Milling is one of the most important post-harvest steps in coffee production. It involves removing the various layers of the cherry to remove the seeds. Wet milling is carried out after harvesting, while dry milling happens once the green coffee reaches the optimal moisture content for export.

Around the world, producers use a number of different types of mills for these steps, which vary from washing stations and large commercial mills to micro mills. Micro mills are small-scale operations which are often family or community-owned.

So, how do micro mills operate and what do they mean for coffee-growing communities? To find out more, I spoke with two coffee professionals based in Africa.

You may also like our article on how micro mills can help producers improve their economic independence.

The stages of coffee processing

There are two main types of coffee milling: wet and dry.

Wet milling covers the removal of the seeds from the flesh of the coffee cherry. The flesh includes the outer skin and the pulp, which are removed using a depulper.

Meanwhile, dry milling covers removing the parchment from the beans, after which they are graded and sorted, packaged, and exported.

After coffee is harvested, it is delivered by farmers to wet mills.

For washed coffees, the cherries are pulped and then fermented in vats or tanks. They are then cleaned to remove any mucilage (a sticky, sugary substance which surrounds the seeds), before being pre-graded and dried.

Alternatively, for honey processing, coffee can be sent directly to drying beds or patios after it has been depulped. Once the beans reach the correct moisture level (between 8% to 12.5%, according to guidelines from the International Coffee Organisation), the beans are dry milled. This is when the husk (the dry skin on parchment coffee) is separated from the green coffee. 

Natural processed coffees, meanwhile, undergo no depulping. Instead, the cherries are delivered straight to drying beds or patios where they are left to reach optimal moisture levels. Producers turn the cherries regularly to avoid mould or overfermentation. 

For any processing method, however, coffee must be destoned and hulled. Destoning machines help to remove small stones and other physical contaminants which can be picked up during harvesting. 

Hulling machines remove the parchment, or in the case of honey or natural processing, the dried cherry as well. This results in green coffee which can then be exported and roasted.

The beans are then polished by removing the silverskin: a thin, papery layer which is difficult to fully remove. This is followed by grading, where the beans are separated according to size and density.

How do micro mills work?

Essentially, micro mills are facilities which process coffee for a small number of producers. They are mainly owned by either a single producer or a group of farmers who use similar processing and quality control methods. 

For a group of coffee producers to set up a micro mill, they will usually form a small co-operative and purchase land on which to build the facility. This way, the farmers can collectively have more control over the harvesting, processing, and trade of their coffees. 

The machinery used in micro wet mills has also significantly improved recently. Previously, large and bulky machines were used, whereas now, farmers have more access to smaller, more modern motorised or handheld pulpers. 

This means it’s possible for smallholder producers to process coffee without having to travel as far from the farm, so they can potentially carry out more experimental processing. However, these lots will usually be small. Handheld pulpers are also useful for farmers living in remote rural areas, who may have little to no access to diesel or electricity.

Peterson Kimathi is the quality control manager for Mai Shayi roasters in Nigeria. He tells me why micro mills can sometimes be appealing for individual producers or small co-ops.

“When smallholder producers have access to a micro mill, they don’t have to pay the charges to commercial millers,” he says. “At Mai Shayi, we have a small mill that carries out the hulling, but grading has to be done elsewhere.”

James Ndegwa is the Secretary Manager for one of the largest coffee farmer co-operatives in Kenya.

“Our co-operative is large, with many washing stations,” he says. “Our mill processes the parchment from all 19 members of local washing stations.”

James explains that micro mills are often appealing to co-operatives who own or operate only one washing station.

“They would need a micro mill that could cater to their needs,” he tells me. “That way, the co-op will be able to control all quality control procedures right from wet milling.”

Micro mills also have benefits for farmers who know their customers want high levels of traceability in their coffees. This is because their coffees can be processed separately from coffees sent from other washing stations, keeping them as individual lots.

