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Water Activity in Coffee by Mike Strumpf


Unless you live completely off the grid, your life relies on data- specifically accurate data. Sometimes erroneous data is very obvious like a phone number with only 6 digits (in Canada) or getting search results for “Columbia” instead of “Colombia.” But sometimes erroneous data is harder to notice. There is an entire industry that focuses on data cleaning, though ideally they do their job so well that we don’t know they exist.

Erroneous data in coffee can come from many sources; maybe your shot timer didn’t start right away, you didn’t tare your scale, or your thermocouple probe offset is wrong. Another source of bad data can be from your lab equipment such as moisture or water activity meters. We think about mise en place when cooking or working on bar, but do you have mise en place for lab work? Consistent preparation of the temperature and humidity of green coffee samples is of paramount importance if you want consistent and reliable results. (And you need reliable results if you want to make good buying and roasting decisions based on your data.)

Things are going to get a little bit technical here, but stay with me.

Water activity in a coffee bean is the measurement of the state of energy of water in the bean. Handheld and benchtop water activity meters often use thin film capacitors to measure the water activity. These capacitors measure the ability to store energy in a closed container of coffee. The more water is present in the air above the coffee in the container, the lower the capacitance, and the higher the water activity. Conversely, the less water is present, the higher the capacitance, and the lower the water activity.

Moisture content in a coffee bean is the measurement of the percentage of water in the coffee bean. Typical moisture meters use capacitance to measure the moisture content in a bean by the ability of the coffee bean to store energy. Moisture meters have the ability to measure the moisture content of many different substances based on the principle that different substances have different dielectric constants- or the ability of a substance to hold energy. The same moisture meter can measure the moisture of wheat, soy or coffee and it uses programmed dielectric constants based on the substance. (Side note, did you know that caffeinated and decaffeinated coffees have different dielectric constants and ideally have different programmed curves on a moisture meter?)

Now that we’re through that, I will plead my case for green coffee mise en place. The most critical elements to control in green coffee mise en place are the temperature and humidity of your samples. Water activity will greatly differ based on the temperature and humidity of your samples due to the Ideal Gas Law- which in a simplified manner states that the vapor pressure is directly related to the temperature. Water activity is measuring the vapor pressure of coffee in a closed container, so if your green coffee is at 15°C you will get a different reading than if your green coffee is at 25°C. Very similarly, the moisture content will vary greatly based on temperature because dielectric constants vary with temperature.

It is a good practice to not accept coffees with too high of moisture content or water activity because of potential quality and toxin risks. What if your coffee actually had acceptable moisture content but you had erroneous data because your sample was warmer than usual? You could be rejecting a lot or paying less for a coffee that was actually deserving of approval or a quality price premium. It isn’t fair to the supply chain before you if you don’t have consistent measurement practices! Out of respect and honesty, you are responsible for keeping the best practices that you can and sample preparation is part of that responsibility.

Everyone’s business and lab are different and I don’t claim to have the answer on the best way to prepare green coffee samples. What I do know is that to get reliable data you should create a procedure for preparing your samples so that you are always reading green coffee at the same temperature and humidity. A relatively simple change in your sample mise en place can have a measurable effect on your data accuracy (and thus your happiness).

For more details watch the full video below.

Mike Strumpf, Director of Coffee at Swiss Water Decaf has an impressive list of coffee credentials – a Licensed Q Grader, Education Coordinator for the Canadian Chapter of the SCA, Authorized SCA Trainer (AST) in Green Coffee and Sensory, member of the Coffee Skills Program (CSP) Green Coffee Creaters Group, SCA Barista Competitions Group Past Chair, and Head Judge for the World Barista Championship to name a few.