How the moisture content of green coffee affects roasting

Esther Gibbs
December 1, 2022
How the moisture content of green coffee affects roasting

Before profiling a coffee, it is important for roasters to determine the moisture content of the beans. 

The moisture within the green coffee will act as a conductor, enabling heat to penetrate the bean. Typically making up around 11% of green coffee’s weight, it can influence a range of characteristics, from acidity and sweetness to aroma and mouthfeel.

For specialty roasters, understanding the moisture content of your green coffee is crucial to creating the best coffee possible.

Measuring the moisture content of green coffee can not only help root out defects in a large batch of beans but also aid key variables during the roast, such as charge temperature and development time.

To learn more about what determines the moisture content of green coffee and how it affects the roast, I spoke with Christopher Richie, the head roaster at Full Court Press in Bristol, UK.

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An image of a coffee roaster turning coffee beans during the drying phase in an article on how moisture content of green coffee affects roasting

What determines a coffee’s moisture content?

Every step along the coffee supply chain has an influence on a coffee’s moisture content, be it processing, shipping, handling, or storage conditions. 

“Coffee travels thousands of miles,” explains Christopher, who is also co-owner of Full Court Press. “This is usually via boats in shipping containers, so there are many things that could ultimately add moisture to the final product. 

Moisture content refers to the measurement of water in a product relative to the total weight of the product, and is expressed as a percentage.

At Roast Magazine’s 2021 virtual event, Monica Traveler and Yimara Martinez of Sustainable Harvest discussed their recent report on Water Activity in Green Coffee.

They say moisture content influences a coffee’s physical properties, including weight, density, viscosity, and conductivity, among others. According to their report, moisture content below 10% is too dry, while above 12% is too wet.

As these leave too little or too much moisture to engage in the desired reactions during a roast, 11% is often considered ideal. 

The moisture level of green coffee is mostly determined by the drying methods used by producers. 

For example, rotating the beans during the drying phase can help ensure the moisture leaves the bean evenly. 

It can be difficult for natural or honey-processed coffees to dry, as the moisture has more barrier to pass through. 

Importantly, a coffee bean must be left to reduce its moisture content for at least four days to prevent the risk of producing mycotoxins. 

“While we are unable to control the factors leading up to the coffee arriving at our roastery, we can maintain the storage conditions of our green coffee,” Christopher says. At Full Court Press, the green coffee is stored between 14°C (57°F) and 16°C (60°F) and a maximum of 55% humidity.

An image of a barista pouring a pour over coffee in a V60 in an article on how moisture content of green coffee affects roasting

What are the risks of inadequate moisture content?

Roasters can use various tools to measure the moisture of their green coffee.

“At Full Court Press, we use the Draminski Moisture Metre, which we calibrate regularly,” Christopher explains. “This tool sends a small electrical charge through the green coffee sample and measures the resistance.”

If the moisture level is too high, there is a risk mould will develop. This frequently leads to unpleasant characteristics in the cup. 

“If the moisture content is too low, there is a risk of a total loss of positive aroma characteristics and muted acidity,” Christopher says. 

Important to note is that moisture content is unlikely to have any direct correlation with cupping scores. A moisture content of 11% is an unlikely indication a coffee will score in the high eighties. 

Moisture and water activity only have a direct relationship with a coffee’s stability, shelf life, and longevity. 

“During a roast, the moisture inside the coffee heats and becomes vapour,” Christopher explains. “This helps conduct heat, creating pathways for heat transfer to the centre of the bean.” 

The vapour is released at first crack when the density of the bean has reduced enough that it can no longer withstand the pressure.

This loss of moisture accounts for the weight loss within a coffee, which is why a light roast will lose less moisture than a darker roast.

An image of freshly roast coffee in a roaster in an article on how moisture content of green coffee affects roasting

How does moisture content affect roasting?

“The higher the moisture content the coffee has, the longer the ‘drying phase’ of the roast will be,” says Christopher. 

“Often a coffee with less moisture will require less heat to get through the drying phase. However, if you don’t account for differing moisture contents of different coffees, you’re often opening the door to roasting defects,” he adds.

Coffee with a low moisture content may taste baked or underdeveloped due to the lack of conductivity. 

That said, coffees with a higher moisture content may be difficult to control during a roast. This is because they can have too much moisture and therefore energy once vaporised.

Christopher, who roasts on a Stronghold S7 Pro, says he tends to use the roasters halogen lamps to aid with drying if the moisture content is too high. 

“Usually I start the roast with 60%,” he adds. “For higher moisture coffee, I’ll opt for closer to 75% to balance out the drying phase, so I can continue with the Maillard and development phases as normal. With dryer coffees, the reverse approach works for me.

Airflow can also help with moisture content. For instance, if there is a lower moisture content in a coffee, the roaster will have to be set with a lower airflow. This is to ensure the moisture is not dried out too quickly, which may leave no energy to create the chemical reactions needed for the roast.

Alternatively, if the moisture content is too high, roasters should increase the airflow to speed up the drying phase. That said, at the end of the roast, roasters should change the drum speed to counteract the energy spike.

Understanding moisture content before roasting coffee can provide insight into how to achieve optimum flavour and avoid roasting defects. 

Checking moisture content regularly enables roasters to achieve a consistent roast profile and ensures their coffee is not deteriorating due to suboptimal storage conditions.

An image of Terbodore coffee in a custom print LDPE coffee bag with a PLA lining in an article on how moisture content of green coffee affects roasting

The materials used to package green coffee must be durable and easy to handle, pack, and stack during storage. It should protect the coffee from moisture and contamination by being airtight and resealable. 

At MTPak Coffee, we offer a range of 100% recyclable coffee packaging options made from renewable materials such as kraft paper, rice paper, or multilayer LDPE packaging with an environmentally friendly PLA lining.

More so, we give our roasters complete control over the design process by allowing them to build their own coffee bags. Our design team is available to help you create the ideal coffee packaging.

Plus, we are able to custom-print coffee bags using innovative digital printing technology, with a quick turnaround time of 40-hours and 24-hour shipping time.

MTPak Coffee also offers low minimum order quantities (MOQs) to micro-roasters who are looking to remain agile while showcasing brand identity and a commitment to the environment.

For more information on sustainable, custom-printed coffee packaging, contact our team.

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