How does the roasting process change coffee beans?

The roasting process turns green coffee beans into the beans that get ground and brewed. Heat drives physical expansion and complex chemical reactions inside the bean, and the timing of first crack and second crack decides much of the final taste. Picking, processing and sorting come first, but roasting is where the flavor code of a cup is largely written.

Drying, browning reactions and development between the cracks shift acidity, sweetness and body. Mechanical picking costs less and suits large, flat farms, while manual picking gives more uniform ripeness. Dry, wet and honey processing also shape what the roaster has to work with before any heat is applied in the drum.

The flavor code of a good cup of coffee is often already written during the roasting stage. From green beans to finished product, coffee beans must go through multiple processes such as picking, processing, sorting, and roasting, and the physical expansion and chemical reactions during roasting are the key to determining the taste. This article will systematically sort out the temperature nodes, color changes, and flavor characteristics of each stage of coffee bean roasting, helping coffee lovers understand the principles behind first crack and second crack, so they can more accurately choose coffee beans that suit their own taste. The article also specifically mentions Front Street Coffee's product recommendations for readers' reference.

[Introduction] If you want to drink authentic and delicious coffee, the roasting stage is the core that cannot be avoided. The final flavor of coffee depends to a large extent on how the roasting process proceeds, and professional roasting is usually divided into several stages. Coffee beans are organic matter, and during heating their internal components undergo extremely complex transformations. With the current understanding of organic chemistry, this still cannot be fully explained.

I. Green Bean Processing

1. Picking

Picking methods are divided into two categories: mechanical and manual. Mechanical picking is suitable for coffee farms with flat terrain and large areas. Brazil is the country that uses mechanical picking the most in the world. Its advantages are low cost and high efficiency, but the harvested beans vary in size and quality. Coffee beans with high quality requirements usually still rely on manual picking, which ensures that the fruit is uniform in size, close in ripeness, and free of impurities, and also creates good conditions for subsequent processing.

2. From Fruit to Coffee Bean

The processing methods from coffee fruit to coffee bean can be roughly divided into three categories: dry method, wet method, and honey process.

Dry processing (natural): The fruit is dehydrated and dried with natural conditions such as sunlight and ventilation, and then the dried fruit is hulled to remove the pulp, skin, and silver skin. This is the most economical and traditional processing method. The resulting coffee beans can retain aroma for a relatively long time, but they also easily absorb environmental odors. Many producing areas in Africa and Asia still use this method, but with technological progress, more and more places are beginning to use dryers to assist drying.

Wet processing (washed): Also called the washed method. The fruit is first soaked in water and screened, then the skin, pulp, and the sticky slippery mucilage on the surface of the silver skin are removed through steps such as fermentation and pressing. After processing, coffee beans that retain the silver skin on the surface are called "Parchment Coffee," and usually only before export are they passed through a huller to remove the dried silver skin. The greatest advantage of wet processing is good appearance quality and excellent retention of original flavor. Its disadvantages are higher cost and large water consumption, so it is generally used for high-quality coffee beans.

Honey process: In English it is called Honey Process or Miel Process. The method is to remove the pulp while retaining the parchment and then dry the fruit.

Transportation

Before Roasting

Before starting roasting, the beans must first be sorted—green bean screening is equally critical, as defective beans will directly affect the flavor of the finished product.

What needs to be removed includes: fully black beans or partially black beans, sour beans or partially sour beans, moldy beans, insect-damaged beans, immature beans, withered beans, shell beans, shell-shaped beans, cracked beans, and non-coffee impurities such as twigs, stones, and copper plates.

Coffee roasting is a discipline. The age, density, hardness, and moisture content of green beans all affect the quality after roasting; even the relative humidity and temperature on the day of roasting cannot be ignored. Roasting is directly heating the object being roasted. It does not require oil, water, or salt or sugar.

Color Changes of Coffee Beans During Roasting

1. Green bean 0:00 – 75°F [23°C]

2. Beginning to turn white 4:00 – 270°F [132°C]

Steaming phase, this is the most important part of the coffee bean's endothermic reaction. The beans gradually expand, the color changes from the green of green beans to white, and moisture slowly evaporates from the center of the bean outward due to heat absorption. Whether the coffee tastes good depends on this stage: if moisture evaporates too quickly, the center of the bean is likely to be underdeveloped and the taste tends to be bitter; if evaporation is too slow, the flavor will be flat and the aroma insufficient. Therefore we insist that the steaming process must never be omitted, and we should also use it to make the coffee more fragrant, richer, and more delicious.

3. Early yellowing 6:00 – 327°F [164°C]

At this point the coffee is still losing moisture through evaporation and has not yet undergone physical expansion. Hot-air roasters can reach this stage earlier because the hot airflow can complete heat exchange efficiently and carry away steam. At this time the coffee has a damp grassy aroma. (All warm-up stages before first crack are endothermic reactions. The coffee beans absorb heat from the environment to store energy for the upcoming audible roasting reaction, namely first crack.)

