Coffee dry mill

How a coffee dry mill works:
7 step breakdown

The coffee dry mill is one of the last stops on the journey your coffee takes from its coffee tree to your cup. While every step on the way plays an important role in coffee’s flavor, what happens at a dry mill is arguably one of the most important.

Many exporters make investments in their coffee dry mills, ensuring they have total control of the process. This allows them to process coffee faster for final export. And at the same time, they can thoroughly control the quality of the final product.

It is here that the beans go through final cleaning, polishing, sorting, and grading – steps that ensure each batch becomes a uniform coffee lot. In this breakdown, we take you deep into the workings of the dry mill. We guide you through the 7 crucial steps within the dry mill so you know what happens to your coffee. But first, why is uniformity important?

One of the primary jobs of a dry mill: making the coffee uniform

Smaller beans tend to roast faster than larger ones. So if a batch of coffee has beans that are not the same size and shape, you end up with beans roasted differently in the same batch. On the other hand, a batch of coffee that is even in shape, size, weight, and color roasts evenly and uniform. It ensures consistent aroma and acidity levels across the batch. A dry mill is the key step wherein uniformity requirements are met.

The operator and customer customize the dry mill process

Operators and exporters automate their modern dry mills to keep human input at a minimum. Once the beans go in one end, they come out the other ready for bagging. This does not mean they produce only one end product. Depending on a customer’s needs, operators set different parameters to customize each batch, starting with the very first step.

Step 1 – Setting the Ratio

Incoming coffee beans move to the dump area within the dry mill. This is also where operators can control the ratio of different types of coffee beans in the batch. Dump areas consist of multiple entry points where operators can empty the sacks. Once the beans are in, they move along the dry mill and pass through each of the following steps along the way.

Step 2 – The dry mill cleaning techniques

Impurities can always seep through the quality checks at the washing station. This part of the dry mill removes every last bit of unwanted material. Only clean coffee beans proceed to the next step. Cleaning itself is a three-part process consisting of pre-cleaning, the magnet, the destoner.


Pre-cleaner: The pre-cleaner is simply a vacuum that sucks any fibers (such as the leftovers from jute bags) or dirt away from the coffee. While the suction is powerful, it is not strong enough to pull away coffee beans that move down to the magnet.


Magnet: This is quite a simple part of the machine. As the coffee beans move past the magnet on a vibrating belt, it takes out any magnetic metallic impurities.


Destoner: Now that the coffee is free of light impurities and metallic impurities, it moves to the destoner. Here, vibrations separate the coffee from any small stones that hide among the coffee beans.

Step 3 – Huller

Not all coffee beans enter the huller. Take natural coffees. In most cases, naturals already passed through a huller. Washed coffees have not and enter the dry mill with the parchment still attached. A bypass system allows operators to choose if the coffee in a particular batch needs to pass through the huller or not. The huller removes the parchment surrounding the coffee beans and reveals the green bean.

Step 4 – Polisher

Now, even if the coffee has been through a huller, there is a chance that dried parchment or mucilage may still cling to the bean in small amounts. The polisher removes any residue leftovers from the huller.

Step 5 – Dry mill sorting

Now that the coffee beans have been cleaned and freed from their parchment, the final sorting begins. Like cleaning, sorting is a multi-step process and begins with grading by size.

Grading by size

Grading by size: Sorting by size is done through an arrangement of multiple sieves of different sizes. Smaller beans make their way down to a sieve with smaller holes till they can’t pass through anymore. Then the beans move to silos that separate the coffee by size.

Grading by weight

Grading by weight (density): The next step is to sort the similarly sized beans by weight. This is done via a densimetric (or air density) table – a type of table that is set at an angle and uses the gravity created by the angle combined with blowing air to sort coffee beans by density.

Grading by color

Grading by color: The final machine-controlled step in the sorting process is color grading. Here, high-speed machines look at every single coffee bean passing through and sort them by color. Operators set the kinds of color the machine should look out for based on client requirements and individual characteristics of a batch of coffee.

Our blog on coffee grades explains more about quality classifications through grading.

Step 6 – Hand sorting

However fast and accurate machines are, they still let a few improperly sorted coffee beans through the system. That is why the final step for specialty coffee is hand sorting. Scores of well-experienced sorters hand sort the coffee beans, picking out any that are not fit for purpose.

Step 7 – Bags filled and sewn

Once hand-sorted, the only step remaining is to bag the coffee. A machine-controlled process. But the customer and operator discuss the parameters before the coffee enters the dry mill. The bagging machine automatically fills bags with the precise weight of coffee beans and sews the bags.

From dry mill to storage and shipping

The dry milling is done. The coffee – now perfectly sorted so that each bag contains a homogenous selection of beans equal in size, weight, and color – awaits shipping in a warehouse. Once it makes its way to a roaster through efficient logistics, all the hard work comes to fruition. Completely uniform in characteristics, the coffee can now be roasted to perfection – releasing its delightful aroma to the world.

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