Dry Milling: Parchment Remover

The task for today was to complete the two remaining videos of the different components within the hulling station at Beneficio El Manzano, and as I began my day, I attempted to look up a credible source or two that might lay out a somewhat scientific explanation of the happenings within two of the machines we were focusing on. 

I scratched and pawed my way through cyberspace, only to find vaguely worded descriptions of this process. Upon emailing the manufacturers however, Emilio and I scratched our heads for several minutes attempting to make sense of the complex description we’d received, of exactly what is happening within the density grading process of green coffee processing. (Seriously, they sent us a book). 

Now, to the layman, we present a scientific explanation of each of the different components within green coffee processing, from parchment to export, without sailing anywhere near the top of your head. 

The first aspect of dry milling we’ll focus on is the hulling, which simply refers to the removing of the outer skin from the green coffee; before it is  graded, categorized, and sorted by other various machines. 

Within hulling the task is two-fold. First, since the coffees are coming to the machinery exactly as they were bagged from the patio, they, must be screened of any foreign objects, be they rocks, leaves, sticks, etc.  Therefore, as the parchment coffee is loaded into a hopper, it is first transported to a machine within acts as a screen and destoner. This machine is a metal plate containing multiple holes, that allow the parchment coffee to pass through, while retaining all foreign objects. 

Secondly, the task within hulling is the strip the green coffee of its external parchment. As coffee leaves the screen/de-stoner, it is transported via bucket elevator to the huller itself, which receives the coffee from a hopper located above. 

There are multiple types of hullers; however the function is to use friction as a means of separating the coffee bean from the parchment without damaging or cracking the bean, and allowing it to continue processing and sorting, while removing and channeling out, the now removed parchment. 

To accomplish this, parchment coffee is put within a cylindrical chamber, which contains a second notched rotating cylinder; meaning the parchment is only able to pass within the space between the inner wall of the stationary cylinder, and the outer, notched wall of the rotating cylinder. It is at this point where it comes in contact with both, and the friction caused by the pressure and rotation, forces the parchment off of the green coffee. 

Gravity naturally pulls the green coffee down, however the parchment is removed by a vacuum positioned above the rotating cylinder which, as the parchment is separated from the bean, draws out the parchment and channels it out of the machinery, and deposits it into a holding tank to await later use. 

As it exits the cylindrical huller, green coffee is sifted by a rotary plate, which sorts out any parchment coffee still remaining in the batch, and cycles it back up into the huller for a second pass. The green coffee continues through the cleaning process, exiting a channel in the rotary plate, and into the machine which will grade the coffee according to weight.