- Email: [email protected]
Dynamic tensiometric characterization of espresso coffee beverage
Food Hydrocolloids 18 (2004) 387–393 www.elsevier.com/locate/foodhyd
Dynamic tensiometric characterization of espresso coffee beverage Luciano Navarinia, Michele Ferrarib,*, Furio Suggi Liverania, Libero Liggierib, Francesca Raverab a
Illycaffe` SpA, R & D, via Flavia 110, Trieste I-34147, Italy Istituto per l’Energetica e le Interfasi, CNR, via De Marini 6, Genova I-16149, Italy
b
Received 18 December 2002; accepted 8 July 2003
Abstract Espresso coffee world-wide success, besides being a phenomenon of fashion, seems to be based on the greater sensory satisfaction it gives to the consumer when compared with coffees prepared with other brewing methods. Conditions normally used in the espresso brewing technique enhance several surface tension-related phenomena such as foam and emulsion formation and stabilisation which strongly affects the organoleptic beverage properties. In spite of the relevant role played by surface tension in several quality characteristics of espresso coffee, little attention has been paid in its determination and its time-dependency has not yet been investigated. In the present work, experimental techniques such as maximum bubble pressure and pendant drop have been used to characterise in a wide time window the dynamic surface tension of air– espresso coffee beverage interface at two different temperatures. The experimental data show a remarkable decrease of the surface tension with time for beverages prepared by using pure arabica as well as pure robusta roasted coffee, with a profile dependent upon the coffee variety. This behaviour is definitely related to the presence of surface active components and is consistent with a system having good wetting properties for oral cavity surfaces. A possible role of some natural surface active chemical components, like lipids, on tensiometric behaviour is discussed. q 2003 Elsevier Ltd. All rights reserved. Keywords: Espresso; Coffee; Emulsions; Foams; Surface tension
1. Introduction Coffee is consumed by a large proportions of the human population (about 70 – 80%) (Schilter, Cavin, Tritscher, & Constable, 2001). The extraction methods for coffee beverage preparation vary significantly on a geographical basis when soluble instant coffee is not used. In general, coffee beverages range from true solutions (e.g. drip filter coffee), to emulsion-like (e.g. Nordic boiled coffee) to thick suspensions (e.g. Turkish style brew) (Petracco, 2001). Espresso coffee world-wide success, besides being a phenomenon of fashion, seems to be based on the greater sensory satisfaction it gives to the consumer when compared with coffees prepared with other brewing methods. Conditions normally used in the espresso brewing technique enhance several surface tension-related phenomena such as foam and emulsion formation and stabilisation which strongly affects the organoleptic beverage properties. * Corresponding author. Tel.: þ 39-10-6475723; fax: þ 39-10-6475700. E-mail address: [email protected] (M. Ferrari). 0268-005X/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-005X(03)00126-7
Italian espresso coffee is properly the beverage (not a coffee roasting degree or a coffee blend) prepared on request from roasted and ground coffee beans (6.5 ^ 1.5 g) by means of hot water (90 ^ 5 8C) pressure (9 ^ 2 bar) applied for a short time (30 ^ 5 s) to a compact roast and ground coffee cake by a percolation machine, to obtain a small cup of a concentrated foamy elixir. The term ‘concentrated’ is well justified if one takes into account the total solids (concentration) of the espresso coffee (typically up to 50 – 60 g/l) in comparison with that of other preparations (10.0 –13.0 g/l for boiled coffee and drip filter coffee; 14.2 g/l for French Press or Plunger coffee). The espresso extraction method results in a polyphasic beverage constituted by a foam layer of small bubbles with a particular tiger-tail pattern, on top of an emulsion of microscopic oil droplets in an aqueous multicomponent solution with dispersed gas bubbles and solid particles. In this particular complex system, interfacial phenomena such as foam and emulsion formation and stabilisation are of crucial importance in contributing to the greater sensory satisfaction espresso coffee gives to the consumer when
388
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
compared with coffees prepared with other brewing methods. It has to be taken into account, that variations in the preparation variables (e.g. coffee blend, roasting, grinding, water temperature and pressure, percolation time and/or beverage volume, etc.) can dramatically alter the ‘cup result’ not only in terms of taste, flavour and mouthfeel (chemical composition) but also in terms of the characteristics of the different phases present in the beverage (foam, emulsion, suspension and solution) and then on the beverage physical properties (Andueza et al., 2002; Maetzu et al., 2001). In Fig. 1 it is possible to observe variations induced by preparation conditions (beverage volume/percolation time) on chemical composition. Moreover it is well known that the foam characteristics is the signature of a perfect preparation being any error immediately denounced by the colour, the texture and the persistence of the espresso foam. This aspect is a further tract of distinction between espresso coffee and other preparation. Several studies have been devoted to characterize the surface properties of beverages in which interface phenomena are strongly related to essential and appreciated organoleptic properties. Beer foam (Evans & Sheehan, 2002) and Champagne wine foam ring (Peron et al., 2001; Peron et al., 2000; Sene`e, Robillard, & Vignes-Adler, 1999) are typical examples in addition to espresso coffee foam (Nunes, Coimbra, Duarte, & Delgadillo, 1997). Beverages are, in most cases, multicomponent and multiphase systems containing many constituents which may show surface activity by themselves or in association with other compounds. Naturally occurring surfactants can be essentially divided into two classes based on molecular weight: low molecular weight compounds (small organic molecules with a molecular weight up to 5000) and high molecular weight compounds (macromolecules and
biopolymers). The coexistence of both classes of surfactants in beverages is more a rule than an exception. In wine, for instance, the surface properties are dominated by ethanol concentration, however, the small difference (approximately 2.5 mN/m) detected between wine surface tension and the corresponding water/ethanol mixture is due to the presence of macromolecules coming from grapes and yeast, mostly proteins and polysaccharides (Peron et al., 2001). From a kinetic point of view, this particular proportion between low and high molecular weight surfactants determines a surface tension decreases less than 0.5 mN/m in the first 20 min of interface formation, and to increase the measurable effect of macromolecules on the surface tension it is necessary to dilute the wine in order to reduce the ethanol concentration (Peron et al., 2000). In beer too, surface properties are due to the presence of low and high molecular weight surfactants. In fact it is generally regarded that fundamental basis for beer foam stability is the interaction between iso-a-acids and beer polypeptides (Dale, Walker, & Lyddiatt, 1993). Surface tension has been also related to other organoleptic food properties including taste (Aroulmoji, Hutteau, Mathlouthi, & Rutledge, 2001; DeSimone, Heck, & Bartoshuk, 1980; Hutteau & Mathlouthi, 1998) and flavor (Larsson & Larsson, 1997) perception and mouthfeel (Bourne, 2002; Lingle, 2001), although the literature is not abundant as in the case of other physical properties. The espresso coffee long lasting after-taste, a sensation perceived for a while (up to 15 min) after having swallowed and emptied the mouth, has been related to the beverage surface properties (Petracco, 2001). It is conceivable that the time-dependent wetting and adsorption properties of the beverage on oral cavity surfaces could be involved in the after-taste but as far as we know, no dynamic tensiometric data has been reported so far. In spite
Fig. 1. Typical analysis of espresso coffee brews (beverage volume/percolation time) Illy and Viani (1995).
