FrontStreet Coffee's Coffee Roasting Course ②: Understanding Raw Coffee Beans! How Do Bean Size, Density, and Moisture Content Impact the Process?

In yesterday's article, FrontStreet Coffee mentioned that learning about roasting allows us to view coffee from different perspectives, thereby understanding it even better! Yes, today FrontStreet Coffee wants to share how to look at coffee beans from a roasting perspective.

For a chef to prepare a dish well, proper heat control is essential! The mastery of heat control, besides skillful operation of equipment, is inseparable from understanding the ingredients! Some ingredients are suitable for slow simmering over low heat, while others are suitable for quick stir-frying over high heat! So let's begin today's FrontStreet Coffee roasting class: Understanding the "Ingredients."

Different beans have different characteristics. For roasting, to find a suitable roasting "recipe," one must understand how the values of bean size, moisture content, and density each bring differences to roasting.

Moisture Content

Let's first talk about moisture content! Moisture content refers to the percentage of water's total weight in green coffee beans. The ideal moisture content for a coffee bean is within the range of 10.5% to 12.5%. Green coffee beans with moisture content within this range can resist quality deterioration during long-term storage and will exhibit rich flavor profiles after roasting.

Moisture content will gradually decrease as green beans are stored. When moisture content falls below 10.5%, coffee beans are easily over-roasted, and most will show bland performance after roasting. When processed coffee beans have moisture content higher than this range, it makes green coffee beans susceptible to mold growth during storage, leading to risks of mold and deterioration, making long-term storage difficult. For coffee roasting, the higher the moisture content, the more heat is required for roasting.

Density

Next is the density of the beans! The more nutrients a coffee bean absorbs, the higher its density! Generally, for the same variety, the higher the growing altitude, the more time it has to absorb nutrients, resulting in greater bean density and hardness. Therefore, most Central American countries use altitude as a grading standard for coffee beans - the higher the altitude beans (the harder), the higher the grade. Similarly, for roasting, denser beans require more heat, and roasting them requires relatively more skill! (Details are written in the size section)

Size

Finally, there's the size of the beans! Under similar other conditions, larger beans require more skill to roast properly. Because large beans have a longer distance between the surface and the core, heat needs time to penetrate to the core to cook it through. But if your heat is too high, it's easy to end up with the surface being cooked while the core remains raw - that is, uneven heating resulting in under-baked beans! Therefore, it's necessary to extend the dehydration period in the early stage using a "slow simmer" approach to ensure even heating between the core and surface! This applies equally to beans with higher moisture content and density!

Of course, there are also certain types of beans that, due to their cultivation and harvesting, result in varying sizes and densities within a single batch, which presents significant challenges for roasting. Examples include Ethiopian heirloom varieties or blended beans.

Defective Beans in Green Coffee

Next, let's get to know the defective beans hidden among green coffee beans. Even in batches of high-quality beans, there will always be some defective beans, which can affect the overall coffee flavor! Therefore, before roasting, we need to screen out these defective beans to ensure the final coffee has a cleaner flavor profile.

Shell Beans/Cracked Beans

Some coffee beans split apart during growth due to developmental issues, with one part being the bean's core and the other being the outer shell. Because the shell shape is similar to that of a seashell, they are called "shell beans."

Cracked beans, on the other hand, are beans that have been damaged during processing or transportation. This type of bean is particularly common in Indonesia's wet-hulling process, as the hulling machine used in wet-hulling is quite aggressive and easily damages coffee beans.

The presence of both types can easily cause unevenness in roasting and extraction. Therefore, we can remove them before roasting.

Insect-Damaged Beans

Because coffee is an agricultural product, during the growth of coffee fruits, they are easily "favored" (gnawed) by pests known as coffee berry borers. These pests gnaw from the fruit to the green bean, leaving small black holes on the bean surface. Insect-damaged beans not only mold easily but also bring medicinal negative flavors. Therefore, they are one of our key targets for removal.

Moldy Beans

Some coffee beans become moldy because they weren't completely dried during processing (moisture content too high) or were stored improperly (humid environment). These beans are what we call "moldy beans"! Moldy beans are easy to distinguish because they all have visible green mold spots.

Moldy beans bring very strong musty flavors and can harm human health. Therefore, they need to be removed before roasting. At the same time, cracked and insect-damaged beans can also lead to moldy beans, because once beans have gaps, moisture and mold can invade, causing deterioration.

Black Beans

As the name suggests, black beans are beans that are relatively darker. Their appearance is characterized by partially or entirely blacker color compared to other regular beans. There are multiple reasons for this type of bean - coffee tree diseases, contamination of coffee beans, or over-fermentation can all produce black beans. It's important to note that black beans contain geosmin, and if not removed, they will harm human health, so they must be carefully identified and selected out.

Quaker Beans

Quaker beans, also known as immature beans, can be considered unripe green coffee beans. These beans result from coffee fruits being harvested before they are fully mature, and the green coffee beans extracted from these unripe fruits are quaker beans. Generally, at the green bean stage, it's difficult to distinguish them from other "mature" green beans, but after roasting, the difference between the two becomes very obvious (color difference)! Unripe green beans haven't accumulated enough nutrients to develop flavor substances, so after roasting, these beans show almost no flavor performance, presenting more of a roasted seed taste! They also easily bring salty and astringent flavors to coffee.

Foreign Objects Other Than Coffee Beans

Besides defective beans, a batch of coffee beans might also be mixed with other foreign objects like twigs, small stones, etc. These all bring negative effects to roasting, so they must all be removed. In summary, it's best to screen out all these defective substances before roasting to ensure the final roasted coffee beans are not affected by their adverse impacts~ Although we will screen for defects again after roasting, by then these flavor-damaging beans won't be so easy to identify!

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