Northwest Botanicals,Inc.
1212 SW 5th Street
Grants Pass, OR 97526-6104

(541) 476-5588
(541) 476-1823 (FAX)

[from Herb Processing Facility 2002, Richard Alan Miller, © 2002. For the complete book, a PDF download is
available from]

by Richard Alan Miller © 2002

There are basic physical principles behind all systems used in agriculture. This includes those used in processing herb and spice crops. The physical principles at work in milling, sifting and cleaning include gravity, air movement, centrifugal force, magnetism, and inertia.

Early man thrashed wheat with a pole and threw it into the wind with a shovel. With tools as simple as a shovel and a pole, this was how grain was separated from the chaff. The principles at work in cleaning wheat are still used in today's cleaning and sifting equipment. Another example is the mortar and pestle: the pounding and shearing forces that operate when grinding down a root into a powder are still used in milling machinery. Today's systems differ mainly in their capacity to process tons of products in volume, not in the principles behind them. Plants and agricultural crops have remained unchanged in their physical characteristics, allowing the same basic laws of physics to continue to be utilized to process herbs and spices.

From the earliest days it was realized that the more pure the final product, the higher the value or price. Both buyer and seller know that value was directly related to quality of flavor, color, or size.

Today, we are not only concerned with flavor and appearance, but also the more aesthetic value and any potential health hazards to the consumer. In other words, not only do we now remove stones, stems, foreign seeds, etc., but also insects, excreta, mold, bacteria, hair and chemicals that may affect purity. Purity has taken on more in its total definition.

There are only four major physical aspects that are currently used for the cleaning agricultural products. They are classified as:

Size and dimension: Herbs and spices (including seed) have many forms and sizes. Even varieties of a given crop can vary in dimensions and weight, like mature or immature seed. Most impurities usually have enough differences in their size and dimension to be easily separated by this characteristic. Overall volume, width, and length are the measures used to size and dimension most products.

Specific gravity is a measure of volume, and is related to the amount of water displaced (volume measurement). When particles are agitated in bulk, the larger sizes will work themselves to the top of the mass of material (specific gravity). Using sieves with a gyrating motion, the smaller particles will work themselves down to the screen surface, and can then be separated via a screen size.

Specific gravity: While many products different in weight, specific gravity and relative density can be used to effectively remove products which weight the same but have a different shape. This is often done by using air blast separation in air screen machines and is known as winnowing. Other forms use gravity or by volumetric variations (like wheat and chaff).

Aspiration: The rate at which a particle falls in still air (velocity) is determined by the force of gravity. As resistance increases (by shape and dimension), a point of terminal velocity is reached. This is when the airflow matches the resistance, and the product is suspended in air. Terminal velocity is determined by weight and surface area (volume) ratio.

Products with compact, spherical or cubical shapes have a higher terminal velocity than more defuse and flake-like shapes. This means most sifting operations use an ascending air current into the material being delivered, rather than still air (free fall).

The velocity of this air current is usually regulated so that shapes of higher terminal velocity will fall, while those of a lower specific gravity are lifted into a collection system (Rotocone). Using this principal particles of chaff, straw, small seeds, dust, stem, etc. that have terminal velocities lower than the product (being aspirated) can be separated. This is called Rotocone separation.

Aspirating the product several times in succession usually does lower separating efficiencies, due to similar shape and weight. Balancing this airflow aspiration is an artform, and needs constant readjustment.

Magnetic properties: Sanitary plates, grates and traps are highly efficient permanent magnet units that remove ferrous metal from dry or liquid processing lines to protect product purity and prevent costly maintenance problems.

The trend in building cleaning equipment is all steel construction, whereas wood and other materials have been used is the past. FDA now requires all food grade materials must only touch stainless steel. Cleaning equipment wants to also be simple in operation, not with gadgets and poor mechanical features.

To use the machines effectively, the processor must know the effective principles involved. He must also understand the physical properties of the product being cleaned, and the physical properties of any contaminates in the crop products. After the differences between the product and its contaminates are known, the processor can then select the specific machines that will make the most efficient separation.

NOTE: This article was taken from Herb Processing Facility 2002, Richard Alan Miller, c2002. For a complete book, a PDF download is available from You can also visit Richard Alan Miller's website at


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1212 SW 5th St.
Grants Pass, OR 97526
Phone: (541) 476-5588
Fax: (541) 476-1823

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In addition, you can visit Richard Alan Miller's home page for a listing of his writings, also containing links to related subjects, and direction in the keywords Metaphysics, Occult, Magick, Parapsychology, Alternative Agriculture, Herb and Spice Farming, Foraging and Wildcrafting, and related Cottage Industries. Richard Alan Miller is available for lectures and as an Outside Consultant. No part of this material, including but not limited to, manuscripts, books, library data, and/or layout of electronic media, icons, et al, may be reproduced or transmitted in any form, by any means (photocopying, recording, or otherwise), without the prior written permission of Richard Alan Miller, the Publisher (and Author).