The two major species of coffee, Coffea arabica and C. canephora, var. Robusta, differ considerably in price, quality, and consumer acceptance. The washed Arabica coffees are characterized by some acidity and intense aroma, while natural, dry-processed Arabica coffees are less acidic and have a less-marked aroma but a richer body. Robusta coffees are characterized by their bitterness and a typical earthy and woody flavor. Blending, which
PLS1 OSC-PLS1
X y x y
LV1 32.9 58.7 40.5 69.4
LV2 55.4 17.6 59.4 28.0
Total 88.3 76.3 99.9 97.4
Table 2.4. Percentage of explained variability for PLS1 and OSC-PLS1 optimal models.
can be done before or after roasting, has the purpose of obtaining coffee brews with a higher quality when compared to their individual counterparts. In espresso, washed coffees bring a fine, intense aroma, and natural coffees add body (Illy and Viani 1995). Torrefacto coffee is widely used in Argentina, Costa Rica, and Spain, where its consumption repre- sents 83 percent of the commercially available coffees in hotel trade. To obtain torrefacto coffee, at the end of the roasting process, sucrose (no more than 15 percent in weight) is added. At high temperatures, sucrose is converted into caramel, forming a burnt film around the coffee bean, making it more bitter and less odorous (Sanz et al. 2002).
The volatile fraction of roasted coffee has been analyzed by many authors (Dart and Nursten 1985; Flament 1989, 1991; Maarse and Visscher 1989; Nijssen et al. 1996), who have identified about 850 compounds (Flament and Bessiére-Thomas 2002). Various methods of extraction have been used to study the aroma fraction of coffee brews. As an alternative to injection of an organic solvent extract, the vapor phase surrounding the brew (headspace) can be directly analyzed. This alternative approach gives the most accurate composition of flavors. However, when a large volume of headspace gas is injected, the carrier gas dilutes the sample. This problem can be solved by injecting the headspace gas directly inside a capillary column—on-column injection (Shimoda and Shibamoto 1990)—using a purge and trap system (Semmelroch and Grosch 1995), using an adsorbent (Pollien et al. 1997), or using a static headspace sampler (Sanz et al. 2001, 2002). In 1990, the solid phase microextraction (SPME) technique was developed for headspace sampling (Arthur and Pawliszyn 1990). It is a simple, rapid, solvent-free, and not very expensive method (fig. 2.15) that has been shown to be suitable for use in coffees (Yang and Peppard 1994; Bicchi et al. 1997, 2002; Ramos et al. 1998; Roberts et al. 2000).
The sensory characteristics of coffee brew depend on the method of extraction used. Petracco (2001) classified the extraction methods from a qualitative perspective: the grouping criterion chosen takes into account both the mode and the time of coffee-water contact. Among the pressure methods, plunger (cafetière) coffee, where the suspension of hot water and coffee powder is pressed through a plunger, and espresso coffee are examples. Some studies have been done on the taste and mouth feel (Maeztu et al. 2001a) and flavor and aroma (Liardon and Ott 1984; Maeztu et al. 2001b) of espresso coffee. At
120 mL flask
Thermostated at 60 °C, during 30 min
Followed by 30 min at 30°C with PDMS 100μm coating fiber for extraction of
volatile compounds
Headspace 80 mL
Liquid phase 40 mL
Fig. 2.15. Experimental procedure for sample preparation and extraction of volatile compounds from coffee brew by headspace–solid phase microextraction (HS-SPME).
Methodologies for Improved Quality Control Assessment of Food Products 31
least 28 compounds were reported as characteristic odorants of ground and brewed coffee (Shimoda and Shibamoto 1990; Blank et al. 1991, 1992; Grosch 1995; Semmelroch and Grosch 1995, 1996; Mayer et al. 2000). The change in the flavor profile from the ground coffee to the brewed is caused by a change in the concentrations of these compounds (Blank et al. 1991; Semmelroch and Grosch 1995) and not by the formation of new odor- ants. The sensory analysis carried out by a trained panel is too cumbersome and costly to be introduced as a routine procedure. Assessors in sensory panels cannot always, con- sistently and objectively, identify the sample, specially a blend. Their perception of the aroma of the coffee will depend on physiological and psychological factors. The coffee industry needs a simple, quick, and objective method to classify, especially, the botanical varieties of coffees (Arabica or Robusta) and/or the type of blends used in brewed coffee preparations.
Aiming for screening and distinction of coffee brews based on the combined technique of headspace–solid phase microextraction–gas chromatography–principal component analysis (HS-SPME-GC-PCA), a methodology has been proposed, based on the defini- tion of the global volatile profiles of coffee brews, that is complementary to that obtained by sensory evaluation and that precludes the identification of the volatile compounds by mass spectrometry (Rocha et al. 2004a). Coffee brews were (1) a blend of natural roasted 80 percent Robusta and 20 percent Arabica (R80 : A20), (2) a 50 percent torrefacto of 80 percent Robusta and 20 percent Arabica (R80 : A20 torrefacto), and (3) a natural roasted 100 percent Arabica (A100).
