June 10, 2019

How Does Moisture Content Affect Coffee Roasters?


At each stage of the coffee supply chain, the moisture content of a green bean must diminish – or the bean might become moldy, defective, and less valuable than before. Ensuring a bean dries correctly is essential in order to optimize its quality potential and minimize the chance of problems.

Roasters, near the end of the supply chain, have two tasks when it comes to managing moisture content. On the one hand, they must maintain the lots they store onsite within a narrow moisture range that is acceptable to their quality standards. They need to hold their coffee in this range for a period that will, hopefully, not be longer than a year.

On the other hand, and in the span of a few minutes, the roaster is responsible for driving the last remaining bits of moisture out of the bean via the application of intense heat and pressure. In these minutes, the coffee is exposed to the most energy it will experience at any point in the coffee supply chain and the roast is set up for either success or failure.

It’s easy to see why roasters should care about the moisture content of their coffee. But how useful is a number supplied by an importer, and how can roasters integrate moisture content readings into their craft? I spoke with Fred Seeber of Shore Measuring Systems, a supplier of moisture content meters, about measuring and making sense of moisture content in green coffee.

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Coffee beans drying to reduce moisture

Coffee cherries dry under the sun at a mill. Credit: Zach Latimore

What Is The “Ideal” Moisture Content?

There is no official standard for ideal moisture content in green coffee, although the ICO recommends 11% as a good target. However, it’s commonly accepted that 10-12% is a reasonable range. Anything less than 10% is likely to result in loss of cup quality, while humidity at higher levels begins to create a risk of mold growth.

Yet a coffee’s humidity is not static. While the pre-export drying process drastically increases a bean’s stability, changes in moisture content are still possible. Environmental factors, such as being in a particularly humid or hot location, are a common cause of this.

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Green beans with little moisture content

Green bean samples with visual differences, due to the drying style and processing method. Credit: Zach Latimore

Measuring Moisture Content: Is It Really Necessary?

Before getting into the technical details of measuring moisture content, it’s worth digging a little deeper into why it’s worth measuring moisture content. Knowing this will help you establish protocols suited to your specific needs.

For roasters of a certain scale, it’s simple: you pay for coffee by weight; the more water in that coffee, the more you’ve paid for water which you’re going to burn off anyway.

Fred describes a common situation roasters find themselves in: “So, [an importer] sends you a sample, and… it’s showing 11.5% moisture in that sample. Then when your container shows up, that’s 40,000 pounds, and all of the sudden you discover it could be 13% moisture. Well, you just got blanked for two percentage points of water of a commodity that’s four bucks a pound… that’s [a lot] of money.”

Measuring green coffee moisture content before roasting a lot. Credit: Edwin Andres Salina

For the smaller, quality-focused roaster, those kinds of calculations may or may not be relevant. But moisture content still plays an indirect role in a roaster’s costs, regardless of whether or not they’re buying a few containers or a few bags.

There is no direct link between a coffee’s quality and its moisture content. A 10% humidity coffee is not necessarily better or worse than a 12% coffee. However, over time, green coffee will gradually lose vibrance. This will eventually result in the dreaded “past crop” flavor, and this process is associated with the drying out of the coffee.

Therefore, even for a small roaster, it’s important to keep track of moisture content. If you paid for an 85-point coffee at 12% moisture, by the time it reaches 10% moisture it may be more like an 83-point coffee. Yet, you still paid 85-point prices for it originally.

By comparing moisture content loss with quality degradation over time, you can make smart buying and consumption decisions with your green lots. And, when combined with water activity measurements, you can even predict the shelf life of your green coffee. Again, precision here is key: you want to track your coffee through a narrow range of percentage points over a long time frame.

Lastly, you may think to yourself that you don’t need to measure moisture content yourself, since your importer supplies those numbers already. Fred cautions against this thinking. He points out that coffee is shipped on water and that ports can often be warm and humid, which will affect moisture readings.

So, if you’re a roaster in a dry part of the United States but your importer is located in New Orleans or Houston, and is taking moisture readings from lots right as they arrive, those numbers might not be applicable to you by the time your coffee arrives at your facility.

Checking moisture content in green beans

Shore Model 920 Coffee Tester used to measure the moisture content of green Brazilian beans. Credit: Shore Measuring Systems

Not as Simple as It Seems: Measuring Moisture Content

According to Fred, three basic controls are needed to ensure accurate moisture readings: accurate quantity, accurate temperature, and an accurate meter.

Having an accurate quantity is pretty straightforward: you want to be testing the same amount of coffee every time you measure moisture content. If you’re working with coffee, you probably have plenty of accurate scales lying around.

