Rye Bread Crust

Dilution analyses show that the following compounds are involved in the aroma of rye bread 

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Schieberle, P. and Grosch, W. 1987. Quantitative analysis of aroma compounds in wheat and rye bread crusts using stable isotope dilution assay. J. Agric. Food Chem. 35 252-257. 

A first approach to analyze such volatiles is the application of the AEDA on extracts prepared by dynamic headspace extraction. An apparatus used for the extraction especially of solid foods is shown in Figure 5 [55]. The powdered material is placed into a rotating cylinder and the volatiles are continuously flushed onto a polymer material (Tenax( )) by using a stream of helium (1 L/min). After 3 hr the volatiles are desorbed from the polymer by elution with a small amount of diethyl ether and evaluated by AEDA after concentration. Since different yields may change the composition of the volatiles during headspace extraction [7], it is essential to sensorially evaluate the flavor of the extracts in comparison with the food flavor itself. The following examples show applications of this method on fresh and stored wheat bread crust [55] and on fresh rye bread crust [P. Schieberle and W. Grosch, unpublished results]. 

In an headspace extract of fresh rye bread crust, 3-methylbutanal, (E)-2-nonenal and methional showed the highest FD-factors (Table 4), while 2-acetyl-1-pyrroline, the key odorant of wheat bread crust (cf. Table 3), did not significantly contribute to the rye (rust flavor. Quantitative measurements established [45, 55] that especially the higher odor activity (cf. 3, this chapter) of the boiled potato-like smelling methional in the rye bread crust in combination with the much lower odor activity of the roasty-smelling 2-acetyl-l-pyrroline mainly contribute to the overall flavor differences in rye and wheat bread crusts. 

Odorants showing high FD-factors in a headspace extract of fresh rye bread crust [P. Schieberle and W. Grosch, unpublished results] ... 

Compounds 3, 5 and 6 shown in Figure 1 were detected in the aroma extracts of the wheat and the rye bread crusts, but on the basis of their relatively low FD-factors we concluded these compounds contribute only to the background flavors of both bread types. Furthermore, there was no indication that the sulfur-containing heterocy-clics 2-[(methyldithioImethylJfuran and 2-acetyl-2-thiazoline (4 and 7 in Figure 1) were of significance to the flavor of the wheat bread crust. 

The use of an isotope dilution assay is the best method to quantify labile and low level odorants. He applied this technique to the determination of 2-acetyl-l-pyrroline and 2-methyl-3-ethylpyrazine, the two compounds which showed the highest FD-factors among the compounds with roasty odor notes in extracts from wheat or rye bread crust, respectively ( 7, 38). The results are summarized in Table III. The high level of the acetylpyrroline in the crusts of the wheat breads was striking compared to the level in the rye breads. These quantitative data confirm that 2-acetyl-l-pyrroline is a character impact odor compound of the wheat bread crust. 

Schieberle P. and Grosch W. (1985) Identification of the volatile flavour compounds of wheat bread crust comparison with rye bread crust. Z. Lebensm. Unters. Forsch. 180, 474-8. 

The composition of the volatile fraction of bread depends on the bread ingredients, the conditions of dough fermentation and the baking process. This fraction contributes significantly to the desirable flavors of the crust and the crumb. For this reason, the volatile fraction of different bread types has been studied by several authors. Within the more than 280 compounds that have been identified in the volatile fraction of wheat bread, only a relative small number are responsible for the different notes in the aroma profiles of the crust and the crumb. These compounds can be considered as character impact compounds. Approaches to find out the relevant aroma compounds in bread flavors using model systems and the odor unit concept are emphasized in this review. A new technique denominated "aroma extract dilution analysis" was developed based on the odor unit concept and GC-effluent sniffing. It allows the assessment of the relative importance of the aroma compounds of an extract. The application of this technique to extracts of the crust of both wheat and rye breads and to the crumb of wheat bread is discussed. 

Rothe (5, , 1 5) calculated the aroma values of some volatiles identified in the crumb of wheat bread and the crust of rye bread. The data listed in Table I indicate that ethanol, isobutanal, iso-pentanal, diacetyl and isopentanol contribute with high aroma values to the aroma of the wheat bread crumb. During baking of rye bread, the two Strecker aldehydes, isobutanal and isopentanal, increased so much in the crust that they showed the highest aroma values of the volatiles investigated. 

Table I. Aroma Values of Some Volatiles Identified in the Crumb of Wheat Bread and in the Crust of Rye Bread 15, 9, 15)...

The aroma extract dilution analysis of concentrates prepared from the crusts of wheat and rye breads revealed fourty-three odorants in rye and thirty-two in wheat extracts (37). 

Rye bread In comparison to white bread the crust only contains small amounts of 2-acetyl-1-pyrroline, but instead more 3-methylbutanal, methional, 2,3-butanedi-one, 2-ethyl-3,5-dimethylpyrazine (see pyrazines), Fu-raneol , phenylacetaldehyde and 4-vinylguaiacol. Ut. l. Agric. Food Chem 44,1515 (1996) Maarse, p. 41 -77 Ohloff. 

Table 15.62. Concentrations of odorants in the crusts of white bread and rye bread ...

Additional odorous components, such as 3-ethyl-2,5-dimethylpyrazine, biacetyl, 3-methylbutanal, 2-phenylethanal and others are found in dark rye bread. Toasted bread contains the same compounds that carry the aroma of bread crust and crumb. A key component is again 2-acetyl-l-pyrroHne, other major components are methional, 4-hydroxy-2,5-dimethyl-2H-furan-3-one (furaneol), ( )-non-2-enal and biacetyl, the odour of which resembles butter. 

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