Discover water activity in specialty coffee

Water activity is understood as the energy state of water in a substance. Essentially, water activity is the amount of free or unbound water in a product. Unlike humidity, which measures the total amount of water present in a substance, water activity measures whether or not that water is capable of chemical reaction.

Water activity is a parameter that is affected by changes in temperature and environmental conditions. For specialty coffee, during the coffee preservation process, it is necessary to maintain water activity at an ideal level to ensure stable green coffee. The higher the water activity, the higher the energy in the grain, and the grain with more excess water can create conditions for mold and fungus to grow. Grains containing more water also accelerate chemical processes that lead to oxidation and rancidity. If the water activity is too low, there will not be enough water for the green coffee beans to maintain important enzymatic reactions. Coffee beans do not have enough water, causing the coffee to have a watery or bland flavor.

When roasting coffee, the water on the surface of the beans is evaporated by heat before the heat-generating chemical process called roasting actually begins. This is called the “drying phase” of the roasting process. Measuring the water activity in green beans can assist in estimating the amount of time and temperature required to begin the actual roasting process. Individual mixture components often have different drying stages that can be more accurately measured through water activity rather than total moisture. If the water activity in roasted specialty coffee increases above the 0.45aw threshold, the aldehydes and ketones in the newly roasted coffee will disappear through oxidation and evaporation. The concentration of methanethiol also decreases, causing furfuryl mercaptan to increase, lipid oxidation occurs quickly, making coffee easily degraded.

In research by Dr. Ted Labuza (University of Minnesota), roasted coffee has a high water absorption ability. Increased water activity can occur when more water is absorbed, causing the coffee to go rancid faster and have a shorter shelf life. Besides, if storage conditions do not ensure temperature, light and packaging materials, increased water activity can increase rapidly, disrupting the flavor-creating reactions of the seeds.

To maintain the quality of roasted coffee, coffee experts recommend storing them at a water activity range of 0.18aw – 0.30aw. If packaged in a polyethylene bag, the range will be on the higher side. On the contrary, if packaged in multi-layer bags, the coffee will have a lower range. In particular, with higher water activity (above 0.25aw), coffee can lose its original flavor and create stale flavors (the smell of paper, bread,…) quickly in about two weeks.

Water activity in green coffee does not affect cupping scores. However, the value of water activity is related to providing an optimal storage range to avoid bacterial contamination, chemical reactions, slow decomposition and flavor. Coffee whose water activity exceeds a certain range will cause fermentation, mold, causing bad taste or toxins that are harmful to health. From there, the quality of green coffee beans will gradually degrade. The seeds have an old  papery, woody or rubbery taste due to the hydrolysis of sucrose.

The optimal water activity range for green coffee beans is 0.45 – 0.55 aw. At water activities less than 0.55, green coffee beans will not support the growth of any microorganisms. Harmful chemical reactions are also limited to the lowest level within the water activity range of 0.45 – 0.55aw. Therefore, storing coffee beans at this water activity range will not improve the cupping quality of green coffee beans but will instead maintain the quality for 6 to 8 months.

The water activity in coffee will change over time, but if it stays within the right range, the beans will retain their unique aromas for a long time. So please pay attention to this parameter!

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