Yerba mate varies from country to country and even within each country. There are several factors the affect each style: leaf size, amount and size of the stems, amount of dust, drying process and the age.
All of these factors effect the taste and effect.
Leaf Size
Affects the brewing speed, an large leaf cut will last longer in a mate with a bombilla and have a softer flavor. A medium to small leaf cut will brew faster and taste stronger.
Stems
The stems are chopped up to the producers specifications before adding back to the final product. They add a little sweetness and soften the flavor and act as a filter when used in a mate with a bombilla. Adding stems also lowers the cost. The amount of stems and size can vary greatly.
Dust
Some dust occurs during packaging and shipping but most if it is added back to the yerba according to the producers specifications. The dust creates a much stronger flavor and effect on the initial brew. It also adds bitterness to the flavor. Added dust is popular in most Paraguayan products because their most popular method of brewing is terere. Which is pouring ice water over the yerba mate and drinking it with a bombilla.
Drying Process
Step 1: When the yerba mate is harvested it is chopped up a bit and is loaded into a large oven that tumbles the yerba and flash dries it. The heat source in this oven affects the flavor significantly. Most producers use wood such as eucalyptus which is easily sustainable. There is a large fan that blows the heat with fumes into the tumbler. In both cases the yerba is not only getting dried it is getting cooked as well. There is a relatively new oven being used now in Brazil where no fumes from the heat source makes contact with the yerba. The flavor is noticeably different from Argentina and Paraguay brands.
Step 2: When the yerba comes out of the first oven it is immediately transferred to very a large drying building. It is slowly moved through these dryers over several hours to completely dry the yerba. Here is where some producers add smoke to the dryer to develop their particular flavor. The yerba mate products in our store will be labeled as smoked if we know this is the case.
Aging
Most yerba mate is aged after drying. Aging softens the flavor and affects the color. Fresh green yerba mate can be very bitter and have a fishy aftertaste especially if it was flash dried with wood. The longer the yerba is aged the less bitter is becomes. Natural aging can be from 9 to 24 months. Many of the large producers in Argentina speed age the yerba over a much shorter time with low heat, this is because they simply do not have the space or the time and they can harvest according to demand. Speed aging is a bit more bitter and dry tasting. If you are new to yerba mate you could not tell the difference. We know of no producers in Paraguay that speed age. We have "born on" dates in our product descriptions for the ones we purchase in large quantities.
Here is an interesting story about finding your favorite. I had a customer purchase a 44 lb bag of our organic yerba and send me a terrible email saying it wasn't fit for compost among many other comments. He made the assumption that all yerba mate tastes the same. Afterwards we had customer that has never purchased from us before order the same 44 lb bag of our organic yerba mate. This time I had to call him to make sure he knew what he was doing. He told me he did not care what it tasted like because he would get used to it. I found this to be true. I will bring a yerba in to try and find it funky tasting but if I keep drinking it I forget what it was about it that I did not like.