Thermally Generated Flavors - American Chemical Society

Chapter 15

New Aroma Compounds in Wheat Bread W. Baltes and C. Song

Downloaded by UNIV LAVAL on February 14, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch015

Institute for Food Chemistry, Technical University of Berlin, Gustav-Meyer-Allee 25, D-1000 Berlin 65, Germany

Over 375 aroma compounds generated in the dough, crust and crumb of wheat bread during the baking process were identified and quantitated using dynamic and static headspace techniques in conjunction with gas chromatography - mass spectrometry. Components generated during baking included 114 evolved volatiles, and 86 aroma products in the crumb and 372 in the crust. The evolved volatiles and those isolated from the crumb fraction contained predominantly fat degradation products (alcohols, aldehydes, ketones), while those in the crust were mostly Maillard reaction products (furans, pyrazines, sulfur compounds). For the first time, 217 compounds were identified as bread aroma components. Most of these were previously found in other food systems, including three kahweofurans (coffee aroma) and nine sulfur-containing furans. Newly identified food aroma components are 5-methyl-4,5-dihydro-3[H]-thiophenone and 1-[H]-pyrrolo-[2,1-c]-1,4-thiazine. Quantitative analyses indicate that aroma components are first formed from fat degradation. After three days storage, their concentration is diminished by vaporization, whereas some of the Maillard products in the crust migrate to the crumb.

The complexity of wheat bread aroma is well established. Initial investigations of the composition of bread aroma were described by Mulders (1-4) who identified 102 compounds, followed by Folkes and Gramshaw (5,6) who reported 97 new compounds. Subsequent studies were performed by Schieberle and Grosch (7-10) and Hiromaka (77). The aim of our research was the identification of aroma compounds generated not only in wheat bread but also those vaporized into the oven during the baking process. This research was combined with attempts to elucidate the rate and mechanism of aroma formation. The main steps of bread production are shown by the following graphic:

0097-6156/94/0543-0192$06.00/0 © 1994 American Chemical Society

Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


15.

BALTES & SONG

New Aroma Compounds in Wheat Bread

Flour

Water

Yeast

Salt

Additives

m i x i n g , kneading Dough R e a c t i o n s of y e a s t Aroma Compounds co 2

Baking Process B r e a d


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THERMALLY GENERATED FLAVORS

Experimental Procedure


Bread Preparation. Bread dough was prepared by combining 500 g of wheat flour, 25 g of yeast, 10 g of sodium chloride, and 290 mL of water, then mechanically kneading the mixture for eight minutes. The dough was fermented at 32°C in a special proofing oven for 30 min., and was subsequently left at room temperature for five min. A 600-g portion of dough was formed into a bread loaf shape and proofed in the same oven for 45 min. to prepare it for subsequent baking. The dough surface was then moistened and, after humidifying the oven, the bread was baked in a hermetically-sealed borosilicate glass "baking tin" in a professional-type oven for 30 min. at an average temperature of 230°C. The baking parameters and the experimental design are shown in Figure 1, including the oven, crust and bread temperatures and the oven humidity. Analytical Methodology. Bread volatiles were purged by a nitrogen stream through a water condenser, which enabled trapping of excess water vapor. (Figure 2) Aroma volatiles were then collected on either a Tenax adsorbent trap or condensed in a liquid nitrogen cold trap. By changing the traps at defined times, it was possible to distinguish the compositions of volatiles which were being formed. In every case, relatively large headspace volumes were injected onto a gas chromatograph, where the aroma compounds were cryofocused onto a retention gap of deactivated fused silica, using liquid nitrogen. The influence of sampling device volume, analyte concentration, and sample matrix have been previously reported (72). Independently of analyzing the volatiles during the baking process, samples of crust and crumb from the freshly baked bread and raw dough were separated, pulverized and analyzed by means of static and dynamic headspace analysis. By this technique, the concentrations of aroma compounds could also be determined. The standard deviations of eight selected compounds were between 6 and 26%, which were higher than those usually observed for purge-and-trap experiments, because of oven temperature deviations (±20°C). Static Headspace Analysis. A 10-g sample was transferred into a 250-mL Erlenmeyer flask (Figure 3) and held at a suitable temperature (e.g. 20°C) for 1 nr. For volatile sampling, a gas-tight syringe (1 or 10 mL) equipped with a deactivated fused silica capillary (25 cm x 0.17 mm i.d.) was directly introduced a few millimeters above the sample. The volatiles were withdrawn at a rate of 0.5 mL/min, then transferred to the gas chromatography column at the same rate. The gas chromatograph was equipped with a retention gap of deactivated fused silica, which was cooled by liquid nitrogen (Figure 4). By injecting large headspace volumes (up to 20 mL) and cryofocusing them onto the G C column, the sensitivity of the technique was increased relative to conventional static headspace analyses. Dynamic Headspace Analysis. A 10-g sample was introduced into a pearshaped two-necked flask as shown in Figure 5 and swept with helium at 50 mL/min. Water was condensed in the first trap, which was cooled with ice water, and the volatile aroma compounds were collected in the second trap, which was cooled with liquid nitrogen. After warming of the second trap at 20°C for 1 hr, 5 mL of the gas phase was injected onto a gas chromatography column as shown in Figure 4. Gas Chromatography. Separations were performed on fused silica capillary columns using one of the following chromatographs: 1) Carlo Erba Model 6000 Vega, Series 2. 2) Carlo Erba H R G C Model 5160 Mega Series with a Model M F C 500 programming unit.