Conversely, at larger facilities, cherries from co-operatives can often end up being grouped together to be milled and harvested. This can then affect traceability, as it becomes more difficult to identify which lot belongs to which individual producer. 

Criticisms of the model

There’s no denying that with micro mills, smallholder farmers can gain more independence and freedom when it comes to how much control they have over their coffees. By controlling their own production schedules and timelines, smallholder producers can focus more on maintaining or improving coffee quality if they wish to do so.

However, there are criticisms of the model. Firstly, they require a significant investment in equipment as well as needing land to be built on. They may not always necessarily be economically sustainable in the long term, as you’re effectively banking on improved quality control balancing out the cost of establishing the mill itself.

It’s also worth noting that dry mills are notably more difficult and expensive to establish than wet mills. This is because they require more machinery, such as cleaners, destoners, hullers, polishers, and graders.

Milling also has an environmental impact, and at smaller facilities, managing this impact can be a lot more difficult; larger and commercial mills often have access to the supply chain and infrastructure to responsibly dispose of waste.

Wet mills also create coffee wastewater, which is difficult to properly dispose of; if it ends up making its way back into the soil, it can even be detrimental to soil health and affect productivity or quality all over again.

Micro mills can make it much easier to process micro and nano lots separately to larger harvests, as well as testing out experimental processing techniques (which may not be possible at larger mills).

However, this is only relevant for a small percentage of coffee farmers, who serve as an exception to the rule. Furthermore, while smaller and more experimental coffee lots can be profitable, they often don’t provide the same kind of scale as far as income is concerned.

Generally speaking, micro mills do have their appeal, but it is important to note that at scale, the model can become much more difficult and complex. With greater volumes of coffee, it can be easier to simply outsource milling to a commercial operation.

Will micro mills become more common at origin?

By definition, micro mills are designed to be used on a small scale. But could more farmers and co-ops be able to operate their own?

“The cost of buying, installing, maintaining, and eventually upgrading the milling equipment has to be accounted for,” James tells me. “Servicing is important for the smooth operation of the mills.

“You can have various options, such as an in-house engineer or a supplier who can visit at scheduled times.”

Micro dry mills in particular need to be affordable for co-operative and smallholder farmers to operate. In order to maintain coffee quality, micro dry mills must also have grading machinery, which can be costly. This is because grading usually requires skilled labour that might not be available at the mill. 

“Before buying a grading machine, farmers should consider how long it will take for them to recoup their investment,” Peterson tells me. “The farmers will save on transport and milling costs, as well as on time.” 

Peterson points out that through operating their own micro dry mills, farmers would have more access to coffee by products, such as husk. This is usually left at dry mills, but it can be repurposed for a number of other uses.

“After dry milling, we have what is referred to as milling loss,” he says. “This is the percentage of green coffee left after hulling, which is the husk.

“The husks can be reused for various purposes, including fuel, compost, or animal feed. When a farmer has their own mill, this is all left with them.”

Ultimately, micro dry mills are likely to be more suitable for groups of smallholder producers or co-operatives who can justify the capital investment, rather than individual farmers.

Nonetheless, there are still concerns that the model is not sustainable for operating at scale, and it’s worth noting that in many cases, working with the larger mills can have benefits for the farmer.

It ultimately depends on how much capital the farmer or co-operative in question has, and exactly what they are planning to do with their operation in the medium and long term.

Milling equipment, as it is specialist machinery, can be an expensive initial investment. However, by bringing it closer to the farm, producers gain more control over the process. But as much as this might sound positive, it is arguably not a solution for everyone.

Ultimately, micro mills are a complex concept. They might be suited to some producers, and arguably won’t work for many others. Before making any kind of investment in milling equipment, it’s worth looking at the medium and long term plan for your farm, and understanding whether or not it really makes sense for you.

Enjoyed this? Then read our article on centralised vs in-farm coffee mills: what are the differences?

Photo credits: Peter Gakuo

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