Odor pyrroline,2-acetyl

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). 

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. 

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). 

PGA-mediated deprotections of l-[iV-(phenylacetyl)amino]-4,5-dioxohexane and 1-[A -(phenylacetyl)amino]-4,5-dioxoheptane were used as the key steps in the synthesis (Fig. 5) of the strong natural roast odorants 2-acetyl-1-pyrroline and 2-propionyl-l-pyrroline [13]. 

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. 

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. 

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. 

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. 

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). 

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]. 

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]. 

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). 

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)...

Fig. 5.8. FD chromatogram of the volatile fraction of French fries. Ordinate n, number of 1 + 1 dilutions. Abscissa retention index (RI) on the capillary SE-54. The following odorants were identified 1 methional, 2 2-acetyl-1-pyrroline, 3 dimethyltrisulfide, 4 l-octen-3-one, 5 phenylacetaldehyde, 6 2-ethyl-3,6-dimethyl-pyrazine, 7 2-ethyl-3,5-dimethylpyrazine, 8 nonanal, 9 (Z)-2-nonenal, 10 2,3-diethyl-5-methylpyrazine, 11 (E)-2-nonenal, 12 2-ethenyl-3-ethyl-5-methylpyrazine, 13 2-isobutyl-3-methoxypyrazine, 14 dimethyltetrasulfide, 15 (E,E)-2,4-nonadienal, 16 (Z)-2-decenal, 17 (E,Z)-2,4-decadienal, 75 (E,E)-2,4-decadienal, 79trans-4,5-epoxy-(E)-2-decenal (according to Wagner and Grosch, 1997)...

Acetyl-1-pyrroline evaporates uniformly (Table 15.58) the intensity of the roasty note is correspondingly stable during storage for 4 hours (Table 15.56). The losses of this odorant in the white bread cmst on longer storage is shown in Table 15.59. 

Yajima identified a-pyrrolidone as a key odorant in Katorimai (scented rice—O. sative japonica) and noted the presence of indole (14). Buttery listed 64 volatile compounds known in rice and identified seven compounds with low odor thresholds octanal, nonanal, (E)-2-nonenal, decanal, (E)-2-decenal, (E,E)-2,4-decadienal, and 2-acetyl-1-pyrroline (2-AP) (8). The latter compound, 2-AP, is the major odorant contributor of scented or popcorn rice (15,16). Although present in most rice at the 1-10 ppb level, in scented rice it can be found at concentration levels in excess of 2 ppm (17). Widjaja, in a comparative study of non-fragrant and fragrant rice, identified (E)-2-decenal, (E,E)-2,4-nonadienal, and (E,E)-2,4-decadienal as having a waxy aroma (13). These three lipid oxidation products are also found in glutinous or waxy rice and contribute to its distinctive odor (18). 

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. 

Figure 10 Chemical structures of the odor-active components identified in the head-space of fresh white bread crust 2-methylpropanal (no. 1), diacetyl (no. 2), 3-methylhuta-nal (no. 4), 2-acetyl-l-pyrroline (no. 11), l-octene-3-one (no. 12), 2-ethyl-3,5-dimethyl-pyrazine (no. 13), and ( )-2-nonenal (no. 15). The numbering corresponds to that in Figure 9.

L. M. Seitz, R. L. Wright, R. D. Waniska, and L. W. Rooney, Contribution of 2-acetyl-l-pyrroline to odors from wetted ground pearl millet, J. Agric. Food Chem. 41 955 (1993). 

The principal impact aroma compounds of freshly prepared popcorn were determined by Schieberle as 2-acetyltetrahydropyridine, 2-acetyl-1-pyrroline, and 2-propionyl-1-pyrroline (49). The crackerlike aroma of the tetrahydropyri-dine, which exists in two tautomeric forms (2-acetyl-1,4,5,6- and 2-acetyl-3,4,5,6-tetrahydropyridine) was previously identified as the character compound of Saltine crackers (50). The decrease of these compounds during storage was directly correlated with staling flavor. Another popcornlike odor note, 2-acetyl-... 

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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