Acetyl bread aroma

In acid-based reactions 2-acetyl-3-hydroxyfuran (isomaltol) (5.76), 3-hydroxy-2-methylpyran-4-one (5.77), and maltol (5.79) are formed. They are responsible for the baked bread aroma. 

C6H9NO, Mr 111.14. Rusty liquid with popcom-like odor, bp. 26-28 °C (10 Pa) rapid darkening of color at 20 °C, limited storage life at-20°C, 1% in pentane olfactory threshold in water 0.1 ppb. TVpical bread and rice flavor. A. is formed from yeast constituents on heating L- ornithine and 2-oxopropanal. The bread aroma 2-acetyl-A A )-tetrahydropyridine [C7H,NO, Mr 125.17,bp. 33-34 °C(0.12 kPa), olfactory threshold 1.6 ppb] is formed analogously from l- proline and 2-oxopropanal. 

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. 

Maltol and isomaltol (1 and 2 in Figure 1) enhanced the fresh bread character of yeast rolls (6) when added at a level of 0.1 %. Other workers (7.) showed that 2-acetyl-l,4,5,6-tetrahydropyridine (3 in Figure 1) restores a cracker-like aroma to bread when added at a level of 6 mg/kg. In the pure state this chemical displays a cracker-like aroma and an odor threshold of 1.6 jig/kg (8). 

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. 

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). 

Rice is an important calorie source mainly in Asia and also throughout the world. Some rice cultivars are especially rich in 2-acetyl-1-pyrroline (119) content and are referred to as Kaori-mai (fragrant rice).142 Bread is also recognized to play a major role in the human diet. The aroma of bread differs substantially between the crust and the crumb, and both of them have been analyzed by AEDA method.143,144 Maize145 is respected as the staple food in Mexico and southern Africa and is also consumed in various forms throughout the world (for instance, popcorn) (Table 15). 

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

Although more than 280 compounds have been identified in the volatile fiction of wheat bread, only a small number is responsible for the flavor notes in the crust and the crumb. Schieberle and Grosch (73) used aroma extract dilution analysis (AEDA) to select 32 odorants in wheat. Among the odorants, 2-acetyl-pyrroline (roasly, bread crust-like) was the most potent aroma, followed by E-2-nonenal (green, tallowy), 3-methylbutanal (malty, nutty), diacetyl (buttery) and Z-2-nonenal (green, fiitty). 

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)...

This characteristic feature appears to be required for a roasted odor. In fact, aU the pyrroUnes and pyridines listed in Table 5.23 as weU as 2-acetyl-thiazole, 2-acetylthiazoline (cf. Table 5.22) and acetylpyrazine (cf. Table 5.23) contain this structural element and have a roasted or cracker-hke odor. However, the thresholds of these compounds vary greatly. The lowest values were found for 2-acetyl-and 2-propionyl-l-pyrrohne. The length of the alkanoyl group also influences the aroma activity because in the transition from 2-propionyl- to 2-hutanoyl-l-pynoline, the roasted note suddenly disappears and the odor threshold increases hy several powers of ten. 2-Acetyl-l-pyrrohne (Apy) is responsible for the typical aroma of the cmst of white bread and it... 

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... 

Bread quality changes rapidly during storage. Due to moisture adsorption, the crust loses its crispiness and glossyness. The aroma compounds of freshly baked bread evaporate or are entrapped preferentially by amylose helices which occur in the crumb. Repeated heating of aged bread releases these compounds. Very labile aroma compounds also contribute to the aroma of bread, e. g., 2-acetyl-l-pyrroline. They decrease rapidly on storage due to oxidation or other reactions (Table 15.59). 

A common basic amino acid is the lower homologue of lysine, L-ornithine (L-2,5-diaminovaleric acid, 2-41), which is an intermediate in the biosynthesis of arginine and an important amino acid of the ornithine (urea) cycle, which has the function of converting toxic ammonia into less toxic urea in mammals. Ornithine formed in dough through the action of yeast Sac-charomyces cerevisiae) is the main precursor of the typical aroma of bread crust, for which 2-acetyl-l-pyrroline together with 6-acetyl-l,2,3,4-tetrahydropyridine and its isomer 6-acetyl-2,3,4,5-tetrahydropyridine are responsible (see Section 8.2.12.4.1). 

Thiazole itself is smelling like pyridine, but substituted thiazoles, such as 2-acetylthiazole and 2,4-dimethyl-5-vinylthiazole, generally have a desirable odour, frequently described as nutty, green, roasted and vegetable-like. For example, the Strecker degradation of cysteine by methylglyoxal yields 2-acetylthiazole in thermally processed foods (Figure 8.83). A possible intermediate of this reaction is 2-acetyl-2-thiazoline with an intense aroma of fresh bread crust, which is similar to the odour of 2-acetyl-... 

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. 

The aroma of cooked rice Oryza saliva, Poaceae), especially of the aromatic basmati rice, is mainly influenced by the formation of 2-acetyl-1 -pyrroline which has the odour of bread crust and popcorn. Its concentration can reach approximately 0.6 mg/kg. Significant compounds are also aldehydes formed by oxidation of fatty acids, especially (2 ,4E)-deca-2,4-dienal, nonanal, hexanal, octanal and ( )-non-2-enal, and phenols guaiacol and 4-vinylphenol resulting... 

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. 

Two compounds that create the characteristic odor notes in the pleasant aroma of wheat bread crust have been identified as the popcomUke 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine (51). The aroma of the bread crumb portion is principally due to lipid-derived unsaturated aldehydes such as E)-2-nonenal and ( , )-2,4-decadienal, which create stale aromas at high levels. The malty notes that predominate in yeast and sourdough breads are attributed to 2-and 3-methylbutanal and Furaneol (51,55). 

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