BALTES & SONG


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280 260 240 220 •200

tenperature of crust

120


tool

average oven tarperai

attune in the hVead 100

Figure 1. Baking humidities and temperatures as a function of time.

(

N,

t

(Bread) y N

H0 9

H

1. Oven with baking tin 2. Cooler 3. Flask 4. Trapping unit

Figure 2. Apparatus for headspace volatile isolation during the baking process.


196



V

Figure 3. Static headspace sampling device for the collection of large headspace volumes: (a) gas-tight syringe, (b) glass restrictor tube, (c) valve, (d) screw-cap with Teflon ferrule, (e) reducer, (f) fused silica syringe needle, (g) 250-mL Erlenmeyer flask, (i) sample.

Figure 4. G C interface for the injection of large headspace volumes by cryofocusing: (1) gas-tight syringe, (2) on-column injector, (3) retention gap, (4) Dewar flask containing liquid nitrogen, (5) capillary union, (6) G C column, (7) flame ionization detector, (8) G C oven.



15. BALTES & SONG


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Figure 5. Dynamic headspace sampling device for purge and cold-trapping of volatiles: (1) flask containing sample, (2) cold trap (ice/water), (3) cold trap (liquid nitrogen); this trapping unit was also connected at position (4) in Figure


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3) Carlo Erba Model 4130 equipped a Model 180 electrometer. Injections were made in the on-column or split/splitless mode at 280°C. A l l gas chromatographs contained a flame ionization detector, which was operated at 280°C. The carrier gas was helium at 2 mL/min. Column 1: 60 m χ 0.25 mm i.d., 1.0 μπι film thickness DB-1 (J&W Scientific, Folsom, CA) fused silica capillary column; initial temperature 30°C, 5 min hold, then programmed at 3°C/min to 260°C, followed by a 60 min hold. (Used in gas chromatographs 1 and 2.) Column 2: 60 m χ 0.25 mm i.d., 0.25 μπι film thickness D B - W A X (J&W Scientific, Folsom, CA) fused silica capillary column; initial temperature 40°C, 5 min hold, then programmed at 2°C/min to 210°C, followed by a 60 min hold. (Used in chromatograph 3.) Gas Chromatography-Mass Spectrometry (GC/MS). A Finnigan M A T Model 4500 mass spectrometer equipped with a Model 2010 interface and an INCOS 2100 data system was used. Transfer line temperature: 240°C (direct coupling); ion source temperature: 120°C; ionization energy: 70 ev. Electron impact: Source pressure: approx. 1 χ 10~ torr.; mass range 35-350 amu, 0.8 s scan rate. Chemical ionization: Methane reactant gas; source pressure: 0.7 torr.; mass range 80-350 amu, 0.8 s scan rate. 6

Results and Discussion Volatiles which were evolved during baking consisted primarily of aldehydes, ketones, esters and some hydrocarbons. A majority of these compounds may have been formed from amino acids in the course of the dough fermentation process. Indeed, the research group of Benedito de Barber (73) recently demonstrated that during fermentation, most free amino acids are considerably degraded. As was also shown by this group, the fraction of free amino acids in the dough, especially serine, threonine, alanine, leucine, phenylalanine, and lysine, was increased by 64% after yeast addition. For example, the amino acid ornithine, present only as a trace in flour, grew to more than 1% of the total amino acid composition. After fermentation, the free amino acid content dropped significantly, and in baked bread it was reduced to about 30% relative to the original concentration in flour. The influence of yeast on bread aroma can be considered in several ways: 1) Yeast is an ingredient of dough, which subsequently can generate aroma compounds. 2) Yeast is capable of producing aroma precursors. 3) Yeast possesses its own aromatics, which can enrich the aroma of baked goods, such as cookies. Figure 6 shows the rates of formation of 14 selected aroma compounds during baking. It was observed that aroma compounds which were formed by yeast such as 1-propanol and 2,3-butanedione were distilled-off very early. Alternately, 3methylbutanal (formed by Strecker degradation of leucine), pyrrole, and the four furan compounds (presumably formed by the Maillard reaction) were evolved towards the end of the baking time. Aroma Compounds Identified in Wheat Bread. By means of static and dynamic headspace analysis, we identified 372 compounds in the crust and 86 in the crumb of white bread. Only three compounds (2,4-heptadienal, 2-dodecenal and 2,3octandione) were formed exclusively in the crumb. The other 83 compounds were also present in the crust. In the crumb fraction, aldehydes (24), alcohols (17) and ketones (13) predominate, while Maillard reaction products such as pyrazines (11) and furans (6)



