is straight forward enough. Solid coffee as whole roasted beans contains soluble and volatile coffee chemicals required to dissolve in water to produce a cupped coffee beverage. Several factors, including temperature of the water, determine speed, amount of solubility and capacity of the water (solvent) to hold enough coffee chemicals (solute) to produce a “strong” (concentrated) and pleasing cup of coffee (solution). With that said, most important is the size of the amount of internal surface area of the coffee beans in direct contact with hot water.
Whichever term is coined to describe the break-up of solid objects (in this case coffee beans), net effect is an increase in surface area proportionate to the size of the particles produced. The surface area produced for interfacing with water, and dissolving out soluble chemicals into the cupped coffee, depends on the size and shape of the particles, and therefore method and mode of breakage as well as condition of roasted beans.
It is easy to see why the word “grind” was adopted for breaking up coffee beans. The earliest documented description of coffee bean breakage in the Middle East and Ethiopia involved pounding beans in a mortar and pestle or grinding with a circular motion between two large flat stones. Even today the simplest manual grinding equipment is the classic hand-powered and operated coffee mill or grinder that utilizes an adjustable, articulated millstone.
This is not the way of commercial coffee grinding. In many ways the term “grinding” is completely misleading when used to describe what actually happens at “bean level” in contemporary equipment for processing whole roasted beans into “ground” coffee. Nevertheless, “grinding” is the established and accepted term whether in the factory, coffee shop or parlour.
At “microscopic” level, other terminologies including fracturing, cracking, cutting, slicing, shaving and flaking are probably more accurate and appropriate to describe how coffee beans are broken up by fast-moving sharp surfaces (blades) used in a range of commercial coffee grinders.
Coffee is most commonly and usefully ground-using cutting rolls available in a wide design range, although impact grinders are utilised for production of ground coffee at the “finer” end of particle size. “Flaking” rolls which generate “flake-shaped” particles claims to provide benefits during brewing.
Coffee grinding is all about trapping the beans between a sharp surface and a “hard place” while exerting the right amount of force and pressure. “Microscopic” is a word very much in order because it describes exactly the dimensions of particles produced, especially for speciality grinds including Turkish and espresso coffees. Coffee particles are measured in microns and one meter is one million microns.
Superficially, there are features that are common to roasting and grinding. The most obvious relates to classification (or rather lack of it) of particle size in “ground” coffee. Just as degree of roast is very much in the “eye” and “mind” of the roaster, grind “size” is not bound by “hard and fast” rules, and certainly not at the international level.
No Rigid Classification
Coffee grinds are qualitatively classed or grouped as “coarse,” “medium” or “fine,” or more commonly (especially in North America) according to the device that will be used to brew the coffee. In this purely functional context, coffee grinds are essentially “graded” using a “horses for (race) courses” system where group and nomenclature relates to intended use, such as “percolator,” “drip pot,” “filter,” “vacuum makers” or “espresso.” With that said, there is absolutely no rigid national or international agreement or consensus over the mean (average) particle size or particle size distribution that corresponds to particular ground coffee, although a range of recommendations are available to classify grinds by screen analysis.
Thus “coarse” or “medium” grinds find application in household percolators, with the coarsest grinds reserved as a resource for large-scale percolation. On the other hand, “fine” grinds are used in filter coffee equipment with the very finest reserved for espresso machines.
Rene Coste, quoting illy (1980), placed ground coffee into two categories. One was called the “regular” category, comprising of grinds of uniformly average-size particles, with the small powdery particle component (fines) separated, isolated and removed. This type of grind, he claimed, was used to make coffee with percolators, filters and all types of equipment giving a fairly diluted coffee brew (50 to 65 gram/liter) and subject to “hydrolytic” effects of water, because the beverage is not drunk immediately after preparation.
The other category covers grinds made up of particles within a wide-size distribution range with 50% being very small powder-like particles and 50% medium-sized and larger particles. This type of ground coffee was used, he claimed, to prepare “espresso” coffee, with ability of the particles to aggregate (cling together), resisting the high pressures (9 to 10 bar) applied by the brewing equipment used.
Particle Size and “Permeability”
Fineness of the grind is one of the most important factors that determine whether or not a “good” cup of coffee is obtained. The overall aim is to extract soluble coffee compounds and volatile chemicals from solid coffee particles in optimal amount and proportions. A crucial factor is not particle size per se but the size of the spaces or pores created between particles in the bed of ground coffee when particles make contact. Footballs packed in a box have much larger spaces between them than tennis balls, which in turn have larger spaces than golf balls.
