Prufrocker: The Blog

IQ & A

Now for the answers to the Quiz from last post. This one goes out to the baristas of Double Shot in Prague who have an in-house competition going on who ever gets the most questions right. (To whomever wins it, I donate a space on a brewing course at Prufrock as a prize.)    A... Read more »

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Now for the answers to the Quiz from last post. This one goes out to the baristas of Double Shot in Prague who have an in-house competition going on who ever gets the most questions right. (To whomever wins it, I donate a space on a brewing course at Prufrock as a prize.) 


  1. A coffee is pulling at 18.5% extraction yield. You score it at 2.5s (on the WBC score sheet: average-good). You’re dosing 18-34g and the timer says 28 seconds. There is some astringency in the shot and it is imbalanced towards bitterness and light bodied. Your brew water is 94degc.

Do you?

a.) drop the dose and grind finer

b.) decrease the brew water temp

c.) coarsen the grind and increase the brew water volume

d.) consider changing the grinder blades


This is the best question of the bunch I reckon. Tasting over-extraction at a lowish level of extraction with a decent sized beverage weight suggests you are producing an excessive amount of  fines which will be reducing flow rates and imparting bitterness but only constitute a small proportion of your over all quantity of grinds, so the answer is probably d.)


  1. You shot tastes sour and thin bodied.

Do you?

a.) increase the dose and grind finer

b.) decrease the dose and grind finer

c.) keep the dose the same and grind finer

d.) keep the dose the same and grind coarser


Dropping the dose needs to be done responsibly. Easy way of saving some money but we restrict our baristas from changes or more than 0.5 of a gram without consensus from the whole team so the best barista would say c.) but the business savvy smart one might say b.)


  1. Your shot reads 20.5% extraction yield and you’ve pulled an 18-38g brew ratio in 30 seconds. You find some sweetness but there is some mild astringency.

Do you?

a.) fine the grind and reduce the dose

b.) coarsen the grind.

c.) coarsen the grind and reduce the yield.

d.) get yourself a new Linea PB


Here I would just b.) coarsen the grind. Keep it simple. Assuming the coffee is not ridiculously under roasted will be caused by a touch too much extraction so loosen up the grind up a bit. Dropping the yield too c.) will have a greater effect of reducing extraction but I prefer to take penny steps. I’d try c.) the very next shot if b.) didn’t work.


  1. You taste bitterness and astringency on a 20-40g brew ratio and a 36 second shot time

Could this be caused by?

a.) over-extraction

b.) under-extraction

c.) channelling

d.) excessive tamp pressure

36 seconds is pretty slow and a 50% EBF (espresso brew formula) is a decent amount of water so probably here you are getting a.) over-extraction. If it was channelling b.) you’d also complain of excessive sourness from the regions you under-extracted in spite of locally over extracted grinds in the region the channel took place.


  1. In your cafe you use the WBC scoring system to maintain quality. You’ve pulled five shots scoring only 3s in each category of the WBC scoresheet and the coffee has been tasting light bodied and sour the whole time with shots falling between 20 and 30 seconds.

Should you

a.) accept the scoring ceiling has been reached for this coffee and remove this coffee from service.  

b.) fine the grind and explore shot times between 30-40 seconds

c.) increase the water temperature.

d.) b and c.


You’re under-extracting so I’d start with b.) and if that didn’t work I’d go a bit hotter too and answer d.). In Prufrock we are frequently pulling shots in the 30-40 second range but have never brewed tasty coffee with water exiting the group above 96degress C.


  1. You’re pulling shots that all taste sour and you are on a 40second shot time and a fine grind. You are on a 50% ebf and yet they still taste sour. There is not bitterness or astringency in the cup and you know the coffee is a good coffee from a reputable roaster and makes a lovely filter.

What will most immediately reduce sourness?

a.) increase water temperature

b.) drop the dose and pull the shot longer

c.) grind finer still

d.) increase pre-infusion


You’re pretty much at what we consider the shot time upper limit based on where we see our scores start to drop lower, so you need to increase the flow rate. More water alone will add to the shot time so dropping the dose will reduce resistance, this then allows you to drop the EBF to reduce sourness and knowing you’re not tasting bitter and astringent, you can push the extraction harder with this coffee too, so the answer is b.)

When brewing coffee that is very bright, the most simple solution to excessive sourness is to go with more water. If I dropped below a 45% EBF and still couldn’t reduce sourness, I’d swap coffees.


  1. Preinfusion helps increase extraction because

a.) it allows you to grind finer

b.) it reduces fines in the cup

c.) it increases overall brewing temperature

d.) it increases flow rate


You are definitely operating on an overall finer grind on a machine with long preinfusion like a lever machine than on a tradition pump machine (we tend to pre-infuse for 10seconds on our lever) so you tend to see much higher average extraction on lever machines with faster flow rates or quicker extraction times after the preinfusion finishes. The coffee in question 5 could benefit from this increase in preinfusion.

