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 through their appearance, we also make an initial judgment about their roast level based on their color. The darker the coffee beans, the deeper the roast level; the lighter the coffee beans, the lighter the roast level.

However, visual assessment is clearly not scientific enough and can lead to communication discrepancies. Therefore, within the industry, people use scientific methods to measure the caramelization value of coffee beans, using this as a reference for roasting.

What is the Caramelization Value?

The caramelization value – known as the "Agtron Number" – is simply referred to as the Agtron value or color value within the industry. During the roasting process, the "Maillard reaction" and "caramelization reaction" are the two main chemical reactions that form the flavor compounds in coffee beans. Besides generating more flavors for the coffee beans, they also increase the color depth of the beans. As mentioned at the beginning, the deeper the roast, the darker the color. Measuring the color value involves using specific instruments (coffee bean roast colorimeters) to measure the color depth of coffee beans, thereby defining the roast level.

The measurement principle is easy to understand: it utilizes the property that carbonized substances in coffee beans absorb infrared light! The specific operation is: the instrument emits infrared light to irradiate the coffee beans. If the infrared light encounters carbonized substances, it will be absorbed, thus reducing the reflected infrared light. The machine then calculates a value based on the degree of refracted light, and finally, we use this value to determine the roast level.

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

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

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

There are two reasons: First, during the roasting process, 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 between these substances, so they all affect the final value obtained.

Second, with different processing methods, the surface of coffee beans may experience different degrees of caramelization during roasting due to varying sugar content. Taking anaerobic and honey-processed coffee beans as examples, these beans are continuously soaked and wrapped in substances like pectin and fruit juice during processing, allowing sugars to adhere to the surface of the coffee beans.

As a result, they undergo a deeper caramelization reaction compared to beans processed by other methods, making their surface color darker. Although they may taste like sweet and sour light-roast coffee, their surface color appears similar to dark-roasted black coffee. Therefore, processing methods can also cause variations in the final measurement results.

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 coffee beans received consistent roasting during the roasting process! The calculation formula is the higher value minus the lower value. For example, if the bean color value is 61 and the ground value is 71, then the calculation is 71-61, resulting in a difference of 10, meaning the RD value is 10.

When the difference value is smaller, it indicates that the interior and exterior of the beans received more uniform roasting, which will be reflected in a more balanced flavor. When the difference value is larger, it indicates a greater gap in the roast level between the interior and exterior of the beans. An appropriate difference helps to express complexity, while an excessive difference suggests that the roasting method may need adjustment. Besides variations caused by processing methods, it might also be due to excessive surface heating of the beans, causing over-caramelization that far exceeds the roasting received by the interior of the beans, which is why the ground value and bean color value have such a large gap.

Of course, the uses of RD values extend beyond this. This is just a simple introduction to the functions and meanings of each value. If we were to elaborate further, it would become a lengthy discussion. So FrontStreet Coffee will save it for next time, with another article to introduce their application methods in detail.

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