LeBrew·2025년 9월 11일

Why is the color value of coffee beans highest at Yellow Point?

Abstract

In the coffee roasting process, the Yellow Point is an extremely important stage, typically occurring at 150–160 °C. At this moment, the color of the coffee beans changes from green to light yellow, the grassy notes noticeably diminish, and subtle cereal-like aromas begin to emerge.

Traditionally, roasters relied mainly on visual observation of color changes and olfactory perception of aroma shifts to determine whether the beans had reached the Yellow Point. With the advancement of detection technologies, such as the LeBrew RoastSee Next Online Color & Crack Analyzer, roasters can now scientifically and precisely capture this point through real-time monitoring. Data show that the color value curve of almost all coffee beans reaches its highest peak at the Yellow Point during the entire roasting process. This naturally raises the question: why is the color value not at its maximum in green beans, but specifically at the Yellow Point?

This article explores in detail the trend of the color value curve, the underlying principles, and the physical transformations that coffee beans undergo at the Yellow Point.

The Trend of the Agtron Curve

Using the LeBrew RoastSee Next Online Color & Crack Analyzer, the color value change trend of coffee beans can be generally described as follows:

Gradually rising from a low point until reaching its peak (Yellow Point).

Then it gradually decreases as the Maillard Reaction and Caramelization intensify, causing the beans to turn dark brown and eventually nearly black. This “rise-then-fall” curve is closely related to the structural changes inside the beans and the progression of chemical reactions.

Changes of Coffee Beans During Roasting

Green Bean Stage: Coffee beans contain about 10–12% moisture, with dense cell wall structures and surfaces covered by chlorophyll and phenolic compounds. Due to the compact structure and strong absorption of green pigments, light is easily absorbed, resulting in relatively low reflectance and low color values.

Green Color of Beans: The characteristic greenish hue mainly comes from chlorophyll and carotenoids, which strongly absorb visible light (particularly red and blue wavelengths), giving the beans a bluish-green appearance.

Yellowing Stage: When the temperature reaches 155–160 °C, much of the moisture has evaporated, and the cellular structure gradually loosens, forming micro-pores. These pores enhance light scattering, making the bean surface appear brighter. Reflectance increases, and as a result, the color value reaches its peak.

Yellow Point: At this point, chlorophyll rapidly decomposes and the green color disappears, while the Maillard Reaction and Caramelization are still in their early stages, not yet producing large amounts of brown pigments. The beans appear light yellow or beige, light absorption decreases, reflectance increases, and the color value reaches its maximum.

Continued Roasting: As the Maillard Reaction deepens, large amounts of browning compounds are formed. The bean color gradually darkens, reflectance decreases, and the color value correspondingly declines.

Principle of Colorimeter Detection

The LeBrew RoastSee Next Online Color & Crack Analyzer, as a representative coffee colorimeter, works on the principle of emitting laser light at specific wavelengths onto the surface of roasting coffee beans, then measuring the reflectance and converting it into numerical values:

High reflectance → High color value (lighter color).

Low reflectance → Low color value (darker color).

Across the different stages of coffee beans:

Green Beans: Green pigments absorb light → reflectance is not high → color value remains low.

Yellow Point: Green pigments disappear + light scattering increases → reflectance is at its highest → color value peaks.

Continued Roasting: Brown pigments accumulate → light absorption increases → reflectance decreases → color value drops.

Thus, from the beginning to the end of roasting, the color value curve of coffee beans follows a trend of rising from low to its highest point at the Yellow Point

Conclusion

The fact that coffee beans reach their highest color value at the Yellow Point is the result of three combined factors: the decomposition of chlorophyll, the formation of micro-pores that enhance light scattering, and the absence of significant amounts of brown pigments.

This stage is not only the peak of the color value curve but also a critical boundary for scientifically identifying the Yellow Point during roasting. Accurately capturing this moment enables roasters to adjust their roasting strategies with greater precision, thereby achieving better control over the development of coffee flavors.