“The question was always how does temperature affect particle size in grinding. What you conclude from the data is up to you”.
— Christopher Hendon
I have a lot to thank Christopher Hendon for. In my writing, he has given me fair, balanced, and valid feedback from low pressure and tamp force, to grinds temperature. He has encouraged my pursuit to improve my coffee through science, even going so far as to collaborate with me on a peer reviewed paper. He has patiently fielded my questions on the mineral content of my water along with my filtration solutions at work. In general he has been incredibly supportive — even with my persistent badgering.
So when he asked if I wanted to try something cool, of course I agreed. And so, around a year ago, trying something cool I started freezing beans at home. And I haven’t looked back since.
Hendon’s paper on grinding physics was the starting point. This peer reviewed paper concluded all coffee statistically shattered in the same way — independent of processing method, degree of roast, and origin of bean — and cold coffee makes a narrower fines spread. Hendon also found while the particle distribution of the grind profile became narrower as the beans cooled the mode, or most occurring value, simultaneously decreased. I’ve been team “we-extract-from-fines,-boulders-are-just-structural” for a while now; a narrower fines spread for me meant greater extraction within my usual espresso parameters, as I’d now be able to extract from a larger amount of even-sized fines.This was the exact opposite of what I was combating with the Mythos, which required a different strategy to mitigate the opposing effect of heat.
With the Mythos, not only was the friction from the burrs creating heat that broadened the fines spread, this expansion occurred in an unpredictable manner. I was getting less fines as the temperature of the grinds increased, with these fines becoming increasingly larger. To successfully and consistently pull great shots, I needed to rely on calibrating grind changes to my beverage mass, while keeping the volume of water used per shot constant. In essence I was using volumetrics and scales combined, and letting shot times take care of themselves. While shot times themselves are important in defining flavour in the cup — solubility of certain flavours still requires time — I couldn’t rely on time calibrated to grind changes defining my beverage mass. I needed a different compromise.
While the Mythos constrained grind temperatures within 35°C and 55°C, this temperature difference still resulted in shot times ranging from 28 to 40 seconds at the highest grinds temperature. From the zero point on my grinder, I’d dial finer as the temperature increased, then dial back to the zero point, continue coarser, before returning back to my zero point at peak heat. Shot times fluctuated via grind changes calibrated to keep my beverage mass constant, while keeping volume of water constant too.
I was relying on changes in the micro grind profile caused by temperature to manage my beverage mass — I was not relying on changes in my shot time to manage my macro grind setting. Within that timeframe this still allowed me to make tasty coffee, and this is how we continue to operate at The Frisky Goat. At high volume, heat will always be a constant given current grinder technology. But freezing beans offered an opportunity to be rid of the inherent variance caused by high temperature fluctuations. Along with the benefits of enabling more fines, a consistent temperature also offered me a consistent grind profile.
Freezing beans offered other benefits as well. Freezing food products has universally been seen as a means of preservation. So by freezing beans, I’m now able to keep a large inventory of beans on hand, without worrying about immediate staling. Combining my beans at home and at work, I’ve around 30 different single origins at last count. My wife now simply accepts we have no room in the freezer for peas. Before freezing, I rest the beans as I would when traditionally storing them, waiting 7 – 10 days post-roast. All my single origins both at home and at work are stored in the freezer until I need them.
I don’t have any actual scientific evidence for this time period beyond my own empirical observations, and experiences during my career. I froze beans 2 – 3 days post-roast, and weeks later when grinding them for the first time, they tasted like beans 2 — 3 days post-roast: roasty, gassy, and lacking in depth and flavour. I’d love to research further the effects of de-gassing combined with freezing, but the preservation aspects of freezing lead me to at least argue that what state the beans are frozen in, will be close to the state they are ground at when removed from the freezer. For me I’ve consistently achieved better results waiting 7 — 10 days post-roast before freezing.
