Cryogenics & the Benefits of Freezing Green and Roasted Coffee | Interview with Christopher H. Hendon

Cryogenics & the Benefits of Freezing Green and Roasted Coffee | Interview with Christopher H. Hendon


Frozen coffee was in the limelight at the 2017 Global Coffee Expo in Seattle. Kyle Ramage became the U.S. Barista Champion using frozen beans, George Howell Coffee presented 2012 and 2013 coffee vintages during the Re;co Sensory Experience, and theoretical chemist Dr. Christopher H. Hendon took the stage at Re;co Symposium to give an engaging talk on cryogenics. Cryogenics refers to "the branch of physics concerned with the production of very low temperatures and the phenomena occurring at these temperatures," according to the Collins English Dictionary. Hendon's talk covered the facts and applications of cooling both green and roasted coffee beans and the results of his research were tangible at Expo—we are beginning to change the way we store, grind, and serve specialty coffee. 


For those not familiar with Hendon, he is the co-author of the ground breaking book Water for Coffee with Maxwell Colonna-Dashwood, and has been deemed Specialty Coffee's Resident Scientist by The Atlantic. Hendon is currently a Post Doctoral Scholar at the Massachusetts Institute of Technology and is about to take on the role of Assistant Professor of Chemistry at the University of Oregon. I am delighted Hendon took some time out following Expo to answer a few questions about the benefits and challenges of freezing coffee...


Christopher H. Hendon at Re;co Symposium. Photo Courtesy of  RGC Coffee

Christopher H. Hendon at Re;co Symposium. Photo Courtesy of RGC Coffee


tLBCC: What compelled you to do research on the influence of temperature on coffee?

Christopher H. Hendon: Initially, the interests were in studying the influence of burrs heating up, and their impact on particle size distributions produced by grinders. This is tangible, of course. In a busy store the grinder gets hot from usage, and inevitably the beverages taste different. We know from some proprietary research from Nuova Simonelli that at higher burr temperatures the number of fines is reduced. We can rationalize why that is: at high T the material is less brittle and doesn’t fracture into as much dust as it does at low temperature. Thus, a hot grinder might be expected to produce less surface area and hence the coffee will taste relatively under extracted (for the same set of given brew parameters). However, I couldn’t actively heat coffee nor the burrs and maintain those temperatures accurately. Instead, I decided to try cooling the beans, making the assumption that cold beans are the same as having cold burrs. Indeed, this is true. We found that not only does cooling of coffee result in a regular formation of the number of coffee particulates formed (i.e. ‘fines’) they are all more or less the same size.


tLBCC: What do you mean when you say coffee is already frozen?

Christopher H. Hendon: Freezing specifically refers to the phase transition between a liquid to a solid. We call conventional refrigeration units that cool to sub 0 ºC ‘freezers’, but that is only because water freezes at 0 ºC (at sea level and 1 atm of pressure). Thus, anything that is already a solid is already frozen. For example, Gold is a solid. It is frozen. We can heat gold to 1064 ºC and then observe a phase transition to liquid (melting). Upon cooling again below 1064 ºC, gold freezes again.

Coffee is a solid at room temperature and pressure. Sure it's releasing some smells (through a phase transition from solid to gas, sublimation), but it is a solid. Solids are by definition frozen. Now, not all materials will melt—some transition to gas directly, others simply decompose upon heating. Coffee does both of the latter.


tLBCC: For the sake of this interview can I continue using the term freezing coffee to mean decreasing the temperature of the beans—even though coffee beans are already technically frozen?

Christopher H. Hendon: Let's be clear that freezing here is used for convention, and refers specifically to the cooling of coffee to at least freezer temperatures—and the word freezing has nothing to do with a phase transition from liquid to solid.  


tLBCC: What is the benefit of freezing green coffee? 

Christopher H. Hendon: Everything happens a lot slower at low temperatures. Freezing green coffee will likely render the seed unable to germinate, but who cares? We roast it immediately after thawing anyway. Anyway, for every 10 ºC you cool something down, most of the processes occurring in the bean occur at half the rate. So cooling of coffee should prevent chemical reactions that occur over time (like staling or aging), by making them proceed extremely slowly. The other key here is that some processes require some input of energy to even get going. Often that happens at room temperature, but something like the staling process may never occur in a freezer if the coffee is stored appropriately.


tLBCC: Is there an ideal temperature and method to store green coffee? 

Christopher H. Hendon: This is a tricky question. The colder you go the longer things last. The Japanese do this with tuna caught offshore by taking the tuna down to dry ice temperatures extremely rapidly. Anyway, the point here is that green coffee contains some H2O, but not ‘reservoirs’. This means that H2O can come and go from the seed as it needs (based on relative humidity of the surroundings). Cold boxes, freezers, fridges, etc. are all moist areas because they are cooler than the atmosphere, so water condenses there. What you do not want is a humid atmosphere around a material that can absorb or release water. Hence, you need to seal the coffee either in grain pro or vacuum bags to prevent water from coming into the seed.


tLBCC: Wouldn’t freezing coffee be a costly endeavor for roasters or green coffee importers?

Christopher H. Hendon: Sort of. It costs about $0.05 USD/kg/month at an industrial storage facility. This price includes the cold transportation to the roasting location.


tLBCC: Are there any limitations to freezing green coffee?

Christopher H. Hendon:  Sure! Some coffee tastes good with a bit of age on it. In other words, if you freeze the coffee then you are going to take a photograph of the chemistry contained therein, and it won’t change. We will never know if freezing the coffee actually was premature and the coffee didn’t reach its full potential. This is unlikely, but you get the idea.


tLBCC: Can you share a little bit about the work the folks at George Howell Coffee are doing with their coffee vintages and what this might mean for the specialty industry moving forward? 

