From the Lab, Resources for Roasters
Monday, July 17, 2023
Consistency in Measuring Moisture Content
Not all moisture measurements are created equal. And those hidden differences can make an impact on the moisture level you receive. The Center’s Tim Heinze delves into how methods for measuring green coffee moisture content can vary and what that means for you.
While it’s fairly common to include moisture content specifications in green coffee contracts, what those measurements really mean is not as straightforward as you’d think. Since it’s entirely impractical to remove every ounce of water from the seed and measure the weight of the water against the total mass, the industry utilizes methods that approximate the amount of moisture present in a given sample of green beans. It’s important that roasters understand exactly how that equipment works and what that means for coffee quality and longevity.
What is Moisture Content?
Unlike most grains, coffee is harvested at a relatively high moisture content. Through post-harvest processing, these moisture levels are brought down, typically to 9.5 to 12.5% moisture “wet basis”. This means that 9.5 to 12.5% of the weight of an individual coffee bean is water and that dry matter (essentially everything else) makes up the remaining.
Moisture content can have an effect on a coffee’s roast profile, freshness and more. It’s important to ensure that you receive beans with the moisture level you need and that your measurements are consistent across the supply chain. If your measurements aren’t consistent with your suppliers, you may have inconsistencies with contract standards. If your contract states 10-12% moisture content, the supplier may believe they are meeting your moisture level requirements but then, because your device measures differently from theirs, your measurements indicate the coffee is actually 13% moisture. This miscommunication can cause delays and add additional costs for you and your supply chain. Ensuring you have calibrated moisture content measurements is an easy way to avoid costly delays.
How We Measure Moisture Content in Green Coffee Beans
The standard across the industry is to use moisture meters that approximate the amount of moisture present in green coffee. Most of them are known as capacitance devices.
Anyone who has used one of these capacitance devices before can attest to the often erratic and/or variable readings from different devices. This is because they are measuring moisture content by measuring something that is related to moisture content, not the moisture content itself. Coffee beans have a specific ‘dielectric constant,’ meaning the ability of coffee beans to store energy. The moisture content in a coffee bean affects its dielectric constant: the more water in a coffee bean, the lower its ability to store energy and vice versa. Capacitance devices measure those beans’ dielectric constant to calculate how much water is inside.
Standards for Measuring Moisture Content
The most accurate way to measure water content is to remove all the water from beans and then calculate the weight difference. While capacitance devices don’t actually remove water from beans, they’re calibrated according to standards for removing water from beans. So, in order to best understand why capacitance devices may measure differently, we need to first understand how those water-removal methods can vary.
One of the most accurate ways to measure moisture content is called the Karl-Fischer Titration (KFT) method. However, since KFT testing completely destroys the sample and requires significant cost, expertise and time, it’s not a very popular method, and, thus, it’s not the method around which standards are created. Instead, the standard upon which most devices are calibrated is the ‘oven-dry’ method.
The oven-dry method heats coffee beans in an oven to slowly evaporate the moisture. Measuring the weight difference between the beans before heating and after makes it possible to determine moisture content.
The International Organization for Standards (ISO), which establishes global standards for a wide range of industries, has two main ISO standards for measuring moisture content in coffee: ISO 6673 and ISO 1447. Both of these standards provide specifications, such as temperature and time, for drying the sample using the oven-dry method to determine moisture content but, of course, the specifications they give are different. Since the two different methodologies produce different results, capacitance devices calibrated to one of the standards may give different results than devices calibrated to the other standard.
The vast majority of capacitance devices are calibrated against ISO 6673. However, one can still find devices that are calibrated against other standards. This means that the 9.5% reading one machine returns could be 11.5% on another device.
If your device is calculated according to a different standard than your supplier, or if there’s variation between your devices or offices, it can affect how you measure your moisture level and may lead to inconsistencies or miscommunication between suppliers and buyers. Inconsistencies in moisture readings from different companies can cause delays, add costs and create general confusion and miscommunication between actors in the supply chain.
What Does This Mean for Roasters and Producers?
There is a lot of discussion in our industry regarding moisture content and water activity. Moisture content continues to remain a very valid standard used for coffee evaluation. However, there are a few suggestions we have in order to ensure consistency and healthy collaboration:
- Always make sure you are using your moisture meter according to manufacturer recommendations. Most of these meters require a consistent volume of sample to be put into the machine.
- Ensure your device is calibrated regularly. We recommend calibrating twice per year.
- Ask the device manufacturer to provide the ISO standard their device is calibrated against and ensure that it is consistent with others you are comparing results against.
- Consider including language in communications concerning the ISO standard you are buying or selling your coffee under. Whether you’re receiving coffee that’s 10-12% moisture content based on ISO 6673 versus ISO 1447 might be an important consideration. In the event there is a discrepancy in a contract, there is an easily identifiable and objective standard to evaluate against.
Additional Resources
https://perfectdailygrind.com/2017/08/roaster-guide-why-is-green-bean-moisture-content-important/
https://www.ctahr.hawaii.edu/oc/freepubs/pdf/EN-3.pdf
Tim Heinze is the Coffee Education Manager for Sucafina and Director of The Center. Since 2009, he has worked across many areas of coffee focusing on the cultivation, production, processing, and trade of coffee. He is an SCA AST and CQI Instructor for sensory and processing programs and holds a Masters of Agriculture from Texas A&M. His passions include training and education across the coffee supply chain. In his spare time, he often can be found on the baseball field coaching or enjoying time with his family.