Green coffee can lose water content along every step of the supply chain, from harvesting and processing, to storage and roasting.
The loss of moisture in green coffee affects not only a roaster’s profit margin, but the quality of the roast. However, a reduction in moisture content is a natural occurrence during roasting.
That being said, it is essential for roasters to track and manage moisture loss as part of their quality control process, as well as to help minimise business risk.
To find out more about the importance of calculating loss in coffee roasting, I spoke with the founder of Sumo Coffee Roasters, Daniel Horbat and head roaster at Café Belleville, Mihaela Iordache.
What does coffee lose during roasting?
Water is an important component of green coffee.
When ripe coffee cherries are harvested, the seed contains around 45% to 55% of moisture. Once the seeds are dried and processed, the moisture content drops to an optimum level of 10% to 12%.
Notably, the International Coffee Organisation (ICO) recommends an acceptable moisture content range of 8% to 12.5%.
During the roasting process, green coffee undergoes a series of chemical and physical changes. This creates hundreds of volatile organic compounds that contribute to the coffee’s flavour.
At the same time, coffee loses its water content. In particular, coffee beans can typically lose between 15% and 18% of their initial weight.
Studies have found the major contributor to coffee weight loss is dehydration. The secondary reason is due to the transformation of dry matter into gases – primarily carbon dioxide (CO2).
Coffee loses water that was already present in the green beans, as well as additional water formed during roasting, which is subsequently lost again as water vapour.
Additionally, research done in the 1980s suggested moisture loss occurs in three different stages during roasting.
Stage one is a slow dehydration, followed by an accelerated rate of dehydration, and finally, a maximal dehydration due to structural changes in the bean.
Separately, a group of researchers in Italy found that within the first six minutes of roasting, coffee beans lose more than 70% of their initial moisture content. This is followed by the gradual release of CO2 and water vapour that developed inside the bean.
The buildup of water and other volatile compounds also leads to an increased internal pressure. Consequently, the beans expand, producing cracks on the surface, causing further weight reduction.
It is clear that moisture plays an important role during the roasting process. Specifically, water acts as a medium for absorbing and transmitting heat, meaning that it enables heat transfer to the centre of the bean.
Insufficient moisture content will lead to the coffee roasting too quickly on the outside, leaving the inner bean raw, which may produce grassy flavour notes.
How does this loss affect coffee roasters?
Weight loss affects coffee roasters because both green and roasted coffee are sold by weight.
As such, moisture loss will directly affect a roaster’s cost of production.
For example, assuming coffee loses 15% to 18% of its weight during roasting, 10kgs (22lbs) of green beans will produce between 8.5kg (18.7lbs) and 8.2kg (18.1lbs) of roasted coffee.
This means a roaster will have paid for 1,500g or 1,800g of green beans that cannot be sold and are lost as moisture.
Daniel explains one of the most significant impacts of moisture loss is on a roaster’s cash flow.
“You lose cash flow, which is one of the most important things for a roasting company,” says Daniel, who is also the 2019 World Cup Tasters Champion. “A roasting company operates by paying for the green beans first, then takes some time, perhaps a few months, before selling the coffee.”
When roasters do not take moisture loss into account when pricing their coffee, there is a risk of facing insufficient cash flow despite having good sales numbers.
“Once you sell the coffee, you will have to buy more green coffee, but your profits will not be what you had expected. So, it will be a continuous cycle of buying and roasting without any growth.”
Furthermore, factors such as processing methods, the varietal of coffee, and the roasting duration may impact the rate of weight loss. For instance, semi-washed coffee has been found to experience greater moisture loss during roasting compared to natural coffee.
Daniel explains that roasting coffee to a lighter degree tends to produce less weight loss as opposed to a darker roast.
However, while roasting lighter may help improve profit margin, it is important to keep in mind that the development of flavours should always be the priority.
“In the business of specialty coffee, the flavour profile will matter the most,” agrees Mihaela, who is also the quality control manager at Café Belleville. “Any decisions motivated by aspects other than the quality of the product will, in my opinion, influence the output negatively.”
Similarly, understanding the properties of the beans can help roasters predict roasting waste better, reducing the risk of financial ramification.
How important is calculating loss in coffee roasting?
Calculating weight loss follows a simple formula.
Roasters can measure the weight of a batch of green beans before and after roasting. The difference in weight indicates the amount of water content that is lost during the roast.
Measuring weight loss can help roasters manage their costs effectively, and inform them on other aspects, such as roast consistency.
Notably, Mihaela confirms weight loss is a useful metric for ensuring consistency and quality control.
This is echoed by coffee consultant Willem Boot, who says coffee roasters will often use a pre-set weight loss percentage (WL %) and assume they know what their output is.
“However, differences in WL % can be telling in terms of production consistency, changes in green coffee, and roast level,” Boot explains in an article for Rob Hoos Coffee Consulting.
For instance, moisture loss can indicate the flavour development of coffee. While using the range of 11% to 13%, Red Fox Coffee Merchants found coffee that loses moisture below this range is often underdeveloped with grainy and cereal-like flavours.
Conversely, a higher loss often provides overdeveloped, roasty, and bitter flavours.
Daniel explains that measuring and tracking roasting waste is important for inventory management and planning. It provides roasters with an accurate picture of how much green coffee is left in stock in every roast batch.
Additionally, managing and controlling moisture loss is another vital aspect of running a coffee roasting company.
Therefore, Mihaela encourages roasters to conduct regular checks on the water density of green beans. This allows roasters to anticipate any instability in the green beans condition, as well as understand the general evolution of the product over time.
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