How Do Agtron Values Measure the Roasting Process?

Judging roast level by eye alone is not rigorous enough. In the industry, a colorimeter measures the caramelization degree value, known as the Agtron number, by shining infrared light at coffee beans; carbonized substances absorb it and the reflected light indicates color depth. Darker readings point to a higher roast level and give a shared reference for the roasting process.

Two readings matter: bean color and ground value. The RD value, calculated from the difference between them, shows how evenly a batch roasted. Coffee beans processing methods and other factors can shift the results, so the shared Agtron roasting table helps enthusiasts read measurements more systematically instead of relying on visual estimates that often cause disagreement.

Judging the roast level of coffee beans by eye alone, simply observing the depth of color, is clearly not rigorous enough. In the industry, a colorimeter is more commonly used to measure the caramelization degree value. This article will start from the measurement principle, introduce the meaning of the Agtron value (color value), the difference between bean color and ground value, and how the RD value calculated from the difference between the two reflects roast uniformity, while also explaining the influence of processing methods and other factors on the measurement results. Front Street will also present the commonly shared Agtron value roasting table to help coffee enthusiasts more systematically understand the scientific basis behind roast judgment.

When observing a coffee bean, besides distinguishing variety and processing method from its appearance, we often also make a rough judgment of its roast level based on the depth of the bean surface color. Beans with a darker color usually indicate a higher roast level; conversely, a lighter color indicates a lower roast level.

However, visual estimation with the naked eye is clearly unscientific, and it is also easy to produce disagreements during communication. Therefore, the industry uses more rigorous methods to measure the caramelization degree value of coffee beans as a reference for the roasting process.

What is the caramelization degree value?

The caramelization degree value is the "Agtron Number," commonly referred to in the industry as the Agtron value or color value. During roasting, the "Maillard reaction" and the "caramelization reaction" are the two key chemical reactions in the formation of coffee flavor compounds. They not only bring richer flavors to coffee beans but also deepen the color of the beans. As mentioned at the beginning, the darker the roast, the darker the color. Measuring the color value means using specialized equipment (a coffee bean roasting colorimeter) to determine the color depth of coffee beans, thereby defining their roast level.

The measurement principle is actually not difficult to understand: it uses the property that carbonized substances in coffee beans absorb infrared rays. Specifically, the instrument emits infrared rays to shine on the coffee beans. When the infrared rays encounter carbonized substances, they are absorbed, and the infrared rays reflected back are reduced accordingly. The machine then derives a value based on the intensity of the reflected light, and we ultimately use the numerical value to classify the roast level.

The darker the roast level, the more carbonized substances the coffee beans contain, the more infrared rays are absorbed, the less light is reflected back, and the final measured value is naturally lower, and vice versa. Based on this value, people created an Agtron value roasting reference table, which is the one Front Street often shares.

I believe everyone has also heard of what the industry calls the "ground value." This is likewise easy to understand: it refers to the color value of coffee grounds, that is, the value measured by the instrument after grinding the coffee beans into powder. Because the overall heating degree of coffee beans varies depending on the roasting method (or tends to be consistent), the color value measured from coffee grounds represents an average roast level of the entire coffee bean.

Next, what needs to be understood is that the real purpose of measuring these two color values is to calculate the "Roast Delta" (roast difference), abbreviated as the RD value, from the bean color and ground value. It is not used to define the roast level of coffee beans (that is not the main purpose), because this method has certain limitations, so the value can only be used as a reference.

There are two reasons for this: First, during roasting, in addition to carbonized substances, the Maillard reaction and caramelization reaction also produce melanoidins, melanin, and other substances that likewise absorb infrared rays. The instrument cannot distinguish among them, so these substances all affect the final measured value.

Second, under different processing methods, the surface of coffee beans will undergo different degrees of caramelization during roasting due to differences in sugar content. Take anaerobic and honey-processed coffee beans as examples. During processing, because these beans are continuously soaked and coated by substances such as pectin and fruit juice, sugars adhere to the bean surface.

As a result, they undergo a deeper caramelization reaction than beans processed by other methods, and the bean surface color becomes darker accordingly. Although they taste like light-roasted coffee with sweet and sour flavors, the bean surface color is close to the black of dark-roasted coffee. Therefore, the processing method can also cause deviations in the final measurement results.

So what is the RD value?

The RD value is the difference between the bean color value and the ground value, mainly used to measure whether the inside and outside of the coffee beans receive a consistent roast level during roasting. The calculation formula is the higher value minus the lower value. For example, if the coffee bean color value is 61 and the coffee ground value is 71, then the calculation formula is 71-61, giving a difference of 10, meaning the RD value is 10.

The smaller the difference number, the more evenly the beans are heated inside and out, and the more balanced the flavor performance will be; the larger the difference number, the greater the roast level gap between the inside and outside of the beans. A moderate difference helps present layering, while an excessively high difference means the roasting method may need adjustment. In addition to differences caused by processing methods, it may also be because the bean surface is heated too much, causing excessive caramelization that far exceeds the roasting received inside the bean, which is why the ground value and bean color value differ too greatly.

Of course, the uses of the RD value are not limited to this. Here we are only briefly introducing the meaning and role of each value. To explain them in detail would probably turn into a long article. So Front Street will leave it until next time and write another article to introduce their applications in detail~

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The above content is compiled by CoffeeHunters, a coffee news website.