more than others. The effects are "obvious" in a numerical
sense, which is why they are significant. However, they have no bearing
on the overall result. As such they simply make the point that on some
very specific combinations of machines, grinders, and coffees, some
tasters will have idiosyncratic preferences. The whole point of using
multiple tasters is to cancel such idiosyncrasies out. However, the
presence of interaction effects disallow blanket statements such as "No
taster will ever detect a difference regardless of machine, grinder and
coffee being frozen." They prove there is a difference between fresh
and frozen that comes out in rare and idiosyncratic circumstances. This
observation in no way counteracts the fact that the overall results
offer no evidence whatsoever that there is a systematic taste
The wording of the last point is taken directly from Jim Schulman's
commentary on the data analysis, for which I am grateful and without
which this study could not have been done.
Finally, some reviewers of earlier versions of this manuscript raised
the issue of extraction percentages and how this might have effected the
results. It is true that the shots tasted in this study tended towards
the ristretto, and the double baskets were "overdosed" with
approximately 18 grams of ground coffee. Because this was done for all 128
shots tasted (64 shot pairs), in order to invalidate the conclusions one
would need to come up with a hypothesis explaining how grinder fineness
and dosing are interrelated with freezing. I cannot produce a feasible
explanation of why this would be the case, and no matter how the shots
were prepared some readers would feel it was not representative of THEIR
own approach. The most important thing is that the preparation was held
constant, so the impact of the technique I used should cancel itself out
in the observed results.
Maximizing your coffee's usable lifespan
Freezing is a viable method of preserving the freshness of very fresh
coffee. Exactly how long the usable lifespan of coffee can be extended
with freezing is unknown, although we do know that if frozen immediately
that lifespan extends to at least 8 weeks. In this experiment, a very
specific methodology was employed, and exactly how far one can deviate
from it and expect to get good results is unclear. The previously frozen
coffee we used was frozen immediately after roasting, within about an
hour, in semi-airtight packaging in a very cold freezer (about -15°F
/ -26°C). It was then defrosted, only once, within the same
packaging before it was exposed to outside air, reducing or eliminating
the possibility of condensation.
If the coffee one contemplates freezing is not "fresh" to begin with,
it is doubtful that freezing will do much of anything positive. To me
this means that freezing is probably of no value when dealing with
purchased coffee of uncertain age. In the case of coffee that has
partially degassed, that is perhaps several days out of the roaster, it
is unclear from this experiment whether freezing will extend shelf life
significantly. Since this is going to be a fairly common scenario for
home espresso enthusiasts ordering online, it deserves further comment.
For the homeroaster or person who can buy fine quality extremely
fresh roasted coffee locally, it makes no sense to degass the coffee at
all before freezing as this adds nothing and the coffee can be degassed
later, after defrosting. On the other hand, if you are buying roasted
coffee that has to be shipped to your door, then you will need to test
for yourselves how well this works with the beans that you purchase. My
opinion, which is not supported by any data (since I have none) is that
freezing fresh coffee that is several days out of the roaster should
extend shelf life by at least a few weeks. I would encourage readers who
use this approach to give us the benefit of their own experiences in
comments you make on the thread that follows this article. Similarly, if
you don't have a very cold freezer, it stands to reason that the amount
of time that freshness in coffee can be preserved will be less. How much
less I do not know, but perhaps some readers will have their own
observations that will prove useful.
If you are concerned about what sort of container you should use for
freezing coffee, it obviously needs to be something that is relatively
airtight and that can tolerate the conditions present in a freezer, and
the temperature stress in going from room temperature to very cold and
back again to room temperature. I generally use Mason type canning jars
or recycled jars from grocery products that will close with a tight
seal; I fill them up as full as possible to minimize the remaining air
that is present. I have also used certain types of commercial plastic
coffee bags that can be sealed and if valves are present I tape over
them. If you purchase coffee that is already packaged in a sturdy valve
bag you could simply tape over the valve and toss it directly into the
freezer. I would however suggest that whatever container you choose, it
be sized to allow you to consume all of the contents within a reasonable
period, say 1 week, without having to open the bag and return some of
the contents to the freezer; doing so risks condensation on the beans
which could theoretically cause damage.