Dynamic headspace analysis of the release of volatile organic compounds from ethanolic systems by direct APCI-MS

Static equilibrium headspace was diluted with a stream of nitrogen to study the stability of the volatile headspace concentration. The headspace dilution profile of 18 volatile compounds above aqueous and ethanolic solutions was measured in real time using atmospheric pressure chemical ionization-mass spectrometry. Under dynamic conditions the volatiles headspace concentration above water solutions decreased readily upon dilution. The presence of ethanol helped to maintain the volatile headspace concentration when the ethanol solution concentration was above 50 mL/L. This effect was such that under dynamic conditions the absolute volatile concentration above an ethanolic solution was higher than that above an aqueous solution, contrary to results observed in equilibrium studies. The ratio of the headspace concentration of volatiles above ethanolic 120 mL/L and water solutions was correlated to their air/water partition coefficient.     

Effect of copper on the volatility of aroma compounds in a model mouth system

Copper is thought to influence aroma perception by affecting volatility of aroma compounds in the mouth through interaction with salivary components, especially proteins. Our objective was to identify the effect of copper on the volatility of aroma compounds and the role of copper-protein interaction in volatile chemistry in the mouth. Copper (2.5 mg/L) and four aroma compounds (hexanal, butyl acetate, 2-heptanone, and ethyl hexanoate, 0.5 microL/L each) were added to model systems containing water, electrolytes, and artificial saliva at different pH levels. Headspace concentration of each volatile was measured using SPME-GC analysis. Copper in the model systems increased headspace concentration of volatiles at pH 6.5, but no change in volatility was observed at pH 7.0. At pH 7.5, the presence of copper in the artificial saliva system containing mucin and alpha-amylase decreased headspace volatile concentration, whereas histatin did not cause any changes in volatility. Effect of copper on volatiles at pH 6.5 may be due to increased solubility of copper at lower pH. Salivary proteins seem to interact with copper at pH 7.5. The interaction may change configuration of binding sites for aroma compounds in mucin.     

Characterization of Citrus unshiu (C. unshiu Marcov. forma Miyagawa-wase) blossom aroma by solid-phase microextraction in conjunction with an electronic nose

The volatile composition of the headspace from Citrus unshiu Marcov. forma Miyagawa-wase blossom was investigated. The volatile constituents were absorbed by a solid-phase microextraction (SPME) fiber and directly transferred to a GC-MS. Volatile compositional changes of C. unshiu blossom prepared via different drying methods (shade, microwave, and freeze-drying methods) were also determined. A total of 96 volatile constituents were confirmed in the headspace from these samples. Monoterpene hydrocarbons were prominent in the headspace volatiles of C. unshiu blossom: fresh, 84.1%; shade-dried, 60.0%; microwave-dried, 88.4%; and freeze-dried, 29.9%. p-Cymene (23.3%) was the most abundant component in the headspace of fresh C. unshiu blossom; gamma-terpinene was the most abundant in shade- and microwave-dried samples (26.8 and 31.2%, respectively) and beta-caryophyllene (10.5%) in freeze-dried sample. By using an electronic nose consisting of six metal oxide sensors, principal component analysis of the volatile compounds showed a clear aroma discrimination of the fresh and all dried blossom samples.     

Verification of a mouth simulator by in vivo measurements

The volatile content of the effluent from the retronasal aroma simulator (RAS) was compared with that of human breath using mass spectroscopy (MS-Nose). The ratios of volatile compounds from the RAS were closely related to those from the panelists' breath with the correlation coefficients ranging from 0.97 to 0.99 from model food systems. A greater sensitivity using the RAS was achieved because higher concentrations of volatiles in the MS-Nose were produced from the RAS than from the breath. In analyzing the effects on volatility of RAS parameters including airflow rate, temperature, saliva ratio, and blending speed, airflow rate had the greatest effect. The correlation coefficients for the real food systems studied ranged from 0.83 to 0.99. The RAS gives a good approximation of time-averaged flavor release in the mouth as defined by breath-by-breath measurements.     

Effects of droplet crystallization and melting on the aroma release properties of a model oil-in-water emulsion

Aroma compounds partition between the dispersed and the continuous phases in emulsions, and phase transitions in the lipid droplets profoundly affect the position of the equilibrium. In the present study, the release of ethyl butyrate, ethyl pentanoate, ethyl heptanoate, and ethyl octanoate from a series of sodium caseinate-stabilized, n-eicosane emulsions was investigated as a function of solid and liquid lipid droplet concentration. For all compounds, headspace volatile concentrations above the solid droplet emulsions were higher than those above the liquid droplet emulsions. The interaction with liquid droplets could be modeled in terms of volume-weighted bulk partition coefficients while the more nonpolar volatiles bound to the surface of solid lipid droplets. The amount of volatiles bound to solid surfaces increased with aqueous concentration up to a critical point and then rapidly increased. The critical point corresponds to the dissolution of the solid lipid in a phase of adsorbed volatile. The binding of volatiles to both solid and liquid eicosane droplets is reversible.     

Effect of beta-cyclodextrin on aroma release and flavor perception

Binding and release of volatile compounds to and from beta-cyclodextrin were measured in model aqueous systems using static equilibrium headspace and dynamic headspace dilution. Beta-cyclodextrin decreased the static equilibrium headspace for some volatiles (e.g., ethyl octanoate and decanone) due to binding, but dilution studies demonstrated that binding was readily reversible. Dynamic release of hydrophobic volatile compounds was similar to that observed from emulsions. When beta-cyclodextrin was added to fat free yogurt, the release of a commercial lemon flavoring was modified and was similar to release from a regular fat yogurt. Sensory difference testing confirmed the release results. The data demonstrate that beta-cyclodextrin can be used to modify flavor delivery in both model and real systems; the effects in the latter are sensorially significant.     

Effect of salivary components on volatile partitioning from solutions

Partition of fourteen volatile compounds, representing the diverse physicochemical properties of aroma compounds, was measured by static equilibrium headspace in solutions containing the components of artificial saliva, either singly or in mixtures. Comparison of a bovine salivary mucin and pig gastric mucin showed no significant difference in partition behavior of the volatiles, so gastric mucin was used. Mucin viscosity changed with pH, but binding of volatile compounds did not show a marked dependence on pH. All combinations of the salivary components were tested for their effect on partition. Three types of behavior were noted. Partition of some compounds was unaffected by mucin, and with other compounds mucin decreased partition, whereas another group showed a decrease with mucin that was affected by the presence of salivary salts and sugar. When volatiles or sugar were added to a mucin solution, the final headspace concentration depended on the order of addition, indicating some competition for binding. These solute-mucin effects are discussed in relation to mucin structure and behavior in solution.     

Model studies on retention of added volatiles during breadcrumb production

Breadcrumb samples were prepared with a range of volatile compounds at known concentrations. The retention of these volatiles was assessed via solvent extraction and quantification by gas chromatography-mass spectrometry. Volatile loss during processing was shown to be substantial and dependent upon the compound's vapor pressure. The influence of initial concentration levels on the retention of volatiles was linear within the bounds of the experimental concentrations (0-300 mg/kg). Comparison of volatile concentration at various stages throughout the production process (by headspace analysis) showed that the greatest losses occurred during the processing stages that involved heat, namely, microwave heating and drying. The production of samples by freeze drying showed an increased average retention of 17% as compared to fluidized bed drying and flat bed drying, which showed the highest volatile losses.     

Quantitative analysis of aroma compounds in carrot (Daucus carota L.) cultivars by capillary gas chromatography using large-volume injection technique

Dynamic headspace sampling was used to collect aroma compounds from raw samples of four carrot (Daucus carota L.) cultivars (Brasilia, Duke, Fancy, and Cortez). The collected volatiles were analyzed by capillary GC-FID and GC-MS using large-volume cool on-column injection (LVI-COC). Of the 36 compounds identified, 6 had not been previously detected in carrots. Significant differences between the carrot cultivars were found for 31 of the identified volatiles as well as for total monoterpenes, sesquiterpenes, and total volatile content. Mono- and sesquiterpenes accounted for about 98% of the total volatile mass in all cultivars. LVI-COC injection was used to determine the loss of carrot volatiles during concentration of headspace samples under a stream of nitrogen. The loss among major monoterpenes in the concentrated samples varied from 16% for p-cymene to >40% for alpha-pinene as compared to nonconcentrated samples. The loss among high-boiling sesquiterpenes varied from not detectable (beta-caryophyllene, alpha-humulene, and caryophyllene oxide) to approximately 7% for (E)- and (Z)-gamma-bisabolene.     

Volatile release from an emulsion: headspace and in-mouth studies

The headspace concentrations of three esters above solutions containing emulsified lipids were more resistant to dilution by a stream of gas than those above water alone. The effect was greatest for ethyl octanoate, and least for ethyl butyrate, with ethyl hexanoate showing intermediate behavior. This correlated with their solubility in the lipid fraction of the emulsion. Headspace analysis (comparing the emulsion with water) underestimated the release of the esters during consumption. The ratios observed between water and emulsion systems were different for the maximum breath concentration compared with headspace analysis. The emulsion appears to have acted as a reservoir for volatile release, counteracting the effects of sample dilution by saliva.     

Flavor volatiles and physical properties of vacuum-microwave- and air-dried sweet basil (Ocimum basilicum L.).

Basil (Ocimum basilicum L.) was dried using conventional hot air or the recently developed vacuum-microwave dryers. The effect of the drying method on the relative abundance of major flavor volatiles, rehydration rate, color, and structural integrity of the plant was evaluated. Dynamic headspace analysis of volatiles present in fresh or dried basil revealed that linalool and methylchavicol (estragole) were the two major headspace volatile compounds of the plant sample. Vacuum-microwave dehydrated basil yielded approximately 2.5 times the linalool and 1.5 times the methylchavicol of the air-dried samples. Furthermore, the vacuum-microwave-treated samples yielded more volatiles than fresh basil, due to chemical reactions during drying. Air-dried samples of basil had darker and fewer green hues than those prepared by vacuum microwave. Vacuum-microwave-dried samples had a higher rehydration rate, whereas the potential of the plant material to rehydrate was hindered in air-dried samples. This is likely attributed to the dramatic and pronounced structural collapse of the air-dried cells as revealed by the scanning electron microscope.     

Aroma release and delivery following the consumption of beverages

Processes controlling aroma release and delivery during and after the consumption of a beverage were studied using real-time physiological and aroma release measurements. The key processes were as follows. During swallowing, a portion of the buccal gas phase was transferred first to the throat and then to the nasal passages via the tidal breath flow. This mechanism accounted for the sharp pulse of aroma seen at the beginning of the swallow breath and on subsequent swallows. The persistence effect was due to liquid-air partition from beverage coated on the throat and was dependent on the concentration of volatile compounds in the beverage. Lipid in the beverage caused a decrease in the intensity of volatile compounds on the breath, but the presence of a thickening agent had no effect on persistence.     

Effects of an Industrial Milling Process on Change of Headspace Volatiles in Yihchuan Aromatic Rice

Headspace volatiles of Yihchuan aromatic rice, a japonica rice cultivar, were extracted by a solid‐phase microextraction (SPME) method and analyzed by GC and GC‐MS. Effects of fiber types and an industrial milling process on the change of headspace volatiles were studied. Of the fiber types, divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), and carboxen/polydimethylsiloxane (CAR/PDMS) were suggested for use in the analysis headspace volatiles in terms of absorption or adsorption ability. Regarding the milling effect, the amount of total headspace volatiles from brown rice to white rice decreased ≈30%, and that of aldehydes and alcohols, the two largest groups of volatiles, decreased ≈20 and 15%, respectively. Comparatively, rice bran contained higher amounts of total headspace volatiles than milled rice grains.     

Computerized apparatus for measuring dynamic flavor release from liquid food matrices

A fully computer-controlled apparatus was designed. It combines a glass reactor with a temperature-controlled hood, in which headspace volatiles are captured. Flavored liquids can be introduced into the reactor and exposed to conditions of temperature, air flow, shear rate, and saliva flow as they occur in the mouth. As the reactor is completely filled before measurements are started, creation of headspace just before sampling start prevents untimely flavor release resulting in real time data. In the first 30 s of flavor release the concentrations of the volatiles can be measured up to four times by on-line sampling of the dynamic headspace, followed by off-line trapping of the samples on corresponding Tenax traps and analysis using GC-TDS-FID. Flavor compounds from different chemical classes were dissolved in water to achieve concentrations typically present in food (micrograms to milligrams per liter). Most of the compounds showed constant release rates, and the summed quantities of each volatile of three 10 s time intervals correlated linearly with time. The entire method of measurement including sample preparation, release, sampling, trapping, thermodesorption, and GC analysis showed good sensitivity [nanograms (10 s)(-1)] and reproducibility (mean coefficient of variation = 7.2%).     

Optimization of extraction of apple aroma by dynamic headspace and influence of saliva on extraction of volatiles

The dynamic headspace procedure of aroma extraction was optimized on Gala apples (Malus domestica). Two parameters affecting the extractability of compounds were studied: temperature and purge time. The influence of artificial saliva was also included. An increase in purge time and temperature caused an increase in the extraction of volatiles from the apple matrix. The optimum point of extraction was 40 degrees C and 70 min of purge. The study also showed that the addition of saliva influenced the extraction of volatile compounds, but this effect was different from one compound to another. To verify that the headspace extracts presented a global odor representativeness of fresh apple under these conditions of extraction, eight assessors compared the odor of extracts with fresh fruit odor for three different cultivars. With regard to the sensory profiles of extracts, the optimal conditions of extraction were suitable for extraction of volatile compounds, even if cooked apple odor appeared in some extracts. The similarity marks of extracts were low but acceptable.     

Volatile composition of sunflower oil-in-water emulsions during initial lipid oxidation: influence of pH.

