Ph. D. Theses 1999
17.03.99, Thomas Jagella (Prof. Dr. W. Grosch):
"Untersuchungen über das Aroma und Fehlaroma von schwarzem und weißem Pfeffer" (only German title provided by the author)
12.04.99, Michael Czerny (Prof. Dr. W. Grosch):
"Investigation of the contribution of carbonyl compounds, pyrazines and furanones to the aroma of Arabica coffee. Influence of provenance and roast degree"
11.10.99, Wolfgang Engel (Prof. Dr. P. Schieberle):
"Odor-active compounds from the Maillard reaction of sulfur-free and sulfur-containing amino-compounds and fructose - aroma formation from the rare amino acids thiaproline and isothiaproline"
10.12.99, Petra Münch (Prof. Dr. P. Schieberle):
"Aroma compounds in thermally treated yeast extracts"
22.12.99, Bettina Fickert (Prof. Dr. P. Schieberle):
"Studies on the formation of aroma compounds during malting of cereals"
Summaries
by Michael Czerny
Sensory investigations proved that Strecker-
3-
A structure-
by Wolfgang Engel
In the first part of this work Maillard-reaction aromas were generated by thermal treatment (either by pressure cooking or roasting) of mixtures from ornithine, arginine, citrulline, lysine, glutamine, ammonia, thiaproline, isothiaproline and cysteamine with fructose. Extracts obtained by ether extraction were assessed by aroma extract dilution analysis. Of 28 detected aroma compounds 25 could be identified by comparison with authentic samples, 4 of them never been identified in foods or reaction aromas before.
The highest flavor impacts of the popcorn and caramel-like smelling reaction aromas, correlated through the corresponding Flavor Dilution (FD) factors, were found for 2-acetyl-1-pyrroline and 4-hydroxy-2,5-dimethyl-3(2H)-furanone. With somewhat lower FD-factors the dicarbonyl compounds butane-2,3-dione and pentane-2,3-dione contributed to the aroma of the mixtures. Apart from the sulphur-containing amino compounds the amino acid ornithine can be considered the best aroma precursor.
The reaction aromas derived from the sulphur-containig amino compounds were also dominated only by a small amount of aroma compounds. Among the sulphur-free compounds only 4-hydroxy-2,5-dimethyl-3(2H)-furanone and in the case of thiaproline also butan-2,3-dione, trimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine und 2-ethenyl-3,5-dimethylpyrazine contribute to the flavor of the mixtures. The importance of sulphur-containing compounds for the overall flavor of the mixtures is especially pronounced in case of cysteamine and isothiaproline. Important aroma compounds are 2-acetyl-2-thiazoline, 2-propionyl-2-thiazoline, 2-(mercaptomethyl)-furane and 5-acetyl-3,4-dihydro-2H-1,4-thiazine. Ethane-1,2-dithiole and the up to now unknown N-(2-mercaptoethyl)-1,3-thiazolidine must be considered as cysteamine-specific aroma compounds. The first open-chain caramel-like smelling aroma compound could be identified as 3,4-dihydroxy-3-hexene-2,5-dione having a lower FD-factor. Another unknown aroma compound, 4-acetyl-3-thiazoline, was detected with high FD-factor in the roasted mixture of thiaproline and fructose being a specific aroma compound of this amino acid. The most pleasant flavor of all mixtures under roasting conditions is generated by the amino acid thiaproline producing an aroma close to roasted almonds. It consists of a well-ballanced combination of only 7 aroma compounds, among them the three aroma-active pyrazines trimethylpyrazine, 2-ethenyl-3,5-dimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine, and 4-acetyl-3-thiazoline, 2-(mercaptomethyl)-furane, butane-2,3-dione and 4-hydroxy-2,5-dimethyl-3(2H)-furanone.
Some important aroma compounds of the mixtures were quantified using stable isotope dilution analysis. For 5-acetyl-3,4-dihydro-2H-1,4-thiazine a strong influence of the pH could be determined with an optimum at pH=7. The same was found for N-(2-mercaptoethyl)-1,3-thiazolidine, having a maximum of formation at a pH=8.
In the second part of the work the formation of the most important aroma compounds was investigated under mechanistic aspects. The complex reaction pathways leading to 2-acetyl-2-thiazoline, 2-propionyl-2-thiazoline und 5-acetyl-3,4-dihydro-2H-1,4-thiazine could be elucidated using labeling experiments with 2-13C-fructose. All of the three compounds can be formed by thermal degradation of 2-acetyl-2-hydroxymethyl-1,3-thiazolidine. This compound found in the reaction aroma of cysteamine and fructose therefore has to be considered as a potential precursor for those aroma compounds. New reaction pathways were introduced for 2-acetyl-1-pyrroline und 4-acetyl-3-thiazoline. For some important aroma compounds possible pathways were discussed. For the first time it was possible to identify 6-acetyltetrahydropyridine in an ornithine reaction aroma ± a compound that up to now was considered to be proline specific.
For the careful isolation of aroma extracts a new method was developed in the course of this work. The so called SAFE-technique (Solvent Assisted Flavor Evaporation) is performed in a also newly developed apparatus. Using this technique it is possible to directly distill aqueous solutions or foods in high vacuum and therefore obtain aroma extracts from food matrices fast and carefully. The method is especially useful with foods like beer, juices and other liquid or semi-liquid foods forming emulsions in case of conventional extraction methods.
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