2-Acetyl-1 -pyrrolin

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. 

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]... 

With regard to a glucose-proline system, Roberts and Acree277 have examined the sensory aspects in much more detail by applying Charm analysis (see the Olfactory Threshold section above). Four compounds provided most of the aroma 2-acetyl-3,4,5,6-tetrahydro-l//-pyridine (burnt, caramel 63%), 2-acetyl-1-pyrroline (popcorn 19%), 2-acetyl-l,4,5,6-tetrahydro-l//-pyridine (burnt, caramel 12%), and UDMF (cotton candy 4%). All Maillard systems of interest need to be submitted to similarly detailed analyses. 

P. Schieberle, The role of free amino acids present in yeast as precursors of the odorants 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine in wheat bread crust, Z. Lebensm. Unters. Forsch., 1990, 191, 206-209. 

R. G. Buttery, L. C. Ling, and B. O. Juliano, 2-Acetyl-1-pyrroline an important aroma component of cooked rice, Chem. Ind., 1982, 958-959. 

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). 

Rice (cooked) 2-Acetyl-1-pyrroline (119), ( , )-2,4-decadienal, nonanal, hexanal, octanal, ( )-2-nonenal 142... 

Maize (cooked) Dimethyl sulfide, dimethyl trisulfide (122), acetaldehyde, 4-vinylguaiacol (123), 2-acetyl-1 -pyrroline 145... 

Tomato (fresh) Solanum 2-acetyl-1 -pyrroline, dimethyl trisulfide, 2,3-diethyl-5-methylpyrazine (Z)-3-Hexenal, hexenal, 1 -octen-3-one, methional 131,169... 

Lobster Trimethylamine, 2,3-butanedione, (Z)-4-heptenal, 1-octen-3-one, 2-acetyl-1-pyrroline, 188... 

Acetyl-1 -pyrroline Oryia sativa (aromatic rice, cooked rice) OD-R (pop-corn) main... 

Researchers at the Australian Wine Research Institute (83) have since identified two additional compounds associated with mousiness and produced by both Brettanomyces and Lactobacillus species 2-acetyl-1-pyrroline (ACPY) and... 

Hofmann, X, Schieberle, P. 2-Oxopropanal, hydroxy-2-propanone, and 1-pyrroline important intermediates in the generation of the roast-smelhng food flavor compounds 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine. J. Agric. Food Chem. 1998, 46, 2270-2277. 

In the case of AEDA, which is mostly applied 7777, the result is expressed as flavour dilution (ED) factor. The ED factor is the ratio of the concentration of the odorant in the initial extract in which the odour is still detectable by GCO [14, 15[. Consequently, the FD factor is a relative measure and is proportional to the OAV of the compound in air. As an example, the analysis of the aroma of the baguette cmst [16] will be discussed. After separation of the acidic fraction, the neutral/basic volatiles were investigated by AEDA. Results listed in Table 6.24 reveal 21 odorants in the FD factor range 32-512, of which 2-acetyl-1-pyrroline (no. 1), 2-ethyl-3,5-dimethylpyra-zine (no. 10) and (E)-2-nonenal (no. 17) showed the highest FD factors. 

To approach the situation in food, OAVs are calculated on the basis of odour threshold values which have been estimated in a medium that predominates in the food, e.g. water, oil, starch. As an example the OAVs of the key odorants of baguette crust are listed in Table 6.26. The highest OAVs were found for the roasty smelling 2-acetyl-1-pyrroline (no. 7), followed by furaneol (no. 20), 2,3-butanedione (no. 2), (E)-2-nonenal (no. 13), l-octen-3-one (no. 9) and methional (no. 6). It is assumed that these odorants contribute strongly to the aroma of baguette crust. 

The compounds listed in Table 6.50 cause the typical smell of popcorn. As a result of its very high OAV, acetyltetrahydropyridine (no. 1) is the character impact odorant followed by 2-acetyl-1-pyrroline (no. 2). The reverse was found for the roasty note of baguette crust [95], The causes for this difference have been clarified by model studies [96]. 

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

Costello, P.J. and Henschke, P.A. 2002. Mousy off-flavor of wine Precursors and biosynthesis of the causative N-heterocycles 2-ethyltetrahydropyridine, 2-acetyltetrahydropyridine, and 2-acetyl-1-pyrroline by Lactobacillus hilgardii DSM 20176. J. Agric. Food Chem. 50, 7079-7087. 

Analytical-grade reference compounds and solvents were obtained commercially (Aldrich Chemical Co., St. Louis, MO), except ( )-P-Damascenone was provided by Firmenich Co. (Princeton, NJ) and 2-acetyl-1-pyrroline was a gift from Dr. R. Buttery (USDA, ARS, WRRC, Albany, CA). Dimethyltetrasulfide was synthesized using a published procedure (72). Reagents used for amino acid analyses were obtained from Beckman Coulter, Inc. (Fullerton, CA). 

Hydroxy-4,5-dimethyl-2(5H)-fiiranone (Sotolon), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (Furaneol) and 2-hydroxy-3-methyl-2-cyclopenten-l-one (Cyclotene) were from Aldrich (Steinheim, Germany). 2-Acetyltetrahydropyridine and 2-acetyl-1-pyrroline were synthesized as previously described (8). 2-Hydroxy-3,4-dimethyl-2-cyclopenten-l-one was a gift from Dr. I. Blank (Nestle, Vers-chez-les-blanc, Switzerland). 

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