Roasting Technology

The main processing steps in the manufacture of roasted coffee are blending, roasting, grinding, and packaging.

Green coffee is shipped in bags weighing from 60 to 70 kg. Prior to processing, the green coffee is dumped and cleaned of string, lint, dust, hulls, and other foreign matter. Coffees from different varieties or sources are usually blended before or after roasting.

Roasting is usually carried out batchwise by contacting beans with hot combustion gases in rotating cylinders. Continuous roasting using long cylinders fitted with helical conveyor flights is also employed. The beans absorb heat at a fairly uniform rate, and most moisture is removed during the first two-thirds of this period. As the temperature of the coffee increases rapidly during the last few minutes, the beans swell and unfold with a noticeable cracking sound, like that of popping corn, indicating a reaction change from endothermic to exothermic. This stage is known as development of the roast. The final bean temperature, 200 to 230°C, is determined by the blend, variety, or flavor development desired. A water or air quench terminates the roasting reaction. Most, but not all, of any added water is evaporated from the heat of the beans.

Theoretically, about 700 kJ of heat are needed to roast a kilogram of coffee beans. However, the hot gas-recirculation rate determines the thermal efficiency achieved. Older roasters that do not use recirculation may have an efficiency rate as low as 25%, requiring as much as 2800 kJ. Roasters with high levels of gas recirculation have an efficiency rate of 75% or more, requiring about 930 kJ. Volatile organic compounds in roaster discharge gases and smoke have to be reduced to tolerable levels. Some reduction can be effected by partial recycling of roaster gas, but to achieve adequate reduction, discharged roaster gas must be passed through an afterburner.

Most roasters are equipped with controls that regulate the temperature of the hot gas used and automatically quench the roast when a chosen end-of-roast bean temperature is reached. The chosen end-of-roast bean temperature correlates well with the desired color of ground roast coffee as measured by photometric reflectance. Roasts are stopped by diverting the heating gas, injecting a controlled amount of cold water, and blowing room-temperature air through the beans. Control systems introduced in the 1990s provide improved control of roasting reactions and reproducible development of coffee flavor by controlling bean temperature versus time behavior during roasting (10).

Conventional roasting by hot combustion gases in rotating cylinders requires 8 to 15 min. Fast roasting technologies patented in the 1980s develop roast flavor in under 5 min, sometimes in 1 to 3 min (11,12).

Fast roasting, whether carried out in a batch or continuous roaster, is achieved by increasing rates of heat transfer into beans. This may be done by using much more heated gas than used conventionally. The heated gas may flow upward through a bed of beans, fluidizing it. The bed may also be fluidized by hot gas reflected upward from a solid surface after issuing from downward-pointed jets. Rapidly rotating paddles and centrifugal devices that throw beans through heated gas are also used to improve heat transfer to beans. Recirculative lifting of beans by heated gas in a spouted bed is also used. When much larger quantities of gas are used to transfer heat to beans, the gas temperature can be lower, that is, 250 to 310°C compared with 425 to 550°C.

Fast-roasted coffees are less dense, retain more chlo-rogenic acid, free acid, moisture, and carbon dioxide than conventionally roasted coffees and release 20% more soluble solids during home brewing. It is further claimed that the lower roasting temperature results in higher aroma retention.

Air must be circulated through the beans to remove excess heat before the finished and quenched roasted coffee is conveyed to storage bins. Residual foreign matter, such as stones and tramp iron, which may have passed through the initial green coffee cleaning operation, must be removed before grinding. This is accomplished by an air-lift adjusted to such a high velocity that the roasted coffee beans are carried over into bins above the grinders, and heavier impurities are left behind. The coffee beans flow by airveying or by gravity to mills where they are ground to the desired particle size.

Small and intermediate-size coffee roasters, after virtually disappearing from the United States, have staged a marked comeback. Now, some 1200 to 1500 small, custom roasters roast roughly 12% of the coffee consumed in the United States.