However, the state they’re placed in the freezer is an important aspect of freezing beans. Once rested, I remove from the original bag, place in a commercial heat grade vacuum food seal bag, vacuum seal, then freeze. Theoretically, the beans should be in an oxygen free environment within any bag sealed with a one-way valve, but it’s not something I’ve found to be consistent. Some beans I’ve just thrown into the freezer after resting still in the original unopened bag they came in. I’ve opened the bag once, days or weeks later, pulled some shots, and been amazed by the flavour. I’ve then transferred the beans to a freezer bag and vacuum sealed, then pulled shots days or weeks after that point. These second shots have always tasted different; not bad, just different. I want a consistent flavour that’s great, not consistently great flavours. As with so many other aspects of coffee, if you want consistency, you need to be consistent in your practices.
But I’ve found vacuum sealing to be the most important aspect of freezing beans. You need to have an oxygen and moisture free environment for your beans in the freezer. Oxygen will lead to degradation of the beans over time, and allow the beans to pick up unwanted flavours from whatever else you’ve stored in the freezer, while moisture can turn into ice crystals as the beans freeze — and water on your burrs as the ice crystals instantly defrost is not a good idea. You can’t just store your beans in a container with a lid on it in the freezer, or in the original resealable bag. Investment in a vacuum sealer is crucial. A cheap version like this has served me admirably, both at home and at work. Vacuuming out oxygen, nitro flushing, then sealing and freezing would be the pinnacle for me, but adds the extra burden of figuring out nitro gas bottles. The guiding principle here is whatever solution you choose, it needs to result in an oxygen and moisture free environment for the beans while frozen.
At home I only really make coffee on the weekends when I’m not working. So I have no need to individually measure and seal doses. I simply open the bag, weigh out my dose, then re-vacuum seal the bag and re-freeze (this ain’t chicken, you can re-freeze). This isn’t sustainable in a busy commercial environment. I make this system work with some of the single origins we have, the under the counter specials that I’ll serve up to our regulars who just want “whatever’s good today”. But for our feature single origin each week I need to have them dosed out already, to enable a workflow that works under high volume.
This part isn’t easy, and I need a better solution. I have a good feeling now for what Colin went through here, only I’ve got to make the fucking bags first. A guillotine, heat sealer, and willing staff have made that job a little easier. We purchase a roll of vacuum sealing bags, cut them into small 5cm x 14cm bags, seal up three of the four sides, then weigh, dose, vacuum seal, and freeze the beans. I treat this task the same as I do with stamping cups, putting away the milk order, or doing dishes — do it first with my staff to show them how this job can be done with dignity … then quietly write it on our prep list for them to do after I head home for the day. Management ain’t a democracy; at best it’s a benign dictatorship.
While this costs a certain amount in labour, there’s also an environmental price to be paid. I’m not a fan of spending this much time making those bags, only to tear it open, pull the shot, then throw the bag away. A more environmentally friendly process is required here, but also one that either allows for vacuum sealing or nitro flushing, or both, and then easy storage and trouble-free freezing. I haven’t found it yet, but I’m optimistic I’ll figure it out.
The workflow itself is relatively simple. Having found a new home for our banana bread in the milk fridge, and I swapped over the direction of the doors on our domestic fridge/freezer combo (which is actually way more of a pain in the ass than you’d think), and started stacking beans in. The fridge/freezer is about three steps away from where I’d stand pulling shots. Not ideal for a super-fast workflow, but close enough to grab a bag in a hurry. The bags are sorted in the freezer with decaf (frozen too, and ground through the EK) and the weeks feature single origin, placed to be easily grabbed in hurry. The bags need to be cut open so scissors are kept nearby, same ones I use to open the kilo bags for our main blend. I grab the bag from the freezer, cut open quickly, dose into the EK hopper, turn the EK on, grind the beans, weigh, dose, distribute, Puk-tamp, then pull the shot. While it may sound cumbersome, I have far more confidence in the output of this workflow than any other system I’ve used. Freezing beans in a commercial environment has given me absolute confidence in the shot I pull.
