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Burr Geometry and Grinding Characteristics
Hand
coffee grinders all use some variant of a conical grinding burr.
Conical refers to the general shape of the inner burr, an overall cone
shape. This cone has larger "scoops" cast or machine cut at the small
end of the cone and smaller "teeth" at the large end of the cone. For
discussion sake think of the scoops as the primary cutters and the teeth
as the secondary cutters. The outer burr is also cast or machined and
the inner surface of the burr is built to create a cone shaped bowl or
cylinder. The outer burr has the secondary cutters (teeth) at the
bottom and primary cutters (scoops) at the top matching the inner burr
geometry. The space between the inner burr and the outer burr
constitutes the "adjustment" of the burr and the smaller the space the
finer the grind and the larger the space the coarser the grind.The
function of the scoops is to initiate the breaking of the bean and to
create a pushing force of the material through the teeth. This action
is determined by the shape of the scoop and the size of the scoop. A
smaller opening between the inner and outer burr creates a smaller
scoop, and this opening must be large enough for at least some part of a
coffee bean to enter the scoop and be broken. A larger scoop will
allow more or the coffee bean to enter the burr for primary cutting.
We think of the small conical burr set as a "nibbler" and a large conical as a "smasher".
Whether
a nibbler or a smasher determines a number of characteristics of the
grinder. With nibbler burr we can expect that the grinding action will
be most noticeably SLOWER. The entire top surface of the burr set is
covered with coffee beans and as the burr rotates the scoops cut or
break off small parts of the beans while the movement of the burr causes
the beans to stir and tumble exposing new areas to cut and break.
Smaller bean bits fall or are pushed into the increasingly narrow gap between
the two burrs where they are broken again and again by the primary
cutters until they are small enough to enter the secondary cutters for
final processing.
The gap between the burrs is a wedge shape
from top to bottom so we see the scoops as processing the beans into
smaller and smaller particles while the teeth groom the particles to an
approximate uniform size. The speed of the process is largely
determined by the rotation speed of the burr in motion and the initial breaking
of the bean. The nibbler burr takes more rotations to process a
coffee bean. A larger burr has larger scoops and more of the bean can
enter on the primary contact, in many cases an entire bean is broken in
one event. The larger burr also has more scoops than a smaller burr.
The process of grinding and grooming the particles is the same for both
types of burrs but the initial action of the scoops is the determining
factor of grinding speed.
When a coffee bean is broken or
smashed by any method the result is a random number or random sized
fragments and notable to this event is the production of "fines" or the
smallest of particles. These fine particles are not effected by the
teeth or cutters and are not groomed but pass through the system. Each
subsequent cracking or breaking of the larger particles creates more
fines. The presence of these fine particles is critical in espresso
extractions, constituting the small particles in what we term the
"bimodal distribution" of particle sizes in an espresso rated grind.
The fines are the glue that support the main target size of particle for
uniform espresso extraction but too many fines will skew the extraction
just as not enough fines will do the same. A conical burr tends to
naturally produce a bimodal particle distribution and as the burrs are
set closer and closer to one another this bimodal distribution becomes
more pronounced. This is the reason that all conical grinding burrs can
and will produce an espresso rated grind, assuming that the stability
of the adjustment system and other factors allow an adequately uniform
target particle size, since the production of fines is automatic and unavoidable.
Given
that the production of micro fines in unavoidable, we concentrated our
design efforts on coarse grinding, with the goal of eliminating not the
micro fines but the sub target particle sizes of coffee that lead to
overextracted portions of any particular extraction method. We
approached the problem by designing a grinder to be of maximum stability
and consistency at the coarse range of the grind an letting the fine
and espresso end of the grinding range "take care of itself" since the
conical burr will and does grind espresso naturally. Most grinders are
designed as espresso grinders (and most consumers test a grinder on
the tightest setting right out of the box) and the coarse ranges are
just an afterthought. To achieve this, we incorporate a dual bearing
system on the burr axle to create a hard set axle for the inner burr in
relation to the outer burr. This creates a fixed burr gap that does
not wander on the coarse setting range. The result of this design on the
OE LIDO is a very stable and consistent grind for coarse extraction
grinding. As one tightens the burr the natural bimodal expression of
grinding is exhibited for espresso.
