For the first time, in my many years of eating lentils, I found a small stone in my lentils while I was eating the curry. It was slightly smaller than a single lentil.
On the bag of the lentils, and in every online recipe describing how to cook lentils, the following warning exists: be sure to rinse your lentils prior to cooking to remove any rocks or debris as modern cleaning equipment cannot. And I do. But I have never actually found a pebble, and so thought this was a cautionary warning that lawyers required. Now, I figured, the process of cleaning the lentils during production likely was efficient and accurate enough now (in the direct contradiction to what the warning tells me). A good self-reflection example of how my prior experience incorrectly outweighs expert knowledge.
“A combination of gravity, screens and air flow is used to clean and sort lentils by shape and density”, is how Wikipedia describes the process. Most of the YouTube videos available focused on splitting the hull from the lentil, post-removing the plant, soil, and rock debris from the collected lentils.
Only a few videos, primarily from industrial companies selling lentil processing machines, showed this cleaning stage. Many of these give very little insight, instead showcasing the machines’ attributes and very poor marketing materials.
Sorting by hand is a tedious process that somehow in videos only takes a few seconds, but the outcome is clear. It takes me longer to peruse the lentils for stones and small debris, though. With green lentils, I use water, but red lentils don’t float for the most part.
A promotional video does a nice job of showing the before and after product, demonstrating what type of debris is removed from the lentils by each machine, but doesn’t demonstrate the mechanics of the cleaning process. The video description is quoted in full below:
Sensitive Cleaning Machine; screens the dust, corp, chaff, etc. from the product and finishes the pre-celaning process. Later; for sorting the high density things, product enters to the stone separator.
Stone Separator is mostly for sorting impurities like; stone, glass, dust, etc. Later the product enters to the Gravity Separator for separating light products.
Gravity Machine is used for separating the light materials from the product. This machine finalize the cleaning process. On the next step; product passes to the color sorting machine.
A much more bountiful process was looking up each of these stages by the machine type, rather than focusing on lentil cleaning specifically, so I changed my research approach.
The lentils are dumped through a series of screens, shaken by an agitator, which allows the lentils to fall through the screen, while larger debris is stuck on top, and steadily shaken off to the side to be dumped on the ground.
The gravity sorting is the most clever aspect to this, sending the grains over a shaking panel that a) has air coming up from the bottom, and b) is slightly titled in one direction. Given the air is only powerful enough to push the lightest objects up, the heavier contaminants stay in contact with the panel and are shook in one direction. The lighter debris is pushed up and over in the opposite direction, going down the tilted panel on the other side. If I understand correctly, the settings to control the tilt, airflow, and vibration (eccentric movement), are set relative to the weight of the desired subtance.
The sorting “dry particles” by separating objects by their density is brilliant, and the speaker in this Oliver Manufacturing video has a great breakdown. Seeing in real time how adjusting the air and tilt settings of the machine affected the separation of contaminants was intuitive. A destoner machine appears to operate on the same principles as the gravity sorting.
1. Honorary mentions go to a nice video on the farming and sustainability of lentils on a Canadian farm. Each lentil plant only grows about a foot tall, and the lentils are found within small pods in the plant. A pod may contain only 1 to 3 lentils each.
2. The word “debris” belongs to that group of words that has no plural and is only plural when describing a plural noun, e.g., piles of debris.