Preparation Of Pomelo Essential Oil Microcapsule And Its Application On The Preservation Of Fresh-Cut Apples

In recent years,fresh-cut fruits and vegetables have gradually been favored by consumers due to their nutritious,healthy,convenient and instant-to-eat characteristics.However,after the fruits and vegetables are mechanically cut,on the one hand,the fruits and vegetables lose the protection of the epidermis,and they are easy to be oxidized and browned;On the other hand,the surface area of fruits and vegetables increased,and the internal cells was damaged to seep out the juice,which is susceptible to contaminated by microorganisms.It provides favorable conditions for their growth and reproduction,so they are extremely perishable.Previous studies have shown that plant essential oils are green and safe,and have good antioxidant and antibacterial properties.It can better maintain the appearance and nutritional quality of fresh-cut fruits and vegetables and extend their shelf life.However,essential oils have strong odor and volatility,which can greatly affect the sensory quality of fruits and vegetables and have a short action time.This can limit essential oils’application.The use of microencapsulation technology to encapsulate essential oils and allow them to be released slowly during application can better overcome the above problems,thereby promoting their application in fresh-cut fruits and vegetables.Therefore,in this study,gelatin and gum arabic were used as the wall materials to microencapsulate pomelo essential oil by the compound coacervation method for the preservation of fresh-cut apples.The effect on the storage quality of fresh-cut apples was evaluated.And essential oil microencapsulation was combined with other natural antioxidants or antibacterial agents to further extend the shelf life of fresh-cut apples.First,after characterizing the quality of pomelo essential oil,we used the microcapsule encapsulation efficiency and average particle size as evaluation indicators to analyze the p H value of the solution,the addition of core material and the mass fraction of wall material in the microcapsule preparation process by single factor experiments.And the essential oil microcapsules prepared under the optimal technological conditions were characterized;then,the fresh-cut apples were treated with the essential oil microcapsule solution prepared under the optimal technological conditions.The effect of the soaking time of the essential oil microcapsule solution on the browning,hardness,respiratory strength,nutritional quality and the total number of colonies of fresh-cut apples;finally,the essential oil microcapsules were combined with citric acid and nisin to treat fresh-cut apples,and the effect of joint preservation on the quality and enzyme activity of fresh-cut apples were investigated.The main conclusions are as follows:The main component of pomelo essential oil is D-limonene.Its scavenging ability to1,1-diphenyl-2-picrylhydrazyl（1,1-Diphenyl-2-picrylhydrazyl,DPPH）free radicals increased with the increasing of its mass concentration.Bacteriostatic zone experiments show that pomelo essential oil has certain antibacterial properties against Salmonella spp,but has no significant antibacterial effect against Bacillus subtilis;single factor experiments can confirm that the best preparation process for microcapsules is:the p H value of solution is 3.9,the amount of core material is0.5%（v/v）,and the mass fraction of wall material is 1%.At this time,the encapsulation efficiency reached 45.3%,and the average particle size is 138.2μm.The microencapsulation was performed by Fourier transform infrared spectra（FTIR）,thermogravimetric analysis（TGA）,differential scanning calorimetry（DSC）,and etc.under this condition.The results show that gum arabic and gelatin underwent complex aggregation through electrostatic interaction.The essential oils was encapsulated,and the thermal stability of the essential oils after encapsulation was improved.It can be observed by scanning electron microscope（SEM）that the pomelo essential oil microcapsules had a regular spherical structure before freeze-drying,and their morphology can be better maintained after freeze-drying..The fresh-cut apples were soaked in the essential oil microcapsule solution for 3 min（EM-1）,5 min（EM-2）,and 7 min（EM-3）,respectively.The control samples was soaked in the essential oil solution with the same concentration of microcapsule solution for 3 min（E0）,and the untreated group was designated as CK.All the groups were stored in a refrigerator at 4°C for quality determination.The results showed that the essential oil solution had almost no fresh-keeping effect on E0 group,and the essential oil taste was so strong,making the sensory scores even lower than the CK group.The essential oil microcapsules treatment can significantly inhibit the growth and reproduction of microorganisms during the first 8 days of storage（p<0.05）,and the total number of colonies of EM-1,EM-2,and EM-3 exceeded the minimum limit of 6 log(CFU·g^-1)on the 12th day,while the CK and E0 group exceeded the standard on the 10th day.In terms of fruit quality and sensory,the EM-1 group was better than the other groups in inhibiting the browning of fresh-cut apples,maintaining their hardness,and nutrient content.This showed that the use of essential oil microcapsule treatment can inhibit the growth of microorganisms in fresh-cut apples and prolong its shelf life.When the fresh-cut apples was soaked in essential oil microcapsules solution for 3 min,the appearance and nutritional quality of fresh-cut apples was best maintained.The fresh-cut apples were soaked in essential oil microcapsule solution（EM-1）,essential oil microcapsule solution with citric acid（EM-C）,and essential oil microcapsule solution with nisin（EM-N）for 3 min respectively.The blank treatment group immersed in ultrapure water for 3 min（BK）and the untreated group（CK）were used as controls.All the groups were stored in a refrigerator at 4°C for quality determination.The results showed that compared with the CK group,the BK group which was soaked in water showed accelerated browning,reduced hardness,and more rapidly loss of nutrients.However,the sensory and nutritional quality of fresh-cut apples can be significantly improved after be soaked in the essential oil microcapsules solution,indicating that the release of the essential oil in the microcapsules can effectively protect the fresh-cut apples.Compared with the essential oil microcapsule treatment group,the combination of microcapsules with citric acid or nisin could further inhibit the reproduction of microorganisms in fresh-cut apples.At the same time,the EM-N group had the most significant effect in inhibiting the browning of fresh-cut apples and maintaining the content of soluble solids,titratable acids,ascorbic acid and total phenols in the apples.On the 8th day,the sensory scores of color,appearance,texture and overall acceptability of EM-N were also higher than other groups.This may be related to the lower POD and PPO enzyme activities in the EM-N group during storage.Therefore,the combination of essential oil microcapsules with nisin had the best preservation effect on fresh-cut apples.This way has great application potential.