Flavour compounds in roasted coffee
Chemical compounds present in roasted coffee can be roughly grouped into volatile and non-volatile, some of the former being responsible for aroma and the latter for the basic taste sensations of sourness, bitterness and astringency (Buffo & Cardelli-Freire 2004). Russwurm reported that carbohydrates, proteins, peptides and free amino acids, polyamines and tryptamines, lipids, phenolic acids, trigonelline, and various non-volatile acids in the green coffee beans were involved in the flavour formation during roasting (Russwurm 1970). For example, chlorogenic acid contributes to body and astringency; sucrose contributes to color, aroma, bitterness, and sourness; minor protein components like free amino acids are highly reactive by interacting with reducing sugars, which make the Maillard reaction happen; triogenlline generates pyridine and may be consequently be responsible for some objectionable flavours; and caffeine can be contributed to the bitterness (Flament 2002).
Maillard reactions have been identified to be the major pathway in the formation of volatile compounds in coffee roasting (Shibamoto 1991). In the Maillard reaction, reducing sugars such as glucose or fructose react with free amino acids to form N-substituted glycosylamine adducts, which are then rearranged to aminoketones and aminoaldoses by Amadori and Heynes rearrangements. A complex reaction cascade of Amadori and Heynes rearrangement products leads to numerous volatile compounds and complex melanoidins.
More than 800 volatile compounds have already been identified in roasted coffee, among which, about 40 compounds are responsible for the characteristic aroma of coffee (Belitz et al. 2009).
(1) Proteins, peptides and amino acids: Crude protein content is relatively stable during roasting, while the free amino acids decrease by 30%, with dark roast espresso reaching up to 50% (Belitz et al. 2009). Protein content plays an important role in espresso coffee as it affects the foamability of the beverage that the foamability increased generally with increase total protein concentration until a maximum value is reached (Nunes et al. 1997). The composition of the amino acids vary dependent on their thermal stability and reactions involved. For instance, changes in glutamic acid content are less dramatic as compared to cysteine and arginine. The latter amino acids tend to deplete rapidly during roasting due to their involvement in Maillard browning reactions (Illy & Viani 2005).
(2) Carbohydrates: Only traces of free mono and disaccharides in green coffee remain after roasting. Cellulose, hemicellulose, arabinogalactan and pectins play important roles in the retention of volatiles and contribute to coffee brew viscosity. It is reported that in espresso coffee, the foam stability is related to the amount of galactomannan and arabinogalactan (Nunes et al. 1997).
(3) Non-volatile lipids and lipid-solubles: Triglycerides, terpenes, tocopherols and sterols contribute to brew viscosity. The lipid fraction tends to be stable and survive the roasting process with only minor changes. Linoleic and palmitic acids are the predominant fatty acids in coffee. Cafestol and kahweol are diterpenes that degrade by the roasting process. Another diterpene, 16-O-methylcafestol, is present in Robusta but not Arabica coffee, making it a suitable indicator for detecting Robusta content in coffee blend (Speer et al. 1991; Belitz et al. 2009).
(4) Caffeine: Caffeine is of major importance with respect to the physiological properties of coffee, and also in determining the strength, body and bitterness of brewed coffee. The caffeine content of green coffee beans varies according to the species that Robusta coffee contains about 2.2%, and Arabica about 1.2%. Environmental and agricultural factors appear to have a minimal effect on caffeine content. During roasting there is no significant loss in terms of caffeine (Ramalakshmi & Raghavan 1999). However, caffeine content per 177 mL (6 oz) of coffee range from 50 to 143 mg, depending on the mode of preparation(Rogers & Richardson 1993; Bell et al. 1996). Bell and others (Bell et al. 1996) reported that more coffee solids, larger extents of grinding, and larger volumes of coffee prepared at a constant coffee solids to water ratio led to significantly higher caffeine content. Home-grinding yielded caffeine content similar to that of store-ground coffee, and boiled coffee had caffeine contents equal to or greater than filtered coffee (Bell et al. 1996).
(5) Acids: Acids are responsible for acidity, which together with aroma and bitterness is a key contributor to the total sensory impact of a coffee beverage. Carboxylic acids, mainly citric, malic and acetic acids are responsible for acidity in brewed coffees. Arabica coffee brews are more acidic (pH 4.85-5.15) than Robusta brews (pH 5.25-5.40) (Vitzthum 1975).
(6) Melanoidins: The final products of the Maillard reaction between amino acids and monosaccharides, are the brown-coloured substances that impart to roasted coffee its characteristic color, possess antioxidant activity, and affect on the flavor volatiles (Hofmann & Schieberle 2001; Del Castillo et al. 2002; Vignoli et al. 2011).
Source : Physicochemical Changes of Coffee Beans During Roasting
JAVA PREANGER COFFEE ADDICT
Fadillah Satria
FTIP TMIP UNPAD
fadilprojectkopi@gmail.com
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