White Bread Crust

The substances which produce the aroma profile of a loaf of French bread (baguette) (Table 15.56) originate from the crust. They are listed in Table 15.57. 2-Acetyl-l-pyrroline 

The aroma of a baguette is not stable. Even four hours after the bread has left the oven, the intensities of the malty and sweet notes in the 

Bread Water Protein Available Dietary Lipid Minerals 

The intensity of each aroma quality was evaluated from 0-3 (strong). Average values of five testers. 

2-Acetyl-1-pyrroline evaporates uniformly (Table 15.58) the intensity of the roasty note is correspondingly stable during storage for 4 hours (Table 15.56). The losses of this odorant in the white bread cmst on longer storage is shown in Table 15.59. 

---

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

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. 

While several reviews on the role of ingredients are available, none of these describe the flavor chemistry of cracker sponge and dough (2, 3, 4, 5). However, some insight into the chemistry of cracker aroma can be obtained from examination of products that have a "cracker-like aroma" such as white bread crust. 

White bread crust Aroma staling Loss of 2-acetyl-1-pyrroline [32]... 

Ci Coch, 2-acetyl-3,4-dihydro-5//-pyrrole [85213-22-5] characteristic odor of white bread crust... 

It had already been found in boiled pork liver flavor (Mussinan and Walradt, 1974) and in white bread of which it constitutes, according to Mulders et al. (1976) the characteristic component, with a threshold concentration in aqueous solution of only 0.04ppb. Indeed, its odor has been described has having a fresh white bread crust (Mulders et al., 1976 Ohloff and Flament, 1978, 1979), but with an alliaceous, burnt flavor (Chemisis, 1968). 

Sizer C.E., Maga J.A. and Lorenz K. (1975) The occurrence of pyrazines in white bread crust and crumb. Lebensm. Hiss. Technol. 8, 267-9,... 

Benzo[fo]thiophenes have been found in unexpected places in cooked potato peel,23 as volatile constituents of white bread crust,24 in off-flavored unblanched green peas,25 in eels which have been exposed to seawater containing crude oil,26 and as components of automobile exhaust soot.27... 

Acetyl-1 -pyrroUne Roasted White-bread crust... 

Fig. 5.4. Headspace analysis of aroma substances of white-bread crust, a Capillary gas chromatogram (the arrows mark the positions of the odorants), b FD chromatogram. Odorants 1 methylpropanal, 2 diacetyl, 3 3-methylbutanal, 4 2,3-pentanedione, 5 butyric acid, 6 2-acetyl-1-pyrroline, 7 l-octen-3-one, 8 2-ethyl-3,5-dimethylpyrazine, 9 (E)-2-nonenal (according to Schieberle and Grosch, 1992)...

Acetyl-1-pyrroline (APy) 0 0.1 White-bread crust, rice, cooked meat, popcorn... 

Acetyltetra- hydropyridine (ATPy) Or X- 0 White-bread crust, popcorn... 

As mentioned in Section II, chromatograms obtained by FID detection and olfactory response are different. Aroma-active compounds usually do not correspond to the major volatile components in the food. As shown in Fig. 9, many important odorants of white bread crust were not visible in the gas chromatogram, for example, 2-acetyl-1-pyrroline (no. 11) (20). This can be explained by the low odor threshold of these compounds. Identification of such minor components (Fig. 10) is a challenging task. 

Figure 10 Chemical structures of the odor-active components identified in the head-space of fresh white bread crust 2-methylpropanal (no. 1), diacetyl (no. 2), 3-methylhuta-nal (no. 4), 2-acetyl-l-pyrroline (no. 11), l-octene-3-one (no. 12), 2-ethyl-3,5-dimethyl-pyrazine (no. 13), and ( )-2-nonenal (no. 15). The numbering corresponds to that in Figure 9.

Schieberle, P. and Grosch, W. 1992. Changes in the concentration of potent crust odorants during storage of white bread. Flavour Fragrance J. 7 213-218. 

In addition to white bread, studies have been done on some exotic breads, such as balady bread, widely consumed in Egypt and other Mid-East countries, by El-Samahy and Tsen (16) and some popular foods such as pizza in the U.S., Italy, and many other countries by Tsen, et a]. (16). Balady bread and pizza crust, like white bread, are prepared from fermented dough, but they are baked at a much higher temperature for a shorter period. 

Since in particular the cracker-like crust odor note was lacking in the synthetic mixture, it was concluded that the character impact compound for this odor note occurs in a concentration in the bread too low to be detected by headspace analysis. Later on the basis of its aroma quality and its very low odor threshold of 0.04 ppb (water), Mulders et al. (19) proposed 2-[(methyldithio)methyl] furan (4 in Figure 1) as the compound which should be responsible for the "golden brown" crust aroma of white bread. 

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). 

Bread flavor. White bread During the baking process the impact substances 2-acetyl-1 -pyrroline and 2-ace-tyl-A (A )-tetrahydropyridine form in the crust, on storage their concentrations decrease rapidly. Precursors are metabolites of yeasL Also of importance are ( )-2-nonenal (see alkenals), 3-methylbutanal (see al-kanals), 2,3-butanedione, and methional. In bread crumbs, degradation products of linolic acid such as ( )- and (Z)-2-nonenaJ and ( , )- 2,4-decadienal predominate. 

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. 

Data are shown in Table 15.45 for white bread prepared with and without papain. There is a rise in the content of both free amino acids in the crumb and volatile carbonyl compounds in the crust when proteinase is used. As long as proteinases are active in a baking process, they release amino acids from flour proteins, which are then changed via Strecker degradation... 

Table 15.59. Decrease (%) in 2-acetyl-l-pyrroUne in the crust of white bread during storage...

The crumb contains the precursors of the roasty odorants 2-acetyl-l-pyrroUne and 2-acetyltetrahydropyridine (cf. 5.3.1.6), but the temperature in the baking process is sufficient to form these substances only in the crust. If white bread is toasted, the two odorants are formed with... 

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

Good bread should have a pleasant odour and a crust which is brownish, shining, uniform and adherent to the crumb. The latter should be more or less white (according to the extent to which the offals are separated), soft, elastic, porous, homogeneous, free from spots, and of pleasant, non-add taste. 

Cornflakes or corn cereal Bread, white, w/o crust Margarine... 

---