Changes in Chemical Compositions during Roasting
Roasting causes a net loss of matters in the forms of CO2, water vapor, and volatile compounds. Moreover, degradation of polysaccharides, sugars, amino acids and chlorogenic acids also occurred, resulting in the formation of caramelization and condensation products. Overall, there is an increase in organic acids and lipids, while caffeine and trigonelline (N-methyl nicotinic acid) contents remained almost unchanged (Buffo & Cardelli-Freire 2004). The main acids present in green beans are citric, malic, chlorogenic, and quinic acids. During roasting the first three acids decrease while quinic acid increases as a result of the degradation of chlorogenic acids (Ginz et al. 2000). Formic and acetic acids yields increase up to the medium roasting degree and then begin to fall as roasting is continued. According to Balzer (Balzer 2001), a rapid increase in titratable acidity during roasting was observed from green to medium roast, followed by a smaller decrease as roasting proceeded.
The reaction products formed are highly dependent on the roasting time-temperature profile used. Excessive roasting produces more bitter coffee lacking satisfactory aroma, whereas very short roasting time may be insufficient to develop full organoleptic characteristics (Yeretzian et al. 2002; Lyman et al. 2003; Buffo & Cardelli-Freire 2004). Although the majority of phenolic antioxidants naturally occurring in coffee bean are lost during roasting, the formation of other antioxidants from Maillard reactions during roasting can enhance the antioxidant activity of coffee. Compared to medium roast coffee, dark roast coffee exhibited lower radical scavenging activity than medium roasted coffee due to the degradation of polyphenol, and thus the antioxidant activity will also depend on roasting severity and type of coffee (Giampiero Sacchetti 2009).
The profile of organic compounds generated during roasting is very dynamic and complex. Using Proton transfer reaction-Mass spectrometry (PTR-MS) technique, Yeretzian et al. (Yeretzian et al. 2002) simultaneously monitored the evolution of 8 volatile compounds at isothermal conditions as a function of time. They observed a distinctive increase in acetic acid, methyl acetate, and pyrazine concentrations in the headspace, all occurred at the same time. Concomitantly, there was a rapid decrease in water vapor and methanol concentrations. Moreover, these peaks shifted in synchronous manner with the roasting condition. For instance, at 190oC, the above observed changes took place at 19 min but shifted to 30 min when the beans were roasted at 180oC (Yeretzian et al. 2002). Similar observations were observed by Hashim and Chaveron, who concluded that methylpyrazine may be used as an indicator to monitor the roasting progress of coffee beans (Hashim & Chaveron 1995). It has been suggested that the pressure buildup within intact bean cells is comparable to inside an autoclave, which can further complicated the chemical reactions occurred in coffee bean during roasting (Buffo & Cardelli-Freire 2004).
Chemical reactions happened during coffee roasting are very complex, which have not been fully understood. Based on the literature reviewed, we can conclude that the quality of roasted coffee cannot be solely described in terms of physical parameters at the start and end point of roasting, but rather it is dependent on the path taken during the roasting process. To reach a specific flavour profile, not only that precise control of roasting time and temperature is needed, the variety/quality of green beans, cooling, and degassing conditions are expected to be important as well.
Source : Physicochemical Changes of Coffee Beans During Roasting
JAVA PREANGER COFFEE ADDICT
Fadillah Satria
FTIP TMIP UNPAD
fadilprojectkopi@gmail.com
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