Influence of physical properties on oxidation in liquid and freeze-dried linoleic acid/sucrose/maltodextrin model systems
The relationship between physical characteristics and lipid oxidation was studied in model liquid and freeze-dried emulsions.
A first part of the study involved the use of liquid emulsions consisting of linoleic acid, phosphate buffer, Tween-20 and sucrose. Iron sulfate and ascorbic acid were added to accelerate oxidation. The effect of variation in the system composition on oxidative behavior was investigated. Oxidation was influenced by the relative concentration of oil and emulsifier and their partition between different phases. When an excess of surfactant was present in the water phase, protection from oxidation was observed. Tween-20 competed with linoleic acid for catalysts and oxygen, and also, when in sufficient amount, provided a more compact coating of the interface. This prevented the contact between catalysts and oil. In the presence of sucrose, the increase in viscosity was expected to reduce oxidation. Although some protection was noticed, it did not correlated with the increase in sugar concentration. A protective action of sucrose other than viscosity, such as scavenging radicals and hydroperoxydes and/or quenching metals, was probably prevalent in this case.
In the freeze-dried emulsions, maltodextrin and sucrose were added in equal amounts to provide a matrix. The effect of storage humidity on linoleic acid oxidation was studied. Lipid oxidation decreased after 32% relative humidity, contradicting the generally accepted oxidation theory that predicts a minimum in reaction rate at a water activity 0.3. Physical changes in the material counteracted the increased mobility and the increased reaction rate expected at the higher moisture. Structural collapse reduced oxygen availability and enclosed some of the linoleic acid that was previously exposed to oxidation. Moreover, sucrose crystallization caused some interface instability and consequent oil droplet coalescence. The coalescence of oil droplet's decreased the surface exposed to oxidation, contributing to the observed delay in oxidation at the higher relative humidity condition.
Glass transition could not predict oxidative behavior in freeze-dried emulsions. The cooperation of the physical changes taking place in the system opposed the effect of glass transition on mobility. Although glass transition is often used to predict food stability, in some cases it may not be sufficient to accurately predict oxidative behavior. It is necessary to take into consideration other physical factors that may counteract its effect.
0749: Agricultural chemicals