World Herb Trade Statistics, Methods of Drying
Answered by: Richard Alan Miller and Conrad Richter
Question from: Sefa Tarhan
Posted on: December 10, 2006

I am working at Gaziosmanpasa University. I am preparing a research proposal on the development of new dryers for the medicinal and aromatic plants. I have a few questions written below;

1. Where can I find statistics about world herb trade (unit price, annual production, annual trade amount etc.)?

[Answered by Conrad Richter:]

World trade statistics for most herbs are hard to find. The main problem is that international trade data sources track only the more important culinary herbs and spices and very few of the medicinal herbs. As far back as 1982 the International Trade Centre called attention to this problem, but little has changed and good crop specific data is still very hard to find.

The United Nations Comtrade database has trade data for medicinal herbs according to the harmonized tariff classification system, which unfortunately only has a few crop specific classifications, namely licorice, ginseng, and hops, and otherwise everything else is put in an "other" category. For example, the Comtrade database indicates that U.S. imports of medicinal plants (H.S. code 1211-90) increased sharply: $130 million (42,000 tonnes) in 2002; $142 million (46,600 tonnes) in 2003; $148 million (46,800 tonnes) in 2004; $165 million (52,000 tonnes) in 2005.

Trade, production, and prices of key culinary herbs and spices can be found in the the International Trade Centre’s report "World Markets in the Spice Trade, 2000-2004" which is available online at

2. What are the common methods to dry basil and echinacea?

[Answered by Richard Alan Miller:]

I have designed more than 12 different drying systems over the years, predicated on specific crop needs. I will presume you have read my chapter in my book "The Potential of Herbs As A Cash Crop" (POH) regarding design considerations and variables.

The scale is important, not the physical size. Many have since built larger designs from my original layout. The design in POH is probably the smallest size most begin. The through-put is limited, and many (ginseng growers, etc.) use larger trays and machinery to slide trays into position or removing dried products.

I have used Hop Kiln Floors (20’ x 20’), where crops can be stacked up to 4 feet in height. Other successful designs also use Grain Bins and Air Shafts. Product is conveyered to the top of an Air Shaft, and allowed to fall through heated air. It most often needs to be re-cycled for complete drying. Larger systems need larger fans, and more heat options.

Your design should reflect your through-put requirements. Obviously someone attempting to dry ten acres is going to need something quite different than one only handling one acre. Some try to time their cutting so that they use maximum efficiency when drying their crops. If you have several different crops, that should be taken into account as to when each will require drying (schedules).

Often a field wilt can speed up artificial drying, like in the case of Comfrey Leaf. Field wilts are usually less than 50% moisture before being removed from the sunlight. In the case of volatile oils, no amount of field drying can be permitted to retain quality and chemistry. Other chemistries (like St. John’s Wort) must also be taken into consideration. Each crop is different.

Wood is probably the cheapest building material, although I have stacked Apple Bins (with screens as their floor) five high. I’ve even seen several use Ag Bags (used for composting) to send air down a horizontal shaft, although this did not work as well as hoped.

Remember, there will a great deal of water coming off the drying product. This will cause absorption by the wood, and at some point, that wood will need to be replaced (usually 3 seasons later). Tin and other non-absorption material is best, of course.

For the design in POH, usually only 180,000 BTU is necessary (on a limited basis). This can be provided by an Air-Tight wood stove, or Propane Salamander. I have never needed anything larger than 400,000 BTU on Hop Kiln floors (20’ x 20’ squares). Heat is used only sparingly, as most of the drying process is accomplished by air flow and vapor pressure changes.

When using propane, usual requirements often demand a large tank reservoir (500 gal), to meet the pressure demands of a Salamander. You don’t need that amount of fuel, but the in-line pressure demands a larger source (Boyle’s Law).

The 17% slant is critical for the exchange of moisture into dry air. This was originally determined from older Prune Dryers and other fruit (leather) operations (1920s). This angle allows moisture to "roll" off the crop with better heat exchange. The angle can vary, with best designs using 15-25% grade.

One fan will work, to mix the heat and dry air with moisture. A second fan, on the top of the dryer acts as an exhaust fan. This vents the wet air into the outside, and creates a better vapor pressure exchange. By pulling a partial pressure on the drying process causes the moisture to be released much faster than normal turbulent flow methods.

Think of it as a move toward "freeze-drying" principles, although the ration is less than 10% ambient. This alone causes the exchange to be sped up more than 10X otherwise. This is a very critical and important aspect in the overall drying operation.

Fan design vary, but the best usually move 10X the volume of the drying chamber every minute. This is the CFM (cubic feet per minute) figure. Larger fans work best, as they can always be scaled downward. Air is far more important in the drying process than heat. Heat is used only during the second and third stage of the four stages of drying.

In the POH design, probably up to 18" depth. With larger systems, I have found 4’ to work. What happens in a Hop Kiln is that a "plate" is formed, where everything below this "plate" is dry, and everything above is wet. The "plate" moves upward at about 1/2-foot per hour when done correctly and uniformly.

When a "break" occurs in the "plate," wet material is then raked over to hold the air under the "plate." Some techniques also like to rotate wet material by turning the trays upside down during the drying process, exposing more wet material closer to the source of air (and heat).` I’ve also seen Tumble Dryers work well with some root crops.

There is a company in Eugene, Oregon which builds these types of dryers on a commercial basis. Designs often include specific crop requirements and can call for expensive engineering. Simple Dryers for new farm have included a warehouse where the crop was laid onto a tarp, and air and heat flowed across it. Of course, this system requires continual turning of the material to expose wet surfaces to the air flow.

There are so many designs nowadays, finding someone with a Mushroom Dryer, or something for Ginseng is quite easy and is probably "next door" or nearby. As stated before, Grain Bins can be used, as they were designed for Seed and cut material (forage harvester).

I also have an unpublished book on drying techniques seeking a publisher. Go to, and realize that are now almost 300 different books on various crops, to include Basils.

This field, including Freeze Drying and Drum Drying techniques, is new and a lot of technology to consider for more efficient systems (and crop requirements). The University of British Columbia (UBC) has a plywood dryer using microwave horns with a conveyor for continuous feed-through processes.

I could probably easily design something inexpensive for specific crops and their volume drying needs as an outside consultant. My physics background has been very useful for these kinds of farm questions. And, I have never been to Turkey...

3. The farmers in Tokat, Turkey, want to grow medicinal and aromatic plants. Where would they market their products?

[Answered by Conrad Richter:]

This question is difficult to answer comprehensively within this forum. Each culinary and medicinal herb crop is a different market and the buyers will be different. Obviously with your proximity to the European market the farmers of Tokay will be looking to that market first. But each crop will likely require a separate effort to find buyers. It is important to note that buyers generally want to become familiar with growers and suppliers before they buy significant quantities of herbs. It is common for buyers to request samples of product prior to placing an order. It is unlikely that they will give new growers a production contract until they have some experience with the new growers. I recommend that any business development project designed for the Tokat farmers should include a budget for a person who is responsible for market development.

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