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2-Methylfuran Pyrrole Furfural 2-Hydroxyfuran Dimethyltrisulfide

Figure 6. Formation rates for 14 selected aroma compounds in wheat bread during baking as a function of time.


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are less represented. Methional was the only sulfur-containing compound identified. While much fewer volatiles were produced from the crumb during the baking process, their compositions are very similar to those identified in the crust fraction. They are predominantly compounds which have been formed in the dough during the fermentation process or which may have been degraded from precursors in this step. This was verified by identification of similar volatile compounds in the dough. On the other hand, a large spectrum of aroma compounds was identified in the crust which were predominantly formed by Maillard reactions: 68 pyrazines, 37 pyrroles, 38 ketones, 28 aldehydes, 21 hydrocarbons, 18 alcohols, 15 esters and 12 pyridines. The predominant result of these investigations were, in our opinion, the identification of 57 sulfur-containing compounds, 44 of which were not described to be present in bread aroma until now. Aliphatic Hydrocarbons and Alcohols. A variety of hydrocarbons were identified in the crust volatiles. Most important from a toxicological viewpoint may be benzene, toluene, xylene, styrene and ethylbenzene, while some aliphatic hydrocarbons such as n-octane, Λ-decane and Λ-dodecane which are derived from fatty acids are not as significant. The aromatic compounds are presumably formed as by-products of sugar degradation. We have identified them following the roasting of sugar/amino acid mixtures (220°C) as well as after pyrolyzing sugars at 700°C (14). The threshold values of hydrocarbons are high, which suggests that they are not important as aroma compounds. In addition, alcohols possess high threshold values, therefore they are not important aroma compounds. We have identified 19 alcohols, six of which were not described in bread aroma up to now. Among them, we have identified (Z) and (E) isomers for both 2-nonen-l-ol and 2-decen-l-ol. The only compound of this class having low threshold values was l-octen-3-ol, which is wellknown to be a character impact compound of mushrooms. C a r b o n y l C o m p o u n d s . Of the 29 aldehydes identified in bread, 3methylbutanal and 2-nonenal seem to be important for crust aroma, while 2-nonenal and 2,4-decadienal dominate as aroma compounds in the crumb. 2,4-Decadienal possesses a green, fatty aroma and has a threshold value of 0.07 ppb. It was previously identified in potato chips, rice and cooked beef. (Έ,Ι-2-Nonenal (0.08 ppb threshold) is responsible for a cucumber-like aroma. At concentrations between 0.4 and 2 ppb, it possesses a wooden note which gives a fresh aroma character to roasted coffee. The structures of these alcohols and aldehydes, some of which are shown in Figure 7, indicate that they have probably been formed by fat oxidation. Among the 38 ketones which were identified in crust and crumb, 17 are described in white bread for the first time. Most of them can be generated by yeast fermentation as well as by Maillard reactions. The same is valid for some hydroxy ketones such as acetoin and hydroxypropanone. Among them were seven cyclic ketones belonging to a group of pentanones and hexanones. Some of these are important aroma compounds in caramel which are formed through Maillard reactions. Oxygen Heterocycles. A group of 63 furans and furanones was identified, 43 which are described to be contributors to bread aroma for the first time. This group contains some derivatives with a long alkyl substituent, such as hexyl furan, which are clearly fat oxidation products. The most abundant compound is furfural (14.9% of all volatiles), and it is the principal component of the volatiles from crust. Furan ketones and diketones are described as prominent aroma compounds in bread and coffee which have caramel-like, burnt aroma notes. Of interest is the identification of some condensed furans, some which were also identified in coffee. Nitrogen Heterocycles. Pyrroles are well-known aroma compounds in foods such as cocoa, coffee, tea and nuts which are formed by Maillard reactions. We



