Acetyl- 1-pyrroline

Table 5.1 Formation of 2-acetyl-2-pyrroline (ACPY) and 2-acetyltetrahydropyri-dine (ACTPY) in model systems221...

Scheme 5.4 Formation of 2-acetyl-2-pyrroline from l-pyrroline and 2-oxopropanal (after... 

Table 5.2 Determination of 2-acetyltetrahydropyridine (ACTPY) and 2-acetyl-2-pyrroline (ACPY) in low-molecular-mass, water-soluble fractions of maize treated in different ways221...

Romanczyk, L.J. et al., Formation of 2-acetyl pyrroline by several Bacillus cereus strains isolated from cocoa fermentation boxes, J. Agric. Food Chem., 43, 469, 1995. 

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). 

Pyridyl)hydrazine (Aldrich), 4-acetylpyridine (Acros), N,N,N -trimethylethylenediamine (Aldrich), methylrhenium trioxide (Aldrich), InQj (Aldrich), Cu(N0j)2-3H20 (Merck), Ni(N03)2-6Il20 (Merck), Yb(OTf)3(Fluka), Sc(OTf)3 (Fluka), 2-(aminomethyl)pyridine (Acros), benzylideneacetone (Aldrich), and chalcone (Aldrich) were of the highest purity available. Borane dimethyl sulfide (2M solution in THE) was obtained from Aldrich. Methyl vinyl ketone was distilled prior to use. Cyclopentadiene was prepared from its dimer immediately before use. (R)-l-acetyl-5-isopropoxy-3-pyrrolin-2-one (4.15) has been kindly provided by Prof H. Hiemstra (University of Amsterdam). 

For 3-acetyl-5-benzylpyrrolin-2-one, the keto-enol tautomer 109 predominates according to spectral data (UV, IR, and NMR) (80HCA121) (for further work on pyrrolin-2-ones see 77JHC681 95MI1). 

C27H3oNaOi3 l-Acetyl-3-benzamido-4-(2,3,4,6-tetra-0-acetyl-/3-D-glu-copyranosyloxy)-3-pyrrolin-2-one (ABGPON)123... 

Tanchotikul, U. Hsieh, T.C.Y. An Improved Method for Quantification of 2-Acetyl-l-Pyrroline, a "Popcom"-Like Aroma, in Aromatic Rice by High-Resolution GC-MS/SIM. J. Agric. Food Chem. 1991,59,944-947. 

The attachment of the pyrrole ring at C-8 was confirmed by noting that the shift of the aromatic H in the H-NMR spectrum moved from 8 5.93 to 6.38 in the acetylated product. This showed that the free proton is ortho to the phenolic OH at C-5 and that the pyrrole must therefore be at C-8. Further proof of the structure of vochysine was obtained when it was synthesized from 5,4 -di-hydroxy-7-methoxyflavan and an alcoholic solution of pyrroline. 

Tanchotikul, U., and T. C. Y. Hsieh. An improved method for quantification of 2 acetyl l pyrroline, a pop-com like aroma, in aromatic rice by high-resolution gas chromatography/ mass spectrometry/selected ion monitoring. J Agr Food Chem 1991 39(5) 944-947. 

Proline and hydroxyproline differ from the other amino acids in that they contain a secondary amino group in a pyrrolidine ring therefore, they do not produce aminoketones and Strecker aldehydes in the reaction with dicarbonyls, ffowever, nitrogen heterocyclics are produced, including 1-pyrroline, pyrrolidine, 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5) [19]. 

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. 

Anodic oxidation of l,3-diaryl-5-methyl-A2-pyrazoline-5-carboxylic acids in CH3CN-Et4NBF4 proceeded with decarboxylation to the aromatized pyrazoles in high yield.414 Similarly, electrochemical oxidation of N-acetyl-2,3-substituted A4-pyrroline-2-carboxylic acids in water-tetrahydrofuran (3 1) containing KOH forms the corresponding pyrroles (80-98%).415... 

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. 

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

Thermoplastic Polyolefin (TPO) One batch of a thermoplastic polyolefin had a roasty off-odor. The important odorants 2,3-butandione, l-hexen-3-one, methional (3-methylthiopropanal), Z-2-nonenal, E-2-nonenal, l-octen-3-one, octanal, E,E-2,4-nonadienal, E,E-2,4-decadienal, and as the most important off-odorant 2-acetyl-l-pyrroline, could be identified (Mayer and Breuer, 2004b, 2006). 

Odor analysis (see Chapter 8) performed using GC coupled with olfactometry has also shown that many food items and household materials are odorant sources (Mayer and Breuer, 2006). Thus, mono-unsaturated aldehydes particularly E-2-nonenal are found in fat, wax, oil finish and lubricants branched aldehydes such as 3-methyl butanal are found in varnish, bread and malt while leather, rice and popcorn are sources of substituted pyrrolines especially 2-acetyl-l-pyrroline. Studies like this are important not only from the point of view of identifying sources of indoor odorants but also from the point view of providing vital information that can help consumers to select products. 

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. 

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). 

In the case of 2-acetyl-l-pyrroline, a quantitative determination established the difference between the crust and the crumb which was found by the aroma extract dilution analyses only 2.5 ug/kg 2-acetyl-l-pyrroline were present in the crumb compared to 78 ug/kg in the crust (Schieberle, P. Grosch, W. in preparation). 

Formation of 2-Acetyl-l-pyrroline and Other Important Flavor Compounds in Wheat Bread... 

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. 

Values were determined by mass chromatography (MS/CI). m/z 112 is the M++l-ion of unlabeled 2-acetyl-l-pyrroline. 

Table III. Formation of 2-Acetyl-l-Pyrroline from Ground Baker s Yeast Cells and Sugar Mixtures...

---