Influence of foam structure on the release kinetics of volatiles from espresso coffee prior to consumption.

The relationship between the physical structure of espresso coffee foam, called crema, and the above-the-cup aroma release was studied. Espresso coffee samples were produced using the Nespresso extraction system. The samples were extracted with water with different levels of mineral content, which resulted in liquid phases with similar volatile profiles but foams with different structure properties. The structure parameters foam volume, foam drainage, and lamella film thickness at the foam surface were quantified using computer-assisted microscopic image analysis and a digital caliper. The above-the-cup volatile concentration was measured online by using PTR-MS and headspace sampling. A correlation study was done between crema structure parameters and above-the-cup volatile concentration. In the first 2.5 min after the start of the coffee extraction, the presence of foam induced an increase of concentration of selected volatile markers, independently if the crema was of high or low stability. At times longer than 2.5 min, the aroma marker concentration depends on both the stability of the crema and the volatility of the specific aroma compounds. Mechanisms of above-the-cup volatile release involved gas bubble stability, evaporation, and diffusion. It was concluded that after the initial aroma burst (during the first 2-3 min after the beginning of extraction), for the present sample space a crema of high stability provides a stronger aroma barrier over several minutes.

Identification of ethyl formate as a quality marker of the fermented off-note in coffee by a nontargeted chemometric approach.

The quality of coffee is influenced by many factors such as coffee variety, agricultural and postharvest conditions, roasting parameters, and brewing. The pleasure of drinking coffee may be affected by off-notes such as burnt, green, earthy, or fermented. Their presence is related to the variety, fermentation during postharvest processing, or over-roasting of the beans. Sensory expert panels trained for the evaluation of coffee are able to detect off-notes and select coffees by well-defined quality criteria. The application of instrumental approaches detecting quality markers related to the perceived off-notes is shown to be useful to assist sensory panels. This paper describes the discovery of a new marker compound related to the fermented off-note occasionally perceived in coffees. The application of untargeted chemometric methods on volatile compounds revealed correlations between individual compounds and the sensory attribute. The new marker compound was identified as ethyl formate, which can be measured in the headspace of roasted and ground coffee by various analytical techniques including online proton transfer reaction mass spectrometry.

When machine tastes coffee: instrumental approach to predict the sensory profile of espresso coffee.

A robust and reproducible model was developed to predict the sensory profile of espresso coffee from instrumental headspace data. The model is derived from 11 different espresso coffees and validated using 8 additional espressos. The input of the model consists of (i) sensory profiles from a trained panel and (ii) on-line proton-transfer reaction mass spectrometry (PTR-MS) data. The experimental PTR-MS conditions were designed to simulate those for the sensory evaluation. Sixteen characteristic ion traces in the headspace were quantified by PTR-MS, requiring only 2 min of headspace measurement per espresso. The correlation is based on a knowledge-based standardization and normalization of both datasets that selectively extracts differences in the quality of samples, while reducing the impact of variations on the overall intensity of coffees. This work represents a significant progress in terms of correlation of sensory with instrumental results exemplified on coffee.

Release kinetics of volatile organic compounds from roasted and ground coffee: online measurements by PTR-MS and mathematical modeling.

The present work shows the possibilities and limitations in modeling release kinetics of volatile organic compounds (VOCs) from roasted and ground coffee by applying physical and empirical models such as the diffusion and Weibull models. The release kinetics of VOCs were measured online by proton transfer reaction-mass spectrometry (PTR-MS). Compounds were identified by GC-MS, and the contribution of the individual compounds to different mass fragments was elucidated by GC/PTR-MS. Coffee samples roasted to different roasting degrees and ground to different particle sizes were studied under dry and wet stripping conditions. To investigate the accuracy of modeling the VOC release kinetics recorded using PTR-MS, online kinetics were compared with kinetics reconstituted from purge and trap samplings. Results showed that uncertainties in ion intensities due to the presence of isobaric species may prevent the development of a robust mathematical model. Of the 20 identified compounds, 5 were affected to a lower extent as their contribution to specific m/z intensity varied by <15% over the stripping time. The kinetics of these compounds were fitted using physical and statistical models, respectively, the diffusion and Weibull models, which helped to identify the underlying release mechanisms. For dry stripping, the diffusion model allowed a good representation of the release kinetics, whereas for wet stripping conditions, release patterns were very complex and almost specific for each compound analyzed. In the case of prewetted coffee, varying particle size (approximately 400-1200 microm) had no significant effect on the VOC release rate, whereas for dry coffee, the release was faster for smaller particles. The absence of particle size effect in wet coffee was attributed to the increase of opened porosity and compound diffusivity by solubilization and matrix relaxation. To conclude, the accurate modeling of VOC release kinetics from coffee allowed small variations in compound release to be discriminated. Furthermore, it evidenced the different aroma compositions that may be obtained depending on the time when VOCs are recovered.

