Imagine shipping exceptional-quality green coffee to a buyer, only to discover that, on arrival, it’s dropped several points on the cupping table. The reason? Water activity, something that has the potential to shorten the shelf life of green beans, create unpleasant flavors in the cup, and even lead to mold.
To find out more about water activity, and how to protect green coffee against it, I spoke to Hanuman Jain, CEO of Ecotact, a manufacturer of hermetic, food-grade packaging for green coffee. Here’s what I learned.
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Moisture Content & Water Activity: What’s The Difference?
Moisture content is a term we hear a lot in the coffee trade. It’s an important thing to know about, since coffee beans hold varying amounts of water during their journey from seed to cup. And this can affect their flavour, quality, and behaviour in the roaster.
In the coffee cherry, the seeds are full of water. After processing, a washed coffee will typically have 45–50% moisture content. And after drying, the ICO recommends a moisture content of 11–12.5% (although many roasters prefer to take it lower, down to 10.5% or even 10%). This means that drying green beans well is critical for quality coffee.
Discover more! Read Why Is Green Bean Moisture Content Important?
Green coffee sits in hermetic plastic bags that will protect it from moisture and pests. Credit: Ecotact
However, not all moisture is the same. Let’s get scientific and look at the different kinds of water and moisture inside your coffee beans:
- Free water: Bite into a ripe pear and watch as the juices run down your hand. This is free water. It’s only loosely bound inside the food item and can be removed by squeezing, cutting, heating (via condensation), and so on. It can be used by bacteria and enzymes, meaning fermentation is possible.
- Bound water: Unlike free water, bound water is held inside the food item, no matter how much you squeeze it. It cannot be used by enzymes and bacteria, meaning it doesn’t take part in fermentation, and it has a negligible amount of vapor pressure. At very high temperatures, it can be evaporated; it can also be frozen, but only at far below zero.
- Vapor pressure: Let’s imagine that you have water in a box that you heat up until the water starts to evaporate. Now, as the evaporated water – the vapor – hits the walls of the box, it then starts to condense. At a certain point in time, and a certain temperature, the rate of condensation will be the same as the rate of evaporation. And at this point, the pressure of the vapor on the water is the vapor pressure.
It’s hard to see how this relates to green coffee. However, vapor pressure indicates how volatile the free water is – something that can indicate how likely fermentation, mold growth, and bean degradation are.
Green coffee beans laid out for quality grading. Credit: Big Island Coffee Roasters
And this brings us onto the important topic of water activity (aW). As Hanuman explains, water activity refers to the vapor pressure within the green beans or any other food or beverage. It is a way of quantifying this pressure. (If you really want to know, it’s calculated by comparing the vapor pressure within the food or beverage to that of pure water.)
In other words: moisture content refers to how much moisture there is inside your coffee. Water activity indicates how likely that water is to result in further transformations, such as fermentation and mold.
Still with us? Good! It’s time to move on from the theory and get practical. Let’s look at how to measure and control water activity in green coffee.
Parchment coffee dries on raised beds in Honduras. Credit: Souvenir Coffee Co
How Much Water Activity Is Okay?
Water activity is measured on a scale from 0 to 1.0, where 0 is dry and 1.0 refers to pure water. Hanuman tells me that there will always be free water in green coffee – meaning that there will always be water activity.
Green coffee beans, he says, should have a water activity that is less than 0.6. Between 0.6 and 0.9 aW, the chance of microbial activity increases, potentially leading to mold and fungus in the beans.
Green coffee and these microbial reactions do not make for good friends. Molds, especially those that contain mycotoxins and ochratoxins, can be a health hazard. Then there’s the impact on flavor: coffees with mold and fungal defects can taste moldy, earthy, and even phenolic.
And even if mold doesn’t develop, Hanuman explains that an unstable water activity may lead to a reduced shelf life for the coffee. The beans may develop “past crop” flavors, with faded notes and aromas.
As Hanuman says, “water activity is important, not just for safety, but it is also an indicator of potential chemical and physical reactions.”
Turning coffee to ensure it dries evenly. Credit: Souvenir Coffee Co
Green Beans at Risk: When Should You Worry?
Green coffee is hygroscopic, meaning that it absorbs moisture from the air. As a result, there is always a risk that its moisture content and water activity might increase.
There are so many elements that can lead to variations in water activity; as Hanuman says, “it’s a mixture of temperature, moisture, humidity in the atmosphere, sea level, and even where the coffee is grown.”
This is one of the reasons quality control is so important during processing and drying, whether it’s washed, natural, or honey coffee. It’s also why a sudden bout of harvest-time rain can be disastrous.
Once the coffee has been dried, poor storage and transit conditions can also pose risks. Ideally, the beans will be kept in a dry, temperature-controlled, and not overly bright location. When coffee is in transit, however, it can be hard to control these factors – especially if it’s a long journey.
A coffee warehouse in Santa Rosa de Copán, Honduras. Credit: Souvenir Coffee Co
How to Control Water Activity
So, what can you do to reduce the risk of increasing water activity in your green coffee beans?
First, you should make sure that you have a tool with which to measure water activity. There are several meters on the market, all of which can give you a result in just five minutes. Hanuman recommends those from Rotronic Measurement Solutions, Meter Group, and Graintec Scientific.
Next, assuming your beans have been uniformly dried to an appropriate level, you should pay attention to the storage conditions. “Good storage conditions are an important factor for controlling water activity,” Hanuman stresses. You should constantly monitor the room temperature, humidity, and light levels.
Moreover, packaging with good barriers against oxygen and moisture will keep the green coffee farm-fresh and stable. “In the modern era, there are so many storage systems where you can control the water activity,” he says.
Burlap or cotton sacks have been used for decades, but they won’t protect your green beans from the temperature and humidity changes that can increase water activity. Hanuman recommends storing green coffee in multi-layer and/or hermetic packaging instead.
He also emphasizes the importance of keeping the bags airtight. This should be done all the way from the producers’ hands to the buyers’. In transit, he recommends against using hooks, as they can break the hermetic bags.
Roasters and cuppers should also be careful when they open bags to collect samples. It’s important to carefully repack and close the bags to keep the green beans from reacting to the humidity and temperature in the air.
Honey processed Geisha packed in hermetic Ecotact bags. Credit: Ecotact
When it comes to specialty coffee, the tiniest of details matter. Pay attention to best practices for drying, storage conditions, and coffee packaging. In doing so, you will protect your coffee from humidity, excessive water activity, and reduced quality.
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Written by Gisselle Guerra.
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