August 2, 2021

Understanding coffee bean density

Coffee bean density is an important data point for roasters, green coffee buyers, and traders alike. It is often viewed as a simple marker of quality, but there’s far much more to it than that.

To truly understand coffee density, you need to appreciate what makes coffees more or less dense, how they are graded and categorised, and how density affects the process of roasting and the final cup profile.

To learn more, I spoke to industry professionals from Kenya and Colombia. Read on to see what they told me, and to learn about why coffee density is such an important metric.

You may also like our article on how to roast hard & soft beans.

different types of coffee beans

What is bean density?

Density is the degree of compactness of a substance. You calculate it by dividing the mass of an object by its volume. In essence, coffee density is the weight of a coffee bean proportional to its volume.

Typically, denser beans can be identified just by looking at them. If the fissure line (the crack down the middle of a bean) is close to straight and only slightly open, the bean is probably less dense. Conversely, if the fissure line is crooked and closed, the bean is probably more dense.

For coffee professionals around the world, bean density is recognised as an important metric for grading and sorting green coffee by quality. Harder, denser beans are generally prized by buyers around the world, and producers often receive a higher price for them as a result.

green coffee beans

What makes coffee more or less dense?

Altitude is one of the main factors that affects the density of coffee beans. At higher altitudes coffee cherries mature and ripen more slowly, resulting in beans that are more dense. Denser beans also contain higher levels of sugar, resulting in more complex and sweeter cup profiles – which is why they are more desirable.

The cherries mature for longer because higher altitudes typically offer consistently cool temperatures all year round. This is important, as the arabica plant grows optimally at temperatures between 18 and 21°C (64 and 69°F). Outside of this range, the fruit will mature too quickly.

The ripening of coffee fruit is caused by something called the Krebs cycle. The process converts acetate (which is derived from the carbohydrates, proteins, and fats in the cherry) into carbon dioxide. The Krebs cycle occurs more slowly at higher altitudes, which is why coffee cherries ripen more slowly.

But altitude and temperature are not the only factors that play a role in bean density. Variety, rainfall, and even nutrient distribution all have a part to play.

Bourbon beans, for example, are generally accepted as being comparatively dense, as are rarer and more exclusive varieties like Geisha and Wush Wush. 

Peter Gakuo is a green coffee quality control professional based in Kenya, and a staff writer at Perfect Daily Grind. He explains that the distribution of nutrients within a coffee plant can also affect bean density. 

He says: “It’s very common for coffee cherries at the end of a branch to be less dense than coffee cherries closer to the trunk, because nutrients are centrally distributed through the middle of the tree. They therefore have a more difficult time reaching the end of the branch.” 

Certain diseases (such as coffee berry disease, for instance) and pests can also affect the cherry maturation phase of a coffee plant’s development, and can cause bean density to fall.

green coffee beans

Measuring coffee density

The main metric used by coffee professionals to grade and measure bean density is “hardness”. Bean hardness is generally split into a few different categories, based on a coffee’s altitude. However, the specific metrics and terminology used do vary from country to country.

For instance, the terms Soft Bean (SB) and Strictly Soft Bean (SSB) generally refer to coffees grown at altitudes of under 1,200 m.a.s.l. or 4,000 feet. 

At these elevations, the less stable temperatures and higher oxygen levels mean the cherries develop more quickly. The beans are therefore less dense, and tend to have a rounded flavour with less sweetness and acidity.

After that, you have Hard Bean (HB) or High Grown (HG) beans, which are coffees grown between 1,200 and 1,370 m.a.s.l. (around 4,000 to 4,500 feet).

At these altitudes, the higher elevation and corresponding low-oxygen atmosphere causes the fruit to develop slowly, resulting in harder, denser beans with more complex and desirable cup profiles.

Above 1,370 m.a.s.l., it starts to get a little more confusing, as the “best” elevation will actually depend on the region and origin in question. Ultimately, coffee grown at these kinds of heights can be referred to as Strictly Hard Bean (SHB) or Strictly High Grown (SHG), which is generally used as a designation for the highest-altitude coffees available.

You can also identify bean density by looking at the physical characteristics of your coffee bean. The bean’s structure plays a key role in identifying density; the more ‘open’ its structure, the softer the bean, and the more compact, the harder the bean

roasting coffee

Roasting for bean density

Bean density is also an important parameter for roasters to keep track of. If you roast beans of different sizes or density, they will develop differently through the roast. For true consistency, it’s important to separate them and roast them individually.

Peter says: “For sample roasting and cupping purposes, I would recommend carrying out green grading and separating different bean densities and bean sizes. The roasting process requires uniformity.” 

Even when your beans are all largely uniform, it’s important to remember that harder and softer beans will behave differently during the roast. To adapt your profile accordingly, you should look at two main factors: charge temperature and airflow.

Softer or less dense beans, for example, contain more air. This slows heat transfer throughout the bean, meaning that at a higher charge (or starting) temperature, you risk “scorching” the outside of the bean (also known as tipping). The solution is generally to start with a lower charge temperature.

Conversely, for harder or denser beans, start at a higher temperature to account for the lack of air and the more compact structure of the bean. 

Daniel Morales Espitia is an independent coffee quality advisor in Colombia. He notes that alongside charge temperature and airflow, density also affects how sugars develop during the roast. As such, he says that denser beans require more of a focus on development time.

He adds: “It’s also important to consider the external environment of roasting, such as the altitude and temperature, because these will affect how the coffees react in the roaster.” 

Denser beans also typically have higher moisture levels. Daniel says that it’s important to account for this, and recommends roasting in smaller batches if you want to maximise precision. 

Ultimately, keeping track of coffee density and understanding how beans of different density behave in the roaster will be crucial when dialling in a roast profile for different coffees. Leveraging your knowledge of density will make sure you can bring the best out of each bean.

Proper storage is also crucial to preserving bean quality. It’s important to maintain constant humidity levels around green beans, as external temperature and humidity can affect quality.

Peter says: “Storing green beans very carefully is crucial, because if they are exposed to high temperatures, coffees can dry out. If they are exposed to moisture, it can also ruin the quality of the green bean or even cause mold.”

light roast coffee beans

Bean density is a complex topic, and just one of a number of metrics used to broadly assess quality, but it’s an important parameter for roasters to keep track of.

By understanding how beans become more or less dense, what influences density, and what this means for the roast, coffee roasters will be able to adapt their profile accordingly and be better prepared than ever to dial in each new lot they buy.

Enjoyed this? You’ll enjoy our guide to fluid bed coffee roasters.

Photo credits: Nick Castellano, Pixabay

Perfect Daily Grind

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