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
of the relevant role played by surface tension (and possibly its time-dependency) in the organoleptic characteristics of espresso coffee, little attention has been paid in its determination, apart from scarce reported data obtained by stalagmometry (Illy & Viani, 1995; Maetzu et al., 2001) that, apart to indicate the presence of naturally occurring surfactants in the beverage, cannot provide information about their kinetics of adsorption. In the present work, dynamic experimental techniques such as maximum bubble pressure (MBP) and pendant drop (PD) have been used for the first time to characterize the air –espresso coffee beverage interface at two different temperatures (20 and 37 8C). Two different beverage samples have been prepared following the standard procedures and under strictly controlled preparation conditions by using the two commercially relevant varieties: Coffea arabica (70% of world coffee production) and Coffea canephora both well known as arabica and robusta, respectively. The aroma profiles as well as the chemical composition of arabica and robusta coffee brews are different. As shown in Fig. 1, where the typical chemical composition of espresso coffee beverage is reported, total lipids (about half in robusta than in arabica brews) and caffeine content (about twice as high in robusta as in arabica brews) are the main differences between the two types of brew. In the present work, the comparison between arabica and robusta has been performed not only because the two varieties are market relevant, but also in the attempt to test the sensitivity of surface properties in detecting the naturally occurring chemical differences of the brews.
2. Materials and methods
389
2.2. Tensiometric techniques The dynamic surface tension measurement has been performed by means of a maximum bubble pressure apparatus (Gammalab, Berlin) and a drop shape tensiometer suitable to study adsorption processes with a characteristic time ranging from 1023 to 10 s and from few seconds to hours and sampling rate of 0.1 s, respectively. These two traditional techniques are described in details (Rusanov & Prokhorov, 1996). The dynamic and the equilibrium surface tension data have been obtained according to the pendant drop method, by using PAT-1 software (Sintech, Berlin), a computer assisted drop shape analysis apparatus, which allows the evolution of the surface tension to be monitored under volume control (Miller, Fainerman, Makievski, Ferrari, & Loglio, 2000a). The equilibrium surface tension data have been evaluated from dynamic surface tension signals long enough to warrant the attainment of the adsorption equilibrium. Pendant drops are formed at the tip of a PTFE coated needle connected to a precision syringe control system.
3. Results and discussion Fig. 2 shows the dynamic surface tension at shortmedium time of espresso coffee –air interface obtained from arabica and robusta, measured by the maximum bubble pressure technique. The experimental data show a remarkable decrease of the surface tension with time for
2.1. Sample preparation Green coffee beans (crop 2000/01) of C. arabica (arabica, Santos, Brasil) and of C. canephora (robusta, Congo) have been roasted at a medium degree of roasting (organic losses: 6.4 –6.5% dry matter) by means of a laboratory roaster (Probat, Germany). Roasted coffee beans have been ground at an appropriate particle size distribution for espresso brewing technique by using a professional grinder (Mazzer, Italy). Espresso coffee beverages have been prepared by using a household espresso coffee machine (Gaggia, Italy) under the following conditions: coffee powder 13.0 g; extract (beverage) mass 50 g according to espresso coffee preparation standard (Petracco, 2001). Extraction yield and refractive index at 20 8C have been determined and both resulted well within the range expected for espresso coffee beverage (24 – 25% and 1.338 – 1.344, respectively). After percolation, the beverages have been maintained at room temperature under constant stirring for 20 min and then at rest for further 20 min.
Fig. 2. Dynamic surface tension at espresso coffee –air interface of arabica (X) and robusta (W) beverages using the MBP at T ¼ 20 8C.
390
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
both beverages, with a profile dependent upon the coffee variety. At a first approximation, given that no differences in the preparation conditions have been used, the difference between arabica and robusta espresso may reflect the more relevant chemical differences of the two brews: caffeine content together with total lipid content (Fig. 1). The presence of high molecular weight (7.000 –50.000 Da) surfactants in hot water extracts of defatted roasted coffee has been recently ascertained (Petracco et al., 1999). Apparently, in espresso beverages there are two classes of polymeric surface active soluble compounds: polysaccharides and melanoidins (Maillard products produced during roasting), independently from coffee species. These compounds have been recognized to be involved in the foaming behaviour of the espresso coffee. No data have been yet reported on possible presence in the beverage of low molecular weight surfactants, although it is highly probable that among the number of chemicals coexisting in coffee some of them could be surface active. Due to the objective difficulties in isolating molecules from the beverage without affecting the integrity of the system and taking into account that to the beverage surface properties could contribute colloidal systems present in the unaltered beverage, dynamic tensiometry offers the opportunity to have an insight into the adsorption kinetics and then to provide information related to the molecular weight of the surfactants. The behaviour reported in Fig. 2, where a strong reduction of surface tension is observed already at short time, suggests that in addition to high molecular weight surfactants, low molecular weight amphipathic solutes may play some role. In fact at very short times the adsorption process is strongly influenced by the concentration and the availability in solution of such surface active molecules. In such a complex mixed surfactant system the presence of short chained surfactants can be significant also in a general description of beverage properties like wetting, foaming, taste. Indeed the surface properties of such molecules can be explained not only by diffusive and kinetic conditions but also by important mechanisms like conformational changes of the hydrophobic and hydrophilic moieties, typical of higher molecular weight substances like protein and other biopolymers (Miller, Fainerman, Makievski, Kragel, Kazakov, & Sinyachenko, 2000b). Other phenomena like phase transitions occurring at the interface even with lower molecular weight surfactants like aliphatic alcohols can influence the adsorption process and the structure of the adsorbed layer in terms of molecular organisation (Ferrari, Liggieri, Ravera, & Massa, 1999; Miller et al., 1999). The surface tension values at about 4 s (46.2 and 48.7 mN/m for arabica and robusta, respectively) are close to those reported in literature (Illy & Viani, 1995) and confirm, under similar experimental condition, that pure
arabica espresso coffee shows surface tension values lower than those shown by pure robusta. From Petracco (2001) caffeine concentration in the beverage varies from 1.2 up to 4.0 mg/ml, depending on cup size and blend composition. Corresponding cup contents range from 60 mg of caffeine (for pure arabica blends) up to double: 120 mg (for pure robusta blends). In the concentration range typical for espresso coffee, caffeine shows appreciable surface activity in water which is enhanced by the presence of sucrose (Aroulmoji et al., 2001), however, in view of the entity of the adsorption phenomenon (for 1.0% pure caffeine at 20 8C monitored up to 20 h surface tension equal to 70.1 and 68.5 mN/m in mixture with 6.0% sucrose) it seems difficult to attribute to caffeine content a relevant role in the observed differences as well as in the evolution of surface tension in espresso coffee. Lipids content in the beverage is higher in pure arabica than in pure robusta espresso coffee (Fig. 1). Triglycerides are preferentially extracted from roast and ground coffee (Petracco, 2001) and the existence of an emulsified lipid fraction (oil droplets of micron size) in espresso coffee is well established (Petracco, 2001). It has also been reported that the lowering of the surface tension in the case of emulsions of triglycerides applied to an air/water interface is mainly caused by spreading of oil (Schokker, Bos, Kuijpers, Wijnen, & Walstra, 2002). It may be hypothesised that something similar could occur in espresso coffee and then the lipid content could play a relevant role in its surface properties. In addition to maximum bubble pressure, pendant drop technique has been also used, in order to explore a wider time scale. As shown in Fig. 3, the results obtained by means of the two techniques are reasonably consistent, within
Fig. 3. Dynamic surface tension at espresso coffee–air interface of arabica beverage at T ¼ 20 8C using the MBP (W) and the PD methods (X).
