Coffee roasting principles: heat conduction, convection and radiation

Baking is a science. To understand the baking principle, we must first understand the heat conduction, radiation on the beam three ways.

Conduction refers to when heat is conducted by heat sources, resulting in molecular vibration of the radius, and the release of heat energy, gradually moving from high temperatures to low temperatures. The conduction of stainless steel has the phenomenon of uneven heating, which easily leads to the situation that some places in the pot are colder and some places are hotter.

Convection refers to when the material is heated, fluid material or gas due to thermal contamination volume expansion, density reduction and rise, its position by the surrounding cooler, denser fluid exchange fill, and then heated up, the surrounding material and fill in. Such repeated heating is called convection.

Radiation does not require any preamble, and heat can be radiated directly to the object to heat it. The most common radiation methods used for food are the use of dielectric, microwave or infrared rays. In the specific operation of baking, depending on the circumstances, two or three ways are used to bake at the same time.

The common problems encountered during the coffee roasting stage are the following:

1. The preheating temperature of the oven is insufficient, and the coffee beans are immediately put into the oven, which delays the roasting time. Therefore, the roasting loss is large, the color of coffee beans is light, this is because of insufficient heat, the skin can not be fully charred, or even lack of luster, and the internal organization is coarse.

Second, the oven temperature is too high, when roasting coffee, the crust is formed prematurely, so that the internal tissue expansion is suppressed, and because of its rapid coloring, the operator mistakenly believes that the product has been roasted and roasted in advance. The bean's interior is sticky and dense, and it doesn't swell as much as it should, and it doesn't have the normal aroma.

Third, the neutral is too long after preheating. Too much heat accumulates in the inner furnace for too long. As soon as the coffee beans enter the furnace, all the heat sources will concentrate on the bean surface at the initial stage of the roasting process, forming too strong a fire, and then the heat will disappear and the rapid cooling will cause the coffee beans to be difficult to ripen.

Fourth, the baking time is too much or not enough. Baking temperature bump, the length of time to adjust with the number of beans. A small number of baking bins have more space, and the heat of metal conduction heat radiation is large, so the temperature should be lower; and when the number is large, the temperature can be higher. The length of the baking time also needs to be adjusted quickly. Most beans into the furnace heating is nothing more than from the outside and around the heat transfer to the center of the beans, so you can see the beans from the appearance of the transformation of its baking situation.

In addition, differences in the type and function of baking machines also cause differences in baking time and temperature. In the past, the direct-fire gas type up-and-down fire system has been widely used at present, and the convection design of energy has been continuously improved. At the same time, the fan can improve the heat convection, so that the color of baked beans is more even, and it can also save electricity and baking time.

In short, coffee roasting when faced with a variety of problems, only familiar with coffee roasting theory, inexhaustible experience, coupled with local conditions, barista can grasp the exquisite skills.