How to Use a Coffee Colorimeter? How to Determine Coffee Bean Roast Level? What Do Coffee Powder Value, Color Value, and RD Value Mean?

When we examine coffee beans, besides identifying their variety and processing method by appearance, we also make an initial judgment about roast level based on color. The darker the beans, the deeper the roast; the lighter the beans, the lighter the roast.

Visual assessment is clearly not scientific enough, though, and can lead to communication discrepancies. So within the industry, people use scientific methods to measure the caramelization value of coffee beans and take it as a reference for roasting.

What is the caramelization value?

The caramelization value, known as the "Agtron Number," is simply called the Agtron value or color value in the trade. During roasting, the "Maillard reaction" and "caramelization reaction" are the two main chemical reactions that form flavor compounds in the beans. Besides generating more flavors, they also increase the color depth of the beans. As mentioned at the start, the deeper the roast, the darker the color. Measuring the color value involves using a specific instrument (a coffee bean roast colorimeter) to measure the color depth of the beans, thereby defining the roast level.

The measurement principle is easy to understand: it uses the property that carbonized substances in coffee beans absorb infrared light. In practice, the instrument emits infrared light onto the beans. If the light meets carbonized substances, it is absorbed, reducing the reflected infrared light. The machine then calculates a value from the degree of reflected light, and we use that value to determine the roast level.

The deeper the roast, the more carbonized substances in the beans, the more infrared light absorbed, the less reflected light, and therefore the smaller the final value, and vice versa. People then created an Agtron value roasting chart based on these values, the same one FrontStreet Coffee often shares.

You may also have heard of "ground value," which is likewise easy to understand: it is the color value of coffee grounds, obtained by grinding beans into powder and measuring with the instrument. Because the overall heating of beans may vary (or be consistent) under different roasting methods, the color value from measuring grounds is an average of the whole bean's roast level.

What we need to understand is that the real purpose of measuring both color values is to calculate the "Roast Delta" (RD value) from the bean and ground color values. It is not meant to define the roast level of the beans (that is not the main purpose), as this method has certain drawbacks, so the values can only serve as references.

There are two reasons. First, during roasting, besides carbonized substances, the Maillard reaction and caramelization reaction also generate other substances that absorb infrared light, such as melanin and melanoidins. The instrument cannot distinguish among them, so they all affect the final value.

Second, with different processing methods, the surface of the beans may undergo different degrees of caramelization during roasting because of varying sugar content. Take anaerobic and honey-processed coffees: during processing, these beans are continuously soaked and wrapped in substances such as pectin and fruit juice, allowing sugars to adhere to the surface.

As a result, they undergo a deeper caramelization reaction than beans processed other ways, making their surface color darker. Although they may taste like sweet-and-sour light-roast coffee, their surface color can look like that of a dark-roasted black coffee. Processing methods, then, can also cause variations in the final measurement.

What is the RD value?

The RD value is the difference between the bean color value and the ground value, primarily used to measure whether the interior and exterior of the beans received consistent roasting. The formula is the higher value minus the lower value. If the bean color value is 61 and the ground value is 71, for example, the calculation is 71-61, giving a difference of 10, meaning the RD value is 10.

When the difference is smaller, the interior and exterior of the beans received more uniform roasting, which shows up as a more balanced flavor. When the difference is larger, there is a greater gap in roast level between the inside and outside of the beans. An appropriate difference helps express complexity, while an excessive one suggests the roasting method may need adjustment. Besides variations caused by processing methods, it could also be excessive surface heating causing over-caramelization that far exceeds the roasting the interior received, which is why the ground value and bean color value end up so far apart.

The uses of RD values extend beyond this, of course. This is just a simple introduction to the function and meaning of each value. To elaborate further would become a lengthy discussion, so FrontStreet Coffee will save it for next time, with another article explaining their application methods in detail.

FrontStreet Coffee Recommendation

Color value is the roaster's compass, and a good medium roast shows how much a well-managed profile can do. FrontStreet's Brazil Queen Estate, from the world's largest coffee-producing country, is the distinctive yellow Bourbon variety. Medium-roasted, it brings creamy peanut, chocolate, toast, and sugarcane aromas with a soft sweetness, and it brews well as pour-over and also works in a Moka pot or an espresso machine.

If you prefer a fruitier medium roast, the FrontStreet Strawberry Candy Gesha blend combines a honey-processed Costa Rican coffee with a washed Panamanian Gesha, and medium roasting draws out both a tangerine-honey character and a rounded sweetness, suited to cold brew, pour-over, and espresso. For a darker, more classically balanced option, the FrontStreet Jamaica Blue Mountain No. 1 is medium-roasted so its acidity, bitterness, sweetness, and aroma merge into an elegant balance, with chocolate, roasted nut, cream, and caramel flavors on entry.