Über den Einfluß von Licht auf den oxydativen Verderb von Speiseölen V: Reaktionskinetik

Abstract

Ausgehend vom Radikalkettenmechanismus der Speiseoleoxydation wird ein Zeitgesetz fur die photochemische Oxydation ohne Hydroperoxidzerfall abgeleitet. Die Korrelation mit Versumsergebnissen fur Sojaol (1. Phase] ermoglimt die B e r e h u n g der Quantenausbeute der photoehemischen Startreaktion und der Kettenlange. Mit Hilfe dieser kinetischen GroDen sowie der Quantenausbeute der Bruttoreaktion wird namgewiesen, daD die Oxydation des Sojaols in der 1. Phase nicht autokatalytisch verlaufen kann. Die autokatalytischen Oxydationen (Sojaol 2. Phase, Sonnenblumen-und ErdnuDol] werden durch Untersuchung ihrer Oxydationsgeschwindigkeit in Abhangigkeit vom absorbierten Quantenstrom diskutiert. Dabei zeigt sich, daD wegen der Anwesenheit von Antioxydantien der Ungesattigtheitsgrad der Ole nieht allein fur die Oxydationsbereitschaft verantwortlich ist. Influence of Light on the Oxidative Deterioration of Edible Oils V: Kinetics of ReactionA time law for the photochemical oxidation without the decomposition of hydroperoxides has been derived from the radical chain mechanism in the oxidation of edible oils. Quantum yield of the photochemical start reaction and of the chain length can be calculated from the experimental results for soybean oil in the first phase. Using these kinetic values as well as from the quantum yield of the brutto reaction it is proven that the first phase in the autoxidation of soybean oil does not proceed by autocatalysis. Autocatalytic oxidations, e. g. soybean oil in the second phase, sunflower oil, and peanut oil,, are discussed on the basis of investigations on the oxidation rate as a function of absorbed quantum current. It is found that owing to the presence of antioxidants, the degree of unsaturation of oils alone is not responsible for the readiness with which the oil is oxidized.

Photodegradation of Foods and Beverages

“…The pigments involved have not really been identified, nor is it clear that all the photooxidations are mediated by singlet oxygen. In fact, kinetic studies (Paul et al, 1972b) show an autocatalytic oxygen uptake on the illumination of some vegetable oils, indicating a possible free-radical reaction. It is well known that some photodynamic sensitizers (such as flavins) sensitize photooxidations via free-radical pathways (Spikes, 1980).…”