Wheat bread crust, formation

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 Formation of Flavor Compounds in Wheat Bread Crust... 

To gain an insight into the origin of the Acp in wheat bread crust, two different experiments were carried out. First, the formation of crust odorants from yeast and from chemically leavened doughs was compared and, secondly, model experiments on the formation of the Acp were undertaken. 

Furthermore, the data presented indicate that a thermally induced rupture of the yeast cells during crust formation is an important step to liberate precursors of the Acp or to liberate the Acp already formed in the interior of the yeast. This would also provide an explanation for the fact that much higher amounts of Acp are present in wheat bread crust compared to the crumb (see chapter by Schieberle and Grosch, this book). 

Schieberle, P. (1990) Occurrence and formation of roast-smeUing odorants in wheat bread crust and popcorn. In Bressiere, Y., Thomas, A.F. (eds.) Flavour Science and Technology. John Wiley, Chichester, pp. 105-108... 

Hie radical cation CROSSPY was identified as a previously unknown type of cross-linking amino acid involved in melanoidin formation during roasting of coffee beans. Also for other browned foods such as wheat bread crust or roasted meat a tight relationship between the formation of CROSSPY and browning development could be observed (Hofinann, unpublished results), thus. 

The flavor compounds of the crust from the chemically leavened model bread were then compared to those recently identified (6) in the crust of a standard wheat bread which was leavened by addition of yeast (Table I). One striking difference was that Acp (No. 16), which showed the highest FD-factor in the yeast-leavened bread showed a very low FD-factor in the chemically leavened bread. This indicated, that the flour contained only minor amounts of the precursor (s) for the formation of Acp. On the other hand, 2(E),4(E)-decadienal, 2(E),4(E)-nonadienal, l-octen-3-one and 2(Z)-nonenal, which are undoubtedly formed by a heat-induced oxidative degradation of the flour lipids, became predominant odorants in the chemically leavened compared to the yeast-leavened bread. 

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