Quantitation of furan and methylfuran formed in different precursor systems by proton transfer reaction mass spectrometry.

Furan has recently received attention as a possibly hazardous compound occurring in certain thermally processed foods. Previous model studies have revealed three main precursor systems producing furan upon thermal treatment, i.e., ascorbic acid, Maillard precursors, and polyunsaturated lipids. We employed proton transfer reaction mass spectrometry (PTR-MS) as an on-line monitoring technique to study furan formation. Unambiguous identification and quantitation in the headspace was achieved by PTR-MS/gas chromatography-mass spectrometry coupling. Ascorbic acid showed the highest potential to generate furan, followed by glyceryl trilinolenate. Some of the reaction samples generated methylfuran as well, such as Maillard systems containing alanine and threonine as well as lipids based on linolenic acid. The furan yields from ascorbic acid were lowered in an oxygen-free atmosphere (30%) or in the presence of reducing agents (e.g., sulfite, 60%), indicating the important role of oxidation steps in the furan formation pathway. Furthermore, already simple binary mixtures of ascorbic acid and amino acids, sugars, or lipids reduced furan by 50-95%. These data suggest that more complex reaction systems result in much lower furan amounts as compared to the individual precursors, most likely due to competing reaction pathways.

Unambiguous identification of volatile organic compounds by proton-transfer reaction mass spectrometry coupled with GC/MS.

Interest in on-line measurements of volatile organic compounds (VOCs) is increasing, as sensitive, compact, and affordable direct inlet mass spectrometers are becoming available. Proton-transfer reaction mass spectrometry (PTR-MS) distinguishes itself by its high sensitivity (low ppt range), high time resolution (200 ms), little ionization-induced fragmentation, and ionization efficiency independent of the compound to be analyzed. Yet, PTR-MS has a shortcoming. It is a one-dimensional technique that characterizes compounds only via their mass, which is not sufficient for positive identification. Here, we introduce a technical and analytical extension of PTR-MS, which removes this shortcoming, while preserving its salient and unique features. Combining separation of VOCs by gas chromatography (GC) with simultaneous and parallel detection of the GC effluent by PTR-MS and electron impact MS, an unambiguous interpretation of complex PTR-MS spectra becomes feasible. This novel development is discussed on the basis of characteristic performance parameters, such as resolution, linear range, and detection limit. The recently developed drift tube with a reduced reaction volume is crucial to exploit the full potential of the setup. We illustrate the performance of the novel setup by analyzing a complex food system.

Comparison of nosespace, headspace, and sensory intensity ratings for the evaluation of flavor absorption by fat.

The goal of this study was to better understand the correspondence between sensory perception and in-nose compound concentration. Five aroma compounds at three different concentrations increasing by factors of 4 were added to four matrixes (water, skim milk, 2.7% fat milk, and 3.8% fat milk). These were evaluated by nosespace analysis with detection by proton transfer reaction mass spectrometry (PTR-MS), using five panelists. These same panelists evaluated the perceived intensity of each compound in the matrixes at the three concentrations. PTR-MS quantification found that the percent released from an aqueous solution swallowed immediately was between 0.1 and 0.6%, depending on the compound. The nosespace and sensory results showed the expected effect of fat on release, where lipophilic compounds showed reductions in release as fat content increases. The effect is less than that observed in headspace studies. A general correlation between nosespace concentration and sensory intensity ratings was found. However, examples of perceptual masking were found where higher fat milks showed reductions in aroma compound intensity ratings, even if the nosespace concentrations were the same.

Proton transfer reaction mass spectrometry, a tool for on-line monitoring of acrylamide formation in the headspace of maillard reaction systems and processed food.

The formation of acrylamide was measured in real time during thermal treatment (120-170 degrees C) of potato as well as in Maillard model systems composed of asparagine and reducing sugars, such as fructose and glucose. This was achieved by on-line monitoring of acrylamide released into the headspace of the samples using proton transfer reaction mass spectrometry (PTR-MS). Unambiguous identification of acrylamide by PTR-MS was accomplished by gas chromatography coupled simultaneously to electron-impact MS and PTR-MS. The PTR-MS ion signal at m/z 72 was shown to be exclusively due to protonated acrylamide obtained without fragmentation. In model Maillard systems, the formation of acrylamide from asparagine was favored with increasing temperature and preferably in the presence of fructose. Maximum signal intensities in the headspace were obtained after approximately 2 min at 170 degrees C, whereas 6-7 min was required at 150 degrees C. Similarly, the level of acrylamide released into the headspace during thermal treatment of potato was positively correlated to temperature.