Water filtering through a bed of ground coffee can be likened to water passing through a soil profile. The bigger the soil particles the larger the spaces created between them. Large spaces offer less resistance to water flow and therefore faster percolation. Thus sandy soils containing relatively large particles of silica (sand) are very permeable because the water flows through easily and quickly. At the other extreme, clay soils hold water and are permanently wet because the spaces between clay particles with tiny dimensions are correspondingly small.
Filter coffee equipment works in the same way. If the grind is too coarse the water will filter through too quickly, not pick up sufficient aromatic principle and therefore taste bland. Conversely, if the grind is too fine the water will stay in the bed of coffee too long and leach out excessive quantities of particular chemicals that impart a bitter taste to the cupped coffee.
Prolonging the filter process cools down the water, preventing sufficient removal of the least soluble chemicals, and adds to the sub-standard taste with a “muddy” as well as bitter cup. Such problems tend to become magnified with the finest sought-after grinds. Mean particle size is already very small so there is less leeway for error from poorly adjusted or blunt blades that cause excessive friction and fineness in the grind.
Last but not least, the “finest of the fines” will be carried through pores in the filter and into the cup. The net result is another type of “grind,” this time by the coffee consumer as he or she moves their jaws in a circular motion with grimace, reflecting a poor tasting cup of coffee and discomfort caused by gritty particles on the palate.
Grinds used for a filter-coffee apparatus should display particle characteristics (dimension and size distribution) intermediate between these two extremes depending on the exact nature of the equipment in use. As a general rule of thumb, household coffee makers, depression equipment, peculators and espresso/Turkish coffee require average; finely ground; special ground; ultra-fine ground coffees, in that order.
Research and development work on brewing coffee, carried out some years ago at the “Coffee Brewing Institute” in New York, discovered and documented definitive information relating to coffee extraction by and during filtration. They found that the optimum flavor and aroma were achieved when 18 to 22% by weight (mass) of the original grounds were “leached out” (dissolved) during filtration. At levels below 18% the resulting cupped coffee beverage possessed a raw-edged taste, while above 22% the coffee drink was distinctly bitter.
Irrespective of whether a grind is “coarse,” “medium” or “fine,” the final measure of quality is the particle size distribution. This is best understood and appreciated graphically by plotting frequency against particle size. An ideal grind will produce a so-called “normal distribution” curve just like that obtained when plotting frequency against height for a group of same-sex, same-age people from uniform genetic and environmental background. Most particles are crowded around the mean (average) particle size at the top of the curve with decreasingly fewer as the graph tails off on either side.
There are two distinct factors that will determine whether or not such grind uniformity is achieved. They are condition of the whole roast coffee beans and condition of “grinding” surfaces (e.g. burrs and rollers) in the equipment. Grinding roast coffee beans which are either too warm or too dry (or both) or with worn, blunt or badly adjusted cutting edges will cause the graph to flatten out meaning there are too many excessively large and/or small particles.
Condition and uniformity of roast coffee beans for grinding and green coffee beans for roasting are comparable. Roasting will only perform and achieve within the limitations of quality and uniformity of the green bean blend, and likewise, grinding within limits of quality and uniformity of roast bean resource.
Ultimate uniformity is best achieved not by relying on “bean passage” through a “one-off” grinding device but through using so-called multi-grinding equipment, equipped with a sequential series of grinding rolls to achieve progressively smaller particles and therefore a progressively finer grind.
Roasted coffee beans have some similarity to roasted beef or lamb. Neither should be “cut” when too warm nor too dry if desired shape and dimension of the cut pieces is to be achieved. Coffee beans straight out of the roaster will clearly record a high temperature and considerably less moisture than when they went in.
Water embedded in the roasted coffee bean matrix affects structure, brittleness and friability, and thus a reaction to the external force (high speed grinder blades) by affecting tension type and level recorded in the bean. Generally speaking hot, dry objects will shatter with a corresponding loss of control over the size distribution of the particles thus produced.
Ideal conditions at the point of impact between the blade and bean to produce optimum grind is logical, especially when extended with a little imagination. Ideally, the friable bean and “razor sharp” blade come together and create a “controlled explosion” in the bean resulting in “granular disintegration.” Net result is millions of particles within the desired size distribution range.
