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Roasting is the process by which processed green coffee beans are cooked and made suitable for brewing coffee. In this process, the coffee bean is heated in a coffee roaster for several minutes to bring the internal temperature of the bean up high enough to cause at least one small pop. Upon completing the roast process, the batch of coffee should be ready for grinding and brewing.

The roasting process

The process of roasting green coffee is fairly simple, requiring only that heat and constant movement be applied to the beans in some form of a coffee roaster. However, depending on the kind of roasting "kiln" used, there is a great degree of skill that goes into roasting suitable coffee.

First crack; pyrolysis begins

When raw coffee beans are first exposed to heat in the roaster, the free moisture in the beans will commence to evaporate as the temperature of the beans rise. As the moisture locked within the beans commences to heat up, the beans commence to expand, and each bean eventually undergoes a small internal explosion called the first crack. At this point pyrolysis, or destruction by heat, begins, the sugars contained within the beans commence to caramelize, and the beans commence to darken appreciably. It is not until this stage of the roasting process that the beans can be removed from the heat and still result in decent coffee.[1] If the beans were employed for brewing before this stage began, then the coffee that resulted would be sour; indeed, this is the characteristic of the cinnamon-degree roast, in which the beans barely reach first crack. Roasting the beans to where they complete first crack, on the other hand, results in the city degree of roasting, the degree to which coffee is most commonly roasted.

Second crack; pyrolysis ends

During the pyrolysis stage, the beans commence to release oils, which results in smoke emanating from the roaster. The internal temperature of the beans now exceeds 350 degrees Fahrenheit. If the roast continues past this point, then the beans will darken further, and the internal temperature of the beans will continue to increase. The full city degree of roasting goes to this stage. Eventually the roast will continue to a quieter series of internal explosions, called the second crack. This is the stage of the French degree of roasting. After the second crack, the roast reaches a darker state at which the beans become increasingly oily and the resulting smoke stronger and more abundant.[2] This is also the stage of the Italian degree and, when continued for longer periods, the Spanish degree of roasting.

Cooling the beans

Nearly as important as the roasting of the coffee beans is the cooling process. It is important that the coffee beans are roasted at a high temperature. Once they are finished roasting, the beans must be brought down to a much cooler (or just slightly warm) temperature as quickly as possible (within two to three minutes of the end of the roast) so that it does not continue to cook at merely high temperature. Some coffee roasting machines accomplish this cooling through a built in cooling fan, however other methods include the use of water to rapidly reduce the temperature of the beans. Cooling roasted coffee with water, called quenching, can be a quicker way of reducing the temperature of the beans so that it does not affect the roast. However, coffee bean roaster which is quenched with more water than what can be evaporated by the heat of the beans will become stale more quickly than properly cooled beans.[3]

Chemical changes

While undergoing roasting, the coffee bean undergoes several chemical changes. The most obvious change at such a high temperature is the loss of water content. Water accounts for between 8 to 14 percent of the weight of unroasted coffee. After roasting, it will account for no more than 3 and as little as .5 percent of the bean's weight, depending on the degree of roast. The carbohydrate content of the coffee beans is also affected during the roast process, becoming caramelized. The resulting material is responsible for the darkened color of the coffee bean and the resulting cup of coffee. Further, the cooking of certain carbohydrates produces the oils responsible for the unique aroma and flavors of the brewed coffee[4].

The total weight loss for an average roast is approximately 16 percent. As part of this loss, the coffee bean loses 9.7 percent of its original fat content and 21.1 percent of its caffeine[5].

Freshness considerations

Once whole-bean coffee has been roasted, it has to be sealed away from air to retain its flavor. This is because gases in air interact with the aromatic oils of the roasted beans, rendering them inert. As previously stated, if the beans are cooled, by quenching, with more water than the heat of the beans can evaporate, then the beans will become stale more quickly. This is because the excess water will wash away some of the aromatic oils of the roasted beans.

Commercially packaged coffee, whether in pre-ground or whole-bean states, is usually so sealed as to allow some gas to escape from the beans while blocking incoming air that would otherwise result in stale beans.

Contrary to popular belief, refrigerating or freezing opened packages of coffee will not extend their freshness and, indeed, actually hastens the process of flavor loss due to the condensation of moisture inside the packaging, in and around the grounds or beans.

See also

References

  1. Kenneth Davids (2001). Coffee: A Guide to Buying, Brewing & Enjoying, Fifth Edition, 104. ISBN 031224665X.
  2. Kenneth Davids (2003). Home Coffee Roasting: Romance & Revival, 11. ISBN 0312312199.
  3. Kenneth Davids (2003). Home Coffee Roasting: Romance & Revival, 190-192. ISBN 0312312199.
  4. William H. Ukers (1922). “Chemistry of Coffee”, All about Coffee, 165. ISBN 0810340925.
  5. William H. Ukers (1922). “Chemistry of Coffee”, All about Coffee, 167. ISBN 0810340925.

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