Mittels isothermer DSC wurde Einfluß isomerer Diglyceride auf die Geschwindigkeit des Phasenübergangs α → β′ von Kakaobutter untersucht. Die Identifizierung der Kristallmodifikationen erfolgte durch Röntgendiffraktometrie. Glycerol‐1,2‐dipalmitat, ‐1,3‐dipalmitat, ‐1,2‐dioleat, ‐1,3‐dioleat und die Diglyceridfraktion eines enzymatisch umgeesterten Palmöles wurden der Kakaobutter zugesetzt. Im Vergleich mit zugesetzten Triglyceriden derselben Fettsäurezusammensetzung verursachten die Diglyceride eine Verzögerung des Phasenübergangs α → β′. Dabei sind 1,2‐Diglyceride wirksamer als 1,2‐Diglyceride.
Insbesondere nach lipasekatalysierter Umesterung verbleiben größere Mengen an Diglyceriden im Fett. Die Auswertung vorliegender Arbeiten ergibt, daß Diglyceride die Kristallisationseigenschaften von Fetten beeinflussen können. Dabei scheinen 1,3(2,3)‐Diglyceride Phasenumwandlungsgeschwindigkeiten stärker zu verändern als die isomeren 1,3‐Diglyceride. Bisherige Untersuchungen konzentrierten sich dabei insbesondere auf die Phasenübergänge α → β′ von Palmöl und Salfett sowie β′ → β von hydriertem Rapsöl. Als Diglyceride kamen vorwiegend Gemische zum Einsatz, deren Fettsäure‐ und Isomerenverteilung häufig nicht bekannt war.
In the selection of an appropriate method for activity determination of lipases existing technical equipment, kind of enzymes, number of samples investigated (e.g. in routine analysis), and expected sensitivity range have to be taken into account. Titrimetric methods and above all copper salt methods with their high detection sensitivity are the most suitable procedures for activity determination of lipases used in laboratories and institutions without equipment for radiochemical analysis.
Types of lipase specificity are as follows: Positional specificity; fatty acid specificity; stereospecificity; substrate specificity (different rates of lipolysis of different glyceride classes. The acylglycerol used for determination of lipase specificity must be so structured, that specificities are not confused and unambiguous results are obtained. Different substrates and methods for detection of specificity are reviewed and advantages and disadvantages are discussed. Positional specificity can be determined with synthetic dialkylacylglycerols and 2,3-dioleoyl butanediol. Stereospecificity can be detected with enantiomeric dialkylacylglycerols or diacylalkylglycerols.
Lipoxygenase catalysed reactions play an important role in the field of lipid peroxidation. The enzyme is characterized concerning its sources, biological importance, isolation, substrates, active centre and inhibition. Extensive explanations should show the complex mechanisms of enzyme catalyses divided into two major ways: dioxygenase and hydroperoxidase reactions. Food science specific importance and expected effects on food systems are discussed and related to reaction products generated during catalysis and enzymatic processes both preceding and following lipoxygenase reaction.
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