OE LIDO GRIND ANALYSIS
To
conduct our grind analysis we used a Ditting KF804 batch grinder with
new tooled 83mm blades to verify the accuracy our sieve system with
hole sizes of 425, 500, 600, 710, 850, 1000, 1400, and 1700 microns.
The Ditting KF804 was dialed in to produce coarse grind coinciding with a
particle size peak of 1000 microns (representing an ideal French Press
extraction grind). The grind sample was placed on top of the sieve
stack and vibrated with an industrial tub vibrator for 30 seconds. The
resultant separation of particle sizes in each sieve was weighed in a
digital scale. The Ditting grinder provided the baseline for
repeatability of the sampling method and verified our assumption of
microfines are produced while grinding on even the "gold standard" of
coffee grinders.
In our tests we used Red Bird Espresso Blend
at 6 days post roast. 10 gram bean samples were hand ground at LIDO
burr settings of .25, .50, .75, 1.0, 1.25, 1.50, 1.75, 2.0, 2.25, and
2.50 rotations of the adjustment screw from our factory set ZERO mark on
grinder. 3 samples were ground, vibrated, sieved, and weighed for each
burr adjustment setting. The results from the 3 trials were averaged.
Results: We have divided the grind results in 2 sections
for graphic simplification. The first set of numbers and graph
represent the mid range fine to coarse/fine range and the second set of
numbers and the graph represents the coarse grinding range. The
vertical axis on each graph is the percentage of each particle size by
gram weight held by a sieve and the horizontal axis is the nominal sieve
size. The blue line in each graph is the Ditting batch grinder
profile.
DISCUSSION AND OBSERVATIONS
If you examine the results of our grind study for the LIDO you can see right away that we lack enough data points to produce a clear picture of a full grind distribution. This we readily admit, but we did not hand grind and sift and sieve perfectly good coffee beans with the illusion of saying that the LIDO is as good as a Ditting batch grinder for producing a consistent grind for a particular brew method. Our goal was to find the sweet spots for various brew techniques and simply put, the coarse grinding range for the LIDO starts at about a setting of 1.5 revolutions of the adjuster from 0 and the farther out you go in the coarse direction, there is a small diminishing of small particles (500-800 microns) with an increasing percentage of particles larger than the target 1mm size. Another observation is that the micro fines (less than 400 microns) is constant throughout the coarse range at about 10% of total weight (this result is the same with the Ditting and we see this number as basically unavoidable due to the factors described previously). The micro fines are often described as inert in the brewing process and some authors describe them as consisting largely of bean hulls and skins. This portion begins to become an increasing percentage of the total as the grind setting is adjusted finer, which is to be expected as the bimodal espresso grind is being set up naturally with this type of burr.
Our personal observations when using the LIDO with various brew methods generally reflect the data which we gathered in this grind study. For French press brewing we generally use a setting of 2 turns from 0 as a central point from which to dial in each direction. Micro fines are fairly constant (reflected as "sludge" in the bottom of the cup) but as the grind setting is diminished the resistance of the plunger of the press increases and the trend toward bitterness (overextraction) becomes apparent. Settings above the approximate setting of 2 result in a very easy plunge of the press but require longer steep times for full flavor. Observations with a Clever dripper are also interesting in that one can directly control the drip down time by adjusting the grind of the LIDO, again with the sweet spot at around 2 for a 1 minute drip down time. And yes, your mileage may vary!
As we have worked with the LIDO mostly on the coarse grinds we have yet to experiment with filter cone methods using the intermediate and fine grinding range so are currently unable to report any observations in this area.
For espresso, the LIDO behaves very much as we anticipated with a forgiving espresso grind generally a few degrees on either side of a setting of 1/2 out from 0. Due to the limitations of our sieve system and the inherent complications of even sifting particles in the sub 500 micron range (sticking together of particles, clogging of screens etc) we have no hard numbers to report. The evaluation of the LIDO in the espresso range will be of course highly subjective and user centric.
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