Characterization of the Key Aroma Compounds in the Crust of Soft Pretzels by Application of the Sensomics Concept.

Soft pretzels show a uniform brown crust color and elicit a characteristic aroma that is clearly different from those of other small types of bread. Data on the odorants responsible for this unique aroma are scarcely available. Application of aroma extract dilution analysis (AEDA) on an extract (distillate) obtained from the crust of freshly baked soft pretzels followed by identification experiments revealed 4-hydroxy-2,5-dimethyl-3(2 H)-furanone (4-HDF, caramel-like) and 2-acetyl-1-pyrroline (2-ACPY; roasty, popcorn-like), which had the highest flavor dilution (FD) factors among the 28 odor-active compounds identified. Quantitation of all 28 aroma compounds by stable isotope dilution assays (SIDA) and calculation of odor activity values (OAV) confirmed 2-ACPY, 4-HDF, and phenylacetic acid as key contributors to the pretzel aroma profile. Compared with other pastry crusts, in particular, the low odor activities of the Strecker aldehydes 2- and 3-methylbutanal, the lipid degradation product ( E)-2-nonenal, and the lack of pyrazines were elucidated as the main reasons for the different aroma profile of pretzel crust. An aroma recombinate, which is among the first established for the crusts of breads and pastries, clearly mimicked the overall odor of the pretzel crust, in particular when a solution in ethanol was sprayed in the ambient air.

Screening for Novel Mercaptans in 26 Fruits and 20 Wines Using a Thiol-Selective Isolation Procedure in Combination with Three Detection Methods.

A selection of 46 fruits and wines were screened for mercaptans using a thiol-selective isolation procedure. The identification was based on three methods: gas chromatography-olfactometry (GC-O), gas chromatography-sulfur chemiluminescence detection (GC-SCD), and two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOF/MS). A database constructed from analytical and sensory data for more than 300 sulfur compounds was used to confirm the identifications. Overall, 11 mercaptans were characterized in the fruits or wines for the first time. In addition, 1- p-menthene-9-thiol identified in kumquat is the first report in any food thus far. Thus, besides 1- p-menthene-8-thiol, p-menthane-8-thiole-3-one, and 3-mercapto-3,7-dimethyl-6-octenyl acetate, a fourth naturally occurring mercapto terpenoid occurs in fruits.

Structure/Odor Activity Studies on Aromatic Mercaptans and Their Cyclohexane Analogues Synthesized by Changing the Structural Motifs of Naturally Occurring Phenyl Alkanethiols.

Following a structure/odor activity approach as previously published, the present study was focused on three aromatic thiols also identified as food odorants, namely 1-phenylethane-1-thiol, phenyl methanethiol, and 2-phenylethanethiol. Their structures were systematically modified to receive 16 new sulfur-containing benzene derivatives. A determination of odor thresholds indicated that none of its homologues elicited a lower odor threshold than 1-phenylethane-1-thiol, and an enantiospecific synthesis, elucidated that its ( S)-enantiomer turned out to be the compound with by far the lowest odor threshold of 0.00025 ng/L in air. Within the homologous series of the ω-phenylalkane-1-thiols as well as the 1-phenylalkane-1-thiols the threshold values increased constantly with an elongation of the side chain. Among the respective cyclohexane derivatives, the tendencies with respect to thresholds and odor properties were comparable. The odor thresholds and odor qualities of the aromatic thiols were quite similar to those of their heterocyclic analogues considered in a previous publication. In addition, spectroscopic data for 28 new sulfur-containing compounds were generated, which might be helpful in the identification of such sulfur containing odorants occurring in trace levels in foods.

Structure-Odor Correlations in Homologous Series of Mercapto Furans and Mercapto Thiophenes Synthesized by Changing the Structural Motifs of the Key Coffee Odorant Furan-2-ylmethanethiol.

Furan-2-ylmethanethiol (2-furfurylthiol; 2-FFT, 1) is long-known as a key odorant in roast and ground coffee and was also previously identified in a wide range of thermally treated foods such as meat, bread, and roasted sesame seeds. Its unique coffee-like odor quality elicited at very low concentrations, and the fact that only a very few compounds showing a similar structure have previously been described in foods make 1 a suitable candidate for structure-odor activity studies. To gain insight into the structural features needed to evoke a coffee-like odor at low concentrations, 46 heterocyclic mercaptans and thio ethers were synthesized, 32 of them for the first time, and their odor qualities and odor thresholds were determined. A movement of the mercapto group to the 3-position kept the coffee-like aroma but led to an increase in odor threshold. A separation of the thiol group from the furan ring by an elongation of the carbon side chain caused a loss of the coffee-like odor and also led to an increase in odor thresholds, especially for ω-(furan-2-yl)alkane-1-thiols with six or seven carbon atoms in the side chain. A displacement of the furan ring by a thiophene ring had no significant influence on the odor properties of most of the compounds studied, but the newly synthesized longer-chain 1-(furan-2-yl)- and 1-(thiophene-2-yl)alkane-1-thiols elicited interesting passion fruit-like scents. In total, only 4 out of the 46 compounds also showed a coffee-like odor quality like 1, but none showed a lower odor threshold. Besides the odor attributes, also retention indices, mass spectra, and NMR data of the synthesized compounds were elaborated, which are helpful in possible future identification of these compounds in trace levels in foods or other materials.

Structure-Odor Activity Studies on Monoterpenoid Mercaptans Synthesized by Changing the Structural Motifs of the Key Food Odorant 1-p-Menthene-8-thiol.

1-p-Menthene-8-thiol (1) has been discovered as the key odorant in grapefruit juice several decades ago and contributes to the overall odor of the fruit with an extremely low odor threshold of 0.000034 ng/L in air. This value is among the lowest odor thresholds ever reported for a food odorant. To check whether modifications in the structure of 1 would lead to changes in odor threshold and odor quality, 34 mercapto-containing p-menthane and 1-p-menthene derivatives as well as several aromatic and open-chain mercapto monoterpenoids were synthesized. Eighteen of them are reported for the first time in the literature, and their odor thresholds and odor qualities as well as analytical data are supplied. A comparison of the sensory data with those of 1 showed that hydrogenation of the double bond led to a clear increase in the odor threshold. Furthermore, moving the mercapto group into the ring always resulted in higher odor thresholds compared to thiols with a mercapto group in the side chains. Although all tertiary thiols always exhibited low odor thresholds, none of the 31 compounds reached the extremely low threshold of 1. Also, none of the synthesized mercapto monoterpenoids showed a similar odor quality resembling grapefruit. Although the saturated and aromatic analogues exhibited similar scents as 1, the aromas of the majority of the other compounds were described as sulfury, rubber-like, burned, soapy, or even mushroom-like. NMR and MS data as well as retention indices of the 23 newly reported sulfur-containing compounds might aid in future research to identify terpene-derived mercaptans possibly present in trace levels in foods.