Christopher H. Hendon: George Howell has been freezing coffee since the mid 2000s. Their vintage coffees are kept frozen at an industrial cooling facility and stored at conventional freezer temperatures (to my knowledge). They then thaw out (note that it is not thawing, but rather reaching thermal equilibrium) the coffee a few days before roasting and then roast it. There are many upsides to having vintage coffees: Quality control from year to year,  demonstration of progression in agricultural techniques, preservation of exceptional coffees for year round excitement, blending becomes easier because the flavor profile doesn’t change dramatically, etc., etc., etc.

A side note is that if the coffee is cooled, warmed, and then left for about 3-4 weeks then roasted, the coffee begins to taste appreciably worse than immediately out of the freezer. The cells are dead and the bean is beginning to decompose.


Sensory Experience at Re;co Symposium Seattle featuring a tasting of three vintage George Howell Coffees:   2012 Guatemala La Esperanza, 2013 Kenya Mamuto AA & 2012 Ethiopia Borboya

Sensory Experience at Re;co Symposium Seattle featuring a tasting of three vintage George Howell Coffees: 

2012 Guatemala La Esperanza, 2013 Kenya Mamuto AA & 2012 Ethiopia Borboya


tLBCC: A few weeks ago Kyle Ramage became the 2017 U.S. Barista Champion. In his winning performance, Kyle referenced your work and used a coffee from Finca Nuguo Panama that he froze to -79° using dry ice. Can you tell us about your research in freezing roasted coffee for espresso preparation and your subsequent work with Kyle and Mahlkonig leading up to the U.S. Barista Championships?

Christopher H. Hendon: Mega congratulations to Kyle. The USBC is a hard competition to win and the work he put in is remarkable. The coffee he used was outstanding, and I think the difference in the coffee with the frozen aspect really kicked it up a level. Attached is the picture he used in the competition.




The summary is that there is a hip in the peak associated with number of fines produced by the grinder (the black lines). Cooling smoothes that out—meaning you can push extractions up without imparting as many flavors associated with under and over extraction (which occur simultaneously from the little fines over extracting and the slightly larger ones under extracting, relative).



Begin watching at 1:06:12 for Kyle Ramage's performance


tLBCC: What are the best practices for storing roasted coffee in the freezer? And how long does freshness stay intact in the freezer?

Christopher H. Hendon: This is also a bit of unknown territory. I think the best thing to do, again, is keep air away from the coffee (because air has H2O in it). Vacuum sealing works a treat. Conventional food grade vacuum sealing devices are cheap and the bags they use are really easy to work with.  


Vacuum sealed coffee for freezing. Photo courtesy of  Michael Cameron

Vacuum sealed coffee for freezing. Photo courtesy of Michael Cameron


tLBCC: Do you think freezing roasted coffee can have practical applications in the cafe?

Christopher H. Hendon: Absolutely. Decaf can be single serve frozen (like the picture below of single doses) and you’ll never waste a shot again. Similarly, you can dial in a coffee from frozen grinding, make it taste great and write that recipe on the coffee. The coffee shouldn’t change much over the time you have the it frozen. Perfect shots every time.  

Of course, work flow and cost are a consideration.


Single doses of vacuum sealed decaf coffee for freezing. Photo courtesy of  Michael Cameron  

Single doses of vacuum sealed decaf coffee for freezing. Photo courtesy of Michael Cameron 


tLBCC: As an avid consumer of specialty coffee, I foresee myself buying and wasting a lot less coffee now that I know I can preserve its freshness and flavor in the freezer. Do you have any thoughts on how consumer knowledge about preserving a roasted coffee’s freshness in the freezer might impact retail sales? Do you think utilizing freezing as a storage method could ultimately add value to roasted coffee? 

Christopher H. Hendon: I think that coffee is extremely valuable and roasters want their coffee to taste good every time the home user brews it. If the coffee shows a dramatic drop off after a few weeks, then the consumer will be unhappy with that. Sure, freezing prolongs the life of the coffee so they might not buy as frequently, but if it means your coffee is better every time then indeed you will prosper. Lots of things to think about here though I can’t really get into it without going down a rabbit hole. I think the key here is that if we up the quality and consistency of every cup, we all win.


Slide from Hendon's talk on Cryogenics at Re;co Symposium

Slide from Hendon's talk on Cryogenics at Re;co Symposium


tLBCC: Many specialty coffee pros have frowned on their moms, grandmas, and friends who have been storing their coffee beans in the freezer for years. Why do you think us specialty coffee professionals and enthusiasts get so easily caught up believing and spreading unsubstantiated myths? How can we do better?

Christopher H. Hendon: Most ‘normal’ folks who freeze things are terrible with their protocol. Lots of water exposure. Lots of poorly sealed bags. If that is the case, the coffee will uptake the smells produced by frozen fish sticks and other things…. not good. Simply being mindful is enough to not have these problems.


tLBCC: Is your freezer full of coffee? If so, what beans make the cut?

Christopher H. Hendon: Absolutely. I have coffee from all over the world that I acquire from trips like the SCA and the World of Coffee shows. What makes the cut? EVERYTHING. I love the diversity of my freezer—coffees ranging from Kyle’s competition winner, to a Mokka grown in Hawaii, to Ethiopians roasted Melbourne. The freezer will eventually be full… but until then its a party in there.

Connect with Christopher H. Hendon: website | twitter

Read our previous interview with Hendon: Coffee Chemistry & the Refractometer

Also, be sure to check out this article in the Atlantic: Specialty Coffee's Resident Scientist