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
Fig. 4. Dynamic surface tension at espresso coffee– air interface of arabica (X) and robusta (W) beverages at T ¼ 37 8C by PD method.
the differences of time and surface tension accuracy of the two techniques. It is evident the tendency to reach an equilibrium value at time higher than 900 s. This behaviour is particularly remarkable at 37 8C and especially for arabica espresso, as shown in Fig. 4. Due to the complexity of the system, the reproducibility of the curves (within 5%) may be considered acceptable as shown in Fig. 5. This behaviour further confirms the existence of higher molecular weight
Fig. 5. Dynamic surface tension at espresso coffee –air interface of two samples of robusta beverages at T ¼ 37 8C by PD method.
391
surfactants adsorbing with a larger characteristic time. The increase of the temperature from 20 to 37 8C shows the expected decrease of surface tension values. The adsorption kinetics profile shows that the robusta sample tends to a steady state more rapidly than the arabica one being the latter influenced by a major content in lipids significantly lowering the surface tension of the water medium (Lee, Kim, & Needham, 2001). As already reported in literature lipids form insoluble monolayers at the interface reaching equilibrium with a slow adsorption process (Kabalnov, Weers, Arlauskas, & Tarara, 1995) rising several discrepancies in the equilibrium surface tension values that are also strongly influenced by other physico-chemical parameters like the lipid phase transition temperature. The overall adsorption behaviour of espresso coffee may be of interest to interpret some peculiarities like the staining ability of the beverage for the oral cavity surfaces and its long-lasting after-taste. It has to be mentioned that human whole saliva at 37 8C is characterised by surface tension values remarkably higher (e.g. 48.5 mN/m at 600 s in Christersson, Lindh, and Arnebrant (2000)) than those of both espresso beverages examined in the present work. Moreover, the critical surface tension of wetting representative of human saliva-coated tooth surface and most restorative materials when exposed in the oral cavity has been reported to be close to 35 – 38 mN/m (Christersson, Dunford, Glantz, & Baier, 1989) and within the bio-adhesive range of 32 – 50 mN/m (Glantz, 1997). Similar studies reported a value of 25 – 27 mN/m for the critical surface tension of salivaconditioned oral mucosa measured in vivo (Christersson et al., 1989). In view of the fact that a surface will be completely wet by a liquid having its surface tension value close to that of the critical surface tension of wetting and considering that the first sip of espresso coffee is generally consumed at high temperature (ca. 60 – 65 8C and accordingly surface tension values lower than those measured in the present work), the dynamic tensiometric behaviour of the espresso coffee beverage is consistent with that expected for systems with good wetting properties for the oral cavity. It has to be stressed that in a study where wetting properties of mouthrinses have been determined through in vivo contact angle measurements on tooth surface, the recruited human subjects were requested not to drink coffee 2 h prior to the measurements (Perdok, van der Mei, & Busscher, 1990). As reported by Shahidzadeh, Bonn, Meunier, and Mavon (2001), lipids in contact with water-based surfaces form a wetting film spreading out by a Marangoni effect and then, due to dewetting instability, breaks up into droplets with a mesoscopic (tens of Angstrom) film in between. From these observations it can be outlined the role played by short and long-range forces: surface active lipids increase the wetting, while the van der Waals forces oppose it.
392
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
Nevertheless, the beverage surface properties show a remarkable adsorption phenomenon at air/beverage interface which can occur at the receptor membrane surface (e.g. taste buds), as well, with clear implication in taste perception and its time-dependency. As a general rule, bitter molecules have a predominantly hydrophobic character whereas sweeteners are rather hydrophilic and both molecules seem to bind to closely located receptors which are coupled to guanidine-nucleotide binding proteins (G-proteins) (Aroulmoji et al., 2001). Due to the importance of the sweetness and bitterness in taste of espresso coffee (it is well known that robusta has a more bitter taste than arabica), the possible role played by lipids could not be confined only in the surface properties but also related to taste perception and possibly duration.
4. Conclusions For the first time espresso coffee beverage has been characterised by dynamic tensiometry. Maximum bubble pressure and pendant drop techniques have been successfully used in spite of the complex nature of the samples. The experimental data, in addition to confirm and extend previously reported scarce data, show a remarkably efficient adsorption at the air –beverage interface for both types of tested beverages with a profile dependent upon the coffee variety especially at higher adsorption times. The arabica espresso coffee is characterised by lower surface tension values than those of robusta beverage and appears to be more efficient as ‘wetting system’ for the oral cavity. The beverage lipid content may play a significant role in this behaviour. In fact the spreading of coffee oil at the air –beverage interface or the emulsified oil droplets present in the beverage may strongly affect the surface properties. Studies on model systems or on beverages prepared by using brewing methods able to extract a lower lipid fraction have been planned in order to clarify this point. The present investigation has been performed by fixing the espresso coffee preparation variables in order to explore the naturally occurring chemical difference of using two different varieties. Further studies could explore chemical differences induced by changes in preparation variables.
Acknowledgements It is a pleasure to thank Marino Petracco for valuable discussions and Bruno Della Pietra for technical assistance.
