Meroasting Kopi!

Roasting of coffee beans

Green coffee beans provide neither the characteristic aroma nor flavour of brewed coffee until they are roasted. Moreover, the roasting process increases the value of coffee beans, by 100-300% of the raw material (Yeretzian et al. 2002). Roasting of coffee beans typically takes place at 200-240°C for different times depending on the desired characteristics of the final product. Events that take place during roasting are complex, resulting in the destruction of some compounds initially present in green beans and the formation of volatile compounds that are important contributors to the characteristic of coffee’s aroma. The chemical compositions of green, roasted, and brewed coffee are shown in Figure 1 (Barter 2004).

Briefly, as temperature increases to about 100oC, green coffee beans undergo moisture loss from 8-12% in green coffee beans to about 5% in the roasted coffee beans (Illy & Viani 1998). The smell of the beans changes from herb-like green bean aroma to bread-like, the color turns from green to yellowish, and the structure changes from strength and toughness to more crumbly and brittle. When the internal temperature of beans reaches 100oC, the color darkened slightly for about 20-60 s due to the vaporization of water. At 160-170oC, the beans become lighter in color for about 60-100 s. As roasting continues at this temperature, Maillard and pyrolytic reactions start to take place, resulting in gradually darkening of the beans (Hernandez et al. 2007). The buildup of water pressure, along with the large amount of gases generated causes the cellulose cell wall to crack, giving rise to the so called “first crack”. As heating continues at the roasting temperature (160-170oC), the coffee becomes darker and more rapid popping of coffee bean occurs (“second crack”) as the carbon dioxide (CO2) buildup exceeds the strength of the cellulosic walls of the bean. Finally, after roasting, the fresh roasted coffee beans are quickly cooled to stop roasting (Yeretzian et al. 2002).

The final quality of roasted coffee is influenced by the design of the roasters and time-temperature profiles used. Although heat transfers during roasting can involve conduction, convection, and radiation, convection by far is the most important mode of heat transfer that determines the rate and uniformity of roasting (Baggenstoss et al. 2008). Coffees roasted in fluidized-bed roaster that is almost exclusive based on convective heating can result in low density and high yield coffee (Eggers & Pietsch 2001). On the other hand, coffees roasted in drum roaster that involves mainly conductive heat transfer have less soluble solids, more degradation of chlorogenic acids, more burnt flavour, and higher loss of volatiles than the fluidized bed roasters (Nagaraju et al. 1997).

The effects of time-temperature profile on coffee aroma properties have been reported by Lyman et al. (Lyman et al. 2003). They observed that the medium roasted process (6.5 min to the onset of the first crack and 1.0 min to the onset of the second crack) resulted in good balance of taste and aroma with citrus flavour. However, the “sweated process” (4.5 min to the first crack and 6.5 min to the second crack) resulted in non-uniform bean color and the coffee was “sour, grassy, and underdeveloped”. Reducing the heating rate further by using the “baked process” (11 min to the first crack and 18 min to the second crack) produced coffee of “flat, woody with low brightness and acidity” (Lyman et al. 2003). In another study, Schenker et al. reported that LHC process (150 to 240oC in 270 s; 240oC for 55 s) resulted in the formation of the highest quantities of aroma volatiles, while the long time low temperature (LTLT) approach (isothermal heating at 220oC for 600 s) generated the lowest aroma volatiles. Moreover, the distribution of the 13 volatile compounds monitored was considerably different depending on the roasting profiles used (Schenker et al. 2002).

Depending on the extent of heat treatment, coffee can be largely categorized as light, medium or dark roasts. Light roast process tends to give non-uniform bean color with sour, grassy, and underdeveloped flavour, while medium roast process produces a balanced taste and aroma with citrus flavour. By contrast, dark roast process produces coffee of low acidity sensory profiles (Lyman et al. 2003). Physical characteristics such as temperature, color, and weight-loss are often used as indicators of roast degree. However, these parameters only allow assessment of the flavour profile for coffee roasted under narrow process conditions (Sivetz 1991; Illy & Viani 1995). Other analytical methods for quantifying the degree of roast include ratio of free amino acids (Nehring & Maier 1992), alkylpyrazines (Hashim & Chaveron 1995), and chlorogenic acids content (Illy & Viani 1995). Fobe and others (Fobe et al. 1968) studied changes in chemical composition of Arabica coffee roasted at 230°C at different process times. They reported that as the roasting time increased, the following changes occurred: (1) sugar contents first increased, and then decreased; (2) minimal change in caffeine content; (3) proteins decreased continuously; (4) free fatty acids increased; and (5) unsaponifiable compounds decreased (Fobe et al. 1968).

Source : Physicochemical Changes of Coffee Beans During Roasting

JAVA PREANGER COFFEE ADDICT

Fadillah Satria

 

FTIP TMIP UNPAD

fadilprojectkopi@gmail.com