I would appreciate someone explaining to me how in an area like here in Honduras where the average humidity is about 80% one can air dry seeds down to 12%…or even 10%.
For roasting coffee, the seed first needs to be down to about 12% moisture. People do that every day all the time here just using the air… no special equipment. How can air with 80% humidity dry something…like coffee or corn or seeds down 12% moisture?
Wow! Your question is incredibly timely. @Tim_Motis, Robert Walle and I were just discussing a tool that Tim found to share with the network that helps put into scope, ambient seed drying. The Seed Information Database has a tool that calculates equilibrium moisture content based on seed oil content, drying temperature (C), and relative humidity of the environment. The database has some presets for oil content of some crops including Coffea arabica based on values from primary literature. It also sites sources. Here is a link to the tool: Seed Information Database.
Based on inputting their preset for coffee and 80% RH, temperatures of around 55-60C to get down to the 12% range. That’s far too hot! Looking at the description of the tool, it’s meant to be used for environments with RH of 10-70% and temperatures of 0-40C so perhaps it’s not applicable. But the tool does explain a bit about how ambient seed drying works (what factors impact it). Tim may be able to elaborate more as well.
The Seed Information Database that Stacy referenced is useful for relating humidity to seed moisture content. The tool accounts for seed composition. At a given level of humidity, seeds high in oil will have a lower moisture content (because oil and water don’t mix) than seeds high in starch.
The higher the air temperature the faster water moves out of seeds. The higher the humidity the higher the temperature needed for drying to occur. Water moving out of the seeds raises the humidity of the surrounding air…at some point drying will stop unless the moist air is vented out/away; this venting would happen naturally with seeds dried in the open air.
These are just some basic principles that apply to drying in general. Recently we had several consecutive days of rainy weather here in Florida, with humidity levels staying above 90% most of the day. That proved to be a good opportunity to test a little temperature-controlled drying box I had made using a Styrofoam cooler (see this link for details: Microcontrollers/W1209 Drying Box at main · ECHOInternational/Microcontrollers · GitHub). Even with ambient humidity above 90%, starting with 413 g of fresh moringa leaves, the leaves lost 6 g (1.45%) of biomass per hour with the drying temperature set at 37 degrees C. Drying rate increased to 2.16% after increasing the drying temperature to 40 degrees C. That turned out to be too slow to avoid yellowing of the leaves (I had them in a mesh bag in too deep of a layer, so they started heating up during the night when I wasn’t around to turn them). I dried a second batch of leaves indoors at an ambient relative humidity of 65% and drying temperature of 37 degrees C. Starting with a smaller quantity of fresh leaves (234 g), and spreading the leaves out more, biomass of the leaves decreased quite a bit faster—3.4% per hour and I ended up with nice, crispy leaves for making moringa leaf powder. All that to say that, while drying will happen faster at low humidity, it can still occur even at higher humidity levels. For this moringa drying example, some key factors were air circulation (a function of air moved by DC fans and the depth of the leaf layer), the ability to warm the air, and the ability to exhaust humid air. In the drying box, I placed mini heat pads close to the bottom and air vents at the top of the box to encourage convection drying.
When drying in open air on a flat surface, temperature experienced by the seeds is probably influenced by the drying surface and the extent to which seeds are piled or spread out. The drying surface would be more of a factor the more spread out the seeds are, with light reaching the drying surface through gaps between seeds. In drying sorghum on a tarp for instance, I noted dead weevils under some of the grain, which clued me into the fact that the tarp got pretty hot.
Lots of interesting things to think about when it comes to drying!
As Dr. Motis stated, surface, air flow, temperature, and relative humidity will all affect how fast something can be dried. You have to remember that AVERAGE humidity is by no means what the humidity is like all the time throughout the day. My guess is that from 2100-0500 in many parts of the tropics, RH is likely 100%. In the tropics of West Africa, its not uncommon to have 30-100% humidity throughout the day. You can pretty much guarantee that with no rain, and no abnormal incoming storm, humidity will peak in the early morning hours 0500-0800 and is typically lowest from 1500-1800. Even during rainy season, people will dry many products, but consistency is a lot harder with the rain storms, which typically then requires everyone to pack up their tarps of drying seeds/grains before they get soaked with rain.