The consistency of the fines spread from frozen beans is for me one of the biggest reasons for freezing beans in a commercial environment, let alone at home. As the temperature is controlled, the shatter transition is predictable. The temperature of our freezer is set to around minus 18°C, which I periodically check with a temperature gun (as with my milk fridge, then recording them all — and so should you!). Checking this temperature again after grinding, consistently gives me a grinds temperature of around 15 – 20°C. This all translates to a consistent dial in. 1.7 on the EK dial this week will be 1.7 on the dial in a month’s time — and 1.7 on the dial in three month’s time (assuming your burrs don’t need cleaning or aligning, which realistically they don’t that often). Our routine set up involves recording the best recipe and dial setting when dialling in, which we simply consult whenever pulling shots from any particular single origin from the freezer. I target a 1:2 ratio of 19g in and 38g out from a 20g VST ridgeless basket, pulled within 28 – 30 seconds, with my brew water temp set to 90°C, pump pressure set at 3 bars, and a controlled tamp force of 11kg, I have a starting point to dial in. Based on the initial appraisal of two or three shots, I’ll decide on the beverage mass I want, balancing the shot time and the grind setting needed to achieve it, and then start going up in bar pressure if I feel it needs it.
What I’m able to achieve through freezing beans is firstly a consistent grinds temperature, giving me a consistent grind profile, which leads to consistent resistance, therefore a consistent flow rate. With some experimentation I’m able to quickly find the optimum recipe for our beans, and then serve it with confidence, knowing it’ll taste as it did last week or last month. You don’t need to piss about pulling three or four shots hoping one of them will taste good. One shot. One kill. Confidently served from your fine selection of beans sitting vacuum sealed in the freezer.
This means with temperature accounted for, if you have your pump pressure lowered, your tamp force set, burrs cleaned and aligned, water and filtration solutions optimised, along with solid technique, and a machine that delivers accuracy and consistency — you can finally get down to the business of appraising beans for their own inherent quality, with a clear view to what that actually is. This is why I freeze beans: overall variance is minimised, while inherent flavour is optimised.
But I still have unanswered questions. If you’re simply using freezing as a means to store beans, then defrosting before pulling shots, I have no answers for you on what effect defrosting will have on those beans. I don’t have solid scientific evidence for the effects of oxidisation increase after defrosting, or if it stays the same relative to not being frozen, or if there are any other possible effects. At home my beans are stored and stay in the freezer until I pull a shot, the bag re-vacuum-sealed and re-frozen. At work before I purchased a second vacuum sealer, I would store my single origins in the freezer in their original 250g bags, take one out when needed, and dose out the entire bag into containers on bar pre-service. This worked, but was vastly inferior to grinding straight from the freezer in terms of flavour and consistency.
How long the beans last for in the freezer is another question I can’t answer. So far I’ve pulled shots from beans that have been frozen for nine months, with spectacular results. Whether this was simply due to me improving my water situation, new burrs on the grinder, or something else, I don’t know. Almost every shot pulled, without exception, from beans frozen longer than six months, have tasted amazing. They give a clarity I can’t achieve with “younger” frozen beans. On aggregate I’m confident in saying you’re not going to get any degradation in flavour from freezing beans for long periods of time.
This hasn’t just been isolated to shots pulled on the Opera at work using the EK43 either. I have a Gina Rossi RR45, manufactured in 1992, pulled on my Rocket R58 set to 5.5 bars, using an Eazytamp 5 Star set to 10kg. This grinder and set up has given me some of the best shots I’ve tasted this year. Mind you a lot of these shots have come from the Barista Hustle subscription, (I love filter beans as espresso!) but I’ve also pulled a huge amount of tasty shots from local roasters both here in Brisbane, nationally, and abroad.
Christopher Hendon is right to explore the fundamentals, present his findings, and leave it to others to conclude what they will from the data. That is the job of the scientist. But man, when he talks I listen, and so far his talking has taken my coffee to stratospheric heights. As long as you follow some basic rules — rest your beans, vacuum seal or nitro flush, single dose and grind while frozen, using good quality input (beans, water, machinery, technique) — you too should benefit from freezing beans.
Now get sealing and get freezing.