Herb Drying Principles
Answered by: Richard Alan Miller
Question from: Hilary Rinaldi
Posted on: December 20, 1999

I read your book "The Potential of Herbs as a Cash Crop" and subsequently have spent the last year growing a test plot in order to go into a full time business growing herbs. I have my degree in Horticulture as well as 15 years in the industry so I am familiar with the plant material, but drying it is a whole other matter. Could you please answer some questions for me regarding building a dryer for herbs? I would appreciate any information you could give me since we want to build it in the next month or so.

I will presume you have read my chapter in "The Potential of Herbs As A Cash Crop" (POH) regarding design considerations and variables. Responding to your questions:

1. Does the dryer have to be the size you specified in the book to work the best?

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.

2. Is there some kind of ratio formula you use to decide the size of a dryer?

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.

3. What material is best to build it with? Wood?

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.

4. What kind of heater do you use and where do you find them? How many BTUs [British Thermal Units]?

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).

5. Do the racks have to be slanted?

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.

6. Will one fan work, or should more be placed around for better circulation?

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.

7. How big should the fan be?

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.

8. How much of each herb should be placed on each rack to dry evenly and quickly?

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.

9. Does someone already have one of these types of dryers built that could be looked at?

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 a book written on drying techniques looking for a publisher. 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).

Thank you so much for your time. I found your book so fascinating that I really want to go into this full time in the future and hopefully make a living at it! I hope to hear from you soon.

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