Open a roaster’s collection chamber for the first time, and the pile of pale brown flakes may be almost as interesting as the coffee.
They crumble easily and lift away in the slightest breeze, rather like tiny pieces of paper.
So what are they?
The short answer is coffee chaff—a normal by-product of roasting that comes mainly from the thin silverskin surrounding the coffee seed.
What Is Coffee Silverskin?
A coffee cherry contains several layers around its seeds, including fruit, mucilage, parchment, and silverskin.
The silverskin is a very thin layer that directly covers the coffee seed. Although much of the surrounding material is removed during coffee processing, some silverskin can remain attached to the surface of the green bean and inside its center crease.
During roasting, the beans lose moisture, expand, and change structure. This loosens the remaining silverskin and causes it to break away.
Once detached during roasting, the light, papery material is commonly called chaff.
You can learn more about these physical changes in our article explaining why coffee beans need to be roasted.
Why Does Chaff Collect in a Hot-Air Roaster?
In a hot-air or fluid-bed roaster, moving air performs two jobs: it transfers heat and circulates the coffee beans.
Because chaff is extremely light, the airflow can carry it away from the beans and into the machine’s chaff collector or filtration system.
The exact collection method depends on the roaster. Some machines collect chaff above the roasting chamber, while others use a separate tray, screen, or cyclone.
Why Do Some Coffees Produce More Chaff?
Switch to a different coffee and you may find a noticeably different amount of chaff in the collector.
The amount can be influenced by:
- Coffee species and variety
- Post-harvest processing
- Dry-milling and polishing
- Bean size and density
- The individual lot
- Roast level
- Airflow and collection efficiency
Natural, washed, honey, and other processed coffees can behave differently, but the processing label alone does not reliably predict exactly how much chaff a coffee will produce.
More chaff does not automatically mean lower-quality coffee, and less chaff does not necessarily mean that a coffee is better.
What Is the Pale Line in the Center of a Roasted Bean?
You may notice a pale line inside the center crease of some roasted coffee beans.
This can be silverskin that remained lodged in the crease during roasting. It is especially noticeable on certain coffees and lighter roast levels.
Its presence does not automatically mean that the bean is under-roasted.
There is no need to roast the entire batch darker simply to make the pale center line disappear. Judge roast development using the complete picture: aroma, color, time, temperature data, first crack, and—most importantly—the flavor in the cup.
Why Should You Clean the Chaff Collector?
Coffee chaff is dry, lightweight, and combustible. Allowing it to accumulate can restrict airflow, increase smoke, and create a fire risk.
After roasting:
- Allow the machine to complete its cooling cycle.
- Switch it off and let it cool.
- Empty the chaff collector.
- Clean related screens and parts according to the manufacturer’s instructions.
When you try another coffee, take a look at the flakes it leaves behind. They offer another small glimpse into the structure and processing of the beans you are roasting.