Study On Heme-protein Triger Algal Oil Emulsion Instability And Lipid Peroxidation

Algal oil,rich in polyunsaturated fatty acids(PUFAs)such as DHA,is a new type of edible oil with high nutritional value.However,its quality and function is easy to be compromised as it is easily oxidized during processing and storage.There are many approaches to process and utilize PUFAs.One of most commonly used forms is micro-/nano-emulsion.Its stability directly affects the shelf life and nutritional value of food rich in PUFAs.It is well known that the nanosizing will bring about new physicochemical properties and physiological effects.Nanoparticles formed during the process may bring about new biological effects.As a nanoscale substance,the effects of oil nanoemulsion in the body are maybe different from oil.Thus,it is necessary to evaluate the physical and chemical stability.Heme protein,as a type of metalloprotein widespread in food material and organisms,however,is an exception.There is few research about the influence of heme protein on the stability of oil emulsion.A previous study revealed catalase(CAT)can trigger the demulsification and oxidation of fish oil emulsion.In this study,catalase(CAT)and bovine hemoglobin(BHb)are selected as the representatives of heme protein,and Hemin was used as the heme monomer control,and bovine lactoferrin(BLF)was used as the ferritin control.They were investigated in Tween 80 emulsified oil-in-water algae oil micro-nano emulsion respectively to explore their effects on physical and chemical stability.The results show that heme protein can cause demulsification and the hemin can both cause a sharp increase the content of superoxide anion radical and lipid peroxidation in the emulsion.The research aimed to reveal the key factors of heme protein affecting the physicochemical stability of algal oil emulsion and to preliminarily explore its mechanism.The main results are following:(1)CAT triggered a sharp increase in the content of superoxide in the algae oil-Tween 80 emulsion and promote lipid peroxidation;high concentration of catalase CAT can cause the emulsion to demulsify quickly and completely within three days.During the demulsification,CAT was simultaneously inactivated,aggregated and precipitated.The heat-inactivated CAT cannot cause demulsification under the same conditions.(2)Compared with CAT,bovine hemoglobin(BHb),which also contains heme structure but does not have enzymatic activity,took longer to initiate the demulsification of the algae oilTween 80 emulsion and self-aggregate precipitation.Hemin did not cause demulsification under the same conditions.Although bovine lactoferrin(BLF)without heme structure can cause demulsification,its speed was lower than that of CAT,with a different demulsification mechanism.(3)Adding BHb and Hemin to the emulsion also triggered a surge of oxygen free radicals and increased the level of lipid peroxidation.Heme is a necessary and sufficient condition for the explosion of oxygen free radicals in emulsion,while heme and lipid peroxidation are not sufficient conditions for demulsification.(4)The results reveal that heme(iron porphyrin ring)has important effects on the physical and chemical stability of emulsions rich in unsaturated fatty acids,and proves that heme protein can trigger the burst of oxygen free radicals,destabilization and demulsification,and protein aggregation and precipitation in the emulsion.The mechanism is: the hydrophobic domain of the protein interacts with lipids on the droplet,while its heme group catalyzes the electron transfer at the oil-water interface,promoting the generation of superoxide and reducing the zetapotential on the droplet surface,subsequently accelerates the process of demulsification,during which the protein itself also undergoes conformational transformation,and finally causes emulsion demulsification and protein precipitation.In summary,the heme protein contained in the food and processing may accelerate the destabilization and oxidation of emulsions rich in PUFAs.The emulsions can also explosively generate reactive oxygen species induced by endogenous catalase and hemoglobin during ingestion,which may rise the risk of oxidative stress damage.This study provides new ideas for re-examining the storage stability,nutritional value and risks of foods rich in unsaturated fatty acids.