References Andueza, S., Maetzu, L., Dean, B., de Pena, M. P., Bello, J., & Cid, C. (2002). Influence of water pressure on the final quality of arabica
espresso coffee. Application of multivariate analysis. Journal of Agriculture and Food Chemistry, 50, 7426–7431. Aroulmoji, V., Hutteau, F., Mathlouthi, M., & Rutledge, D. N. (2001). Hydration properties and the role of water in taste modalities of sucrose, caffeine and sucrose–caffeine mixtures. Journal of Agriculture and Food Chemistry, 49, 4039–4045. Bourne, M. (2002). Food texture and viscosity (2nd ed.). London: Academic Press. Christersson, C. E., Dunford, R. G., Glantz, P.-O., & Baier, R. E. (1989). Effect of critical surface tension on retenti oral microorganism. Scandinavian Journal of Dental Research, 97, 247 –256. Christersson, C. E., Lindh, L., & Arnebrant, T. (2000). Film forming properties and viscosities of saliva substitutes and human whole saliva. European Journal of Oral Sciences, 108, 418 –425. Dale, C., Walker, S. G., & Lyddiatt, A. (1993). Dynamic changes in the composition and physical behavior of dispersed beer foam. Journal of the Institute of Brewing, 99, 461–466. DeSimone, J. A., Heck, G. L., & Bartoshuk, L. M. (1980). Surface active taste modifiers: a comparison of gymnemic acid and sodium lauryl sulfate. Chemical Senses, 5, 317 –330. Evans, D. E., & Sheehan, M. C. (2002). Don’t be fobbed off: the substance of beer foam—a review. Journal of American Society of Brewing Chemistry, 60(2), 47 –57. Ferrari, M., Liggieri, L., Ravera, F., & Massa, A. (1999). Molecular reorientation in the adsorption of some CiEj at the water–air interface. Colloids and Surfaces A, 156, 455 –463. Glantz, P.-O. (1997). Interfacial phenomena in the oral cavity. Colloids and Surfaces A, 123 –124, 657 –670. Hutteau, F., & Mathlouthi, M. (1998). Physicochemical properties of sweeteners in artificial saliva and determination of a hydrophobicity scale for some sweeteners. Food Chemistry, 63, 199 –206. Illy, A., & Viani, R. (Eds.). (1995). Espresso coffee: The chemistry of quality (p. 183). London: Academic Press. Kabalnov, A., Weers, J., Arlauskas, R., & Tarara, T. (1995). Phospholipids as emulsion stabilizers. 1. Interfacial-tensions. Langmuir, 11, 2966–2974. Larsson, M., & Larsson, K. (1997). Neglected aspects of food flavor perception. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 123–124, 651–655. Lee, S., Kim, D. H., & Needham, D. (2001). Equilibrium and dynamic interfacial tension measurements at microscopic interfaces using a micropipet technique. 2. Dynamics of phospholipid monolayer formation and equilibrium tensions at the water – air interface. Langmuir, 17, 5544–5550. Lingle, T. R. (2001). The coffee cuppers handbook: A systematic guide to the sensory evaluation of coffee flavor (3rd ed.). Long Beach, California: Specialty Coffee Association of America. Maetzu, L., Andueza, S., Ibanez, C., Paz de Pena, M., Bello, J., & Cid, C. (2001). Multivariate methods for characterisation and classification of espresso coffees from different botanical varieties and types of roast by foam, taste, and mouthfeel. Journal of Agriculture and Food Chemistry, 49, 4743–4747. Miller, R., Aksenenko, E. V., Liggieri, L., Ravera, F., Ferrari, M., & Fainerman, V. B. (1999). Effect of the reorientation of oxyethylated alcohol molecules within the surface layer on equilibrium and dynamic surface pressure. Langmuir, 15, 1328–1336. Miller, R., Fainerman, V.B., Makievski, A.V., Ferrari, M., & Loglio, G (2000). Measuring dynamic surface tensions. In Handbook of applied surface and colloid chemistry. New York: Wiley. Miller, R., Fainerman, V. B., Makievski, A. V., Kragel, J., Grigoriev, D. O., Kazakov, V. N., & Sinyachenko, O. V. (2000b). Dynamics of protein and mixed protein/surfactant adsorption layers at the water/ fluid interface. Advances in Colloid and Interface Science, 86(1/2), 39 –82. Nunes, F. M., Coimbra, M. A., Duarte, A. C., & Delgadillo, I. (1997). Foamability, foam stability and chemical composition of Espresso
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393 coffee as affected by the degree of roast. Journal of Agriculture and Food Chemistry, 45, 3238–3243. Perdok, J. F., van der Mei, H. C., & Busscher, H. J. (1990). Physicochemical properties of commercially available mouthrinses. Journal of Dentistry, 18, 147–150. Peron, N., Cagna, A., Valade, M., Bliard, C., Aguie-Beghin, V., & Douillard, R. (2001). Layers of macromolecules at the champagne/air interface and the stability of champagne bubbles. Langmuir, 17, 791–797. Peron, N., Cagna, A., Valade, M., Marchal, R., Maujean, A., Robillard, B., Aguie-Beghin, V., & Douillard, R. (2000). Characterisation by drop tensiometry and by ellipsometry of the adsorption layer formed at the air/champagne wine interface. Advances in Colloid and Interface Science, 88, 19– 36. Petracco, M. (2001). Beverage preparation: brewing trends for the new millennium. In R. J. Clarke, & O. G. Vitzthum (Eds.), Coffee recent developments (p. 140). Oxford: Blackwell Science.
393
Petracco, M., Navarini, L., Abatangelo, A., Gombac, V., D’Agnolo, E., & Zanetti, F. (1999). Isolation and characterization of a foaming fraction from hot water extracts of roasted coffee. Proceedings of the 18th ASIC Colloquium (Helsinki), 95–105. ASIC Paris, France. Rusanov, A.I., Prokhorov, V.A (1996). Interfacial tensiometry. In Studies in interface science. Amsterdam: Elsevier. Schilter, B., Cavin, C., Tritscher, A., & Constable, A. (2001). Health effects and safety considerations. In R. J. Clarke, & O. G. Vitzthum (Eds.), Coffee recent developments. London: Blackwell Science. Schokker, E. P., Bos, M. A., Kuijpers, A. J., Wijnen, M. E., & Walstra, P. (2002). Spreading of oil from protein stabilised emulsions at air/ water interfaces. Colloids and Interfaces B: Biointerfaces, 26, 315– 327. Sene`e, J., Robillard, B., & Vignes-Adler, M. (1999). Films and foams of champagne wine. Food Hydrocolloids, 13, 15–26. Shahidzadeh, N., Bonn, D., Meunier, J., & Mavon, A. (2001). Wetting of biological lipids on aqueous substrates. Physical Review E, 64(2).
