What is the difference between coffee beans with different roasting degrees?

Coffee beans will change in different stages of roasting. After the raw coffee beans absorb heat, the color begins to change from green to yellow, and the moisture content of the beans begins to decrease. The heat energy converts the moisture inside the coffee beans into steam → carbon dioxide is produced inside the coffee beans → the pressure inside the coffee beans reaches 25 bar (pressure unit)

First explosion: The characteristic popping sound of coffee beans occurs during the roasting process.

Flavor Development: The longer the roasting time, the lower the sourness and the higher the bitterness.

After dehydration, the coffee beans begin to release heat and turn brown, and also take off a layer of silver skin. The volume of roasted coffee beans will increase by 1.5 to 2 times, and the weight will lose at least 11%.

Continuing to roast the coffee beans will continue to produce carbon dioxide → second crack.

The color of the coffee beans deepens, entering the medium-dark roasting stage. The weight loss at this time is 22% of the original weight. After the second crack, the coffee beans will produce oil on the surface due to pyrolysis reaction and begin to carbonize. If you are not careful, it is very likely to burn.

Three to four aromas begin to develop during the drying phase. The aroma and flavor of coffee are released through two chemical reactions:

• Menard reaction: When the moisture content of coffee beans is less than 5%, sugars and amino acids undergo chemical changes to degrade proteins.

• Caramelization reaction: A chemical reaction between water, sugar and sucrose.

As the roasting time increases, the sourness will gradually decrease, the bitterness will increase, and many aromatic molecules will begin to evaporate.

At the end of roasting, nearly 800 kinds of aroma substances will develop, and the flavor, acidity, sweetness and mellowness of the coffee beans will also mature . However, roasting may sometimes bring out the unpopular flavor of coffee. In the final stage, the aroma substances will be destroyed, replaced by bitterness, the acidity will disappear, and the mellowness will also decrease.

Friends often ask the following questions:

1. Can the degree of roasting be determined from the color of coffee beans?

There is no uniform color when roasting coffee beans that can be used as a basis for the degree of roasting. A more accurate method is to determine the time of the first crack based on the total roasting time.

2. The longer the roasting time, the higher the caffeine content?

The caffeine content of green coffee beans hardly changes with the degree of roasting (caffeine loss rate is about 10%). However, the longer the roasting time, the more weight the coffee loses (weight rate is about 11-22%), in which case the proportion of caffeine naturally increases.

3. How do roasters determine the aroma of coffee beans?

When coffee beans are roasted to a certain degree, the aroma will gradually be covered by the taste of roasting. Only proper roasting can retain the obvious coffee fragrance. Coffee roasters must make a choice to decide which aroma characteristics to leave.

For example, if the roaster wants the coffee to have a lot of aroma and a sour taste, then he or she will have to give up some of the body of the coffee.

Moderate roasting can highlight the natural sourness of coffee beans, but the heat energy brought by roasting will destroy more than 40 kinds of chlorogenic acid in coffee beans and decompose them into quinic acid and caffeic acid ; both of these substances are astringent and may affect the taste.

Most other organic acids, such as citric acid and malic acid, reach their maximum value when lightly roasted, and then gradually decrease with the increase of roasting time. This explains why fast roasting and light roasting can better express the distinct acidity of coffee beans.

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