Preparation Of Carvacrol Emulsion And Its Application In Strawberry Preservation

Essential oils,as aromatic substances of natural origin,have strong antibacterial and antioxidant activities,but their insolubility in water,volatility and poor stability limit their application in food.Normally,emulsion systems be used to emulsify essential oils to solve these problems.However,essential oil emulsion systems are thermodynamically unstable and highly susceptible to Ostwald ripening,instability,and easy delamination.Therefore,how to prepare well-stabilized essential oil emulsions is one of the hot topic at present.In this study,the essential oil with the strongest antibacterial activity was carvacrol,and different types and ratios of cosolvents and surfactants were used to prepare carvacrol emulsions,and the carvacrol emulsions were added to gelatin/chitosan coating solution to prepare a bacterial inhibition film for strawberry preservation.The main studies are as follows:Firstly,the minimum inhibitory concentration（MIC）of eight essential oils（carvacrol,thymol,eugenol,cinnamaldehyde,citral,anisaldehyde,geraniol and peppermint）against Staphylococcus aureus,Escherichia coli,Salmonella typhimurium,Bacillus subtilis,and Aspergillus Niger was determined.The MIC of carvacrol against these tested bacteria were0.3125,0.6250,1.2500,0.6250 and 0.6250 mg/m L,respectively,with the best antibacterial effect.The emulsions were prepared by high pressure homogenization method,and the emulsifiers were selected by particle size,polydispersity index（PDI）and Zeta potential,and the emulsifier concentration,shear rate,shear time,homogenization pressure and homogenization times were optimized.The optimal formulation and process were determined as follows:emulsifier Tween 80,emulsifier concentration 5%（w/v）,shear rate 1×10⁴ rpm,shear time 2 min,homogenization pressure 300 Bar,and homogenization times 3 times.Under the optimal preparation process,the particle size,PDI and zeta potential of the carvacrol emulsion were 265.67 nm,0.40 and-37.20 m V,respectively.Secondly,the effect of the composition of the dispersed phase on the carvacrol emulsions was investigated.The dispersed phases consisted of cosolvents（ethanol,medium chain triglycerides and corn oil）and carvacrol,and different carvacrol emulsions were prepared under different dispersed phase compositions,and the differences in stability,bacterial inhibition and antioxidant activity were investigated.The results showed that the addition of either co-solvent significantly reduced the particle size and PDI value of the carvacrol emulsions and enhanced the storage stability of the emulsions.The results of bacterial inhibition showed that the carvacrol emulsions without the addition of cosolvents had the strongest inhibition,with the inhibition circle sizes of 21.97,16.85,17.91 and 18.00 mm for Staphylococcus aureus,Escherichia coli,Salmonella typhimurium,and Bacillus subtilis,respectively,and the inhibition decreased with the decrease of carvacrol content in the dispersed phase,and the addition of medium-chain triglycerides and corn oil significantly reduced the The addition of medium-chain triglycerides and corn oil significantly reduced the bacterial inhibition of the carvacrol emulsion.When the carvacrol content was 5 mg/m L,the DPPH radical scavenging rates of the carvacrol emulsion without the addition of cosolvent and the carvacrol emulsion with the addition of anhydrous ethanol,medium-chain triglycerides,and corn oil were 79.66%,79.40%,77.35%,and 78.26%,respectively,and the addition of anhydrous ethanol did not affect the antioxidant activity of the carvacrol emulsion,while medium-chain triglycerides and corn oil caused the emulsion The antioxidant activity of the emulsions was reduced by medium chain triglycerides and corn oil.Considering the practical application,while improving the stability of the carvacrol emulsion,the antibacterial and antioxidant activity of the carvacrol emulsion should be maintained as much as possible.Therefore,the dispersion phase was selected as 75%（w/w）carvacrol+25%（w/w）ethanol.Then,Tween 80 and sodium dodecyl sulfate（SDS）were used as composite emulsifier to prepare carvacrol emulsions.The differences of stability and antibacterial activity of carvacrol emulsions prepared by single emulsifier and composite emulsifier were investigated,and the minimum inhibitory concentration and antioxidant activity between carvacrol and carvacrol emulsions were explored.Results showed that adding SDS in the carvacrol emulsion prepared with Tween 80 could significantly reduce the particle size and PDI value,enhance the zeta potential value,increase the content of the dispersion phase from 20%（w/v）to 40%（w/v）,and enhance the bacterial inhibition.Additionally,the particle size of the carvacrol emulsions by preparation of Tween 80 and SDS was not changed significantly when carvacrol emulsions were stored at 4℃and 25℃for 45 days,respectively.The MIC values for carvacrol emulsion against Staphylococcus aureus,Escherichia coli,Salmonella typhimurium,and Bacillus subtilis was 0.3125 mg/m L,and the antibacterial activity of carvacrol emulsion was significantly enhanced compared with carvacrol.When the concentration of carvacrol reached 5 mg/m L,the DPPH free radical scavenging rates of carvacrol and carvacrol emulsion were 76.50%and82.54%,respectively,which indicated that the carvacrol emulsion system enhanced the antioxidant activity.Finally,carvacrol emulsion was added to gelatin/chitosan to prepare antibacterial films,and the differences in thickness,tensile strength,elongation rupture rate,water vapor transmission rate,carvacrol retention rate and antibacterial properties of antibacterial films with different carvacrol emulsion additions were investigated,and the antibacterial films were applied to strawberry preservation.The results showed that the addition of carvacrol emulsion increased the thickness of the film from 45.67μm to 69.67μm,decreased the tensile strength from 33.23 MPa to 25.04 MPa,increased the elongation at break from 36.58%to 47.65%,and decreased the water vapor permeability from 7.32×10^-11 g/（m·s·Pa）to 6.39×10^-11 g/（m·s·Pa）,resulting in enhanced bacterial inhibition.When the antibacterial film was applied to strawberry preservation,the rot rate was reduced from 61.11%to 19.44%,the weight loss rate was reduced from 29.86%to 15.67%,the soluble solids content was increased from 5.73%to 7.60%,the hardness was increased from 1.08 N to 1.47 N,and the sensory evaluation score was increased from 1.92 to 3.83,when compared with the blank control group at 7 days of storage at room temperature.The sensory quality of strawberry could be improved significantly after the treatment of antibacterial film.