Dynamic tensiometric characterization of espresso coffee beverage Luciano Navarinia, Michele Ferrarib,*, Furio Suggi Liverania, Libero Liggierib, Francesca Raverab a
Illycaffe` SpA, R & D, via Flavia 110, Trieste I-34147, Italy Istituto per l’Energetica e le Interfasi, CNR, via De Marini 6, Genova I-16149, Italy
b
Received 18 December 2002; accepted 8 July 2003
Abstract Espresso coffee world-wide success, besides being a phenomenon of fashion, seems to be based on the greater sensory satisfaction it gives to the consumer when compared with coffees prepared with other brewing methods. Conditions normally used in the espresso brewing technique enhance several surface tension-related phenomena such as foam and emulsion formation and stabilisation which strongly affects the organoleptic beverage properties. In spite of the relevant role played by surface tension in several quality characteristics of espresso coffee, little attention has been paid in its determination and its time-dependency has not yet been investigated. In the present work, experimental techniques such as maximum bubble pressure and pendant drop have been used to characterise in a wide time window the dynamic surface tension of air– espresso coffee beverage interface at two different temperatures. The experimental data show a remarkable decrease of the surface tension with time for beverages prepared by using pure arabica as well as pure robusta roasted coffee, with a profile dependent upon the coffee variety. This behaviour is definitely related to the presence of surface active components and is consistent with a system having good wetting properties for oral cavity surfaces. A possible role of some natural surface active chemical components, like lipids, on tensiometric behaviour is discussed. q 2003 Elsevier Ltd. All rights reserved. Keywords: Espresso; Coffee; Emulsions; Foams; Surface tension
1. Introduction Coffee is consumed by a large proportions of the human population (about 70 – 80%) (Schilter, Cavin, Tritscher, & Constable, 2001). The extraction methods for coffee beverage preparation vary significantly on a geographical basis when soluble instant coffee is not used. In general, coffee beverages range from true solutions (e.g. drip filter coffee), to emulsion-like (e.g. Nordic boiled coffee) to thick suspensions (e.g. Turkish style brew) (Petracco, 2001). Espresso coffee world-wide success, besides being a phenomenon of fashion, seems to be based on the greater sensory satisfaction it gives to the consumer when compared with coffees prepared with other brewing methods. Conditions normally used in the espresso brewing technique enhance several surface tension-related phenomena such as foam and emulsion formation and stabilisation which strongly affects the organoleptic beverage properties. * Corresponding author. Tel.: þ 39-10-6475723; fax: þ 39-10-6475700. E-mail address: [email protected] (M. Ferrari). 0268-005X/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-005X(03)00126-7
Italian espresso coffee is properly the beverage (not a coffee roasting degree or a coffee blend) prepared on request from roasted and ground coffee beans (6.5 ^ 1.5 g) by means of hot water (90 ^ 5 8C) pressure (9 ^ 2 bar) applied for a short time (30 ^ 5 s) to a compact roast and ground coffee cake by a percolation machine, to obtain a small cup of a concentrated foamy elixir. The term ‘concentrated’ is well justified if one takes into account the total solids (concentration) of the espresso coffee (typically up to 50 – 60 g/l) in comparison with that of other preparations (10.0 –13.0 g/l for boiled coffee and drip filter coffee; 14.2 g/l for French Press or Plunger coffee). The espresso extraction method results in a polyphasic beverage constituted by a foam layer of small bubbles with a particular tiger-tail pattern, on top of an emulsion of microscopic oil droplets in an aqueous multicomponent solution with dispersed gas bubbles and solid particles. In this particular complex system, interfacial phenomena such as foam and emulsion formation and stabilisation are of crucial importance in contributing to the greater sensory satisfaction espresso coffee gives to the consumer when
388
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
compared with coffees prepared with other brewing methods. It has to be taken into account, that variations in the preparation variables (e.g. coffee blend, roasting, grinding, water temperature and pressure, percolation time and/or beverage volume, etc.) can dramatically alter the ‘cup result’ not only in terms of taste, flavour and mouthfeel (chemical composition) but also in terms of the characteristics of the different phases present in the beverage (foam, emulsion, suspension and solution) and then on the beverage physical properties (Andueza et al., 2002; Maetzu et al., 2001). In Fig. 1 it is possible to observe variations induced by preparation conditions (beverage volume/percolation time) on chemical composition. Moreover it is well known that the foam characteristics is the signature of a perfect preparation being any error immediately denounced by the colour, the texture and the persistence of the espresso foam. This aspect is a further tract of distinction between espresso coffee and other preparation. Several studies have been devoted to characterize the surface properties of beverages in which interface phenomena are strongly related to essential and appreciated organoleptic properties. Beer foam (Evans & Sheehan, 2002) and Champagne wine foam ring (Peron et al., 2001; Peron et al., 2000; Sene`e, Robillard, & Vignes-Adler, 1999) are typical examples in addition to espresso coffee foam (Nunes, Coimbra, Duarte, & Delgadillo, 1997). Beverages are, in most cases, multicomponent and multiphase systems containing many constituents which may show surface activity by themselves or in association with other compounds. Naturally occurring surfactants can be essentially divided into two classes based on molecular weight: low molecular weight compounds (small organic molecules with a molecular weight up to 5000) and high molecular weight compounds (macromolecules and
biopolymers). The coexistence of both classes of surfactants in beverages is more a rule than an exception. In wine, for instance, the surface properties are dominated by ethanol concentration, however, the small difference (approximately 2.5 mN/m) detected between wine surface tension and the corresponding water/ethanol mixture is due to the presence of macromolecules coming from grapes and yeast, mostly proteins and polysaccharides (Peron et al., 2001). From a kinetic point of view, this particular proportion between low and high molecular weight surfactants determines a surface tension decreases less than 0.5 mN/m in the first 20 min of interface formation, and to increase the measurable effect of macromolecules on the surface tension it is necessary to dilute the wine in order to reduce the ethanol concentration (Peron et al., 2000). In beer too, surface properties are due to the presence of low and high molecular weight surfactants. In fact it is generally regarded that fundamental basis for beer foam stability is the interaction between iso-a-acids and beer polypeptides (Dale, Walker, & Lyddiatt, 1993). Surface tension has been also related to other organoleptic food properties including taste (Aroulmoji, Hutteau, Mathlouthi, & Rutledge, 2001; DeSimone, Heck, & Bartoshuk, 1980; Hutteau & Mathlouthi, 1998) and flavor (Larsson & Larsson, 1997) perception and mouthfeel (Bourne, 2002; Lingle, 2001), although the literature is not abundant as in the case of other physical properties. The espresso coffee long lasting after-taste, a sensation perceived for a while (up to 15 min) after having swallowed and emptied the mouth, has been related to the beverage surface properties (Petracco, 2001). It is conceivable that the time-dependent wetting and adsorption properties of the beverage on oral cavity surfaces could be involved in the after-taste but as far as we know, no dynamic tensiometric data has been reported so far. In spite
Fig. 1. Typical analysis of espresso coffee brews (beverage volume/percolation time) Illy and Viani (1995).
