Situation:
Rosco manual grinder takes about 50 seconds to grind 20g of beans at an espresso suitable grind
Pharos manual grinder takes about 25 seconds (or less) for 20g at the same grind level
Impression: the coffee from the Pharos tastes better. Why?
The work done is the same in both cases. It must be by definition, as the same amount of beans are ground to the same degree
Power P = WD/time
P (Rosco) = WD/50
P (Pharos) = WD/25
Thus P (Pharos) = 2 P (Rosco)
If you assume the burr pattern was the same in both the grinders (angle of attack of the cutting face to the bean & pitch & length of the grind path) then if both grinders turned at the same speed, yet one ground the same mass of beans in half the time then it could only be because the surface area of the grinding faces in the faster grinder was twice that of the slower grinder
So whilst the faster grinder needed twice as much power and therefore generated twice as much heat, that heat is occurring over twice the surface area, so the Pharos does not heat the coffee any more than the Rosco does
Secondly the larger mass of the larger burr set has a greater thermal capacity – i.e. it can conduct away heat for longer, but because the burrs are turning so slowing in a manual grinder the power (heat) generated by either grinder is very low, and so heat can not be used to explain the difference it taste, which seems noticeable
So this is a situation where we can’t attribute the improvement in taste to a reduction in heat transferred to the coffee, so what is it?
A coffee bean with a diameter of ‘a’ around the waist of the bean (if we had it standing upright) needs to be reduced to a coffee ground with an average diameter of ‘b’
The effort required to achieve this is the work done (WD) = force (F) x distance (d)
If we assume WD is a constant, if we increase ‘d’ then F must reduce
We increase ‘d’ by lengthening the grind path. Typically when the diameter of the burrs increases the thickness of the burrs also increases. An increase in the thickness of the burrs increases the length of the grind path.
I don’t know for sure but I think you will find that as the thickness of the burr increases the combination of;
1. increasing the mass of coffee held in each bean channel of the female burr that you need to reduce the pitch of the grind path otherwise there will be too much resistance between the coffee lying in the bean channel of the female burr and the surface of the bean channel for the spinning male burr to be able to drive the bean down the length of the tapering bean channel. This reduction in pitch will further increase the length of the grind path.
2. whilst not applicable for manual grinders, the reduced rpm as you move to large burrs on a electric grinder will also demand a reduction in the pitch of the grind path i think, otherwise the coffee will not feed properly through the burrs. Reduction in pitch increases the length of the grind path
So an increase in the diameter of the burrs can give you a significantly longer grind path in terms of a percentage improvement
In lengthening the grind path the reduction of ‘F’ that results, progressively improves the action of the grinder from one of grinding/pulverising/smashing to one of milling, where the bean sits in the tapering helical female ‘bean channel’ is held against the cutting face of the rotating male burr for more or less the entire length of the grind path and the bean is progressively reduced in diameter in a smooth continuous milling action without the bean breaking or crumbling, being driven down the tapering helix of the female burr (‘the bean channel’) by the rotation of the driven male cutting burr. You are reducing the size of the bean from diameter ‘a’ to diameter ‘b’ over a greater distance. This reduces the likelihood of the bean being crushed/shattering as it travels down the bean channel
Note that i used the word ‘progressively’. Im not saying that small diameter conical burrs are smashing all the beans as happens in a blade grinder, but they will still be crushing/smashing some of the beans, rather than milling them. As you lengthen the grind path the percentage of beans smashing in the process gradually reduces, with a corresponding percentage increase in the number of beans that have been milled in the optimum way. I doubt if anyone in the world knows what those actual percentages are, the important thing is to understand the change in manner in which the beans are ground as the length of the grind path increases
OK, so this is slightly theoretical in that beans will break even as the make their way through the finest conical burr grinder in the world, but i think it is important to set the scene, and illustrate that as few breakages as possible is the goal – it is also why lightly roasted beans are harder to grind – they are much less likely to fracture as their cell structure is much more intact than in a more darkly roasted bean
The trouble is I suspect most manufacturers of burrs also change the angle of attack that the cutting face of the burr presents itself to the bean – better grinders will have a shallower angle of attack as this reduces the chance of the bean fragmenting, and instead being milled, or shaved if you prefer
Note: As the circumference between the burrs increases the number of extra ‘bean channels’ that can be milled in the female burr increases, and this very quickly demands motors that are exponentially more powerful or the resistance presented by the beans will prevent the grinder from turning