| Animal liver has high protein content,low price and rich source.However,animal liver has large molecular weight,compact structure and poor functional characteristics of protein.In addition,in order to facilitate storage and transportation,proteins usually need to be made into dry powder.Most proteins are prone to excessive aggregation during extraction and drying,and their functional properties are further reduced.From the perspective of application scenarios,the functional properties of proteins under neutral conditions are generally poor.Therefore,the processing and utilization of animal liver protein has been greatly limited.Artificial modification of natural proteins by scientific methods can change the structure of proteins,and may improve their solubility,emulsification,foaming and other functional properties.In order to improve the solubility,emulsification,foaming and other functional properties of porcine liver protein,and make it be used as a natural surfactant(emulsification,foaming)in the food industry,this paper took porcine liver protein(PLP)as the research object,and investigated the effects of three treatment methods on the structure and functional properties of porcine liver protein,including ultrasonic combined with pH-shifting treatment,pH-shifting treatment and mild heat assisted pH-shifting treatment,The main results are as follows:(1)The effects of ultrasonic combined with pH-shifting on the structure and function of porcine liver protein were studied.Under the conditions of pH3,pH7 and pH9,the porcine liver protein was treated with 300 W ultrasound for 5min,and then the neutral environment was restored.The results showed that,compared with the control,the turbidity and particle size of PLP in pH3-7 treatment group were larger,and the absolute values of solubility and potential were lower.Ultrasonic treatment would reduce the turbidity and particle size of PLP,and increase the absolute values of solubility and potential,but the difference was not significant.The protein particle size of ultrasonic treatment(U),pH9-7 and pH9U-7 treatment groups decreased,the turbidity of the solution decreased,the solubility of the protein increased,and the absolute value of potential was higher.All treatments reduced the fluorescence intensity of PLP and the content of active sulfhydryl group.The electrophoretic results showed that pH3-7 may cause the disulfide bond aggregation of PLP,pH9-7 had no significant effect on the electrophoretic strip,and the addition of ultrasonic treatment would not further change the electrophoretic strip.Compared with the control group,ultrasonic treatment(U),pH9-7 and pH9U-7 groups all improved the emulsifying activity,lotion stability and foaming ability of PLP;However,pH3-7 and pH9U-7 groups decreased the emulsifying activity and the stability of lotion,but did not improve the foaming property,but improved the stability of foam.The above results showed that only ultrasound treatment under alkaline pH-shifting conditions could synergistically improve the functional properties of porcine liver proteins.(2)In order to further understand the process of pH-shifting,the changes of structure,physicochemical,emulsifying and foaming properties of porcine liver protein treated by broad pH-shifting(pH3-11)after unfolding refolding were studied.The results showed that the pH of the solution tended to approach the neutral environment during the 30 min unfolding process,indicating the unfolding of the protein structure.Compared with the control group,acid and alkaline pH-shifting had significantly different effects on PLP.Acid pH-shifting had larger turbidity and particle size distribution,lower absolute value of potential and solubility;The alkaline pH-shifting reduces the turbidity and particle size of PLP,and improves the absolute value of potential and solubility.PLP after pH-shifting has higher surface hydrophobicity,lower fluorescence intensity and active sulfhydryl content,indicating that the unfolding refolding process induced by pH-shifting has a significant effect on the tertiary structure of PLP;The FTIR results showed that the pHshifting treatment had little effect on the secondary structure of PLP.The electrophoresis results showed that the acidic pH-shifting may cause the disulfide bond aggregation of protein,and the alkaline pH-shifting did not have a significant effect on the molecular weight of PLP.Alkaline pH-shifting treatment can effectively improve the emulsifying activity of PLP and the stability of lotion,and delay the stratification of lotion.Within the range of test conditions,pH11-7 group emulsions have good emulsifying properties,and the emulsifying properties become worse after acidic pH-shifting.Alkaline pH-shifting improved the foaming ability of PLP,but decreased the foam stability;The foaming property of PLP was not improved after acid pH-shifting,but the foam stability was significantly enhanced.The above results show that the greater the acidic pH shift,the greater the degree of protein aggregation.The direct cause of protein aggregation is the structural expansion and passing through the isoelectric point under acidic conditions.The main force of protein aggregation is hydrophobic interaction.The greater the alkaline pH-shifting,the greater the protein structural expansion,which is more conducive to the exertion of functional properties;Alkaline pH-shifting may be an effective method to improve the functional properties of porcine liver proteins.(3)The effects of mild heat treatment(25,40,50 ℃)on the structure and function of porcine liver protein under alkaline pH-shifting conditions(pH7,pH9,pH11)were studied.The results showed that although the turbidity and particle size of protein decreased after heat treatment alone,the changes of solubility,sulfhydryl content,surface hydrophobicity and polydispersity(PDI)showed that heat treatment alone could not effectively unfold the structure of protein,but accelerated the collision of protein molecules and reduced the particle size;Heat treatment under alkaline condition further reduced the particle size,fluorescence intensity,active sulfhydryl content,surface hydrophobicity and absolute potential of the protein.Secondary structure indicates that after heat treatment the content of α-helix and β-turn structure decreases while the content of β-fold increases,the effect of heat treatment on the secondary structure under alkaline condition has the same trend,and the change range is smaller.With the increase of alkaline pH-shifting,the solubility,emulsification and foaming properties of porcine liver protein can be improved with the increase of heat treatment temperature.Among all treatment groups,pH11-50 group has the best solubility,the smallest lotion droplets and the best lotion stability.Under alkaline pH conditions,the protein structure expands in advance and the strong charge effect makes the heat treatment more mild,and the protein structure expands further,which may be the reason why mild heat assisted pH-shifting improves the functional properties of pig liver protein.The above results show that mild heat treatment under alkaline pH-shifting can significantly improve the functional properties of PLP,and the improvement effect is related to the degree of alkaline pHshifting and heat treatment temperature. |
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