Raw Coffee Bean Water Activity | How to Protect Coffee from Excessive Water Activity

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For a long time, it has been understood that food spoilage has a certain relationship with the water content in food. However, when considering food quality and safety, the water content in food is not a reliable standard. Compared to water content in food, water activity (AW) is considered more important as a scientific indicator that is closely related to the chemical, physical, and biological characteristics of food and its raw materials.

Imagine that high-quality green coffee beans reach the buyer, only to find that the cupping score/quality has decreased. The reason is related to water activity, which can potentially shorten the optimal shelf life/flavor period of green beans, produce undesirable flavors in the cup, and even lead to mold growth.

The Difference Between Moisture Content and Water Activity

Moisture content is a term we often hear in coffee trade. From farm to cup, the water stored inside coffee beans varies. This can affect taste, quality, and later roasting.

Coffee seeds in coffee cherries are filled with water. After processing, washed coffee typically contains 45-50% moisture. After drying, ICO recommends coffee beans have a moisture content of 11-12.5% (although many roasters prefer 10.5% or even 10%). This means proper drying is crucial for coffee quality. However, not all water is the same. Let's scientifically examine the different types of water inside coffee beans.

Free Water:

Refers to water that can flow freely within organisms or between cells and within cells. The vast majority of water in cells exists in free form (accounting for approximately 95.5% of all water within cells). When people eat pears, the juice that drips from their hands is free water.

Characteristics of Free Water:

1. Free flowing in tissue spaces with mobility.
2. Can act as a solvent.
3. Will evaporate and be lost when heated.
4. Can be utilized by microorganisms (bacteria and enzymes) for growth and reproduction. (This means the possibility of fermentation.)
5. Can freeze.

Bound Water:

No matter how hard you squeeze a pear in your hand, you won't get bound water. Bound water combines with proteins, polysaccharides, and other substances within organisms or cells, losing its mobility. Bound water is an important component of cell structure, cannot dissolve other substances, and does not participate in metabolic processes. It cannot be utilized by enzymes and bacteria, meaning it does not participate in fermentation. Its vapor pressure is negligible, and under extreme temperatures, it can evaporate or freeze.

Vapor Pressure:

Under certain external conditions (such as heating water in a pot until it begins to evaporate), liquid molecules in the liquid will evaporate into gaseous molecules, while gaseous molecules will also hit the liquid surface (and the inner wall of the pot) and return (condense) to liquid state. At a certain point in time and at a certain temperature, the condensation rate and evaporation rate can reach equilibrium. At equilibrium, the content of gaseous molecules reaches its maximum value. The pressure generated by these gaseous molecules hitting the liquid is simply called vapor pressure.

What does vapor pressure have to do with green coffee beans? Vapor pressure shows the volatility of free water, which is related to fermentation, mold growth, and decomposition of substances inside coffee beans. This leads to an important topic of water activity. Water activity (aW) (referred to in this article) is the vapor pressure inside green coffee beans or other foods, and the water activity index is a quantitative expression of this vapor pressure. In other words: water content refers to how much water is in the coffee. Water activity indicates the relative humidity when coffee beans reach equilibrium with the surrounding space. Water activity (degree) is expressed as 0...1.0aw.

How to Measure and Control Water Activity in Coffee Beans

What should the water activity of coffee beans be?

Water activity is measured on a scale of 0 to 1.0, where 0 is dry and 1.0 is pure water. There will always be free water in green coffee beans - which means water activity will always exist. The water activity (degree) of coffee beans should be below 0.6. When between 0.6 and 0.9, the possibility of microbial activity increases, which may lead to mold and fungi entering the coffee beans.

Water Activity and Microbial Growth

Molds, especially those containing mycotoxins and ochratoxins, can harm health. Additionally, there are flavor impacts: mold and fungal defects in coffee can taste musty, earthy, or even phenolic. Even without mold growth, unstable water activity can lead to reduced shelf life of coffee, affecting the color, taste, vitamins, components, and aroma of coffee beans.

Green coffee beans are hygroscopic, meaning they can absorb moisture from the air. Therefore, their moisture content and water activity may increase. Many elements can cause changes in water activity, such as temperature, humidity, atmospheric humidity, altitude, and even coffee origin. This is one of the reasons why quality control is so important in coffee processing and drying, whether for washed, natural, or honey-processed coffee. A sudden rain shower could be disastrous.

Once coffee drying is completed, poor storage and transportation conditions can also pose risks. Ideally, coffee beans should be stored in a dry, non-direct sunlight environment with controlled temperature. However, when coffee is in transit, it's difficult to control these factors.

How to Control Water Activity to Reduce Risk

First, you should ensure you have a tool to measure water activity. There are several brands on the market such as Rotronic, Measurement Solutions, Meter Group, and Graintec Scientific.

Second, assuming your beans have been fully dried to the appropriate level, you should pay attention to storage conditions. Good storage conditions are an important factor in controlling water activity, frequently monitoring room temperature, humidity, and light.

Additionally, green bean packaging should be able to block oxygen and moisture. Jute or cotton bags have been used for decades, but they won't protect your green coffee beans from temperature and humidity changes (which lead to increased water activity). It's recommended to store green coffee beans in multi-layer and/or sealed packaging.

Roasters and cuppers should also be careful when opening bags to collect samples. Carefully reseal to prevent green coffee beans from being affected by moisture and temperature in the air.

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