Study On Almond Protein Isolates And The Stability Of Its Oil-in-Water Emulsion

Lipids are important constituents in food products.Lipids mainly present in food systems as oil-in-water(O/W)emulsions.The lipids presenting in emulsion as the form of droplets are easily oxidized,resulting in food deterioration.,proteins,the most common food emulsifiers in O/W emulsions,also have antioxidant activity.However,proteins are susceptible to be attack by active reaction oxygen and be oxidized.Therefore,how to effectively inhibit the oxidation of oils and proteins in emulsions is a subject that scholars pay more attention to in recent years.Almond proteins are a kind of high quality plant protein resource,but the lack of in-depth research on almond proteins currently limits its application in the food industry.In view of this situation,in this paper,almond protein isolates(API)were used as materials and firstly the influence of ultrasound was explored on the functional characteristics of API.Subsequently,O/W emulsions are prepared by using API as emulsifier and walnut oil as oil phase,and the effects of ultrasound and the natural small molecule surfactant tea sapon in on the oxidation stability of oils and proteins in emulsion systems were investigated by monitoring the oxidation products of proteins and oils.The current study would provide theoretical support for the development of protein emulsions with higher stability.Firstly,the impact of ultrasound treatment on API molecular structure,physicochemical and functional propertie was studied.API(0.5 wt%)was extracted by alkali-soluble acid precipitation,and the aqueous API suspensions were treated at different ultrasonic powers and times to determine the changes in API structures.SDS-PAGE electrophoresis showed that ultrasonic treatment did not change the molecular weight of the protein,and the peptide bonds did not break.However,circular dichroism showed significant changes in the secondary structure of the protein.Ultrasound treatment reduced the ?-helix and ?-turn content of the protein molecules,and increased the content of random coils.After sonication,the content of free sulfhydryl groups in API increased,and the fluorescence intensity decreased.Ultrasound has a greater effect on the tertiary structure of API molecules.SEM images showed that the intact lamellar structure of API molecules was destroyed and the particle size of API was significantly reduced after ultrasonic treatment.Morever,the solubility of the protein increased by 58%,the emulsifying ability and emulsifying stability increased by 188%and 93%,as well as foaming capacity and foaming stability increased by 75%and 45%,respectively.These results suggest that the degree of improvement in API functional characteristics is related to ultrasound power and time.Secondly,the effects of ultrasonic treatment on the physical stability(laser confocal microscope,particle size,potential)and oxidative stability(oxidation of fats and proteins,oxidation)of the emulsions were studied.By using API(0.3 wt%)as emulsifier and walnut oil(5%v/v)as oil phase,O/W emulsion system was prepared,and the coarse emulsion was treated at different ultrasonic powers(300 or 600 W)for 20 min.The study found that the prolonging of the oxidation time led to the coalescence of emulsion droplets,an increase of the emulsion droplet size,a large number of aggregates were observed by laser confocal scanning microscopy.The absolute value of the ?-potential decreased,and the electrostatic repulsion between the droplets weakened.The physical stability of the emulsion gradually decreased.However,compared with the non-ultrasonic emulsions,the emulsions after ultrasonication had smaller droplets and more uniform distribution.The contents of the primary and secondary oxidation products of the sonicated samples were significantly lower than those of the control group.The content of carbonyl groups decreased and the content of sulfhydryl group increased after ultrasonic.The contents of carbonyl groups in control group(0 W,5d)and sonicated(600 W 20 min,5d)emulsion samples were 14.954±0.09(?mol/g),12.909±0.48(?mol/g),respectively,and sulfhydry content were 31.715±0.04(?mol/g),39.607±0.05(?mol/g),respectively.Meanwhile,it was found that sonication significantly reduced the degree of change in the fluorescence intensity and the growth rates of the schiff base content of the emulsions.Moreover,it was found that the intensity of electrophoretic bands of the non-ultrasonic emulsions were lower.Therefore,sonication was beneficial to enhance the stability of the API emulsion system.Finally,the effect of tea saponin(1%,2%wt)on the stability of API emulsion was explored in different pH(3.0 and 7.0)environment.The results showed that lipids were more stable in acidic systems than in neutral environments,while proteins were more stable in neutral environments.With the increase of tea saponin concentration,the oxidation products of lipids were decreased,at the same time,increasing tea saponin reduced the formation of carbonyls and schiff base.Also,the content of sulfhydryl groups,and the fluorescence intensity of the emulsions were increased with the increasing levels of tea saponin.Furthermore,analysis of almond protein components by SDS-PAGE electrophoresis also found that tea saponin enhanced the oxidation stability of the emulsions to a certain extent.