What are the basic knowledge of coffee roasting? How to judge the process and color change of coffee beans roasting

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Coffee roasting can be divided into five stages:

Warm machine: preheat the coffee roaster boiler and set the temperature to the beginning of baking, which is called "bean temperature".

Qianjie Coffee talk about the roasting process and color change of coffee beans

When judging the roasting degree of coffee beans, the roasting degree can be judged by sound, color and aroma.

Its color from light gray to dark brown, aroma from light and elegant gradually rich; the sound is issued twice burst sound, for example, heat-resistant raw beans, about 7 points began the first explosion, lasting about 2 minutes, its voice is low, indicating the beginning of pyrolysis, starch into caramel, water evaporation, exhaust, around 12 minutes issued a second explosion, the sound is violent and sharp, indicating that the cell fiber is broken, the deeper the baking.

The famous American Fine Coffee Association (SCAA) promotes the determination of the degree of baking by the caramelization analysis value (Agtron number) determined by the red inside. the higher the value, the lower the caramelization, the grayish color, and the lighter the baking; conversely, the lower the value, the higher the caramelization, the brighter the color, and the deeper the baking.

Dehydration period: the water content of raw coffee beans is usually 8-12%, and water is the heat conduction medium. with the increase of temperature, the free water in the beans evaporates slowly, and gas is produced inside. The so-called "dehydration" does not remove the water completely. In the process of baking, water is also a heat conducting medium. With the increase of temperature, the free water of the bean evaporates slowly, which produces gas inside, exerting pressure on the tissue of the bean and causing the bean to expand gradually. At the same time, the bean will become elastic with the change of temperature and humidity, until the water activity decreases and then returns to the hard and brittle vitrification state. At this time, a large part of the cell has been transformed into gas, and the outer layer is thinned by heating and expansion. In the case of baking too fast, the evaporation of water from bean surface and bean core is different, which is easy to make the moisture of coffee beans uneven, resulting in vitrification of the skin and undercooked bean core, resulting in astringent coffee.

Mena reaction period: when most of the free water is removed, the water activity drops below 0.7. the temperature and pressure inside and outside the bean are at about the same stage, when the bean expands rapidly, and the raw bean changes from green to white to yellowish brown. And it smells a bit like the aroma of toast, followed by the so-called Mena reaction and caramelization stage. Mena reaction refers to a series of reactions between reducing sugars in food and protein / amino acids when heated to form brown-black macromolecules, also known as flavor compounds.

Development period: turn to a darker brown, and the temperature climbs to close to 185 ℃ (different temperature measurement points, there is a deviation in the value), so you should pay attention to the coming burst sound. The first explosion is the reaction in which the carbon dioxide and water in the raw bean are expanded by heating, and it is also the key point that affects the flavor. After the beans began to burst for the first time, it entered the stage of development, the Mena reaction has entered the end, and caramelization is accelerating. During the explosion, a large amount of water vapor and carbon dioxide in the cells will be discharged in a short time, causing the bean temperature to drop slightly and the humidity to rise rapidly; when the free water is completely released at the end of the explosion, the humidity will drop again. There are many reactions taking place in a short time, so the regulation of air volume and firepower in the later stage of Mena reaction has a great influence on the final flavor of beans.

Bean cooling: when baked to the baking degree or temperature of your choice, the beans can be cooled. Cooling is the last key to coffee roasting. Reduce the heat of the coffee as soon as possible so that the flavor of the coffee can be locked in the temperature and bean condition you decide.