15. BALTES & SONG


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identified 27 of them in the crust, 19 which were found in bread for the first time. Of interest is the identification of some pyrrolizines. The aroma impression of pyrroles are pyridine-like, while acylpyrroles possess mostly a bread-like aroma note. 2Acetylpyrroline has been described to be a character-impact compound of wheat bread crust. It is formed by the reaction of the amino acids proline and ornithine with sugars in the course of the Maillard reaction. 2-Acetylpyrroline was initially identifed in rice and is responsible for popcorn-like aroma notes. Its threshold value is in the range of 1 ppb in water. Twelve pyridines were identified, seven of which were found in bread for the first time. One of these is 2-pentylpyridine, which is formed starting with the reaction of 2,4-decadienal and ammonia (Figure 8, R = C 5 H n ) . It possess a fatty, tallow-like aroma note and a threshold value of 0.6 ppb in water. The corresponding thiophene is formed by reaction with H2S. 2-Acetyltetrahydropyridine was identified by Hunter (75) in white bread. It possesses a cracker-like note and a low threshold value (1.6 ppb), and is formed by reaction of proline with pyruvic aldehyde. Through aromatization, 2-acetylpyridine is formed; it possesses a higher odor threshold but similar sensory character as 2-acetyltetrahydropyridine. Pyrazines are not discussed in this chapter, even though 38 new pyrazines were identified in bread for the first time. However, a huge literature is available concerning pyrazines in foods. For instance, we have previously described 123 pyrazines obtained through the heating of serine and threonine with sucrose. (76) Sulfur Compounds. Of special significance is the identification of 44 sulfurcontaining compounds which are described as bread aroma products for the first time. Methionine and cysteine are precursors to a variety of aliphatic sulfur compounds, some of which are shown in Figure 9. Heterocyclic sulfur compounds such as thiophenes, are generated by reactions of the corresponding aldehydes with H2S or with mercaptoacetic aldehyde (the Strecker degradation product of cysteine.) Examples of these compounds identified in bread aroma include the previously unknown 5-methyl-4,5-dihydro-3-[2//]-thiophenone (26) and the thiophenones 24 and 25 which are already known from coffee aroma (Figure 10). Thiophenes with alkyl substituents in the 2-position may be derived from 2,4-dienals through fat degradation. Thiazoles represent another chemical class, the precursors of which appear to be cysteamine (formed by reaction of suitable carbonyl compounds with H2S and amino compounds). Strecker degradation plays a dominant role in the formation of thiazoles. 2-Acetyl-2-thiazoline, which possesses a strong fresh bread aroma character and was designated as a bread aroma compound by Folkes and Gramshaw (5), was not identified in our experiments. Of special interest are furans, which are easily substituted with sulfur groups. A variety of these types of compounds can be formed by reactions of furfural or pentoses with H2S. Ten sulfur-substituted furans were identified in bread, nine for the first time (Figure 11). Furfuryl methyl disulfide (29) has previously been identified by Mulders (4) as a character impact compound of bread. Its threshold is approximately 0.04 ppb in water. Furfuryl mercaptan is important in coffee aroma. Difurfuryl sulfide and disulfide are described to possess a white bread crust-like aroma. In our opinion, the identification of kahweofuran derivatives in bread aroma is a distinguishing result. Compound 34 was first identified by Stoll in coffee. (Figure 8) In suitable dilution, it possesses a smokey-like odor. Compounds 35 and 36 were also recently identified in coffee. Their aroma qualities are reminiscent of sulfur-like, meaty, or mushroom-like notes. They were formed in a model system reaction between mercaptopropanone and vinyl hydroxymethyl ketone in 50% yield (77). Other sulfur-containing compounds identified include methional, methionol, oxidation products of methyl mercaptan, and 1,2,3,4-tetrathiane. A new compound



202


Figure 7. Some selected aldehydes and alcohols identified in wheat bread aroma.

R - CH : CH - CH : CH - CHO

CH - CH R-CH

. C H = CH

^CH

I

R-CHo-CH

I

,CH NH

Η

CH II

0

Figure 8. Formation of pyridines and thiophenes with long-chain alkyl substituents via reaction of 2,4-dienals.


15.

H S - C H - C H — NH 2

CH —

s

3

" (

C H

2)3

—

N H

2

3

2

2

2

HS-CH -CH-C00H 2

NHo

NH

Methionine

C H - S - C H - C H - CHO 3

2

-

2

Cysteine

Strecker-Degradation

J

2

Cysteamine

C H - S - C H - C H - CH - COOH


203


BALTES & SONG

|

HS - C H - CHO

2

2

HoO

CH =CH-CH0 2

C H - HS 3

H,S

H,0 Figure 9. cysteine.