Note: Fines migration is called into question and difficult to prove it can be prevented by preinfusion but I would answer a.) and or d.) here but I’m waiting for Gordon Howell (UK Brewer’s Cup Champ 2015) to call me out on this one as he does lots of work on flow rates with Dalla Corte on their Mina Machine and is always getting his fancy Acaia scale involved on the drip tray. He can clarify this one for the next post. I can hear Gwilym’s voice in my ear saying something like pre-infusion is bollocks, your customers are just having to wait longer and he wouldn’t be wrong. And I can hear Gordon’s voice saying it is much more important to control the incoming flow rate at the start of the shot which the DC Mina machine does like a dream with it’s micro adjustable flow restrictor controller (imagine a extremely tiny  automatable camera shutter)


  1. Your are getting excessive channelling.

Would this be most likely be cause by

a.) your tamper being slightly convex

b.) the way you insert the portafilter into the group

c.) flow restrictors being too small

d.) overdosing the filter basket


b.) I’m going to say 95% of baristas I observe put the handle into the group too hard and cause channels between the steel of the basket and the coffee bed. If it makes a noise it’s too hard is my rule of thumb.


  1. You are pulling big split triples at 24-55g. You detect over-extraction.

Would it be most efficient to

a.) drop the dose and grind finer

b.) increase the dose

c.) decrease the beverage size

d.) coarsen the grind


c.) would be most efficient. At the end of the shot you pick up approximately 0.1% extraction yield per gram of water so you can drop half a percent with 5g less. This will vary but it is certainly easier to adjust water quantities than grind. No purging required.


  1. You detect over-extraction in a brew

Would the most likely reason for this be?

a.) one of the ingredients in your blend is lower density

b.) one of the ingredients in the blend is castillo variety

c.) the water you are brewing with has dropped in TDS to 50ppm

d.) your roaster has just employed a new head roaster


a.) A Colombia-Brazil blend (very common in Australia for milk blends) can do this to you. We observe a need at the start of the year when Brazils start cropping up in the Red Brick that we need to target slightly lower extraction yields and increased bitterness can be an issue. I suspect it is a case more of fewer solubles in lower altitude coffees rather than lower density so perhaps none of these answers are correct. That said d.) could certainly be the answer ;D but d seems to become a good deal less of an issue after a couple of months. This emphasises the need for a polite dialogue with your roaster.


  1. You are scoring what you think is a 3.5 out of 6 on the WBC sensory score sheet but you sense there’s a 4 there just around the corner.

Should you?

a.) just serve it

b.) ask the customer if they’d like you to serve this one or if they want to wait a second while you tighten up the grind

c.) nutate a little more

d.) remove quakers from the hopper and winnow out the chaff from the centre cut.


In this instance, I would just serve it. A 3.5 will get you into the semifinals of the UKBC quite probably and this is where we pitch our serving standard minimum.


  1. Your company has established a scoring minimum of 3.5 in all categories on the WBC sensory score sheet and they expect you to hit this target in 4 shots. You have pulled 4 shots during dialling in and none of them have hit the 3.5 you’re after.

Do you?

a.) kept pulling shots on different shot times until you hit the god shot you always knew was there.

b.) just serve it

c.) take the head barista aside and suggest the current blend might be hitting a ceiling below your quality threshold so they better contact the roastery to negotiate a better coffee for the next order.

d.) send this coffee back


c.) Smart and pragmatic. Sending coffee back is drastic, it needs to be terrible. Great cafe QC is about response time rather than gun slinging tactics and keeping you roaster happy is part of this but also identifying problems when they occur is also needed. 4 shots to calibrate a grinder is a basic expectation we have to reduce waste but it could also be your coffee is one of those very hard to work with coffees with very little margin of error and it needs more work.


  1. Your coffee tastes a tiny bit roastier than the previous batch from last week.

Do you?

a.) pull the shots longer

b.) pull the shots a touch shorter and reduce extraction and increase brew strength

c.) increase the dose and grind finer.

d.) decrease the dose and grind finer


b.) Go shorter. Even Starbucks do this with their triple ristretto flat whites. Perceived roastiness is less at higher concentration somehow. Gwilym beat Pete Licata in the WCE All Stars with this approach when they were given a roasty coffee to work with. Trouble is, your customers aren’t stupid and they notice big shifts in brew strength so mostly going short makes roasty coffee taste better because it is generally devoid of fruitiness and at higher concentrations you perceive more acidity in the cup, which seems to balance quinic and caramelly bitterness. I draw a limit to this tactic at 10% TDS. If I have to go stronger than this to balance a coffee, then I might get on the phone to the roaster and try to parley.


  1. The best coffee equipment is that which

a.) achieves the highest extraction on a given shot time

b.) slows down the rate of extraction

c.) has the lowest astringency ceiling

d.) is the most temperature stable


Definitely a.) VST baskets for example are better because they extract more evenly and thus give higher extraction than all other baskets which gives you more control. Allows you to drop the temperature, or the pressure or brew more efficiently. The same is true of the Pergtamp. Nice and wide, never any vacuum effect, reduces channelling and so extracts higher than a crappy 57.8 mm tamper.


14.The best grinder burrs are those that

a.) reduce fines production

b.) reduce boulders

c.) increase sweetness

d.) have a higher astringency ceiling


I’ll say d.) but that probably is on account of a.)