Grinding

Roasted coffee beans are ground to improve extraction efficiency in the preparation of the beverage. Particle size distributions ranging from about 1100 //m average (very coarse) to about 500 //m average (very fine) are tailored by the manufacturer to the various kinds of coffee makers used in households, hotels, restaurants, and institutions.

Finer grinds are provided for espresso coffee (300 to 400 /¿m), and Turkish coffee (<100 //m).

Most coffee is ground in mills that use multiple steel cutting rolls to produce the most desirable uniform particle size distribution. After passing through cracking rolls, the broken beans are fed between two more rolls, one of which is cut or scored longitudinally, the other, circumferentially. The paired rolls operate at controlled speeds to cut, rather than crush, the coffee particles. A second pair of more finely scored rolls, installed below the main grinding rolls and running at higher speeds, is used for finer grinds. A normalizer section is then used to distribute particles uniformly.

Packaging

Most roasted and ground coffee sold directly to consumers in the United States is vacuum packed in metal cans; 0.45, 0.9, or 1.35 kg are optional, although other sizes have been used to a limited extent. After roasting and grinding, the coffee is conveyed, usually by gravity, to weighing-and-filling machines that achieve the proper fill by tapping or vibrating. A loosely set cover is partially crimped. The can then passes into the vacuum chamber, maintained at about 3.3 kPa (25 mmHg) absolute pressure, or less. The cover is clinched to the can cylinder wall, and the can passes through an exit valve or chamber. This process removes 95% or more of the oxygen from the can. Polyethylene snap caps for reclosure are placed on the cans before they are stacked in cardboard cartons for shipping. A case usually contains 10.9 kg of coffee, and a production packing line usually operates at a rate of 250 to 350 0.45-kg cans per minute.

Though slight losses in fresh roasted character occur due mostly to chemical reactions with the residual oxygen in the can and previous exposure to oxygen prior to packing (9), vacuum-packed coffee retains high quality for at least one year.

Coffee vacuum packed in flexible, bag-in-box packages has gained wide acceptance in Europe and the United States. The inner liner, usually a preformed pouch of plastic-laminated foil, is placed in a paperboard carton that helps shape the bag into a hard brick form when vacuum is applied (13). The carton also protects the package from physical damage during handling and shipping. This type of package provides a barrier to moisture and oxygen as good as that of a metal can.

Inert gas flush packing in plastic-laminated pouches, although less effective than vacuum packing, can remove or displace 80 to 90% of the oxygen in the package. These packages offer satisfactory shelf life and are sold primarily to institutions.

Some coffee in the United States, and an appreciable amount in Europe, is distributed as whole beans, which are ground in the stores or by consumers in their homes. Whole-bean roasted coffee remains fresh longer than unprotected ground coffee and retains its fresh roasted flavor for several days longer than ground.

Roasted whole beans are often packed in bags that contain a one-way valve to allow escape of carbon dioxide gas produced during roasting and prevent air from entering package.

Modified Coffees

Coffee substitutes, which include roasted chicory, chickpeas, cereal, fruit, and vegetable products, have been used in all coffee-consuming countries. Although consumers in some locations prefer the noncoffee beverages, they are generally used as lower-cost beverage sources, rather than as coffee.

Chicory is harvested as fleshy roots, which are dried, cut to a uniform size, and roasted. Chicory contains no caffeine and, on roasting, develops an aroma compatible with that of coffee. It gives a high yield, about 70%, of water-soluble solids with boiling water and can also be extracted and dried in an instant form. Chicory extract has a darker color than does normal coffee brew (14). The growing technology for the processing and use of roasted cereals and chicory is evidenced by the introduction on the market of coffees extended with these materials.

Roughly 2% of the coffee sold in the United States contains added flavors, for example, vanilla, chocolate, and nut flavors. These coffees are manufactured by spraying flavors dissolved in alcohol on roasted beans and drying the beans.

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