4. Tan stage 6:30 – 345°F [174°C]

The coffee beans begin to show a tan color, and marbling appears on the surface, but they still have not expanded. A slightly damp toasted wheat smell can be detected. Some varieties appear brighter at this stage, such as Costa Rica and Mexico.

5. Light brown stage 8:00 – 370°F [188°C]

First crack is approaching. A few individual beans show slight expansion, the center line opens, and the silver skin attached to the center line begins to fall off.

6. Brown stage 9:00 – 393°F [200°C]

At this point it has reached the threshold of first crack. The coffee beans rapidly turn brown in a short time. Part of the reason is the caramelization reaction, but more critically it is the "Maillard reaction" (which is also why steak turns red).

7. First crack begins 9:20 – 401°F [205°C]

At this time sporadic crackling sounds similar to popcorn can be heard, clearly different from the dense and intense sounds of second crack. Once again, the temperature indication is only the environmental temperature and differs greatly from the actual temperature inside the bean. Through a relatively complex method, the internal bean temperature at this time was measured at 356°F.

8. First crack in progress 10:00 – 415°F [213°C]

While first crack is in progress, the surface color of the coffee is still mottled and uneven. The bean body begins to crack, expands in volume, steam escapes from the inside, the center line opens fully, and the remaining silver skin falls off completely. Because first crack is an exothermic reaction, it causes the beans to cool, but it quickly turns into an endothermic reaction again, which means that if the roaster fails to supply heat in time, roasting will stall here. This is not good. Once the caramelization reaction begins (bean internal temperature 340–400°F), loss of temperature will produce a bitter "baked" taste, possibly because long-chain polymerization reactions are interrupted. The melting point of sucrose is 370°F, exactly the same as the caramelization reaction temperature indicated here.

9. First crack ends 10:40 – 426°F [219°C]

This stage is called City Roast. The surface of the expanded beans becomes relatively smooth, but still retains dark marks like

10. City Roast+ 11:05 435°F [224°C]

The stage between first crack and second crack is a brief endothermic period. When a certain temperature is reached, the lignocellulosic matrix structure composing the coffee bean begins to break down, and second crack begins. You can observe the color darkening rapidly in a very short time, while the temperature change is much less noticeable than in the previous stage.

11. Deep City Roast 11:30 – 444°F on the verge of second crack [229°C]

The image shows a lighter deep city roast. At this point the coffee is on the edge of second crack; the internal bean temperature at second crack is usually around 446°F. In fact, second crack is harder to predict than first crack—why is that? Based on my experience, first crack is a physical expansion reaction of the bean body caused by the release of water vapor and carbon dioxide, whereas second crack is a process of physical fracture of the fiber structure. Coffee from different origins, varieties, and altitudes varies in size and density, and you can imagine that their fiber structures also differ; an ordered lignocellulosic structure reacts more readily than a disordered one.

12. Deep City Roast+ 11:50 – 454°F second crack begins [234.5°C]

The image shows a heavier deep city roast; at this point the coffee beans have almost entered second crack. Roasting was stopped 10 seconds after hearing second crack. By comparing with the full image of the deep city roast, the significant difference can be easily seen, but a more reliable method is to judge by sound.

13. Vienna – Light French Roast 12:15 – 465°F second crack in progress [240.5°C]

Vienna (European-style) – Light French Roast is where the coffee's original characteristics begin to be masked by roast characteristics. If you buy coffee for different flavors, then a heavy roast may not be suitable for you. But there are exceptions; certain coffees may perform quite excellently at this stage.

It should be noted that: Espresso is not a type of roast, but Northern Italian-style espresso is often roasted to an internal temperature of 440–446°F, while Southern Italian is generally equivalent to a light French roast or darker.

14. Deep French Roast 12:40 – 474°F second crack rapidly nearing its end [245.6°C]

Sugars are highly caramelized (it looks as if they have been burned) and simultaneously decompose. The woody structure inside the bean begins to carbonize, and the volume continues to increase while the weight becomes lighter and lighter. Aromatic compounds, oils, and soluble substances in the bean begin to burn, turning into the smoke that fills your room. My limit is 465°F.

15. Fully Carbonized 13:00 – 486°F [252°C]

Some people call this Italian or Spanish style, which is an insult to these two roasting styles. At this stage, more than 25% of the coffee beans have turned to ash, and the rest are nothing but carbonized, dead, burnt beans.

16. Internal Combustion 13:30 – 497°F [258°C]

At this point the beans are on the verge of combustion; in fact, if the batch size is large enough, sufficient oxygen supply will cause flames to ignite the moment the beans are dropped. If it is a large roaster of 2–3 bags, please contact your local fire department before attempting 497°F.

The above content is compiled by CoffeeHunters, a coffee news website.