The best grinder is the easiest to

a.) the easiest to clean

b.) the most ergonomic

c.) maintains a stable shot time and dose

d.) all of the above


d.) obviously. The easiest question of the bunch.


  1. The best barista

a.) works the hardest

b.) tastes the most

c.) needs to make the fewest changes to the grinder

d.) makes the most changes to the grinder


Silly question. Easy to say a.) but actually the workflow of a great barista with modern equipment using tech like the puq-press is so effortless, they make coffee quickly without seeming to be working hard. So they can enjoy the company of their customers more. I’m going to say b.) It’s fine if you spit it out, but you can’t work as a barista at Prufrock if you regularly taste coffee. Don’t assume a temperature grinder like the Mythos does’t overheat. Have you noticed Starbuck Reserve alternates the shots on paired up Mythos. At crazy high volume, all grinders will overheat and then the barista can’t rely on the observations made during dialling in. They have to recalibrate for excessively warm beans which will tend to lead to increased extraction which will then require a temp drop in water or a coarsening of the grind.


  1. The best dosing and tamping technique achieves the

a.) lowest extraction over a given shot time

b.) achieves the highest extraction over a given shot time

c.) achieves the lowest flow rates at the highest pump pressure

d.) distributes the coffee the fastest.


b.) The highest extraction over a given shot time. Just like the best coffee equipment answer in question 13.


  1. The best roasted coffee

a.) gives the broadest extraction and shot time range for balanced shots

b.) tastes the most full bodied

c.) extracts the most easily

d.) tastes the most balanced


For brewed coffee I might say a.) but for espresso we must say it’s d.) but really I’d love to just work with a.) all the time but those geshas and Pacamaras sometimes just won’t behave and require very specific treatment I find.


  1. The best espresso machine

a.) dispenses the most stable water temperature

b.) is the easiest to service

c.) is low profile so you can see the customers and ergonomic and looks fantastic

d.) is under counter


The one I’d buy is c.) but I think it’s a case of a.) and c.) with a little b.) and very little of d.)


  1. An espresso is balanced when

a.) it has no bitterness

b.) there is an harmonious balance of sweetness, acidity and/or bitterness

c.) it is the sweetest

d.) when it is 18-22% extraction


For me it’s c.) Am I a bit juvenile? Coffee isn’t like wine for me, the more dessert-[wine]-like, the better. I should say b.) which is the WCE terminology and this answer also reflects the laudable approach of the wine industry in establishing that sweeter doesn’t immediately mean better but still, for coffee this tends to be the case in my experience.


  1. The best tamper

a.) has a ripple base

b.) allows for the best pre-infusion

c.) reduces channelling

d.) promotes the highest extraction yield


d.) we’ve covered this but it’s achieved through reducing c.) and for the record, that ripple design from RB was so ridiculously cool.


  1. You grinder burrs probably need replacing

a.) every 250kg

b.) when the grinder starts heating up above 45 degrees during busy periods

c.) when you taste over extraction when your average extraction yield is under extracted

d.) when the grinder takes more than 3.5 seconds to produce 18g of grinds


c.) Increased fines production is likely to impart over-extracted tastes in an average lower yield. Get me? If you’re finished reading the grinder research paper and concluded, so I don’t have to worry about fines any more, my opinion is, think again.


  1. Your coffee tastes sour. Is your water likely to be

a.) too high in calcium

b.) too high in KH

c.) too low in sodium

d.) too low in KH


d.) Brewing with distilled water which has no KH definitely makes coffee taste more sour. London water, about 100 ppm of KH makes it taste boring and like a crappy chocolate biscuit (specifically a Tiny Teddy for any Australian readers; too much baking soda)


23.) Brewing at 11 bars of pressure will

a.) increase flow rates

b.) permit me to grind coarser

c.) reduce espresso beverage size

d.) Increase channelling


c.) Reduce beverage size due to compressing the coffee (I’m guessing). Scott Rao covers this topic in his E book. But the answer is probably also b.) which for me is interesting as I’d prefer to be operating on a coarser grind. Less fines, more grinder efficiency and less burr wear. Same reason Dalle Corte have always preferred the 53mm basket width and why Prufrock prefer our 24-60 recipe in 22g VSTs for bit split quad shots.


  1. Hotter grinds during busy periods will

a.) decrease extraction

b.) increase extraction

c.) increase aromatic intensity

d.) increase astringency


d.) Increase extraction. Gwilym learned this during his work on the Mythos project. We had a prototype that went up to 70deg. Very interesting idea: Take the coffee beyond it’s hottest possible high temp level so it will be stable. Tasted bad though.


25.) The best distribution of coffee grinds is

a.) the method that promotes the fastest flow rate

b.) the method that moves coffee equally into the north, south, east and west of the basket

c.) the weiss distribution technique

d.) the method that promotes the highest extraction yield


d.) which might well be the Weiss method (you know, the tooth pick one) but thisreally should be confined to the domain of the home barista. The possibility has always existed that home baristas can make better coffee than us, as they have the time to do the Weiss.