Prufrocker: The Blog

Re-Hulling

Answering a need for cleaner coffee and advancing of existing techniques.

by  |   |  Coffee Science


Petra Strelecka re-hulling

Part one: Perforated Plastic Egg Jackets

When it comes to coffee innovation, I use a bench mark to put new ideas up against. I encountered an inventor around the Christmas of 2005 in an East London squat party. He was a rough sleeper most of the year and would come indoors in the heart of winter when possible. This man was certainly not always rational but had intense moments of conviction and came up with a lot of design ideas.

He told me he often corresponded with Virgin Enterprises regarding his inventions and one idea he pitched to them was a machine that would peel uncooked eggs and replace the shell with a perforated plastic wrapper. And I argued with him, suggesting people don’t like plastic; they think it’s unnatural. He was adamant that cutting the wastage in broken eggs would save millions. Apparently and to Virgin’s credit, they wrote back and thanked him for his suggestion but gave him the knock back.

I argued with him that his idea was rejected because one reason people love eggs is they love the shell; love hearing it crack. It’s a remarkably strong structure too. It can withstand the weight on an entire chicken. But he was focussed on hygiene, efficiency, cutting wastage. His idea was all head and not enough heart and so I remember this guy when I’m coming up with ideas and I concede, his was the voice of progress! And progress is something difficult to find support for in the world of grinder design and competition sponsorship.

Me and my dear friend and two time Czech Brewer’s cup champion Petra Strelecka have made it no secret that we are opposed to the grassy roasting flavours imparted by excessive silverskin in the cup. Manually stripping all the silverskin from the beans centre cut in the 2014 World Brewer’s Cup was one solution: 45min per 15g! We looked for a way to solve the impracticalities highlighted by Petra’s judging panel from last year where the lack of a ‘real world service’ had incurred some points deductions in training for the 2015 Worlds.

In Petra’s national championships, she spurred us on with the comment, ‘a lot of work has already gone into improving this coffee by the farmer and the roaster, so I wanted to have my chance to contribute.’ This was a notion from the heart.

Part 2: Working backwards, let me introduce you to the notion of re-grinding.

Prufrock had already made some headway on this issue leading into the Czech comp. One of Petra’s rivals to the Czech Brewers title, Prufrocker Hana Dunajova helped us pioneer this idea which she took to the Czech Brewer’s Cup and got to 4th place. She had taken inspiration from Petra’s performance from the year before and made it more efficient, albeit a touch less comprehensive. This was the premise:

The basis of re-grinding is that first you manually pre-break your coffee on your grinder’s coarsest setting (actually, it’s probably better to go coarser than its coarsest setting but this isn’t that easy. It requires grinder recalibration for most bag grinders.), which sends through the grinds at an incredible speed which seems to generate more static effect than the normal rate of grinding and this means a lot more chaff gets stuck inside. It looks like about 80% less silver skin comes through with an EK43 on grind setting 22. The idea of course is that you don’t thwack the thwacker so be prepared to lose some portion of your grinds but the majority of this is chaff and fines..

Note: The auger in a grinder like an EK or a Bunn G2 or a Mythos 1 for that matter (the twirly thing that draws the whole beans into the burrs) does some pre-breaking of the beans for you and this makes it easier for the blades to get purchase on the beans. The beans feed in better. And the pre-breaking presents a particularly opportune moment to get at the silver skin before it is smashed to pieces through the burrs. But current grinder design much like the World Brewer’s Cup rules are based on real world efficiency for the mass market.

Then all that remains is to remove the residue, then grind on your target setting. Adam Obratil, co-owner of Industra coffee in Brno brought a Henry Hoover backstage at the Worlds for this purpose which was fantastically funny to see. And he commented that there was an excess of coffee caught in the blades. The real solution to this is to make your coarsest grind coarser which is risky. The ideal would be to have one grinder calibrated for pre-breaking and then another for target grinds. This is about grinder development; something that forcing everyone at the world competition to use the same grinder will not at all facilitate…but fortunately, there are other exciting coffee making opportunities in life, including this fantastic competition (;D)

I’m going to theorise that the pre-breaking process reduces the overall fines count as I expect the smooth outsides of the beans are scraped and abraded before being drawn into the blades, so breaking the beans into quarters or tenths makes sense. Also if you manually pre-break, the static is also collecting a quantity of fines along with silverskin in the grinder throat so this is an interesting opportunity for anyone to correct problems of under extraction if you’re stuck with some older equipment or worn burrs as you’ll be able to grind finer overall with fewer fines making fewer blockages.