Having a consistent, accurate ambient testing temperature and sample temperature is critical. Many meter companies, such as Shore Measuring Systems, automatically compensate for ambient temperature, so whether you like your lab warm or cool shouldn’t affect your readings. If in doubt, though, opt for a cooler lab. “If you’re really paying attention to quality,” says Fred, “I really like the notion of lower [temperatures].”

Additionally, if there is a large difference between the temperature of your sample and the temperature of the room in which you test, Fred cautions that you will have an issue of “the surface moisture [of the sample] condensing.” Therefore, your best bet is to wait for your samples to come up or down to the same room temperature that your device is in before taking measurements.

Green coffee stored in a roastery. Credit: Gregory Bodnar via Flickr, CC BY-SA 2.0

Of course, meter accuracy is critical. If your readings are consistently inconsistent with the numbers supplied to you by your supplier, it’s worth checking your meter’s calibration. Fred says that “most of the manufacturers will send you a sample of a known moisture or a loaner meter” which you can use to calibrate your own meter.

Lastly, it’s important to consider how you pull samples to measure and how often. Roaster Andrés Salinas of Endorphin Beans tells me that his team “read and check the moisture and density of our green beans every time before we roast; also, when [we] request samples, pre-shipment and after [arrival].”

As for where you pull samples from, pulling directly from the lot is the best practice. A sample taken from a lot and stored separately will almost certainly dry out differently and no longer be representative. If your roaster has the capacity to roast more than a full bag of coffee at a time, Fred recommends taking a sample from each bag you will use and then averaging them.

Shore Model 930 Coffee Tester used to measure the moisture content of green beans in storage. Credit: Shore Measuring Systems

Roasting Wet & Dry Coffees

Like all things roasting, there are so many variables that go into the roast process that it’s hard to isolate just one factor – such as moisture content – and roast around it. However, with a solid grasp of the fundamentals of the roast process and precise, consistent moisture readings, you can definitely identify patterns and develop your own strategy.

In Modulating the Flavor Profile of Coffee, Rob Hoos states that the beginning of the roast cycle consists of building up enough heat and pressure to begin driving water vapor out of the beans: “For lack of a better terminology, I often refer to this as the drying phase. There are no real significant chemical reactions happening; rather, water vapor is being driven off, pressure is beginning to build and the thermal momentum of the batch is being established.”

If you are roasting a coffee with a higher moisture content, then you will need to apply more thermal energy during this phase. Your three options are to charge at a higher temperature (without going so high as to scorch the outside of the bean), use a higher flame during the drying phase, or accept or aim for a longer drying phase.

While there are always different approaches, some more creative than others, if your goal is consistency in your drying times, a higher charge temperature and flame height is the best starting point. Roaster David Wilson of Coffeebar tells me, “The more moisture in there, the more heat the bean’s going to be able to retain, the better heat transfer you’re going to have, so it can take a higher heat through the initial phase of the roast.”

A sample coffee roaster. Credit: Steven Brooklyn via Wikimedia Commons, CC BY-SA 3.0

If you’re roasting an especially moist coffee, all of that free moisture that you expel during the initial minutes is going to increase the humidity of the drum. This released moisture has a cooling effect, meaning you’ll need more gas to overcome it and maintain proper levels of thermal energy. So, a reliable corollary to a higher flame is more airflow at the beginning in order to relieve some of that humidity.

Then again, for David, roasting in a dry, high-elevation climate, the opposite is true:

“I do lower fan speed and closed drum for the charge to try and keep moisture in the system, pretty much through drying. At that point, I’m trying to conserve the moisture in the system to improve our heat transfer.”

Whatever your approach is, with a high-moisture coffee, the fundamentals at work are that you need more energy to expel that water and, once expelled, that moisture is going to have a cooling effect. The important thing is to overcome the water vapor and establish sufficient momentum.

As David says, “I’m looking to create as much momentum in the bean as I can. And you can create that early on, you can’t make up for that later.”

As for roasting drier coffees, there’s no need to overthink it: “lower temperature, lower flame,” David says. While it’s worth knowing the context of the bean’s dryness – is it a high-altitude, dense Ethiopian, or a formerly moist coffee that has aged and dried out a bit? – at the end of the day, the physics are the same: less moisture will require less thermal energy.

Therefore, a more delicate approach is called for, with the same goal of achieving consistency in the drying phase and entering into the yellowing period with appropriate momentum.

Ultimately, your own roaster and roasting conditions will determine how you handle coffees with moisture contents outside of the standard range. By closely tracking the moisture content of the beans you roast, you will be able to experiment and come up with the best solution for your own context.  

Enjoyed this? Read How to Design Your Coffee Roastery For Maximum Efficiency

Feature photo credit: Shore Measuring Systems

Please note: This article has been sponsored by Shore Measuring Systems.  

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