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Coffee grinding issues and answers.

Frequently, we are approached by our customers and clients with questions and issues regarding the coffee grinding facet of their operations. After almost fifty years of manufacturing and reconditioning coffee grinders we have, in all probability, heard most of the grind-related topics before and are able to lend some assistance in these matters.

In an effort to lend that same assistance on a more industry-wide basis, we have assembled a few of the more common questions, issues and problems, along with our explanations and recommendations in this regard.

Coffee Grind Varies from Time to Time: "Sometimes I get a good grind and a little while later it has changed!" There are several areas of possibilities given this scenario. One: the grind changes because of the "condition" of the whole bean coffee going into the grinder.

Think of a grinder as being nuts and bolts and, while things can break, typically the grinder will do the same thing every time.

One of the factors that can affect the whole bean condition includes the amount of time that has accrued between roasting and grinding and, if too short (and the roasted beans are still warm), the friability, or "grindability," changes. Typically, a coffee roaster will quench the coffee at the end of the roast cycle to "check" the development of the roasting process, and moisture remains on the outside of the coffee bean for a short period of time. Thereafter, this moisture, as well as other chemical transformations that are occurring in the coffee bean, become factors in the required cooling and holding of the coffee beans for a period of time after roasting and before grinding.

To better understand this concept, it should be noted that "grinding" is a misnomer and the process properly described is more of a "fracturing/cracking" of the coffee bean between rotating surfaces, at which point an "explosion" occurs and the resultant particle granulation produces a particle distribution or grind. When coffee beans of reduced friability are introduced into the grinder, such as those still hot from the roaster, they will fracture (grind) less and a larger grind will result. But wait four hours and that same coffee will be able to produce, again, its optimal grind.

Grinder wear/condition.

If the coffee grinding equipment is worn, and the rollers or burrs need reconditioning or replacing, the grind achieved will be increasingly "uncontrolled." Regardless of the grinding methodology, sharp edges are used to fracture/crack/cut the coffee bean at a precise rare. If these corrugations (rolls) or ridges (discs) are worn-out, the only way that the coffee grind can be achieved is by crushing and friction, neither of which produces a good grind.


Grind measurement methodology.

If we consider any quantity of ground coffee as having a population of differently-sized particles which, as an average, comprise a standard "grind size", then it becomes clear that in measuring that quantity of coffee, that same exact percentage of differently-sized particles, in proportion to each other, is vital to certifying a proper grind. Whether a ro-tap, laser or other method is used for testing the ground coffee, it is critical to maintain that same proportion of particles throughout the transporting or sample preparation process. For instance, if coffee is merely "poured" into a container for testing, that simple act of pouring can "declassify" the various particles that make up that grind, giving it an erroneous test result.

Coffee Sediment: "I find fine coffee particles that look like "sediment" at the bottom of my coffee cup." If we consider that ground coffee is made up of a wide variety of particle sizes, and these particle sizes can range from 10-1,500 microns.


One can see that the more uniform the particle size along that spectrum, the greater the brewing performance will be. For instance, if we want a 700 micron (drip) average particle size, but we have a high percentage of 200 or 1,000 micron particles as well, then the brewed coffee will be over-extracted or under extracted, respectively.

Now, if we introduce a more extreme coffee grind, where the 700 micron (target) grind is populated by some 10 micron particles, which were generated by the friction and crushing method in 1 (B) above, those particles will migrate through the filter medium to appear as sediment in the cup, a result that goes far beyond appearances. Unfortunately, that sediment will create a bitter sensation due to not only the over-extraction that has occurred as a result of the particle size, but also as the result of the sediment that will reside on the palate which, in addition to the appearance in the bottom of the cup, causes an inferior taste.

This particular symptom is important as grinds tend to become finer which will, by definition, result in a higher percentage of finer particles. If not controlled, these particles will include the very small micron variety.

Pods: "I don't get the same brew performance from my pods as my portion brewed method."

Pods make grinding requirements even tougher, since this application requires a greater control of the grind distribution and, typically, a smaller particle size (grind). At the same time, the product's bulk density is important since pods need to be a certain size for packaging and brewing consistency. It has been commonplace for pod equipment manufacturers to affix "disc grinders" to the top of their pod machines to provide the required ground coffee. Although this is convenient, the ability of any disc grinder to produce this required grind is limited and, as an alternative, roller-style grinders are far superior in these applications.


In as much as the particle size required in pod manufacturing is often 300--500 micron, the generation of finer particles that could produce coffee sediment, bitterness and increased brew time will be a factor and should be avoided at all costs.

Coffee Darkening: "My coffee gets darker during grinding. Is that normal and, if not, how can it be avoided?" There are two possibilities for this condition, one due to heat generation in the grinder/normalizer/mixer during grinding and the other being a perceived difference due to color measurement technology. Grinder heat and the resultant temperature increase of ground coffee is a function of the amount of energy required to grind a certain quantity of coffee. Remember physics? All energy eventually returns to its final form, which is heat. There are a few grinding relationships that reflect this:

As coffee is ground finer, more total energy is required, hence more heat.

In addition to the above, as coffee is ground finer, less coffee is produced and, consequently, the energy required per pound produced goes up even higher. Think of it as, say:

As any coffee grinder wears the otherwise sharp teeth and corrugations become dull and, like a knife, more energy is required to fracture/crack/cut the coffee than when they were sharp. Basically, it's the same as cutting a piece of meat. When the knife dulls, it takes longer and the amount of energy required (muscle) increases. The end result is more energy (heat) and an inferior cut.

The densifier/normalizer/mixer, which is part of a commercial coffee grinder, or Gran-U-Lizer, is used to reduce the chaff" as well as to modify density. Since energy is being consumed to perform this function, this is another area for heat generation and coffee darkening.

(Note." One of the benefits of water-cooled grinders is that color change is minimized (and flavor, volatiles and aromatics retention(s) are maximized) because the heat generation in the grinder is dissipated away from the grinding rolls.)

The alternative indication of color change involves instrumentation measurement differences.

When coffee is ground on, say, a disc grinder in the laboratory, and a color sample is established through a colormeter, with particle shape, consistency and a myriad of other Factors playing into the final tally, or numerical color shade.

Now, if we take that same whole bean and grind it through a roller-style grinder, many of those factors that contributed to the color shade are different and an inconsistent color match will be inevitable. Often, the roller-style grind will measure dark when, in Fact, the color is consistent and that particular grind is superior in every way, including grind uniformity, heat generation, etc.

Color standards must match "apples and apples" and a lab-ground whole bean sample cannot be expected to be color comparable to a Gran-U-Lizer ground product; however, a particular color standard can be established for each grinding methodology, which can then be certified for future color control purposes.

It's clear that coffee grinding significantly affects both brew quality and taste. In fact, coffee grinding is one of the two most important processing functions affecting the quality and taste of coffee, the other being roasting. This makes it a critically important factor in our business and one that merits the highest level of attention.

Good luck and good grinding!

Daniel Ephraim is the president of Modern Process Equipment, Inc., manufacturers of grinding equipment and representatives for roasting equipment. Dan extends the courtesy to our readers that if any issues or questions were not addressed, please contact Modern Process Equipment at: Tel: (1)(773) 254-3929, Fax: (1)(773) 254-3935 or E-mail:
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Author:Ephraim, Daniel
Publication:Tea & Coffee Trade Journal
Geographic Code:1USA
Date:Nov 20, 2004
Previous Article:Is my traceability performance "good enough"?
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