I get this one at least several times per week – how should I store my coffee?
For me, the answer has been obvious for a long time. You should store your coffee, well packaged, in the freezer. My answer was the result of my own empirical observations; my coffee tastes better that way. I’ve tried it many different ways, and many times have wound up (inadvertently) with the same batch of beans, some stored in the freezer and some on the counter. I usually drink espresso, and my observation is that the freezer beans require a grinder setting close to what is needed for fresh, while the counter beans need a significantly finer grind to result in an acceptable pull. Then there is the taste – the frozen ones just taste better.
Often, this conversation tails off in the comment from the customer, “I’ve heard so many things I’m not sure what to do”. And this is after I just answered the question – they are still not sure! And so I realized that my answer to this question to date has been weak. Sure I have lots of anecdotal data that leads me to believe the freezer is correct, but where is the rigorous data, and the understanding of the underlying mechanisms? In my life as an engineer and scientist, my little bush league experiments would have been viewed with great skepticism, as well they should. So I decided to do what I should have done a long time ago – read the academic literature on the subject and make an informed conclusion based on data and peer-reviewed publications.
Turns out, perhaps unsurprisingly, that this is a pretty well-studied topic. I had a bunch of papers to wade through, and they all cost money to read. So I decided to read all the abstracts and rely on the papers that made their core arguments in fields where I’m competent. That ruled out the chemistry based papers – while I can follow along well enough on those, I lack the deep domain expertise in that subject that would allow me to critically assess what the authors were saying. Instead, I would up focusing on papers that relied on statistics and material science, fields where I do have domain expertise.
One paper in particular I found to be very illuminating: Cardelli and Labuza, Application of Weibull Hazard Analysis to the Determination of the Shelf Life of Roasted and Ground Coffee, published in Food Science and Technology, Volume 34, Issue 5, August 2001, pages 273-278. The Cardelli paper attracted me because I made extensive use of Weibull analysis for my master’s thesis. I refreshed my understanding of Weibull Hazard Analysis specifically with Bryan Dodson’s excellent book, The Weibull Analysis Handbook. I have the first edition; I’m sure there are subsequent updates. I also read some papers from a person whose work I followed in a parallel life – Constantin Yiannoutsos, Professor of Medicine at Indiana Univerity Medical School. Dr. Yiannoutsos’ research includes changes in survival risk over time for patients with HIV and cancer; certainly more serious stuff than figuring out when your coffee will go stale, but analytically it’s the same procedure. I pulled one of the references in the Cardelli paper, Dugle, J., Note on Experts Versus Consumers: a Comparison, Journal of Sensory Studies, 12, 147-153 (1996). Finally, I came across a nice masters’ thesis (in addition to my own!) from Ross Larsen at BYU titled “Food Shelf Life: Estimation and Experimental Design” submitted for his degree completion in 2006; Larsen used SAS for his analysis, and as a SAS programmer myself I was very comfortable with his methods and results.
The Cardelli methodology was to grind the coffee (Colombian Arabica was the most specific description of the coffee), then have a panel of taste testers try the coffee at t=0 and then again at subsequent time points. Tasters were equally male and female, and the inclusion criterion was that they be habitual black coffee drinkers (I didn’t realize there were that many left out there!). It is fair to question whether non-professional consumers can accurately rate the coffees, at least I thought it was a fair question. Turns out that is the subject of the Dugle paper, which concluded that your taste buds are just as good as mine, on average. Certainly I know I have many discerning customers, so this was no surprise to me.
In Cardelli, storage conditions were rigorously controlled: six O2 partial pressures, three water activity levels (loosely, humidity) and three temperatures were used (some other parameters were controlled, as well).
I wish I could reproduce some of the graphs here to illustrate the points I’m about to make, but I don’t have that permission; I emailed to see if I can get permission. See me at the shop if you want to read the paper.
The first important takeaway was that this paper was written for industrial coffee producers – people who vacuum seal coffee in cans. The most important of the controlled conditions, O2 partial pressure, is important in determining shelf life only when controlled at levels below 5 kPa. O2 partial pressure at sea level is about 21 kPa. In other words, home stored roasted/ground coffee has no chance whatsoever of lasting beyond about 7 weeks, and that would be well packaged and frozen. On the counter in the summer you’re looking at a week.
The variables you can control are exposure to water, and temperature. Both are extremely important. On humidity the best you can do is package the coffee in a high vapor barrier bag or jar. On temperature, I quote Cardelli, “storage of an open can in the freezer would increase shelf life over that at room temperature by about 70%”.
When a coffee goes “stale” what’s actually happening is oxidation of lipids. Lipid oxidation has a high temperature dependence. Diffusion controls the rate at which O2 can diffuse to the lipids. When temperature is increased, the amount of oxygen available for reaction is what limits the reaction rate, hence low O2 partial pressure is advantageous.
The significance of the Larsen analysis is that it takes a second look at the Cardelli analysis. Without getting into the technical details, I found several aspects of the Cardelli analysis (his actual statistical analysis, the math itself) to be a little on the… let’s say “weak” side. Larsen takes another run at it much the same way that I would, and concludes that maybe Cardelli overstimates shelf life. This may not be terribly surprising given that Cardelli is employed by Big Food.
Like any good piece of academic research, this work raises many questions. One of the big ones is the difference between whole bean and ground coffee. A later paper Flavour and Fragrance entitled “Coffee Flavour: An Overview” proposes that CO2 diffusion is more rapid in ground coffee due to glass transition… I’m not even sure that I support the notion that coffee exists in a glassy state and that it has a glass transition temperature over the normal ranges of storage and use.
But you want the bottom line – what does all this mean to the home user? What should your takeaways be from all this?
1. You have no realistic hope of keeping coffee fresh beyond about 10 weeks, no matter what you do. And that is the absolute upper limit. About the most you can realistically hope for given the data I’ve seen is maybe four weeks if all other conditions are exactly right.
2. Buy whole beans and grind when needed. O2 has to diffuse further in whole beans than it does on ground coffee to oxidize the lipids, so whole bean will last longer (disclaimer: this is my logical leap from the data, it is not explicitly proven by the studies I’ve read. But it makes sense.)
3. Store your beans in bags or jars with high vapor barrier – ziploc, mason jars, etc.
4. Keep your well-protected beans in the freezer.
5. Buy your coffee freshly roasted every week or every other week.
For me, the question is now definitively answered, with data published in peer-reviewed journals. And the published results match my own experience. I hope you are convinced, too.