| Walnut is an important woody oil crop and ecological economic tree species in China,and the planting area and annual output rank first in the world.Walnut is rich in fat,protein and other nutrients,and it has anti-aging,brain health and other functions.However,there are still many defects in the methods used to produce walnut oil in our country,such as squeezing method,solvent extraction,and the comprehensive utilization rate of walnut is very low.Therefore,in this study,single factor and orthogonal experiments were used to optimize the process parameters of simultaneous preparation of walnut oil and walnut polypeptide(WP)by aqueous enzyme method.The quality differences of walnut oil extracted by aqueous enzyme method,cold press method and soxhlet method were compared in terms of physicochemical properties,chemical composition and antioxidant activity.Finally,the effect of WP on the physical and oxidative stability of oil-inwater(O/W)emulsion and the improvements of oxidized chlorogenic acid(OCA)and oxidized tannic acid(OTA)on the emulsification properties of WP were investigated.The main research results are as follows:(1)The optimum conditions for simultaneous preparation of walnut oil and WP by aqueous enzymatic method were as follows:papain:cellulase=2:1,pH 5,time 3.0 h,temperature 60℃,enzyme dosage of 3.0%,solidliquid ratio of 1:5(g/mL).Under these conditions,oil extraction rate of walnut was 53.37%,and WP yield was 4.01 mg/g.WP prepared by aqueous enzymatic method showed a concentration dependent scavenging ability of free radicals in vitro.At the same concentration,WP had better scavenging ability on ABTS,hydroxyl and DPPH free radicals,but poor scavenging ability on superoxide anion free radicals.(2)The yield of walnut oil from the three extraction methods was ranked as follows:soxhlet method(70.39%)>water enzyme method(57.08%)>cold pressing method(32.70%).The quality of walnut oil extracted by cold pressing method and aqueous enzymatic method was superior to that by soxhlet method,mainly manifested by the fact that the acid value of walnut oil extracted by cold pressing method was significantly lower than that by soxhlet method and aqueous enzymatic method(P<0.05),the peroxide value and iodine value of walnut oil extracted by soxhlet method were significantly higher than that by cold pressing and aqueous enzymatic method(P<0.05),and the saponification value was significantly lower than that by cold pressing and aqueous enzymatic method(P<0.05).In addition,the three extraction methods had little effect on the composition of fatty acids and triglycerides in walnut oil,and the ratios of saturated fatty acids,monounsaturated fatty acids and polyunsaturated fatty acids in walnut oil were close to 1:2.18:8.38.The total unsaturated fatty acids in walnut oil extracted by the three processing methods were all as high as 91%,and the triglyceride content was all as high as 97%.The contents of total phenols and flavonoids in walnut oil extracted by cold pressing method were significantly higher than those by soxhlet method and aqueous enzymatic method(P<0.05).The antioxidant activity of walnut oil extracted by cold pressing method and aqueous enzymatic method was better than that by soxhlet method.According to the antioxidant evaluation model of walnut oil,linoleic acid and total phenols were the key chemical components that affect the antioxidant activity of walnut oil.(3)The physical and oxidative stability of O/W emulsions could be im proved with different WP additions.When the addition of WP was 10 mg/m L,the physical and oxidative stability of the emulsions were better,which was mainly manifested by the highest potential of emulsion,the smallest pa rticle size,the lowest flocculation index(FI)and coagulation index(CI),th e lowest content of conjugated diene(CD)and malondialdehyde in emulsio n,the largest distribution of WP on the interface,the smallest oil droplets i n emulsion and the best dispersion,and the stability of emulsion was also b est during the 10 days of storage period.(4)Both OCA and OTA significantly improved the emulsification characteristics of WP,thus enhancing the physical and oxidative stability of O/W emulsions.When the addition of OCA was 1.5%or OTA was 1.0%,the emulsions had the highest potential,smallest particle size,lowest FI and CI,lowest CD value and malondialdehyde content,the most WP distribution at the interface,and the smallest emulsion oil droplets with the best dispersion.In addition,the stability of the emulsion with 1.5%OCA addition was better than that of the emulsion with 1.0%OTA addition.In conclusion,the aqueous enzymatic method could prepare high quality walnut oil and WP simultaneously,and WP played an important role in improving the physical and oxidative stability of O/W emulsions.This study provides a theoretical basis for the industrialization of the simultaneous preparation of walnut oil and WP by aqueous enzymatic method and the application of WP in food processing,which is of great significance to promote the comprehensive utilization of walnut resources and the sustainable development of walnut processing industry. |
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