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
of the relevant role played by surface tension (and possibly its time-dependency) in the organoleptic characteristics of espresso coffee, little attention has been paid in its determination, apart from scarce reported data obtained by stalagmometry (Illy & Viani, 1995; Maetzu et al., 2001) that, apart to indicate the presence of naturally occurring surfactants in the beverage, cannot provide information about their kinetics of adsorption. In the present work, dynamic experimental techniques such as maximum bubble pressure (MBP) and pendant drop (PD) have been used for the first time to characterize the air –espresso coffee beverage interface at two different temperatures (20 and 37 8C). Two different beverage samples have been prepared following the standard procedures and under strictly controlled preparation conditions by using the two commercially relevant varieties: Coffea arabica (70% of world coffee production) and Coffea canephora both well known as arabica and robusta, respectively. The aroma profiles as well as the chemical composition of arabica and robusta coffee brews are different. As shown in Fig. 1, where the typical chemical composition of espresso coffee beverage is reported, total lipids (about half in robusta than in arabica brews) and caffeine content (about twice as high in robusta as in arabica brews) are the main differences between the two types of brew. In the present work, the comparison between arabica and robusta has been performed not only because the two varieties are market relevant, but also in the attempt to test the sensitivity of surface properties in detecting the naturally occurring chemical differences of the brews.
2. Materials and methods
389
2.2. Tensiometric techniques The dynamic surface tension measurement has been performed by means of a maximum bubble pressure apparatus (Gammalab, Berlin) and a drop shape tensiometer suitable to study adsorption processes with a characteristic time ranging from 1023 to 10 s and from few seconds to hours and sampling rate of 0.1 s, respectively. These two traditional techniques are described in details (Rusanov & Prokhorov, 1996). The dynamic and the equilibrium surface tension data have been obtained according to the pendant drop method, by using PAT-1 software (Sintech, Berlin), a computer assisted drop shape analysis apparatus, which allows the evolution of the surface tension to be monitored under volume control (Miller, Fainerman, Makievski, Ferrari, & Loglio, 2000a). The equilibrium surface tension data have been evaluated from dynamic surface tension signals long enough to warrant the attainment of the adsorption equilibrium. Pendant drops are formed at the tip of a PTFE coated needle connected to a precision syringe control system.
3. Results and discussion Fig. 2 shows the dynamic surface tension at shortmedium time of espresso coffee –air interface obtained from arabica and robusta, measured by the maximum bubble pressure technique. The experimental data show a remarkable decrease of the surface tension with time for
2.1. Sample preparation Green coffee beans (crop 2000/01) of C. arabica (arabica, Santos, Brasil) and of C. canephora (robusta, Congo) have been roasted at a medium degree of roasting (organic losses: 6.4 –6.5% dry matter) by means of a laboratory roaster (Probat, Germany). Roasted coffee beans have been ground at an appropriate particle size distribution for espresso brewing technique by using a professional grinder (Mazzer, Italy). Espresso coffee beverages have been prepared by using a household espresso coffee machine (Gaggia, Italy) under the following conditions: coffee powder 13.0 g; extract (beverage) mass 50 g according to espresso coffee preparation standard (Petracco, 2001). Extraction yield and refractive index at 20 8C have been determined and both resulted well within the range expected for espresso coffee beverage (24 – 25% and 1.338 – 1.344, respectively). After percolation, the beverages have been maintained at room temperature under constant stirring for 20 min and then at rest for further 20 min.
Fig. 2. Dynamic surface tension at espresso coffee –air interface of arabica (X) and robusta (W) beverages using the MBP at T ¼ 20 8C.
390
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
both beverages, with a profile dependent upon the coffee variety. At a first approximation, given that no differences in the preparation conditions have been used, the difference between arabica and robusta espresso may reflect the more relevant chemical differences of the two brews: caffeine content together with total lipid content (Fig. 1). The presence of high molecular weight (7.000 –50.000 Da) surfactants in hot water extracts of defatted roasted coffee has been recently ascertained (Petracco et al., 1999). Apparently, in espresso beverages there are two classes of polymeric surface active soluble compounds: polysaccharides and melanoidins (Maillard products produced during roasting), independently from coffee species. These compounds have been recognized to be involved in the foaming behaviour of the espresso coffee. No data have been yet reported on possible presence in the beverage of low molecular weight surfactants, although it is highly probable that among the number of chemicals coexisting in coffee some of them could be surface active. Due to the objective difficulties in isolating molecules from the beverage without affecting the integrity of the system and taking into account that to the beverage surface properties could contribute colloidal systems present in the unaltered beverage, dynamic tensiometry offers the opportunity to have an insight into the adsorption kinetics and then to provide information related to the molecular weight of the surfactants. The behaviour reported in Fig. 2, where a strong reduction of surface tension is observed already at short time, suggests that in addition to high molecular weight surfactants, low molecular weight amphipathic solutes may play some role. In fact at very short times the adsorption process is strongly influenced by the concentration and the availability in solution of such surface active molecules. In such a complex mixed surfactant system the presence of short chained surfactants can be significant also in a general description of beverage properties like wetting, foaming, taste. Indeed the surface properties of such molecules can be explained not only by diffusive and kinetic conditions but also by important mechanisms like conformational changes of the hydrophobic and hydrophilic moieties, typical of higher molecular weight substances like protein and other biopolymers (Miller, Fainerman, Makievski, Kragel, Kazakov, & Sinyachenko, 2000b). Other phenomena like phase transitions occurring at the interface even with lower molecular weight surfactants like aliphatic alcohols can influence the adsorption process and the structure of the adsorbed layer in terms of molecular organisation (Ferrari, Liggieri, Ravera, & Massa, 1999; Miller et al., 1999). The surface tension values at about 4 s (46.2 and 48.7 mN/m for arabica and robusta, respectively) are close to those reported in literature (Illy & Viani, 1995) and confirm, under similar experimental condition, that pure
arabica espresso coffee shows surface tension values lower than those shown by pure robusta. From Petracco (2001) caffeine concentration in the beverage varies from 1.2 up to 4.0 mg/ml, depending on cup size and blend composition. Corresponding cup contents range from 60 mg of caffeine (for pure arabica blends) up to double: 120 mg (for pure robusta blends). In the concentration range typical for espresso coffee, caffeine shows appreciable surface activity in water which is enhanced by the presence of sucrose (Aroulmoji et al., 2001), however, in view of the entity of the adsorption phenomenon (for 1.0% pure caffeine at 20 8C monitored up to 20 h surface tension equal to 70.1 and 68.5 mN/m in mixture with 6.0% sucrose) it seems difficult to attribute to caffeine content a relevant role in the observed differences as well as in the evolution of surface tension in espresso coffee. Lipids content in the beverage is higher in pure arabica than in pure robusta espresso coffee (Fig. 1). Triglycerides are preferentially extracted from roast and ground coffee (Petracco, 2001) and the existence of an emulsified lipid fraction (oil droplets of micron size) in espresso coffee is well established (Petracco, 2001). It has also been reported that the lowering of the surface tension in the case of emulsions of triglycerides applied to an air/water interface is mainly caused by spreading of oil (Schokker, Bos, Kuijpers, Wijnen, & Walstra, 2002). It may be hypothesised that something similar could occur in espresso coffee and then the lipid content could play a relevant role in its surface properties. In addition to maximum bubble pressure, pendant drop technique has been also used, in order to explore a wider time scale. As shown in Fig. 3, the results obtained by means of the two techniques are reasonably consistent, within
Fig. 3. Dynamic surface tension at espresso coffee–air interface of arabica beverage at T ¼ 20 8C using the MBP (W) and the PD methods (X).
