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Grinding technology.

Grinding Technology

Reduction of roasted coffee whole beans to the appropriate granular size for the most efficient extraction in brewing equipment is a key step in its marketing, and is under continual study for efficiency and improvement. The industry has come a long way since the early days of crude cracking and grinding rolls; and even greater progress may be envisioned during the next century.

In October of this year, one of the oldest U.S. manufacturers of grinding equipment, Micron Powder Systems of Summit, N.J.-held an all day seminar for members of the technical press, giving an overview of the industry with discussions of the various factors and markets for powdered products and ingredients. Topics included: unit operations relating to powders; size reduction and powder technology; model installations for powder research; critical issues in powder technology; estimate of market potential for such equipment.

Micron Powder Systems has been operating as a manufacturer of grinding equipment for some seven decades. Early, it originated the Mikro Pulverizer, a simple continuous hammer mill which can regulate particle size by varying exit screens. It is now the American division of Hosokawa Mikron, International headquartered in Japan with branches in all major countries.

Market analysis, conducted by an independent agency, indicated the total world market in 1988 for powder processing equipment was $1.2 billion per annum. Breakdown of these data for coffee and tea processing indicated sales of $18 million; growing at a steady 4% rate to an estimated $23 million in 1995.

The theory and practice of size reduction and particle technology has already been viewed as something of a curiosity by members of the engineering profession. It is highly scientific and a thorough understanding requires comprehension of the basic principles of physics, aerodynamics, mechanical engineering, entropy, in addition to chemistry and chemical engineering. Yet it is visualized as a defined area of expertise by very few advocates in the scientific academic community, where it would be an appropriate study for fundamental research.

An aspect of grinding that merits intensive study is the energy expenditure. Size reduction is fairly energy intensive and the finer the grind the more energy required. Unfortunately, it must be noted, only a small portion, less than 10% of the energy supplied to the size reduction equipment is utilized efficiently for reducing the size of the individual particles. The lion's share of the supplied energy is converted to heating up the product. This heat serves to volatilize flavor, causing a significant loss of aroma. Thus grinders should be chilled or even refrigerated. Air cooling is not appropriate as the air carries away aromatic flavor.

Size reduction is also characterized by the hardness of the product ground. The word "hardness" is used to indicate the resistance a body presents against penetration by another body. This resistance, which also plays an important role in wear analysis, is generally indicated by reference to standard hardness scale. This is a 10 stage scale so that a substance in one stage can scratch a body in a lower stage and will itself be scratched by a body in the next higher stage.

In this scale, talcum is number one and will be scratched by everything. Diamond is number 10 and is superior in hardness to all known materials. Salt crystals are number 2; while quartz or sand is a high number 7: when even a small amount of sand contaminates coffee, it will cause serious abrasive wear upon grinding and other metals that it comes into close contact.

Particle failure is the aim of size reduction. During grinding, the particles may be subject to three types of contact forces: Forces involving contact with a machine component; forces between particles and a grinding medium; or between the particles themselves. These contact forces deform the particles and generate stress, which, when strong enough, brings about fractures or cracks. The number and direction of these fractures determine the size and form of the fragments. Grinding thus becomes dependent on the internal state of strain of the particles.

Deformation behavior under grinding conditions may be either elastic, plastic or viscous. "Brittle" deformation implies that a product like roasted coffee beans behave inelastically, fracturing as a result of time-dependent strain. Green coffee will deform elastically, when being ground, until such time as the fracture occurs. Splits, cracks, hollow parts, foreign inclusions and displacements within the particle have the effect of increasing internal source of inelastic deformations and fractures.

With decreasing particle size, elastic behavior decreases and plastic activity increases. A grindability limit is reached when particles go below a millionth of an inch in diameter. With plastic behavior, permanent deformation increases, thus reducing possibility of fractures. Fine powders tend to agglomerate and are only temporarily separated by grinding.

Various types of grinding mills were described with their fields of application. This included Jaw Crushers, several types of roller mills, high compression, table, bowl, pendulum and high impact mills; hammer mills; disintegrators, cage mills, jet mills, classifiers, ball mills, dispersers, colloid mills and industrial comminution systems.

Myriad Factors Involved

Micron Powder Systems Co. maintains a laboratory for initial study of the best procedure for grinding various materials as well as a pilot plant for the preparation of semi-commercial batches. Before selecting, optimizing and operating a size reduction process, a thorough study of the properties of the material is pursued. Over a dozen different factors are important including toughness, abrasiveness, cohesivity, adhesivity, particle shape and structure, softening and melting points, fat and aromatic content; toxicity, flammability, explosive potential, particle size distribution, agglomeration tendency, bulk density, etc. Whether this laboratory could be of aid in routine grinding of roasted coffee is open to question. However, it could be of inestimable value for special grinding problems such as Turkish coffee, green coffee or coffee for non-beverage purposes, e.g. candy, baked goods, etc.
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Title Annotation:Micron Powder Systems; techniques and equipment for coffee grinding
Author:Lee, Samuel
Publication:Tea & Coffee Trade Journal
Article Type:column
Date:Dec 1, 1990
Words:973
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