Professional Roasting of Coffee
Roasting coffee is the last stage in the process of transforming of coffee fruit to coffee grinds which are most commonly used when brewing coffee. However, roasting of coffee is relatively new in the history of consumption of coffee. Coffee was consumed for many centuries before it was discovered that roasting the beans is the best way of extracting the natural flavors of the plant. Before this discovery there where other methods used other then the roasting to extract these flavors and some are still used today. The berries can be fermented to make a wine, for example, or the leaves and flowers cured and steeped in boiling water to produce a coffee tea. In parts of Africa, people soak the raw beans in water and spices, then chew them like candy. The raw berries are also combined with bananas, crushed, and beaten to make a sort of raw coffee and banana smoothie.
In Yemen, where coffee was first cultivated as a commercial crop, the husks of the dried coffee fruit are boiled with spices to produce a sweet, light beverage called qishr. It is served cool as a thirst quencher in the afternoon, much like we might serve iced tea.
The key to the success of the current mode of coffee making is the roasting process, to which we owe the delicately flavored oils which extenuate the natural flavors of coffee beans.
During the roasting process the bean loses a good deal of its moisture, which results in a reduction of the weight of the bean. It loses some protein, about 10 to 15 percent of its caffeine, and traces of other chemicals. Sugars are caramelized, which contributes color, some body, and sweetness, complexity, and flavor to the cup.
Roasting is simple in theory; the beans must be heated, kept moving so they do not burn or roast unevenly, and cooled, or quenched, when the right moment has come to stop the roasting. Coffee roasted too long or at too high a temperature is thin-bodied, burned, and industrial-flavored.
During the early part of the roast the bean merely loses free moisture, moisture which is not bound up in the cellular structure of the bean. Eventually, however, the deep bound moisture is forced out, expanding the bean and incidentally producing a snapping or crackling noise. So far, the color of the bean has not changed appreciably (it should be a light brown), and the oil has not been volatilized. Then, when the interior temperature of the bean reaches about 370°F, the oil suddenly begins developing. This process is called pyrolysis, and it is marked by darkening in the color of the bean.
The beans cannot be allowed to cool of their own accord or they may overroast. With smaller scale equipment the cooling may be managed by fans that pull room-temperature air through the hot beans while they are stirred, a procedure called air quenching. Larger roasting machines may permit the roastmaster to water quench the beans, or kick off the cooling process with a brief spray of water. If the water quenching is done properly and discreetly, the water evaporates immediately from the surface of the hot beans and does not adversely affect the flavor of the coffee. In fact, coffee that has been tactfully water quenched often tastes better than air-quenched coffee because the cooling is more decisive.
Equipment used in Professional Roasting:
The exact nature of the equipment used to roast coffee depends on the capacity of the roaster. Most large commercial roasters resemble a gigantic screw rotating inside a drum. The screw works the coffee down the drum; by the time the coffee reaches the end of the drum, it is roasted and ready to be cooled by air or water. The temperature is controlled automatically, and the roaster includes equipment that monitors both the air temperature and the temperature inside the moving mass of beans to monitor their progress. Such roasters are called continuous roasters for obvious reasons and are inappropriate for spec
ialty coffee roasting because they cost too much, roast too much coffee at a time, and cannot be easily stopped to reload with new and different kinds of coffee.
The average specialty roaster uses a batch roaster, which simply means any machine which roasts a batch of coffee at a time rather than the same coffee for most of the day. The most common design of batch roaster consists of a rotating drum above a heat source, usually a gas flame. Operating like a Laundromat clothes drier, the rotating drum tumbles the beans, ensuring an even roast, while convection currents of heated air move through the drum. Drum roasters may be as large as five or six feet in diameter, or as small as a small waste can set on its side.
In all cases, the goal of the technology is to offer the operator maximum control over the process, and to keep the beans from touching hot metal for anything longer than a split second at a time to prevent scorching or uneven roasting. Proponents of the various styles of batch roaster all make cogent arguments in favor of their favored system and in mild deprecation of rival approaches, but to my knowledge no conclusive comparative tests have ever been conducted to validate any of these claims and counterclaims.
Roasting by Instrument:
The key to technical roasting is an electronic thermometer or heat probe designed to rest inside the bed of beans as they roast. Since degree or darkness of roast precisely reflects the internal temperature of the roasting beans the heat probe permits the roastmaster to follow the development of the roast inside the machine with confidence. A second heat probe registers the temperature of the air inside the roasting chamber. The roastmaster, using various formulas that define the optimum relationship between these two temperatures plus information regarding air velocity inside the roasting chamber, profiles the roast, adjusting temperature ratios and air velocity to control the intensity of the pyrolysis and the length of the roast.
Roasting by Experience:
Craft roasters also profile coffees, adjusting temperature and air velocity inside the roasting chamber as the roast progresses. However, rather than on formula, their adjustments are based on long experience with roasting generally, with roasting specific coffees, and with the peculiarities of their machines.
Internal bean temperature is not the only way to tell what is going on inside a roasting machine. Roasting coffee signals its internal changes by a number of rather dramatic external signs. As well as changing in color, roasting coffee speaks to the roaster by emitting a crackling sound at two very predictable moments in its development -- the "first crack" when pyrolysis begins, and the "second crack" when the woody matter of the bean begins to transform and the beans start to enter the pungent, bittersweet realm of darker roasts. When craft roasters speak about a specific green coffee and how to roast it, for example, they speak in terms of the crack -- just before the second crack, just at the second crack, just into the second crack, well into the second crack, and so on.
The changing smell of the roasting smoke also tracks the development of the roast, starting with a bready smell before the first crack, to a fuller, sweeter, more rounded scent between the first and second cracks, to a pungent, sharper, oiler odor during the second crack. The best old time craft roasters can control the roast quite accurately based on the smell of the roasting smoke alone.
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