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
Fig. 4. Dynamic surface tension at espresso coffee– air interface of arabica (X) and robusta (W) beverages at T ¼ 37 8C by PD method.
the differences of time and surface tension accuracy of the two techniques. It is evident the tendency to reach an equilibrium value at time higher than 900 s. This behaviour is particularly remarkable at 37 8C and especially for arabica espresso, as shown in Fig. 4. Due to the complexity of the system, the reproducibility of the curves (within 5%) may be considered acceptable as shown in Fig. 5. This behaviour further confirms the existence of higher molecular weight
Fig. 5. Dynamic surface tension at espresso coffee –air interface of two samples of robusta beverages at T ¼ 37 8C by PD method.
391
surfactants adsorbing with a larger characteristic time. The increase of the temperature from 20 to 37 8C shows the expected decrease of surface tension values. The adsorption kinetics profile shows that the robusta sample tends to a steady state more rapidly than the arabica one being the latter influenced by a major content in lipids significantly lowering the surface tension of the water medium (Lee, Kim, & Needham, 2001). As already reported in literature lipids form insoluble monolayers at the interface reaching equilibrium with a slow adsorption process (Kabalnov, Weers, Arlauskas, & Tarara, 1995) rising several discrepancies in the equilibrium surface tension values that are also strongly influenced by other physico-chemical parameters like the lipid phase transition temperature. The overall adsorption behaviour of espresso coffee may be of interest to interpret some peculiarities like the staining ability of the beverage for the oral cavity surfaces and its long-lasting after-taste. It has to be mentioned that human whole saliva at 37 8C is characterised by surface tension values remarkably higher (e.g. 48.5 mN/m at 600 s in Christersson, Lindh, and Arnebrant (2000)) than those of both espresso beverages examined in the present work. Moreover, the critical surface tension of wetting representative of human saliva-coated tooth surface and most restorative materials when exposed in the oral cavity has been reported to be close to 35 – 38 mN/m (Christersson, Dunford, Glantz, & Baier, 1989) and within the bio-adhesive range of 32 – 50 mN/m (Glantz, 1997). Similar studies reported a value of 25 – 27 mN/m for the critical surface tension of salivaconditioned oral mucosa measured in vivo (Christersson et al., 1989). In view of the fact that a surface will be completely wet by a liquid having its surface tension value close to that of the critical surface tension of wetting and considering that the first sip of espresso coffee is generally consumed at high temperature (ca. 60 – 65 8C and accordingly surface tension values lower than those measured in the present work), the dynamic tensiometric behaviour of the espresso coffee beverage is consistent with that expected for systems with good wetting properties for the oral cavity. It has to be stressed that in a study where wetting properties of mouthrinses have been determined through in vivo contact angle measurements on tooth surface, the recruited human subjects were requested not to drink coffee 2 h prior to the measurements (Perdok, van der Mei, & Busscher, 1990). As reported by Shahidzadeh, Bonn, Meunier, and Mavon (2001), lipids in contact with water-based surfaces form a wetting film spreading out by a Marangoni effect and then, due to dewetting instability, breaks up into droplets with a mesoscopic (tens of Angstrom) film in between. From these observations it can be outlined the role played by short and long-range forces: surface active lipids increase the wetting, while the van der Waals forces oppose it.
392
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393
Nevertheless, the beverage surface properties show a remarkable adsorption phenomenon at air/beverage interface which can occur at the receptor membrane surface (e.g. taste buds), as well, with clear implication in taste perception and its time-dependency. As a general rule, bitter molecules have a predominantly hydrophobic character whereas sweeteners are rather hydrophilic and both molecules seem to bind to closely located receptors which are coupled to guanidine-nucleotide binding proteins (G-proteins) (Aroulmoji et al., 2001). Due to the importance of the sweetness and bitterness in taste of espresso coffee (it is well known that robusta has a more bitter taste than arabica), the possible role played by lipids could not be confined only in the surface properties but also related to taste perception and possibly duration.
4. Conclusions For the first time espresso coffee beverage has been characterised by dynamic tensiometry. Maximum bubble pressure and pendant drop techniques have been successfully used in spite of the complex nature of the samples. The experimental data, in addition to confirm and extend previously reported scarce data, show a remarkably efficient adsorption at the air –beverage interface for both types of tested beverages with a profile dependent upon the coffee variety especially at higher adsorption times. The arabica espresso coffee is characterised by lower surface tension values than those of robusta beverage and appears to be more efficient as ‘wetting system’ for the oral cavity. The beverage lipid content may play a significant role in this behaviour. In fact the spreading of coffee oil at the air –beverage interface or the emulsified oil droplets present in the beverage may strongly affect the surface properties. Studies on model systems or on beverages prepared by using brewing methods able to extract a lower lipid fraction have been planned in order to clarify this point. The present investigation has been performed by fixing the espresso coffee preparation variables in order to explore the naturally occurring chemical difference of using two different varieties. Further studies could explore chemical differences induced by changes in preparation variables.
Acknowledgements It is a pleasure to thank Marino Petracco for valuable discussions and Bruno Della Pietra for technical assistance.
