Physical Changes During Coffee Roasting

Roasting will transform coffee beans from raw beans into ripe beans with rich aroma and flavor, but do you know what happened during the roasting process?

When we are roasting beans, it is mainly divided into physical and chemical changes. Let us first take a look at what physical changes happen to the coffee beans during roasting.

The Importance Of Physical Structure

The layered structure of coffee beans has a great influence on roasting the flavor we want. Without a specific physical structure, the chemical reactions necessary for flavor and aroma will not occur.

Changes During Roasting

Green coffee beans are seeds with a high density and compact structure, but they will change their original state once roasted. Let's take a look at the physical changes that occur during roasting.

Color Change

Perhaps the most obvious change during roasting is the color. Before roasting, the fresh green coffee beans will appear blue-green, and then they will turn brown due to melanoidin production. These are polymers formed when sugars and amino acids are combined under heating. During the roasting process, part of the silver skin will also fall off. The silver skin is the outer paper material closest to coffee beans.

Roasters and consumers will use color as the quality criterion for coffee beans and roasting results.

Changes in moisture content and quality

After the drying process, the moisture content of the green coffee beans is about 10-12%, but after roasting, the moisture content will drop to about 2.5%. In addition to the moisture in the green beans, additional moisture is produced through chemical reactions, but this will evaporate during the roasting process.

The loss of moisture and the conversion of some dry matter into gas are the reasons for the overall quality reduction of green beans after roasting. On average, the weight of coffee beans will be reduced by 12-20% before and after roasting. Bakers often record the weight loss ratio to determine which batches of green beans may require additional quality monitoring.

Different baking curves will affect the time when dehydration occurs. At different time points of baking, the changes in water activity may represent differences in chemical reactions, which may impact the final baking curve.

Changes In Volume And Pores

The strength of the cell wall of coffee beans is at the top of the plant kingdom. They have a tough outer layer that increases their stiffness and strength.

When coffee is roasted, the rising temperature and converting moisture into gas will increase the pressure inside the coffee bean. These conditions will change the cell wall structure from rigid to rubbery because coffee beans contain polysaccharides Molecular (bound sugars Molecular ).

The internal material pushes out toward the cell wall, leaving a gas-filled void in the center. It means that as the quality decreases, the volume of the beans will expand, and most of the gas accumulation is the carbon dioxide released after roasting.

Roasting also increases the pores of the coffee beans, reducing the density and making them more soluble. Of course, this also has a lot to do with turning them into delicious drinks.

Changes In Grease

Coffee beans contain fats and oils. During the roasting process, the high pressure inside causes these compounds to move from the center of the cell to the surface.

Oil helps to retain volatile compounds in the cells. Volatile compounds are chemical substances with high volatility characteristics at room temperature. These substances are indispensable for the aroma and aroma of coffee. If there is no oil, these molecules May dissipate quickly.

The longer the roasting time, the more obvious the structural transformation will be, the density of the coffee beans will continue to decrease, and more gas will be produced. The longer the roasting time, the more oil on the surface of the coffee beans.

These developments explain why the taste of dark-roasted coffee is different from that of light-roasted coffee. Still, there are also important chemical changes that affect the roasting result. We will look at this part in another article.

What Happened At Each Stage Of Roasting

Different roasting methods will affect the final coffee's flavor, aroma, and mouthfeel because chemical changes occur at different time points during roasting.

But no matter what kind of baking method, baking will mainly be divided into three stages: dehydration, Maynard (caramelization) reaction, and flavor development. These terms actually describe the different stages of chemical and physical changes.

1. Dehydration

The dehydration process will occur at the temperature recovery point. The temperature recovery point is when you put the green beans into the roasting machine, the heat inside the device will decrease before it rises again, and the point at which the temperature starts to rise is called the temperature recovery point. During the dehydration stage, the moisture in the raw beans will begin to evaporate, and pressure will start to build up inside the beans.

2. Maynard reaction

When the coffee beans start to turn brown, the Maynard reaction has begun, which occurs when heated to about 150°C. This process will produce many gases, including carbon dioxide, water vapor, volatile gases, etc. When the internal pressure is large enough to break through the cell wall, it will expand, called the first burst.

The development of flavor will also occur in the Maynard reaction at the same time. In addition to the change in the color of the beans, it will also affect the final coffee flavor.

3. Flavor development period

When the roast reaches the first burst, the roasting changes from an endothermic reaction (absorbing heat from the drum) to an exothermic reaction (the beans release heat). At this stage, the physical changes will continue, the pores on the surface of the beans will continue to increase, the oil will continue to move from the core to the surface, and the color will continue to darken. Many chemical reactions will also occur at this stage, and this will be discussed in another article.

Although it may seem simple, coffee roasting is a highly complex procedure because many physical and chemical changes occur in the roasting process simultaneously. But due to the unique structure of coffee beans, all of this has become a reality.

Before buying a bag of coffee beans, you can think about what will happen to each bean when it is roasted to this degree.

The following is a coffee roasting video sharing from Youtube: