https://orcid.org/0000-0003-4572-3053
Abstract
A headspace-proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF/MS) system that combines PTR-TOF/MS with a dynamic and reactive flavor-release monitoring system was constructed to measure the orthonasal aroma components in beer before and after foaming. Some aroma components in the headspace of beer after foaming in real time, such as m/z 145.1150 and m/z 173.1536, are thought to be derived mainly from ethyl hexanoate and ethyl octanoate, respectively, resulting in a fruity and sweet aroma. They showed significantly higher volatilizations than those in the headspace before foaming. The ratio of the concentration of headspace m/z of beer after foaming and before foaming showed a significant correlation with the partition coefficient of each assuming the main aroma component, indicating that the volatility of beer aroma with foaming depends on the hydrophobicity. The aroma components with high partition coefficients were believed to be able to associate with isohumulones and some proteins and to move to foam during foaming. They might be more volatilized by collapsing bubbles than those with low partition coefficients. Beer foam plays a novel role as a carrier of aroma, specifically promoting the release of some characteristic aroma compounds.
Introduction
The role of foam in beer has been studied by many researchers.[Citation1–6] Beer foam is extremely important not only for its appearance but also for its deliciousness. The contrast between the foam and liquid color formed by beer foam is a symbol of beer’s beauty. The foam prevents beer from losing its flavor due to changes in its composition caused by exposure to air, and it also acts as a “lid” to prevent the escape of gaseous carbon dioxide. Creamy foam provides a unique mouthfeel on the upper lip when we drink it, and so it is thought to play a role in making beer delicious. The taste of draft beer is the smoothness of the creamy foam and the refreshing sensation when we drink it.
The generation and disappearance of foam in beer are considered as follows;[Citation1–6] bubbles are generated by the impact when beer is poured into a glass. In beer, the bubbles that reach the liquid surface do not burst immediately, while those generated from below accumulate steadily. In the initial stage of formation, the bubbles rise while being entwined in the liquid, and the liquid is relatively separated downward; thus, the boundary between the liquid and bubble layer quickly moves upward. However, the bubbles that have reached the surface of the bubble layer shrink and become finer bubbles. The bubbles in this period contain a large amount of carbon dioxide gas, but, because the carbon dioxide gas can permeate the membrane of the bubbles, it is thought that the difference in partial pressure from the outside air causes carbon dioxide permeation and shrinkage of bubbles. Furthermore, in the vicinity of the surface of the bubble layer, large bubbles immediately collapse to some extent. In this process, a layer of fine bubbles with a uniform diameter remains on the liquid beer. In the foam, hydrophobic components in the beer (mainly the hydrophobic portion of the protein surface) are oriented on the side of the gas phase, and the liquid portion gradually drains downward according to gravity. In this process, proteins and the bitterness of hops (isohumulones) are left behind in the form of cross-linked proteins in the remaining part of the foam film, resulting in a more stable state. Even in the formed bubbles, the liquid continues to flow down, and when the liquid film on the surface becomes thin, the bubbles collapse and burst. On the inner surface of the bubble layer, liquid flow occurs toward the plateau boundary in which the liquid films meet. When the thinned liquid film breaks, the surrounding bubbles combine to form a larger, more unstable liquid, and the bubbles gradually collapse and disappear.
It should be noted how beer aroma components behave during the above processes. Delvaux et al.[Citation7] described the foam as perhaps one of the most appealing beer qualities, which is not surprising because the foam acts as an efficient gas exchange surface funneling aromas toward the drinker’s olfactory sensors, and it provides a drinker’s first tantalizing entrée as to the quality of the beer’s flavor, freshness, refreshingness, and wholesomeness. However, very few reports have focused on the role of foams in beer aroma. This is probably because it is extremely difficult to capture the aroma in beer foam because it undergoes continuous cycles of bubbling and collapsing. Gas chromatography (GC) is the most commonly used method for measuring aroma components in beer.[Citation8,Citation9] However, measuring the aroma components in beer foam that repeatedly foams and collapses in real time is difficult using GC because GC analysis requires a separation operation that takes several minutes to several tens of minutes.
Recently, researchers have focused on real-time measurements by directly introducing aromatic components into mass spectrometers.[Citation10,Citation11] Real-time aroma measurement techniques such as a proton-transfer-reaction time-of-flight/mass spectrometer (PTR-TOF/MS) are characterized by their ability to measure aroma components in food and beverages in real time. Some studies have reported the uses of PTR-TOF/MS to characterize foods based on their released aroma compounds.[Citation12,Citation13] In the mass spectrum measured by PTR-TOF/MS with soft ionization, each mass peak corresponds directly to a substance. Therefore, it is possible to relate it to the evaluation by a sensory test and to express it numerically. If the time profile is also considered during characterization, information closely related to the sensory evaluation can be obtained. The headspace-PTR-TOF/MS system may be able to measure in real time with high sensitivity the changes in the orthonasal aroma of beer after foaming.
In this study, a headspace-PTR-TOF/MS system was constructed that combines PTR-TOF/MS with a dynamic and reactive flavor-release monitoring system. The orthonasal aroma components in beer were measured before and after foaming, and the novel role of beer foam is discussed.
Experimental
Beer
Regular Japanese beer brewed with adjuncts (brand A) and all-malt Japanese beer (brand B) were purchased from the local Japanese market. Beers were stored at room temperature (20 °C) or in a refrigerator (5 °C).
Apparatus
A PTR-TOF/MS (Ionicon Analytik GmbH, Innsbruck, Austria) was used to analyze the aroma components of beer. Dynamic and reactive flavor-release observation equipment (Shoreline Science Laboratory, Inc., Tokyo, Japan) was used to analyze the orthonasal aromas in beer. An ultrasonic washing machine (Model AU-26C, Aiwa Medical Industry Co., Ltd., Tokyo, Japan) was used for the uniform foaming of the beer.
Beer foaming
To avoid foaming and loss of carbon dioxide, beer measured with a scalpel cylinder was carefully transferred to a sample container for measurement. The sample container was placed in an ultrasonic cleaner, sonicated for 2 min to achieve uniform foaming (), and then left at room temperature for 1 min before being set up in the headspace-PTR-TOF/MS measurement system (). During the operation, the sample container was open to the atmosphere so that the aroma components generated in the container’s headspace during the ultrasonic treatment were released from the container.
Figure 1. Beers before and after foaming.
A can of beer (brand B) stored in a refrigerator (5° C) was removed at room temperature (20° C). Samples of 50 mL (1) or 65 mL (2) of beer were transferred to a measuring container and bubbled by ultrasonic waves for 2 min in an ultrasonic washing machine.
(I) Before foaming and (II) after foaming.
Figure 2. A proton-transfer-reaction time-of-flight mass spectrometer with a dynamic and reactive flavor-release monitoring system.
For the control (beer before foaming), the beer was transferred to the sample container for measurement without foaming as described earlier, placed in an ultrasonic cleaner for 2 min without ultrasonication, left at room temperature for 1 min, and then set up in the headspace-PTR-TOF/MS measurement system.
Subsequently, the measurement began, and after confirming that the state of the aroma components in the mass spectrometer was stable for 30 s, the stopper connecting the headspace air and the mass spectrometer was opened. The x-axes of the indicate the time from the start of measurement.
Figure 3. The behavior of orthonasal aroma components of 65 mL beer A sample before and after foaming.
(1) m/z 43.0542, (2) m/z 45.0355, (3) m/z 57.0699, (4) m/z 60.0525, (5) m/z 71.0855, (6) m/z 89.1050, (7) m/z 131.1067, (8) m/z 145.2139, (9) m/z 173.1536, (10) m/z 201.1848
Figure 4. The behavior of orthonasal aroma components of 65 mL beer B sample before and after foaming.
(1) m/z 43.0542, (2) m/z 45.0355, (3) m/z 57.0699, (4) m/z 60.0525, (5) m/z 71.0855, (6) m/z 89.1050, (7) m/z 131.1067, (8) m/z 145.2139, (9) m/z 173.1536, (10) m/z 201.1848
Orthonasal aroma measurement
A headspace - PTR-TOF/MS measurement system was constructed by connecting a dynamic and reactive flavor-release observation system to the PTR-TOF/MS system (). Moreover, 50, 65, and 75 mL of beer were placed in a sample container (125 mL), and the flavor components in the headspace were introduced into the PTR-TOF/MS system with nitrogen gas to measure the behavior of the flavor components. In this experiment, a mixture of the headspace sample and nitrogen gas was introduced into the PTR-TOF/MS system at a ratio of 1:1.
The PTR-TOF/MS measurement is relatively fragmentation-free due to ionization by hydronium ions (H3O+) and detects the m/z, which indicates the ions of each molecule attached to hydrogen ions (). Even under such conditions, alcohols can desorb water ions; therefore, in measurements targeting propanol, butanol, and pentanol, the m/z shows ion molecules with desorbed water and added hydrogen ions.
Table 1. Target compounds in beer.
The concentrations of each aroma component are theoretical values converted from the concentrations of hydronium ions (H3O+) introduced into the detector as concentrations in the detection cells. The detection limit of this analyzer is several tens of ppt of the gas in the measurement cell, so it is estimated that it can detect odor components drifting in the nasal cavity at a level of several tens of ppt or more.
Each result represents the mean value of six or more repeated measurements.
Results and discussion
Headspace aroma components associated with beer foaming
shows the m/z values of the target compounds in this experiment, which are the main aroma components of beer. The PTR-TOF/MS system is one of the most effective techniques for the real-time measurement of aroma components. However, it has a major disadvantage in that it is difficult to identify isomers because it is a mass measurement system that does not involve separation operations.[Citation11] Therefore, the m/z in may contain isomers.
When drinking beer, the canned or bottled beer is taken out of the refrigerator and poured into a glass or mug. The real-time behaviors of aroma components in the headspace air of the measurement vessels in the headspace-PTR-TOF/MS system were measured. and show the behavior of the orthonasal aroma components in 65 mL of brand A or B before and after foaming. All aroma components immediately increased after the introduction of the aroma components into the PTR-TOF/MS. They reached a maximum in about 30 s and then decreased. A relatively stable aroma concentration curve was maintained after approximately 100 s. In this experimental system, the temperature of the beer gradually increased from 13 °C to 16 °C during the measurement. The same behaviors of aroma components in the headspace of the beer were observed for the 50 mL and 75 mL samples of beer. In the measurement, the measurement container containing the beer was set to the measurement system and stabilized by introducing nitrogen gas so that the inside of the container was in a pressurized state. At the start of the measurement, the aroma compounds in the headspace were simultaneously introduced into the PTR-TOF-MS. This may be similar to the volatilization of aromas immediately after opening a bottled beer or a canned beer. Because of the aroma components in the headspace may be unstable for some time after opening, each integral value of the aroma components was compared before and after foaming, from 100 to 300 s after the start of measurement; over this period, we believed that the aroma components in the headspace may have stabilized. All of the integral values of the orthonasal aroma compounds in the 65 mL samples of brands A and B after foaming were significantly higher than those before foaming ( and ). Those in 50 mL and 75 mL samples of brands A and B after foaming were also significantly higher than those before foaming.
Figure 5. Comparison of the amount of headspace aroma components in 65 mL beer A sample with and without foaming.
The integral values of the concentration of each aroma component from 100 to 300 s of measurement are shown.
(1) m/z 43.0542, (2) m/z 45.0355, (3) m/z 57.0699, (4) m/z 60.0525, (5) m/z 71.0855, (6) m/z 89.1050, (7) m/z 131.1067, (8) m/z 145.2139, (9) m/z 173.1536, (10) m/z 201.1848
Figure 6. Comparison of the amount of headspace aroma components in 65 mL beer B sample with and without foaming. The integral values of the concentration of each aroma component from 100 to 300 s of measurement are shown.
(1) m/z 43.0542, (2) m/z 45.0355, (3) m/z 57.0699, (4) m/z 60.0525, (5) m/z 71.0855, (6) m/z 89.1050, (7) m/z 131.1067, (8) m/z 145.2139, (9) m/z 173.1536, (10) m/z 201.1848
To understand the real-time changes in the amount of orthonasal aroma components before and after foaming, the concentration ratios of the aroma components in beer before and after foaming (concentration of aroma components after foaming/before foaming) for each measurement (every second) were plotted ( and ). As mentioned earlier, it was thought that the volatilization of the aroma components immediately after introduction to the PTR-TOF/MS system was not stable. Therefore, the behavior of the concentration ratio of each m/z component in the beer before and after foaming, from 100 to 300 s after the start of measurement, was observed. Most of the aroma components in the two beer brands were one to two times higher after foaming than before foaming. However, the components of m/z 145.1230 (mainly ethyl hexanoate) and m/z 173.1536 (mainly ethyl octanoate) showed 2.5 to 7.0 times higher values after foaming than before foaming. The concentration ratio of m/z 131.1067 (mainly isoamyl acetate) and m/z 201.1848 (mainly ethyl decanoate) in the beer was shown to be more than 2.5 times in some samples.
Figure 7. The behavior of headspace aroma component concentration ratio in 50 mL (1), 65 mL (2), and 75 mL (3) samples of beer A before and after foaming.
Figure 8. The behavior of headspace aroma component concentration ratio in 50 mL (1), 65 mL (2), and 75 mL (3) samples of beer B before and after foaming.
The changes in headspace aroma components due to the foaming of beer may be due to the decrease in the headspace volume caused by the foam in the sample container. Therefore, the concentration ratio (headspace volume before foaming/headspace volume after foaming) due to the decrease in headspace volume caused by foaming was calculated (). The concentrated rates of aroma components were 1.3 to 1.9, owing to the decrease in headspace volume caused by foaming. In addition, the concentration ratios of most of the aroma components gradually decreased with measurement time, suggesting that the concentration rates of aroma components before and after foaming approached 1.0 ( and ). Therefore, we inferred that the increase in the concentration of many aroma components due to foaming may be due to a decrease in the headspace volume caused by foaming. However, the concentration ratios of m/z 145.1230 (mainly ethyl hexanoate) and m/z 173.1536 (mainly ethyl octanoate) were 2.5 to 7.0 times higher, which could not be explained by the decrease in the headspace volume by foaming. The concentration ratios of the m/z 131.1067 (mainly isoamyl acetate) and m/z 201.1848 (mainly ethyl decanoate) were also above 2.5 in some samples. Therefore, it was speculated that the components of m/z 145.1230 (mainly ethyl hexanoate) and m/z 173.1536 (mainly ethyl octanoate) were highly volatilized by the foaming of beer. Ono et al.[Citation14] studied the influence of foaming and its various effects on the flavor of beer through sensory, physical, and chemical analyses and showed the transfer of some compounds into the foam. Ethyl hexanoate, ethyl octanoate, and ethyl decanoate decreased in the liquid beer and increased in the foam. Some esters were thought to be transferred and concentrated in the foam, supporting our findings.
Figure 9. The headspace volume concentration ratio by foaming. Each value was calculated as I volume/II volume.
The components of m/z 145.1230 (mainly ethyl hexanoate) and m/z 173.1536 (mainly ethyl octanoate) are known to have a fruity and sweet aroma.[Citation15,Citation16] Kishimoto et al.[Citation17,Citation18] attempted to reconstruct beer aroma by calculating the odor activity value (OAV: concentration/threshold) from the concentrations of 76 aroma components identified by GC-O analysis. Ethyl hexanoate and ethyl octanoate were classified among the 25 components with the highest OAV (OAV = 3.58 to 0.35, Group A). Xu et al.[Citation19] studied the effect of esters on the flavor of lager beer and found that ethyl octanoate had a sweet brandy aroma. This aroma is desirable and scores the highest in sensory evaluations in the moderate concentration range in beer (0.16 − 0.41 mg/L). However, it has an unpleasant solvent-like odor at very high concentrations. Therefore, it was expected that beer foaming might volatilize some favorable aroma, thereby increasing the attractiveness of the beer.
Kosin et al.[Citation20] described that, typically, hydrophobic characteristics, such as hop bitterness and aroma oils, are favorably concentrated in the foam, along with spices such as coriander and orange peel. One downside is that the sweet corn taint (dimethyl sulfide) and papery (trans-2-nonenal) aged flavor are also concentrated in the foam, which can negatively affect lager- and pilsner-style beers. Conversely, the absence of foam accentuates hydrophilic flavors such as malt, caramel, and fruity esters such as banana (isoamyl acetate), while enhancing the perception of undesirable butterscotch (diacetyl) flavors. In this experiment, m/z 63.0190, which was mainly derived from dimethyl sulfide, was detected in the beers, although the m/z 141.1201 component considered to be mainly derived from trans-2-nonenal could not be detected in the stored beer (data not shown). The average concentration ratio of the m/z 63.0190 component before and after foaming (concentration of aroma components after foaming/before foaming) was 1.9 for the 65 mL samples of beers A and B. It appears that the increase in m/z 63.0190 caused by foaming might be due mainly to the decrease in headspace volume caused by foaming, although it was slightly higher. It is unlikely that the odor of dimethyl sulfide would be significantly strengthened by foaming.
As cited in the introduction, Delvaux et al.[Citation7] showed that foam is perhaps one of the most appealing of beer’s qualities, which is not surprising because foam acts as an efficient gas exchange surface directing aroma toward the drinker’s olfactory sensors. It provides the drinker with the first tantalizing entrée regarding the quality of the beer’s flavors, freshness, refreshingness, and wholesomeness. Our results partially support these claims, and it is proposed that beer foams have a novel role as aroma carriers that evoke the consumers’ desires to drink beer.
Volatilization of aroma components in beer associated with foaming and their physicochemical properties
To clarify the reproducibility and mechanism of the increase in the diffusion of some aroma components by foaming in the absence of beer temperature change, the volatilization behavior of aroma components before and after foaming of beer stored at room temperature was studied. The aroma components in the headspace of 50, 65, and 75 mL beers A and B stored at room temperature before and after foaming showed behaviors similar to those shown in and . The concentration of aroma compounds was higher after foaming than before foaming. Similar to and , the integral value of each aroma component, before and after foaming from 100 to 300 s after the start of the measurement, was compared. All the integral values of the orthonasal aroma compounds in brands A and B after foaming were significantly higher than before. In beer stored at room temperature, in addition to the aroma compounds of m/z 145.1230 (mainly ethyl hexanoate) and m/z 173.1536 (mainly ethyl octanoate), the aroma compounds of m/z 131.1067 (mainly isoamyl acetate) and m/z 201.1848 (mainly ethyl decanoate) after foaming also showed concentration ratio behaviors that were 2.5 to 9 times higher than those before foaming ( and ).
Figure 10. The behavior of the headspace aroma component content ratio in 50 mL (1), 65 mL (2), and 75 mL (3) samples of beer A before and after foaming.
Figure 11. The behavior of the headspace aroma component concentration ratio in 50 mL (1), 65 mL (2), and 75 mL (3) samples of beer B before and after foaming.
The volatilization of the aroma component in beer appears to be related to the partition coefficient.[Citation21–24] When a compound is added to the two solvent phases of 1-octanol and water to reach an equilibrium state, the concentration ratio of the chemical substance in the two phases is denoted as Pow. Pow = Co/Cw, in which Co is the concentration of the test substance in the 1-octanol layer and Cw is the concentration of the test substance in the water layer. The common logarithmic value of Pow is the 1-octanol/water partition coefficient (Log Pow value), which is used as an index to evaluate hydrophobicity. According to the International Chemical Safety Cards (ICSCs)[Citation24] the 1-octanol/water distribution coefficient of the aroma component in beer targeted in this study was as follows: 1-propanol:025, acetaldehyde:0.63, dimethyl sulfide:0.84, n-butanol:0.90, acetone:-0.24, isoamyl alcohol:1.28, ethyl acetate:0.73, isoamyl acetate:2.13, ethyl hexanoate:2.64, ethyl octanoate:4.47, and ethyl decanoate:4.79. The concentration ratio of each aroma component in 65 mL beer before and after foaming and the partition coefficient showed a significant correlation (r = 0.912 (P < 0.01, n = 10) for beer A, r = 0.980 (P < 0.01, n = 10) for beer B (), indicating that the concentration ratio of aroma components after and before foaming increased with the hydrophobicity of the aroma components. The concentration ratio of each aroma component in 50 mL and 75 mL beer samples (after foaming/before foaming) and the partition coefficient also showed a significant correlation (55 mL beer A: r = 0.912 (n = 10), 75 mL beer A: r = 0.843 (n = 10), 50 mL beer B: r = 0.918 (P < 0.01, n = 10), 75 mL beer B: r = 0.967 (P < 0.01, n = 10)).
Figure 12. Relationship between the headspace aroma component concentration ratio of 65 mL beer A (1) or B (2) samples before and after foaming and the partition coefficient of the aroma component.
Proteins and isohumulones in beer play crucial roles in foam formation, and the affinity between the hydrophobic residues of proteins and hydrophobic isohumulones plays an important role in foam stabilization. Ono et al.[Citation14] showed that when the foam fraction and liquid fraction were separated after pouring beer, isohumulones were concentrated in the foam of the beer. Miyamae et al.[Citation25,Citation26] studied the molecular characterization of beer surfaces using vibrational sum-frequency generation and found that, on the surface of the beer, hop-derived iso-α-acids and isoxanthohumol molecules were aligned such that their hydrophobic groups pointed toward the air. They believed that the hop-derived molecules interacted with the hydrophobic proteins at the beer surface to form more surface-active complexes and concluded that the improvement in foam stability by increasing the number of hops was due to an increase in the number density of surface-active species formed with the hop-derived molecules and hydrophobic proteins on the beer surface. Moreover, they observed a decrease in the number of alcohol molecules on the surface owing to an increase in the amount of hop-derived components. Combining the results of this study with those of the above reports, it seems that during foaming and its maintenance, some aroma compounds with high partition coefficients may be associated with hydrophobic proteins and isohumulones in the liquid beer foam. When the foam collapses, isohumulones and hydrophobic proteins remain in the liquid beer, but the aroma components may be released into the atmosphere along with carbon dioxide, leading to an increase in some orthonasal aroma compounds by foaming.
On the basis of the results presented thus far, it appears that aroma components with high partition coefficients, such as ethyl hexanoate and ethyl octanoate, might be associated with isohumulones and some proteins and may move to the foam during foaming, where they might be more volatilized by bursting bubbles than low partition coefficients (). The slight differences in the headspace aroma behavior of beer before and after foaming between refrigerator storage and room temperature storage may be due to the associating ability of the aroma compounds with isohumulones and proteins and the volatility at beer temperature because the partition coefficient is temperature dependent.[Citation27]
![Figure 13. Diagram of the high distribution coefficient aroma components in the gas-liquid interface of beer foam. The figure was drawn with reference to the Summary of AIST press release on August 10, 2018[Citation9] and the report by Suzuki.[Citation6]](/cms/asset/0e3650bf-2563-4425-a966-403e9136761c/ujbc_a_2215686_f0013.jpg)
Conclusions
In this study, using a headspace PTR-TOF/MS measurement system that can determine the changes in aroma components in beer in real time, we succeeded in understanding the changes in the orthonasal aroma components during the continuous foaming and collapse of beer foam and clarified a novel role of beer foam as a carrier of aroma. Beer foam can promote the release of specific and attractive aromas to encourage beer drinking.
Acknowledgments
We would like to thank Tsukasa Oritae, Harumi Matsufuji, Yuka Suetsugu, and Kousuke Takemoto for providing technical support.
Disclosure statement
No potential conflict of interest was reported by the authors.
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