References Andueza, S., Maetzu, L., Dean, B., de Pena, M. P., Bello, J., & Cid, C. (2002). Influence of water pressure on the final quality of arabica
espresso coffee. Application of multivariate analysis. Journal of Agriculture and Food Chemistry, 50, 7426–7431. Aroulmoji, V., Hutteau, F., Mathlouthi, M., & Rutledge, D. N. (2001). Hydration properties and the role of water in taste modalities of sucrose, caffeine and sucrose–caffeine mixtures. Journal of Agriculture and Food Chemistry, 49, 4039–4045. Bourne, M. (2002). Food texture and viscosity (2nd ed.). London: Academic Press. Christersson, C. E., Dunford, R. G., Glantz, P.-O., & Baier, R. E. (1989). Effect of critical surface tension on retenti oral microorganism. Scandinavian Journal of Dental Research, 97, 247 –256. Christersson, C. E., Lindh, L., & Arnebrant, T. (2000). Film forming properties and viscosities of saliva substitutes and human whole saliva. European Journal of Oral Sciences, 108, 418 –425. Dale, C., Walker, S. G., & Lyddiatt, A. (1993). Dynamic changes in the composition and physical behavior of dispersed beer foam. Journal of the Institute of Brewing, 99, 461–466. DeSimone, J. A., Heck, G. L., & Bartoshuk, L. M. (1980). Surface active taste modifiers: a comparison of gymnemic acid and sodium lauryl sulfate. Chemical Senses, 5, 317 –330. Evans, D. E., & Sheehan, M. C. (2002). Don’t be fobbed off: the substance of beer foam—a review. Journal of American Society of Brewing Chemistry, 60(2), 47 –57. Ferrari, M., Liggieri, L., Ravera, F., & Massa, A. (1999). Molecular reorientation in the adsorption of some CiEj at the water–air interface. Colloids and Surfaces A, 156, 455 –463. Glantz, P.-O. (1997). Interfacial phenomena in the oral cavity. Colloids and Surfaces A, 123 –124, 657 –670. Hutteau, F., & Mathlouthi, M. (1998). Physicochemical properties of sweeteners in artificial saliva and determination of a hydrophobicity scale for some sweeteners. Food Chemistry, 63, 199 –206. Illy, A., & Viani, R. (Eds.). (1995). Espresso coffee: The chemistry of quality (p. 183). London: Academic Press. Kabalnov, A., Weers, J., Arlauskas, R., & Tarara, T. (1995). Phospholipids as emulsion stabilizers. 1. Interfacial-tensions. Langmuir, 11, 2966–2974. Larsson, M., & Larsson, K. (1997). Neglected aspects of food flavor perception. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 123–124, 651–655. Lee, S., Kim, D. H., & Needham, D. (2001). Equilibrium and dynamic interfacial tension measurements at microscopic interfaces using a micropipet technique. 2. Dynamics of phospholipid monolayer formation and equilibrium tensions at the water – air interface. Langmuir, 17, 5544–5550. Lingle, T. R. (2001). The coffee cuppers handbook: A systematic guide to the sensory evaluation of coffee flavor (3rd ed.). Long Beach, California: Specialty Coffee Association of America. Maetzu, L., Andueza, S., Ibanez, C., Paz de Pena, M., Bello, J., & Cid, C. (2001). Multivariate methods for characterisation and classification of espresso coffees from different botanical varieties and types of roast by foam, taste, and mouthfeel. Journal of Agriculture and Food Chemistry, 49, 4743–4747. Miller, R., Aksenenko, E. V., Liggieri, L., Ravera, F., Ferrari, M., & Fainerman, V. B. (1999). Effect of the reorientation of oxyethylated alcohol molecules within the surface layer on equilibrium and dynamic surface pressure. Langmuir, 15, 1328–1336. Miller, R., Fainerman, V.B., Makievski, A.V., Ferrari, M., & Loglio, G (2000). Measuring dynamic surface tensions. In Handbook of applied surface and colloid chemistry. New York: Wiley. Miller, R., Fainerman, V. B., Makievski, A. V., Kragel, J., Grigoriev, D. O., Kazakov, V. N., & Sinyachenko, O. V. (2000b). Dynamics of protein and mixed protein/surfactant adsorption layers at the water/ fluid interface. Advances in Colloid and Interface Science, 86(1/2), 39 –82. Nunes, F. M., Coimbra, M. A., Duarte, A. C., & Delgadillo, I. (1997). Foamability, foam stability and chemical composition of Espresso
L. Navarini et al. / Food Hydrocolloids 18 (2004) 387–393 coffee as affected by the degree of roast. Journal of Agriculture and Food Chemistry, 45, 3238–3243. Perdok, J. F., van der Mei, H. C., & Busscher, H. J. (1990). Physicochemical properties of commercially available mouthrinses. Journal of Dentistry, 18, 147–150. Peron, N., Cagna, A., Valade, M., Bliard, C., Aguie-Beghin, V., & Douillard, R. (2001). Layers of macromolecules at the champagne/air interface and the stability of champagne bubbles. Langmuir, 17, 791–797. Peron, N., Cagna, A., Valade, M., Marchal, R., Maujean, A., Robillard, B., Aguie-Beghin, V., & Douillard, R. (2000). Characterisation by drop tensiometry and by ellipsometry of the adsorption layer formed at the air/champagne wine interface. Advances in Colloid and Interface Science, 88, 19– 36. Petracco, M. (2001). Beverage preparation: brewing trends for the new millennium. In R. J. Clarke, & O. G. Vitzthum (Eds.), Coffee recent developments (p. 140). Oxford: Blackwell Science.
393
Petracco, M., Navarini, L., Abatangelo, A., Gombac, V., D’Agnolo, E., & Zanetti, F. (1999). Isolation and characterization of a foaming fraction from hot water extracts of roasted coffee. Proceedings of the 18th ASIC Colloquium (Helsinki), 95–105. ASIC Paris, France. Rusanov, A.I., Prokhorov, V.A (1996). Interfacial tensiometry. In Studies in interface science. Amsterdam: Elsevier. Schilter, B., Cavin, C., Tritscher, A., & Constable, A. (2001). Health effects and safety considerations. In R. J. Clarke, & O. G. Vitzthum (Eds.), Coffee recent developments. London: Blackwell Science. Schokker, E. P., Bos, M. A., Kuijpers, A. J., Wijnen, M. E., & Walstra, P. (2002). Spreading of oil from protein stabilised emulsions at air/ water interfaces. Colloids and Interfaces B: Biointerfaces, 26, 315– 327. Sene`e, J., Robillard, B., & Vignes-Adler, M. (1999). Films and foams of champagne wine. Food Hydrocolloids, 13, 15–26. Shahidzadeh, N., Bonn, D., Meunier, J., & Mavon, A. (2001). Wetting of biological lipids on aqueous substrates. Physical Review E, 64(2).