| Sichuan pepper essential oil(SPEO)is a natural plant essential oil extracted from the pericarp of Sichuan pepper fruit.It has strong aroma,unique flavor,as well as potent anti-oxidation and antibacterial effects,and is widely used in food and medicine fields.However,the defects of high hydrophobicity,high volatility,low bioavailability and intense sensory stimulation limit the application of SPEO in food,especially in food with high water content.As a kind of "thermodynamically stable system",nano-emulsion has been widely used as a carrier of many hydrophobic substances,which is an effective method to solve the problem of "poor water-solubility" of essential oil.Currently,the extensive researches on SPEO mainly focus on the extraction,composition analysis,biological activity identification,and food preservation application.In this study,oil in water(O/W)type Sichuan pepper essential oil nano-emulsion system(SPEO-NE)was prepared by high-pressure homogenization technology.Additionally,the stability,antioxidant and bacteriostatic properties of SPEO-NE were investigated.Finally,the influence of nano-emulsion on the storage quality of flavoring vinegar was discussed.The specific research contents are as follows:(1)SPEO-NE(NE-1 and NE-2)was prepared by high-pressure homogenization method with the particle size,polydispersity index(PDI),zeta potential and turbidity as the indexes.Wherein,Tween-80(T-80)and caprylic capric triglycerride(ODO)were used as the emulsifiers of NE-1,while Tween-60(T-60)and polyglycerol fatty acid ester(PGFE)were used for NE-2 preparation.Additionally,the effects of the ratio of emulsifier,the amount of SPEO addition,the amount of emulsifier addition and the homogenization times on the stability of the nano-emulsion were investigated.Zeta potential represents the surface charge properties of the nano-emulsion,and the larger the absolute value is,the more stable the emulsion system will be.Therefore,the formula and preparation process of SPEO-NE were optimized by response surface test was the zeta potential as an indicator.The optimal formulation and process of NE-1 were emulsifier ratio(ODO:T-80,m/m)of 7:3,3.0% SPEO,2.5% emulsifier and homogenization for 3 times.The particle size and zeta potential of NE-1were 124.57 ± 2.43 nm and-33.12 ± 0.79 m V,respectively.The optimal formulation and process of NE-2 were emulsifier ratio(T-60:PGFE,m/m)of 2:1,3.0% SPEO,8.0% emulsifier and homogenization for 4 times,which led to the particle size and zeta potential of NE-2 of23.12 ± 1.47 nm and-29.74 ± 0.83 m V,respectively.(2)The properties of NE-1 and NE-2 nano-emulsions prepared by adding different emulsifiers were investigated,the effects of emulsification on the flavor compounds of SPEO were analyzed by electronic nose.The changes in volatile flavor compounds of SPEO in NE-1 and NE-2 were quantitatively analyzed by GC-MS internal standard method.Using particle size,zeta potential,turbidity and centrifugal stability(Ke)as indexes,the p H,thermal and storage stability of the nano-emulsion were investigated.The antioxidant capacity and bacteriostatic growth curve were measured to investigate the antioxidant and bacteriostatic activities of SPEO in aqueous environment before and after emulsification.The results showed that the loss of volatile flavor compounds in NE-2(6.79%)was less than that in NE-1(18.83%).When the p H decreased or the heat treatment temperature increased,the variation range of each index of NE-2 was smaller than that of NE-1.Then,the microstructure of the nano-emulsion was observed.After heating at 100℃ for 30 min,small droplets in NE-1converged to large droplets,thus causing the dispersion variation;while there was no significant change in NE-2.In the storage experiment,NE-1 and NE-2 stored at 4℃ showed good stability,the particle size and zeta potential did not change significantly during the storage period.With the increase of storage temperature,the degree of changes in nano-emulsion increased.After storage at 37℃ for 31 d,demulsification occurred in NE-1,but NE-2 was still relatively stable and had no essential oil precipitation.Therefore,NE-2 was considered to have better p H,thermal and storage stability.The results of antioxidation and bacteriostatic experiments showed that the bioavailability of SPEO could be improved by preparing SPEO-NE via high pressure homogenization method,and NE-2 was superior to NE-1 in all aspects.(3)In order to determine the applicability of the prepared nano-emulsion in seasonings,NE-2 was applied to flavoring vinegar.Electronic nose was used to determine the effect of the addition amount of NE-2 on flavor of vinegar,and the effects of NE-2 on physicochemical properties of flavoring vinegar at different storage temperatures were investigated using sensory,color,transmittance,Ke and total acid as indexes.The results showed that NE-2 and vinegar had good mutual solubility,and the addition of 1.0% NE-2 could provide the vinegar with good flavor while keeping the original flavor of the later.NE-2 could inhibit the changes in physicochemical properties of vinegar.At 25℃,the changes in L*,a*,b* value,transmittance and Ke of NE-2 group(vinegar + NE)were 0.99,0.09,0.13,2.79% and 0.6%,which were all smaller than those of the group without NE-2(vinegar).Moreover,the indexes at 35℃ and 45℃ showed the same change rule to that at 25℃,but the addition of NE-2 had no significant effect on the total acid content of vinegar.The antioxidant capacity and microbial changes of vinegar and vinegar + NE during storage were also discussed.It was concluded that the antioxidant capacity of vinegar and vinegar + NE decreased gradually with the increase of time or temperature,but the antioxidant capacity of vinegar + NE was always better than that of vinegar.In the whole storage test,all samples at all temperatures met health standards. |
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