One of the most important things a roaster must check when assessing green beans is moisture content.
Typically making up around 11% of a green bean’s weight, moisture content can influence a range of characteristics, including acidity and sweetness.
During the roasting process, coffee beans will lose the majority of their moisture content, and their weight will drop by around 18%.
If roasters do not account for this the beans can become mouldy, causing issues to quality control and cupping scores. This could affect profits as a result.
Read more to find out how roasters can minimise these potential problems and control coffee weight loss.
Why does coffee lose weight during a roast?
Like any fruit, the coffee cherry is filled with moisture – as is the green bean. And, since coffee is a plant, water is essential for its growth.
As with any natural matter or organism, water represents a significant proportion of the green bean.
According to the International Coffee Association (ICA), moisture content contributes around 8% to 12.5% of a green coffee’s weight, which is the ideal moisture volume for specialty coffee.
There is clear science underpinning this precise range of moisture content. If the green bean has less than 8% moisture content, it is likely the roasted coffee will be of a low quality.
However, the higher the moisture levels are, the greater the chance of mould growing within the beans.
Some sources suggest this range of moisture content should be a minimum of 10%, but the ICA permits a larger range. The range of 8% to 12.5% was chosen as a medium, which balances these two risks.
One of the main benefits of coffee processing methods is they keep the green beans’ fluctuating moisture rate as stable as possible.
Before green beans are exported and purchased, they will have gone through a drying process. Whether it is washed or naturally processed will depend on the type of bean and the methods of the coffee farm.
When the ripe coffee cherry is picked, it typically has a moisture content of around 34% to 55%. It is during the drying phase that this falls to the required 8% to 12.5%.
By removing the fruit, coffee processing limits the fluctuation of the green beans’ moisture. However, it is important to realise that it is not stable, and the moisture content of a green bean is not fixed.
These moisture levels will naturally fluctuate, and the beans’ external environment can affect its moisture.
For instance, if they are stored in a humid environment, moisture can seep into their porous structure. Therefore, it is essential that roasters carefully consider how they store green beans to prevent them from spoiling.
How much weight is lost in a typical roast?
In a typical roast, coffee beans can lose between 15% and 18% of their weight.
Using this as a rough estimate, when a 5kg (11lbs) bag of green beans is roasted, its weight will drop to between 4250g (9.4lbs) and 4100g (9lbs) of roasted coffee beans. This means that between 750g (1.6lbs) and 900g (2lbs) of weight is lost during the roast.
This predominantly occurs as moisture evaporates. However, the loss of the outermost layers of the bean – known as parchment and silverskin – also has an impact.
When the bean is roasted, these layers dry out and fall away from the rest of the bean. When the roast is complete, these waste products are often repurposed.
Typically, the darker the roast, the more moisture evaporates and so, the weight lost through this process is greater.
Different types of coffee beans can go beyond the typical range of weight loss in roasting. For instance, Indian Monsooned coffee beans naturally have a higher level of moisture.
How does this affect roasters and what can they do to manage it?
Weight loss caused by roasting can cause problems for roasters, as they have paid for the green beans by their weight. Furthermore, they will sell the roasted beans by their weight.
Therefore, any weight lost during the roast directly results in reduced profit margins.
The impact is two-fold, as moisture levels in green beans may directly affect their value. If the moisture levels fluctuate during transit, the roaster may pay more for a poor quality product.
For instance, if a roaster purchases a bag of green beans with 12% moisture content, this falls within the range recommended by the ICA. This percentage tends to produce the best aromas and the highest cupping score.
However, once they arrive, the beans are actually at 14% moisture. This means the roaster has paid the same price for less coffee, as this extra moisture causes a small amount of beans to weigh more. Additionally, the roasted coffee may be of a lower quality.
This is a common problem for roasters as green beans are often shipped by boat and travel in humid conditions. They also tend to spend a lot of time waiting at ports.
Roasters can manage this by measuring the moisture content of green beans independently. It is recommended that roasters include moisture content as a condition in their contract with the importer.
It is further recommended that roasters set a minimum and maximum moisture content for coffee. If the coffee falls out of that range, roasters will be in a stronger position to negotiate prices with the supplier.
An additional way to manage moisture content is to change the roast profile. Opting for lighter roasts may reduce the weight loss during roasting.
However, it is critical the roast profile suits the type of bean chosen. If not, the roaster may risk compromising the quality of their product.
One of the best ways roasters can avoid issues with their green beans is to monitor how they are stored at the roastery.
It is vital the roastery is kept at a stable, controlled temperature and humidity. The ideal temperature would be around 20ºC (68ºF).
Using hermetically sealed bags to store green coffee can also be beneficial. Furthermore, roasters should not store green coffee bags directly on the floor. The bags should be rotated occasionally to ensure air can circulate the beans evenly.
While the issue of moisture cannot be eliminated altogether, it can certainly be managed with careful planning.
At MTPak Coffee, we offer a range of sustainable coffee packaging for both roasted and green coffee beans. We can help you across every step of the process from concept and design, to printing and manufacturing.