Volatile profiles, expressed as a relative percentage of the GC peak area for the different chemical classes of volatile compounds ensuing from the headspace SPME analysis by gas chromatography–mass spectrometry (GC-MS), are shown in figure 2.16 (Rocha et al. 2004a). The relative percentage of GC peak area for the different chemical classes of the R80 : A20 natural and torrefacto coffees were similar for both espresso and plunger coffees, although the natural R80 : A20 coffee had a more intense aroma than the torrefacto. The decrease of volatile compounds in the torrefacto brew may be due to the fact that, in this blend, a fraction (6 percent) of the coffee had been replaced with sugar. Also, torrefacto coffee is usually submitted to a lower degree of roast to avoid flavors produced by the burnt sugar. The presence of sucrose during the roast may also contribute to this decrease by hindering the volatile compounds in a caramel interface. The relative percentage of the GC peak area for furans, in both espresso and plunger coffees, was higher in A100 than in R80 : A20 coffees, and conversely, the relative percentage of GC peak areas for pyrazines was lower. Pyrazines are products obtained from the Maillard reactions, and they are more abundant in Robusta coffee (Sanz et al. 2002; Ho et al. 1993), which is consistent with the lower content of free amino acids in Arabica coffees (Illy and Viani 1995).
Figure 2.17 shows the PCA scores scatter plots of PC1 × PC2 (fig. 2.17a) and PC1 × PC3 (fig. 2.17b), which contain 82 percent of the total variability of SPME volatile profiles of espresso and plunger coffees.
Figure 2.18 represents the corresponding loadings plots that establish the relative importance of each volatile component according to its retention time. The PC1 × PC2 scores plot (fig. 2.17a) shows the distinction between the Arabica coffees (PC1 negative and PC2 positive) and Robusta coffees, both R80 : A20 and R80 : A20 torrefacto (PC1 positive and PC2 negative).
According to the corresponding loadings plots (fig. 2.18a, b), the Arabica brews are characterized by the compound with the retention time of 44.9 min (furfurylacetate), and
Robusta brews are characterized by the compounds with the retention times of 12 min (2-methylbutanal), 38.2 min (2-ethyl-5-methylpyrazine), 39.3 min (trimethylpyrazine), and 41.1 min (3-ethyl-2,5-dimethylpyrazine). These results are in accordance with the distinc- tion between Robusta and Arabica espresso coffee based on the amount of aldehydes and pyrazines (Petracco 2001), as furfurylacetate (PC1 negative and PC2 positive) was twice higher in Arabica than in Robusta brews and 2-methylbutanal, 2-ethyl-5-methylpyrazine, trimethylpyrazine, and 3-ethyl-2,5-dimethylpyrazine (PC1 positive and PC2 negative) were up to three times higher in Robusta than in Arabica brews.
Fig. 2.16. Volatile profile, expressed as a relative percentage of the GC peak area for the dif- ferent chemical classes of volatile compounds, of (a) espresso and (b) plunger coffees using a polydimethylsiloxane (PDMS) SPME coating. R80 : A20—80 percent Robusta and 20 percent Arabica blend; R80 : A20 torrefacto—80 percent Robusta torrefacto and 20 percent Arabica blend; A100—100 percent Arabica coffee (Rocha et al. 2004a).
0 10 20 30 40 50 1 Aldehydes 2 Ketones 3 Furans 4 Pyrazines
5 Pyridines 6 Phenolic compounds 7 Indoles 8 Others
R80:A20 R80:A20 torrefacto A100
2 3 4 5 6 7 8 1 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 espresso coffee 0 10 20 30 40 50 GC p e a k a re a ( % ) 1 Aldehydes 2 Ketones 3 Furans 4 Pyrazines
5 Pyridines 6 Phenolic compounds 7 Indoles 8 Others
a
R80:A20 R80:A20 torrefacto A100
2 3 4 5 6 7 8 1 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 espresso coffee 0 10 20 30 40 50 1 Aldehydes 2 Ketones 3 Furans 4 Pyrazines
5 Pyridines 6 Phenolic compounds 7 Indoles 8 Others
R80:A20 R80:A20 torrefacto A100
2 3 4 5 6 7 8 1 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 plunger coffee 0 10 20 30 40 50 G C p e ak ar ea (% )
b
R80:A20 R80:A20 torrefacto A100
2 3 4 5 6 7 8
1 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
plunger coffee
PC1 (52%) PC2 (1 6 % )
espresso R80:A20 espresso A100 espresso R80:A20 torrefacto plunger R80:A20 plunger A100 plunger R80:A20 torrefacto
arabica robusta a PC1 (52%) P C 3 ( 14% )
espresso R80:A20 espresso A100 espresso R80:A20 torrefacto plunger R80:A20 plunger A100 plunger R80:A20 torrefacto
espresso
plunger
b
Fig. 2.17. PCA scores scatter plot of the chromatographic SPME areas of coffee volatile
compounds. (a) PC1 × PC2 and (b) PC1 × PC3 (axes cross each other at the origin) (Rocha et al. 2004a).
The PC1 × PC3 scores plot (fig. 2.17b) shows the distinction between plunger (PC1 positive and PC3 positive) and espresso (PC1 negative and PC3 negative) coffee brews. According to the corresponding loadings plots (fig. 2.18a, c), the plunger coffee brews are characterized mainly by the compound with the retention time of 27.5 min (pyridine) and espresso coffee brews are characterized by the compound with the retention time of 10.1 min (2-methylfuran). For the three coffees studied, pyridine (PC1 positive and PC3 positive) was, in fact, 23–43 percent higher in plunger than in espresso coffee brews, and 2-methylfuran (PC1 negative and PC3 negative) was 10–62 percent higher in espresso than in plunger coffee brews.
The volatile profile of espresso and plunger coffee brews obtained by SPME-GC-MS seems to be established mostly by the botanical variety (Arabica or Robusta) than by the process of preparation of the brews (espresso or plunger). Furthermore, the use of the variability given just by the GC areas and respective retention times, combined with the PCA, allowed for the observed distinction. The combined technique of HS-SPME-GC- PCA, when compared with the conventional techniques based on GC-MS identification of volatile compounds, can be proposed as a lower-cost, fast, and reliable technique for screening and distinction of coffee brews (Rocha et al. 2004a).