Ph. D. Theses 1998
16.03.98, Susanne Antes (Prof. Dr. W. Grosch):
"Reoxidation of High and Low Molecular Weight Subunits of Glutenin and their Effects on Rheological Properties of Dough and on Breadmaking Quality"
24.04.98, Robert Karl Wagner (Prof. Dr. W. Grosch):
"Untersuchungen über das Aroma von Pommes Frites und über die Beziehung zwischen Struktur und Geruch daran beteiligter Alkylpyrazine" (only German title provided by the author)
16.06.98, Petra Anne Pfnür (Prof. Dr. P. Schieberle):
"Untersuchungen zum Aroma von Schokolade" (only German title provided by the author)
22.12.98, Andrea Büttner (Prof. Dr. P. Schieberle):
"Wichtige Aromastoffe in frisch gepressten Citrusfruchtsäften aus verschiedenen
Orangenvarietäten (citrus sinensis (L.) Osbeck) sowie Grapefruit (citrus paradisi Macf.)" (only German title provided by the author)
Summaries
by Susanne Antes
HMW subunits of wheat are important for the aggregation of wheat glutelin and thus, for the rheological properties of dough and for breadmaking quality. Whereas the reoxidation behaviour of HMW subunits of wheat was already investigated, only a few informations are available for LMW subunits which are also integrated into glutenin polymers. There are also no informations about the reoxidation behaviour of HMW subunits of rye which are homologous to wheat HMW subunits, but do not form a gluten-like material. For the investigation of reoxidation behaviour, HMW and LMW subunits were isolated from flours of the wheat cultivar Rektor (REK) and of the rye cultivar Danko (DAN) by a specific extraction/precipitation procedure. The fractions obtained (HMW-REK, HMW-DAN, LMW-REK) were then reoxidised with KBrO3 and KIO3 under different conditions. During reoxidation the thiol content of the fractions obtained was analysed with Ellman's reagent and the molecular weight distribution of reoxidised proteins was determined by gel permeation chromatography. The results demonstrated that HMW-REK, HMW-DAN and LMW-REK were reoxidised much faster by KIO3 than by KBrO3. The reoxidation behaviour of HMW-REK and HMW-DAN was similar. Both fractions produced more polymeric proteins during oxidation with KBrO3 compared with KIO3 whereas LMW-REK formed higher amounts of polymeric proteins when was oxidised with KIO3. The amount of polymeric proteins could be increased using higher concentrated protein solutions for reoxidation.
Because of its different number and position of cysteine residues, a different tendency to form intermolecular disulfide bonds was ascribed to x-type and y-type HMW subunits. To investigate these differences, HMW subunits were isolated with Acid PAGE and reoxidised with KBrO3. The molecular weight distribution of reoxidised proteins was again determined by gel permeation chromatography. Subunit 5 (x-type) showed the highest tendency to form intermolecular disulfide bonds. Subunit 7 (x-type) also forms high polymeric proteins whereas the cysteine residues of subunit 10 (y-type) are linked intramolecular.
To investigate the influence of the different reduced and reoxidised protein fractions on the rheological properties and baking quality, extensigrams of dough and mikro-baking tests after adding protein fractions to flour were done. The maximum resistance of dough could be increased adding reoxidised HMW subunits and mixtures of reoxidised HMW and LMW subunits and also adding reduced HMW subunits. The addition of reoxidised LMW subunits led to a decrease of the extensibility whereas the maximum resistance stayed constant. The extensibility of dough was increased adding gliadin and reduced glutathione and the maximum resistance was decreased adding reduced LMW subunits.
The breadmaking qualities were also influenced by adding different protein fractions. LMW subunits which were oxidised with KBrO3 were decreasing the loaf volume whereas HMW subunits reoxidised with KBrO3 increased the loaf volume. The loaf volume could also be increased adding gliadin. Gliadin isolated from flour of the wheat cultivar Rektor (REK) which contains less ω-gliadin was much more actual than gliadin isolated from flour of the wheat cultivar Apollo (APO) which contains high amounts of ω-gliadin. The results of the mikro-baking tests are only related to the results of the rheological investigations concerning the glutenin subunits not concerning the gliadins.
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