Like many crucial processes in coffee production, wet milling exemplifies a fundamental truth that has emerged over recent decades in the industry: Quality in coffee is created through a series of intentional mechanical interventions, each designed to achieve specific biochemical outcomes.

Geeky, I know, but when we understand these mechanics and their effects, we can better appreciate how wet milling shapes the coffee we ultimately experience in our cups.

Wet mills are commonly used in regions known for high-quality Arabica coffees, such as Ethiopia, Colombia, and Central America. The process is preferred when clarity and acidity in cup profile are desired.

The Essential Principles

At its core, wet milling is just a basic mechanical separation process that removes the coffee cherry’s outer layers from the internal the seed (bean). This stands in stark contrast to dry milling, where the entire cherry is dried before removal of the outer layers. As you can imagine, each method yields startlingly different flavour results from the other. Our focus here is on wet-milling

A “wet mill” (also known as a washed processing station) is a facility where, as mentioned, freshly harvested coffee cherries are processed using water to remove the outer fruit layers, leaving behind clean coffee beans. This method, known as “washed” or “wet processing”, enhances the coffee’s acidity, clarity, and brightness in flavour.

To consistently achieve optimal results, producers need:

• Clean, abundant water (approximately 130-150 gallons per 100 pounds of cherry)
• Well-maintained pulping equipment with correctly adjusted pressure settings
• Fermentation tanks with proper drainage
• Accurate temperature monitoring capabilities
• Reliable quality control protocols

How a Wet Mill Operates in Coffee Processing

The wet mill follows a series of steps to convert ripe coffee cherries into parchment coffee (dried beans still encased in a protective husk):

Step 1. Cherry Reception and Sorting

• Freshly harvested coffee cherries are delivered to the wet mill.
• They are sorted by density using flotation tanks—ripe, denser cherries sink, while underripe or defective ones float and are removed.
• Some mills also use manual sorting or mechanical sifters to ensure only high-quality cherries proceed.

Step 2. Pulping (Removing the Outer Skin)

• The cherries pass through a mechanical pulping machine that removes the outer fruit skin, leaving behind the beans covered in sticky mucilage.
• The removed pulp is often collected for composting or other agricultural uses.

Step 3. Fermentation (Mucilage Removal)

• The beans, still covered in mucilage, are placed in fermentation tanks for 12-48 hours, depending on temperature and altitude.
• Natural enzymes break down the mucilage, making it easier to wash off.
• Some modern mills use mechanical demucilagers to speed up the process.

Step 4. Washing

• After fermentation, the beans are washed in water channels or agitation tanks to remove the loosened mucilage.
• High-quality wet mills use clean, fresh water to ensure proper removal and prevent defects. NOTE: This need for increased fresh water can create challenges for the surrounding community.

Step 5. Drying

• The clean, washed beans (now called parchment coffee) are spread out on drying patios, raised beds, or mechanical dryers to reduce moisture to around 10-12%.
• Sun drying can take several days, while mechanical drying speeds up the process.

Step 6. Storage and Hulling

• Once dried, the parchment coffee is stored in a cool, dry warehouse to stabilize before further processing.
• Later, the outer parchment layer is removed in a dry mill to produce green coffee beans ready for export or roasting.

Why These Steps Matter

Each step in wet milling serves multiple purposes that directly impact quality:

Floating Separation: By exploiting density differences, producers can separate ripe cherries (which sink) from unripe or damaged ones (which float). This simple yet effective quality control measure ensures processing begins with optimal raw material. It is generally the case in areas where wet milling is appropriate for the conditions, that producers who diligently perform this step achieve notably higher cup scores.

Pulp Removal: The mechanical removal of the outer cherry must occur within hours of harvest to prevent unwanted fermentation. Modern pulping machines use precisely calibrated pressure to remove the fruit while leaving the delicate parchment layer intact. This is critical – damage at this stage can lead to defective beans that roasters will later reject.

Controlled Fermentation: Here’s where the biochemistry becomes fascinating. During fermentation, naturally present microorganisms break down the sticky mucilage layer adhering to the parchment. This process typically takes 12-36 hours, depending on ambient temperature and desired flavour outcomes. Through careful monitoring, producers can develop unique flavour profiles while avoiding the risk of over-fermentation.

Key Benefits of Wet Milling

• Produces clean, high-quality coffee with bright, well-defined flavours.
• Allows better control over fermentation, reducing defects.
• Enhances consistency and uniformity in the final coffee beans, values much appreciated by roasters.

Challenges of Wet Milling

• Requires a significant water supply, which can be a concern in drought-prone areas.
• Needs proper waste management to handle coffee pulp and wastewater responsibly, all of which adds to the cost of production.
• Requires high initial investment in infrastructure and equipment.

Impact on Cup Quality

The effects of wet processing on final cup quality can be profound and measurable. What you are likely to see are:

• Enhanced clarity and brightness in the cup
• Cleaner, more defined acidity
• Reduced body compared to natural or dry processing
• More consistent flavour profiles between batches (much preferred by roasters)
• Lower risk of fermented or “funky” notes

The Path Forward

As climate change (and demand) continues to impact coffee production globally, precise control over processing becomes increasingly crucial. Wet milling, when properly executed, provides producers with greater control over quality outcomes while using water resources efficiently.

For further reading on this topic, I recommend reviewing the Specialty Coffee Association’s Processing Handbook and for those of you far enough down the rabbit hole, the comprehensive research published in the Journal of Agricultural and Food Chemistry (Vol. 62, Issue 12) on the biochemical changes during coffee processing.

Remember: Understanding these processes not only makes us better coffee professionals and enthusiasts but also more informed consumers who can support producers implementing best practices.