The formation of odor active compounds resulting from initial lipid oxidation in sunflower oil-in-water emulsions was examined during storage at 60 degrees C. The emulsions differed in initial pH, that is, pH 3 and 6. The volatile compounds were isolated under mouth conditions and were analyzed by gas chromatography/sniffing port analysis. The lipid oxidation rate was followed by the formation of conjugated hydroperoxide dienes and headspace hexanal. The initial pH affected the lipid oxidation rate in the emulsions: the formation of conjugated diene hydroperoxides and the hexanal concentration in the static headspace were increased at pH 6. Pentanal, hexanal, 3-pentanol, and 1-octen-3-one showed odor activity in the emulsions after 6 days of storage, for both pH 3 and 6. Larger amounts of odor active compounds were released from the pH 6 emulsion with extended storage. It was shown that this increased release at pH 6 was not due to increased volatility because an increase in pH diminished the static headspace concentrations of added compounds in emulsions.     

Headspace solid-phase microextraction method for the study of the volatility of selected flavor compounds

Changes in the volatility of selected flavor compounds in the presence of nonvolatile food matrix components were studied using headspace solid-phase microextraction (HS-SPME) combined with GC-MS quantification. Time-dependent adsorption profiles to the SPME fiber and the partition coefficients between different phases were obtained for several individual volatiles, showing that HS-SPME analysis with a short sampling time can be used to determine the "true" headspace concentration at equilibrium between the headspace and a sample matrix. Equilibrium dialysis followed by HS-SPME/GC-MS was carried out to confirm the ability of HS-SPME extraction for monitoring the free volatile compounds in the presence of proteins. In particular, a short sampling time (1 min) avoided additional extraction of volatiles bound to the protein. Interactions between several selected flavor compounds and nonvolatile food matrix components [beta-lactoglobulin or (+)-catechin] were also studied by means of HS-SPME/GC-MS analysis. The volatility of ethyl hexanoate, heptanone, and hexanal was significantly decreased by the addition of beta-lactoglobulin compared to that of isoamyl acetate. Catechin decreased the volatility of ethyl hexanoate and hexanal by 10-20% and increased that of 2-heptanone by approximately 15%. This study indicates that HS-SPME can be a useful tool for the study of the interactions between volatile compounds and nonvolatile matrix components provided the kinetic and thermodynamic behavior of the volatiles in relation to the fiber chosen for the studies is carefully considered.     

Emission of volatile chemicals from flowering dogwood (cornus Florida L.) flowers

Reproduction of flowering dogwood trees occurs via obligate out-crossing, and U.S. native bees have been suggested to be primary pollinators of this ecologically and economically important deciduous tree. Whether floral volatiles play a role in reproduction of the dogwood remains unclear. Objectives of this study were to identify principal volatile chemicals emitted from dogwood flowers and to assess a temporal volatile emission profile and volatile consistency across four cultivars. Inflorescences with intact bracts and 5 cm flower pedicel were removed from dogwood trees and subjected to headspace volatile collection. Six principal volatile compounds were detected from the flowers of the cultivar 'World's Fair' with 3-formylpyridine as the most abundant constituent. Subsequent headspace analyses performed using inflorescences without bracts or floral pedicels alone indicated that 3-formylpyridine, E-beta-ocimene, S-linalool, and ketoisophorone were mainly emitted from inflorescences. Experiments were also conducted to determine whether volatile emissions differed across time and between different cultivars of flowering dogwood. When volatile emission was analyzed for 48 h using 12 h light/dark cycles, the emission of several volatile compounds displayed diurnal patterns. Finally, whereas florets in inflorescences of four different dogwood cultivars emitted similar levels of the six principal floral volatile chemicals, 'Cherokee Brave' flowers alone yielded 4-methoxybenzaldehyde and germacrene-D. The implications of the findings of this study to dogwood breeding programs are discussed.     

Automated dynamic headspace/GC-MS analyses affect the repeatability of volatiles in irradiated Turkey

Although a dynamic headspace/gas chromatography-mass spectrometry (DH/GC-MS) method is an effective tool for determining volatiles of irradiated turkey meat, the profile of volatiles may be changeable depending upon the availability of oxygen in the sample vial and sample holding time before purge. The objective of this study was to evaluate the effects of helium flushing and sample holding time before purge on the volatiles profiles of irradiated raw and cooked turkey breast meat. Vacuum-packaged turkey breasts were irradiated at 2.5 kGy, and the volatiles of irradiated raw and cooked samples were analyzed using a DH/GC-MS with different holding times up to 280 min. The amounts of dimethyl disulfide and dimethyl trisulfide decreased as sample holding time in an autosampler (4 degrees C) before purge increased, whereas those of aldehdyes increased as holding time increased due to lipid oxidation. Helium flush of sample vials before sample loading on an autosampler retarded lipid oxidation and minimized the changes of sulfur volatiles in raw meat but was not enough to prevent oxidative changes in cooked meat. Although DH/GC-MS is a convenient method for automatic analysis of volatiles in meat samples, the number of samples that can be loaded in an autosampler at a time should be limited within the range that can permit reasonable repeatabilities for target volatile compounds.