Some evidence of this reduction of fines beyond hand sieving samples is that we have observed espresso shot times decrease after manual pre-breaking even without sieving the pre-broken portion. More blogging about this to follow. We’re doing some pre-broken experiment shots at our Oakley Pop-up if you want to give it a taste. Really it’s obvious. Consider each cut of a saw gives you saw dust so the saw dust produced during the first phase of grinding is not getting through to the cup quite as much, as it is trapped by static in the grinder throat or indeed, you can sieve it out. The ‘real world’ practical side of this process is that you can do it in bulk. You can pre-break an entire hopper’s worth before the start of service. You can sieve it and use static to reduce the chaff. You can reduce the amount of torque required to get your grinder moving and you should see an improvement in your grinders volumetric timed dosing too as the beans are feeding in fragment by fragment rather than bean by bean. We’ll report on this. But beyond this, you can hull it…again…

Part 3: Next step is the entertaining process of re-hulling.

Petra happens to have an enviably perfect piece of Tupperware for this purpose from her mum. It’s an hand powered food processor with a kind of blobby thing shaped like an oar, perfectly suited to spinning coffee around. The insides of coffee beans are rather protected, in part let’s assume, by pressurised CO2. The outside is in contact with all sorts of gunk during processing and shipping and roasting. Take a look at Oli Bradshaw’s instagram here for example after cleaning day. And this is at perhaps the world’s cleanest roastery.

I met up with ex Tim Wendelboe Roaster Mans Akne who together with Magnus Lindskog, roasted the coffee for the World Brewer’s Cup Champion, Odd-Steiner Tollefsen and also for the Swiss Champion, Benjamin Prager who took third place. I think this is an incredible achievement for a roaster. He advised 6 monthly full clean outs of the flues and daily hoovering of the roasting device. And he commented on Twitter later:

‘The construction of the drum is important for flavour, how you can apply heat/roast and also how often it needs to be cleaned. If you roast very dark and roasty coffee it will affect how clean the lighter coffees will taste / If roasted directly after. And you are 100% correct in your conclusion that dirty roasters roast dirty tasting coffee. Cleaning is so very important to make coffee tasty.’

Many of our top roasters in Europe use old cast iron roasters which require seasoning or they will rust. And my experience of of any roasting device and indeed any seasoned cast iron cookware is that it has some odour, and best practice as it stands at Prufrock and the London Barista Resource is that any odour coming from anything in the brewing process that isn’t freshly ground coffee is not desirable.

I’m guessing one reason roasters can get just about get away with not detailing the inside of the drum every day like we would an espresso machine group, is the abrasion of the beans themselves. Whilst this leaves some ashy residue, it will help do some scraping clean of the drum. To us virtuous baristas wiping out our portafilters every half an hour, this sounds a bit like putting your jeans in the freezer. Defend yourselves roasters! I contend that you don’t clean your roasters because you can’t. It’s too hard to get them open every day. A bit like a K30. They’re not designed for daily cleaning and this is certainly not sustainable from a staffing perspective.

Our contention is, if you just exfoliate the outermost surface of roasted coffee, like scraping your toast into the sink, you’ll lose some some carbony taste. You’ll remove the portion of the beans most affected by odours on the surface of the drum. You’ll remove oxidized oils I’m guessing too, but you’ll create a decent amount of dust in the process. We solved this problem perhaps a bit heavy handedly with the Henry Hoover. Perhaps this affected Petra’s aroma scores which weren’t spectacular. Re-hulling might not become part of your next fancy grinder’s spec. and customers are not wanting to throw away bean mass that their customers have been happily consuming all these years. But, we do peel carrots. Should we? Some people don’t, but if you don’t, they taste a bit earthier and they’ve got a different texture? We buff our apples on our t-shirts. And backstage at the worlds in the compulsory service, you need a point of distinction, and Petra weighing in at #9 in the world this year and #6 in the world last year has made headway into demonstrating this point of distinction in her brewing. It stands up to the benchmark of Christmas 2005, and is answering a need for cleaner coffee and is a natural advancement of existing techniques. It is trying to be a part of the the process. And it comes from the heart.