Grinding quality of roasted coffee beans is determined by degree of brittleness, in turn governed by internal and external temperature, moisture content and degree of roasting. Grind quality is enhanced by adding water beforehand to an optimum level corresponding to 7% weight/weight (w/w), a procedure especially beneficial to coffee, which is ground to serve large-scale percolation procedures. Coffee roasters are usually equipped with a built-in facility to “quench” the coffee at the end of the roast cycle to curtail development of the roasting process. Some of this moisture will stay on the surface of the bean and assist in the cooling, holding and conditioning process occurring between roasting and grinding.
Worn out surfaces create friction. Energy resources are diverted away from a precise pinpoint impact on the bean at the expense of a controlled explosive disintegration. The net result is excess heat and a broader particle size distribution. Heat thus generated may be the cause of coffee darkening during grinding.
Extra fine grinding to around 50 micron particle size is rarely required for domestic brewing or industry-scale percolation, but may be required for say Turkish coffee and to satisfy particular and exacting specifications and needs of roast beans used to manufacture soluble (instant) coffee. If required, such fine grinding is often best carried out under cryogenic (sub-zero temperature) conditions using materials such as solid carbon dioxide or liquid nitrogen, especially when using impact-type mills.
Coffee grinding under cryogenic conditions is additionally recommended for enhancing retention of volatile aroma compounds that reside in the ground roast coffee. They are released and escape under the influence of conventional grind processes, especially when heat is generated from frictional forces between moving blade and roasted coffee bean.
Grinder Gas and Chaff Release
Another factor documented for coffee grinding is so-called “grinder-gas,” which can be collected by installing condensation equipment at appropriate positions within the exit passage of the grinder. These condensates will possess usable volumes of volatile compounds important for maintaining inherent flavor and aroma qualities of a particular roasted origin or blend. They may be reincorporated later in the manufacturing process or when the R & G coffee is packed.
That said, by far the biggest proportion of grinder gas is carbon dioxide, residual in the whole beans after roasting, and some water vapour. As soon as the bean is broken up during grinding at least 50% of the carbon dioxide gas will be released from the matrix, creating potential pressure hazards unless it is vented sufficiently rapidly. The remaining portion of carbon dioxide gas will remain in the ground coffee to be released gradually over a period of time.
Residual chaff that was left after most “flew off” the green beans during roasting is another material released during grinding, either from the actual outside surface or the “centerfold” of the whole bean. This supernumerary chaff is generally disposed of, but some sources suggest advantages are obtained by incorporation of this chaff into the ground coffee under the influence of mixing blades in a “normaliser.”
Utilisation of residual chaff in this way is claimed beneficial for dark roast coffees through absorbing and holding coffee oil that seeps onto the surface, and additionally for keeping ground coffee in a free-flowing condition to make subsequent handling much easier. Appropriate adjustment of the blade speed and disposition, they say, can offer some control over bulk density of the ground coffee product.
There are two basic methods used to determine the bulk density of ground coffee, by “free fall” or “packed measurement.” In the “free fall” method, mass (weight) of coffee needed to fill the chosen container (under specified conditions and after any heap formed at the neck of the container is levelled) is calculated. For the “packed measurement” method, mass (weight) of coffee that just fills the same container after “jogging” (vibrating) the jar to allow settlement is determined.
Bulk density will clearly vary according to particle shape, size and size distribution that is governed before, during and after the grinding process by a whole range of sequential factors including green bean blend, degree of roasting and grinding, moisture level in the beans and treatment in a normaliser.
Variations in bulk density value can ultimately be traced back to condition of the whole roast beans used for grinding. Bulk density values decrease with severity of roasting and increase with fineness of the ground coffee. As a general rule of thumb, a roast and ground coffee product described and defined as “finely ground” is expected to possess a bulk density within the range of 0.40 – 0.45 g/ml determined with coffee in “free fall.”
Dr. Terry Mabbett has been covering the tea, coffee and cocoa industries for decades. He resides in England.
Dr. Terry Mabbett is a technical writer with a PhD degree in Tropical Agriculture. He has worked in crop production and processing throughout the tropics- India, South East Asia, West Africa and the Caribbean – and in his home country of the U.K. Dr. Mabbett has been writing professionally for over 20 years.