0H-CH -CH0 2

CH3-S-S-CH3

Formation o f typical degradation products from methionine and

12

13

14

CHO

'

17

16

^CHO 18

ο 21

15

19

20

Ο

ο 22

çf° ςχ ç£° 0

23

Ο

24

25

xf 26

Figure 10. Selected thiophenes and thiophenones formed i n wheat bread aroma.




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15. BALTES & SONG

New Aroma Compounds in Wheat Bread 205

which was not described in food systems until now is l-[//]-pyrrolo-[2,lc]-l,4thiazine (33) which we identified in the crust volatiles (Figure 11). It was initially described by Werkhoff in a model system consisting of cysteine and ribose (18). Its formation is presumed to proceed by reaction of an aminoketone with 3,4dideoxypentosene, or by reaction of furfural with mercaptoacetic aldehyde. Because of the high concentration of furfural in the bread volatiles, we favor the latter mechanism.


Conclusions The majority of volatiles identified in wheat bread were found in the crust portion. While the crust was found to contain predominantly Maillard reaction products such as furans, pyrroles, pyrazines and sulfur compounds, the aroma products in the crumb were generated through yeast fermentation processes and included alcohols, aldehydes and ketones. Overall, the most abundant volatiles in bread were alcohols, furans and pyrazines. During storage of bread for 1, 2 and 3 days, the spectrum of aroma compounds changed. During this time, the concentrations of aroma compounds formed by fermentation such as 2,3-butanedione, 2-butanone, ethyl acetate, 3-methylbutanol and 2,3-pentanedione decreased in the crust. A possible explanation may be diffusion out of the bread together with evaporation of water. Alternatively, the concentrations of pyrazines and furans reached a maximum on the second day and subsequently dropped. The opposite was observed in the crumb. While the concentrations of fermentation products remained constant, the levels of Maillard reaction products increased. It became apparent that Maillard reaction products in the crust are able to migrate into the crumb, as was determined by measurement of the concentrations of selected aroma compounds at a 5-millimeter distance from the crust. Literature Cited 1. Mulders, E. J.; Maarse, H.; Weurman, C. Ztschr. Lebensm. Unters. Forsch. 1972, 150, 68. 2. Mulders, E. J.; ten Noever de Brauw, M. C.; van Straten, S. Ztschr. Lebensm. Unters. Forsch. 1973, 150, 63. 3. Mulders, E. J. Ztschr. Lebensm. Unters. Forsch. 1973,151,310. 4. Mulders, E. J.; Kleipool, R.; ten Noever de Brauw, M. C. Chem. & Ind. 1976, 613. 5. Folkes, D. J.; Gramshaw, J. W. J. Food Technol. 1977,12,1. 6. Folkes, D. J.; Gramshaw, J. W. Progr. Food Nutr. 1981, 5, 369. 7. Schieberle, P.; Grosch, W. Ztschr. Lebensm. Unters. Forsch. 1983,177,173. 8. Schieberle, P.; Grosch, W. Ztschr. Lebensm. Unters. Forsch. 1984, 178, 489. 9. Schieberle, P.; Grosch, W. Ztschr. Lebensm. Unters. Forsch. 1985, 180, 474. 10. Schieberle, P.; Grosch, W. Ztschr. Lebensm. Unters. Forsch. 1991,192,130. 11. Hiromaka, Y. Cereal Chem. 1986, 63, 369. 12. Bohnenstengel, F.; Soltani, N.; Baltes, W. J. Chromatogr., in press. 13. Prieto, J. Α.; Collar, C.; Benedito de Barber, C. J. Chromatogr. Sci. 1990, 28, 572. 14. Baltes, W.; Schmahl, H., J. Ztschr. Lebensm. Unters. Forsch. 1978, 167, 69. 15. Hunter, I. R.; Walden, M. K.; Scherer, J. R.; Ludin, R. E. Cereal Chem. 1969, 46, 189. 16. Baltes, W.; Bochmann, G. Ztschr. Lebensm. Unters. Forsch. 1987, 184, 485. 17. Gorzynski, M.; Rewicki, D. Liebigs Ann. Chem. 1986, 625. 18. Guntert, M.; Bruning, J.; Emberger, R.; Kopsel, M.; Kuhn, W.; Thielmann, T.; Werkhoff, P. J. Agric. Food Chem. 1990, 38, 2027. RECEIVED August 5, 1993 Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Thermally Generated Flavors - American Chemical Society

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