Bread crust

Brot masse, /. breadstuff. raffinade, /. loaf sugar. rindei /. bread crust. teigt m. bread dough, -wurzel, /. cassava yam. zucker, m. loaf sugar. 

The characteristic aroma of wheat bread crust has been attributed to 2-acetyl-1-pyrroline, and its formation depends on the presence of bakers yeast [31]. In model systems it was demonstrated that the acetylpyrroline is formed from the reaction of proline with pyruvaldehyde or dihydroxyacetone. Other compounds with bread-like aromas formed in the reaction of proline with pyruvaldehyde include l-acetonyl-2-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5). These compounds are unstable, which explains why the characteristic aroma of freshly baked bread disappears quickly during storage. 

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. 

Gluten.—100 grams of the bread (crust and crumb in proper proportions) arc moistened with water and left for about half an hour to swell up thoroughly the moist dough is then placed on a very fine sieve and manipulated under a water jet in the way described for the separation of gluten from flour (q.v., section 7). 

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of this method on fresh and stored wheat bread crust [55] and on fresh rye bread crust [P. Schieberle and W. Grosch, unpublished results]. 

On the basis of high FD-factors (Table 3) the sensory significance of 3-methylbutanal and 2-acetyI-l-pyrroIine with malty, roasty odors previously identified as the key odorants in fresh wheat bread crust [21] was established. During storage for 4 days the FD-factors of both odorants decreased significantly, while especially butanoic acid (rancid) and (E)-2-nonenaI remained unchanged. The fatty, green note of the latter odorant especially contributes to the stale note detectable in the overall crust flavor of the stored wheat bread. 

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] ... 

The caramel-like smelling HDF has been established as a main contributor to the flavors of several processed foods (Table 17). In addition, it should be noted that in all these foods, on the basis of a high FD-factor, HDF was also by far the most important caramel-like smelling odorant. In the following, the strategy in the HDF precursor analysis will be shown using wheat bread crust, popcorn [88] and malt as the examples. Quantitative measurements were performed by using a stable isotope dilution assay (cf. Section 3.2.). 

Acetyl-1 -pyrroline Wheat bread crust Ornithine/2-Oxopropanal [104]... 

O Fresh white bread crust O Roasted bread crust O Freshly baked bread 0.04 ppb/water F 6, 7 35 36... 

Comparing the Nutritive Values Between Fermented Doughs Before and After Baking and Between Bread Crust and Crumb... 

Tsen, et 1. (14) have recently observed the deleterious effect of baking by feeding rat with diets prepared from fermented and proofed dough before and after baking and from bread crust and crumb. PERs (adjusted) were found to be 1.34... 

In another study by Tsen et al (14), a substantial PER difference was observed between diets with bread crust (0.36) and bread crumb (1.35). Palamidis and Markakis (12) also found that the PERs of their bread and its crumb were 0.46 and 0.91, respectively, while the crust showed a negative PER, -0.23. Hansen, et al.(18) reported that bread crumb and crust had respective PERs of"T.36 and 0.62. The difference in PERs for bread crumb or crust reported by the three groups of investigators is largely due to different raw materials and processing conditions for the breads. Nevertheless, the marked difference in PER between bread crumb and crust indicates clearly that the browning reaction can reduce the nutritive value of bread (Table IV). 

As shown in Table IX, the lysine availability (%) showed changes for the three samples. However, the unavailable lysine (total lysine minus available lysine) contents in bread (whole), bread crust and crumb were only 0.04, 0.05, 0.03%, respectively. Table 7 shows that the unavailable lysine contents for all pizza crusts, baked and unbaked, varied only from 0.02 to 0.03%. These data indicate the reduction of lysine caused by baking is mainly shown by the total lysine analysis. It appears then that there is no need to run available lysine determinations for such bakery foods. This finding also suggests that the nutritive loss of bread and pizza crusts was primarily due to the destruction of lysine in those products to a lesser extent baking caused it to become unavailable. 

Table XX. Total and available lysine in bread, bread crust, and bread crumb...

As would be expected from the results of studies on model systems, it has been found that foods with a high starch or sugar content may form genotoxic substances, but at a much lower level than meats or fish. Spingarn et al (65) showed that several common foods, in addition to beef, contained mutagens active for TA98 in the presence of S9 (Table V) Pariza et al. (66) found mutagenic activity in basic fractions of chicken broth, beef broth, rice cereal, bread crust, crackers, corn flakes, toast and cookies. 

Sizer at al. (20) observed that the compounds causing the pleasant odor which resembled bread crust occur in the basic volatile fraction of white bread. Identification experiments yielded the five pyrazines listed in Table II. A comparison of the odor threshold of each pyrazine in water to its concentration found in bread indicated that 2-ethyl-3-methylpyrazine and 2-methyl-6-propylpyrazine were present at concentrations above their odor thresholds. The authors described the odors of these two pyrazines as "butterscotch, nutty" and "burnt, butterscotch" notes (Table II). 

GC-effluent sniffing of wheat bread aroma concentrates has shown the presence of low level volatiles that smell like the fresh bread crust. As discussed in the preceeding sections, these compounds (3-7 in Figure 1) were proposed to be responsible for this odor note. 

The HRGC chromatogram of the neutral/basic volatiles obtained from wheat bread crust showed a region with an intense cracker-like. 

The low odor threshold (0.1 pg/kg water) of this compound and its odor description as "popcorn-like" (27) agrees with its strong crusty character. Furthermore, the statement of Buttery et al. (27) that "2-acetyl-l-pyrroline seems to be the most potent of the cracker-like group of odor compounds" (which includes 3, 5 and 7 in Figure 1) underlines its importance for the flavor of the white bread crust. 

In the case of wheat bread, 2-acetyl-l-pyrroline appeared with the highest FD-factor, followed by 2(E)-nonenal, 3-methylbutanal, diacetyl and 2(Z)-nonenal. These results confirm that the 2-acetyl-l-pyrroline is the "character impact compound of the wheat bread crust odor. 

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. 

A comparison of the most important aroma compounds present in the wheat bread crust (3 7) with those identified in the crumb (Table IV) revealed two striking differences 2-acetyl-l-pyrroline and 3-methylbutanal, which appeared as potent flavor compounds respectively responsible for the roasty and malty aroma note in wheat bread crust, showed low FD-factors in the wheat bread crumb and are not listed in Table IV. On the other hand, carbonyl compounds with fatty aroma notes like 2(E),4(E)-decadienal, 2(E)-nonenal and 2(Z)-nonenal predominated in the crumb (Table IV). 

We recently identified 2-acetyl-l-pyrroline (Acp) with a crackerlike odor as the most intense flavor compound of wheat bread crust (4). Tressl et al. (5) reported that small amounts of this compound were formed when model mixtures containing proline and monosaccharides were heated. 

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