Aroma acetyl-pyrroline

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). 

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. 

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. 

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. 

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). 

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. 

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). 

Proline is a particular case in the Maillard reaction because it is, unlike other amino acids, a secondary amine. It represents an important precursor for popcom-like aroma compounds in the reaction with reducing sugars such as glucose [46]. The relevant aroma compounds in this reaction are 2-acetyl-l-pyrroline (22), l-(l,4,5,6-tetrahydro-2-pyridyl)-l-ethanone (23) and its isomer l-(3,4,5,6-tetrahydro-2-pyridyl)-l-ethanone (24) [47], The formation pathway (Fig. 3.25) shows the important intermediates 2-oxopropanal (7) and 1-pyrroline (25) [48, 49[. Another amino acid precursor for 22 is ornithine, an amino acid in yeast or yeast extracts [49[. 

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. 

On the basis of high OAV, 2-acetyl-l-pyrroline (no. 1), 2-furfurylthiol (no. 2), 2-phenylethylthiol (no. 3) and furaneol (no. 4) are the most important contributors to the overall roasty, caramel-like aroma of the moderately roasted sesame. The two thiols nos. 2 and 3, but not the unstable 2-acetyl-l-pyrroline (no. 1), were also identified as key odorants of white and black sesame seeds which had been longer roasted and which elicited intense burnt or even rubbery odour notes ]97, 98]. 

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

Acetyl-l-pyrroline was reported to be the major contributor to mousy off-flavor (Herderich, et al., 1995), with an aroma impact of one order of magnitude greater than ATHP (Buttery et al., 1982), but it is a relatively unstable compound and was found in wine in trace quantities up to 7.8pg/L (Grbin et al., 1996). At the pH of wine these compounds are not volatile and as a consequence they have a low sensory impact. However, when mixed with the neutral pH of saliva they can become very apparent on the palate as mouse cage or mouse urine (Snowdon et al.,2006). 

BUTTERY, R.G., LING, L.C., JULIANO, B.O., Cooked rice aroma and 2-acetyl-1-pyrroline,/. Agric. Food. Chem., 1993,31, 823-826. 

Sesame seed (roasted) flavor. According to aroma extract dilution analysis, 2-furylmethanethiol, guaiacol, 2-phenylethanethiol (CgHn is, Mr 138.23), and Furaneol have the highest dilution factors in the flavor of roasted sesame seeds, besides 4-vinyl-guaiacol, 2-pentylpyridine (C,oH,5N, Mr 149.24), 2-acetyl-l-pyrroline, 2-acetylpyrazine, and 2-ethyl-3,5-dimethylpyrazine (see pyrazines). 

A -heterocyclic compounds other than pyrazines such as pyrrolines, pyrrrolidines, piperidines and pyrroles are also very important flavor compounds. The formation of pyrrolines and pyrrolidines are reported to be generated from the reaction of proline with glucose (Shigematsu et aL, 1975 Tressl et aL, 1985a). The pyrrolidines possess smoky and roasty aromas while 2-acetyl-1-pyrroline was reported by Tressl et al. (1985b) to have a cracker-like odor. The pyrrole rings from proline and hydroxyproline are present in many of their reaction products. N-acetylpyrrole exhibits a cookie-like and mushroom-like odor (Tressl et ai, 1986). 

Chan and Reineccius [35] also have published some kinetic work on other aroma compounds (isovaleraldehyde, phenylacetaldehyde, 2-acetyl-l-pyrroline, 2-acetyl-furan, and di(H)di(OH)-6-methyl pyranone). All of these volatiles followed pseudo zero order reaction kinetics in their early stages of reaction (Table 5.2). The fact that the concentrations of isovaleraldehyde, phenylacetaldehyde, and the methylpyr-anone reached plateaus late in heating suggests that a first order fit, as proposed by Jusino et al. [32], might be more appropriate. 

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)...

In the AEDA of UHT milk (Table 10.38), 5-decalactone, which contributes to the aroma of butter (Table 10.40) as well as unripened and ripened cheese (cf. 10.3.5), is the predominant aroma substance. Apart from other lactones, 2-acetyl-l-pyrroline, methional, 2-acetyl-2-thio-azoline and 4,5-epoxy-2-decenal are among the identified aroma substances. 

Unlike in Europe and USA, some rice varieties popular in Asia develop a popcom-like aroma on cooking. This is due to the formation of 2-acetyl-1-pyrroline, which is present in concentrations of 550-750 pg/kg in aromatic varieties of rice (cooked) and <8 pg/kg in lowaroma varieties. 

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). 

The compounds (Z)-3-hexenal, methanethiol, (Z)-l,5-octadien-3-one, dimethyltrisulfide, 3-iso-propyl-2-methoxypyrazine and 3-5 c-butyl-2-methoxypyrazine contribute to the aroma of the fresh vegetable. In cooked spinach, (Z)-3-hexenal decreases and dimethylsulfide, methanethiol, methional and 2-acetyl-1-pyrroline are dominant. 

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