| Winter jujube(Zizyphus jujuba Mill.cv.Dalidongzao)is known as the best fresh jujube variety worldwide for its good taste quality and high nutritional value.However,the alcoholic softening during cold storage and shrinkage under ambient temperature led to the serious consumption of winter jujube,restricting the development of winter jujube industry.It is urgent to explore efficient storage technology for jujubes.Researches have shown that X-ray irradiation is a highly automated and environmentally friendly non-thermal processing technology.It can inhibit sprouts and microbial growth,delay aging and prolong the shelf life of food under the premise of minimizing nutrient and sensory loss.In this study,winter jujubes harvested at the white ripeness stage were irradiated using X-ray and packaged with 0.015 mm PE microporous film.The quality and physicochemical indexes were determined during cold storage and ambient temperature storage in order to explore the preserving potential of X-ray irradiation and establish the prediction model of shelf life.The main research contents and results are as follows:(1)0,0.3 and 0.5 k Gy X-rays irradiated winter jujube at the white ripe stage was stored at 0±1℃and the changes of quality indexes was measured every 10 d to explore the appropriate irradiation dose and irradiation process.It was found that 0.3 k Gy X-ray extended the storage period to 70 d by preventing 8.20%of fungal and 29.12%of aerobic bacterial growth.It can preserve the volatile substances content of winter jujube from damage,and maintain complete cell membrane structure,low relative conductivity level and malondialdehyde content,high brightness(L*)and excellent appearance,and significantly delay the reddening.0.3 k Gy was the optimal X-ray irradiation dose.(2)In order to explore the reasons that 0.3 k Gy X-ray irradiation delayed the reddening of winter jujube during cold storage,the changes of color and pigment content,anthocyanin biosynthesis related enzyme activities and relative gene expression levels of jujubes were determined.The results showed that 0.3 k Gy X-ray could stimulate the production of total phenols and flavonoids and promote their antioxidant effects.In addition,the activities of phenylalanine ammonia-lyase and dihydroflavonol reductase were restricted in 0.3 k Gy X-ray treated jujubes,and the relative expression of MYB transcription factors(LOC107415776 and LOC107404478)and b HLH transcription factors(LOC107407443,LOC107423582 and LOC107423649)were down-regulated,causing the reduced anthocyanin and quercetin content.Together with the alleviated chlorophyll degradation levels delayed the reddening of irradiated jujube.This research demonstrates the potential of0.3 k Gy X-ray in delaying winter jujube reddening.(3)The winter jujube was transferred to 20±1℃from cold store on 0 d,30 d and 60 d and the quality assessment was carried out every 3 d to determine the corresponding shelf life.The changes of polysaccharide content and degrading enzyme activity in cell wall of winter jujube were measured,combined with the scanning electron microscope(SEM)and transmission electron microscope(TEM)to explore the effect of X-ray irradiation on softening of winter jujube.Results indicated that the later winter jujube out of the cold store has a shorter shelf life.0.3 k Gy X-ray extended the shelf life of jujube transferred from 0 d,30 d and 60 d of cold storage to 15 d,12 d and 3d,respectively,which kept the decay rate at the lowest level compared with other treatments throughout the storage.Furthermore,it could delay the ripening and senescence of winter jujube,reduce the damage degree of fruit cells and membrane permeability,prevent pectin decomposition(0.21%on the 60 d),and block the lignification process.The highest inhibition rates on the activities of polygalacturonidase,cellulase andβ-glucosidase were 47.32%,74.71%and 68.00%,respectively,which alleviated the softening.SEM and TEM observation validated the morphological integrity of peel,cuticle,stomata and the density of cell arrangement in the0.3 k Gy X-ray irradiated jujubes.(4)Based on the chemical reaction kinetics equation,Arrhenius equation and BP-artificial neural network,the shelf life prediction model of irradiated winter jujube was established,the results showed that the trends of decay rate and weight loss rate,hardness and overall quality were in line with the first-order and zero-order reaction kinetics models,respectively.The relative error of the model based on decay rate was 0.45%,indicating the highest prediction accuracy and the best effect.By Pearson correlation analysis based on 11freshness indexes and optimizing the number of hidden layer neurons,it is determined that when the number of hidden layer neurons is 8 and the input parameters are L*,a*,ΔE*,the model has the best prediction performance(R~2>0.90).Model based on(35)E*had higher prediction precision and the minimum error(0±1℃:R~2=0.98842,RMSE=0.65;20±1℃:R~2=0.99176,RMSE=2.10).The results show that the established shelf life prediction model can provide guidance and help for farmers’production practice. |
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