Acetyl-l-pyrroline

Biosynthesis of Heterocycles From Isolation to Gene Cluster, First Edition. Patrizia Diana and Girolamo Cirrincione. 

FIVE-MEMBERED HETEROCYCLIC RINGS AND THEIR FUSED DERIVATIVES 

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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. 

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. 

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...

The results reveal that baker s yeast is a potent source for precursors of 2-acetyl-l-pyrroline. It appears likely that the flavor compound is formed in the yeast cells from proline and dihydroxyacetone phosphate via 1-pyrroline and pyruvaldehyde. This is corroborated by the results of c-labeling experiments which showed that the acetyl group in the Acp stems from a sugar degradation product and that the pyrroline ring was derived from proline. 

Scheme 5.5 Formation of 50% labelled 2-acetyl-l-pyrroline from [1 -13C]glucose and proline via -deoxy-2,2-diketose and dihydrodiacetylformoin223...

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[. 

Fig. 3.25 Formation of 2-acetyl-l-pyrroline and 2-acetyltetrahydropyridine from proline and 1-deoxyglucosone (adapted from [59])...

In the example of baguette crust (Table 6.25), analysis started with a headspace volume of 20 mL and 13 odorants were revealed by GCO. The headspace sample was then reduced in a series of steps to determine the most potent, highly volatile odorants. GCOH of volumes of 10 and 2.5 mL indicated only 10 and 7 odorants, respectively (Table 6.25). After reduction to 0.2 mL, only 2,3-butanedione was found. According to this experiment, 2,3-butanedione was the most potent highly volatile odorant of baguette cmst. A comparison of Tables 6.24 and 6.25 shows that some odorants were detected by both GCOH and AEDA (e.g. l-octen-3-one, dimethyltri-sulphide, 2-acetyl-l-pyrroline). 

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]. 

Figure 8.17. Chemical structures of A-heterocycles responsible for mousy off-flavor of wines (8) 2-acetyl-l-pyrroline (APY), (9) 2-acetyltetrahydropyridine (ATHP), (10) 2-ethyltetrahydropyridine (ETHP).

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., and Juilano, B.O. (1982). 2-Acetyl-l-pyrroline an important aroma component of cooked rice, Chem. Ind. (London), 958-959. 

Costello, P.J. and Henschke, PA. (2002). Mousy off-flavor of wine Precursors and biosynthesis of the causative X-h etc recycles 2-ethyltetrahydropyridine, 2-acetyltetrahydropyridine, and 2-acetyl-l-pyrroline by Lactobacillus hil-gardii DSM 20176, /. Agric Food Chem, 50(24), 7079-7087. 

FIGURE 17.15 Analogues of 2-acetyl-l-pyrroline and 2-acetyl-l-thiazoline. 

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