| Lingwu Long-Jujube,as a "National Geographically Protected Product",is an excellent and characteristic fresh jujube species in Lingwu area of Ningxia.It has a pivotal position in the local forest and fruit economy in Ningxia.The edible rate of the fruit is as high as 94.6%,and the quality of fresh food is excellent,which is very popular among consumers.However,after harvesting,Lingwu Long-Jujube is very prone to dehydration,softening,shrinkage,reduced freshness and brittleness,and other texture quality deterioration,which seriously affects its nutritional characteristics and fresh food value.With the continuous expansion of the cultivation area of Lingwu long jujube and the continuous development of planting technology,it is particularly important to alleviate the decline in the texture and quality of fresh fruit during the storage,transportation and processing in the course of its sale.Therefore,it is necessary to detect the texture of Lingwu Long-Jujube in real time through non-destructive testing after picking for rapid evaluation and classification,and also to use low-cost methods and extend the storage period of jujube fruit within the scope of ensuring food safety so as to maximize local economic benefits.Since the quality of fresh fruit is closely related to its mechanical properties,in order to explore the feasibility of using rheological parameters to evaluate the texture index of Lingwu Long-Jujube,this paper first used the rheological theory to analyze the stress relaxation characteristics of Lingwu Long-Jujube.During the storage period,the stress relaxation and TPA index of Lingwu Long-Jujube fruit samples were measured and the correlation between the rheological characteristic parameters of Lingwu Long-Jujube and TPA indexes was analyzed,and the mathematical model of stress relaxation parameters to predict the texture of Lingwu Long-Jujube was established.The seven-component Maxwell model can well describe the stress relaxation behavior of Lingwu Long-Jujube whole jujube(R2>0.9995).The original test data of the stress relaxation curve under this model was closer to the measured value,and the dispersion of each Predictive value can also be minimized,and the stress relaxation behavior of Lingwu Long-Jujube was mainly described by viscous elements and elastic elements of three Maxwell's,and there was no significant correlation between the stress relaxation time? and each index of TPA(P>0.05).The prediction model of each index established by stepwise regression analysis had good verification results,the method of predicting the texture of Lingwu long jujube by stress relaxation was proved feasible,provided an effective and fast detection method for the quality evaluation of Lingwu Jujube without pretreatment and destruction.Aiming at the texture deterioration of Lingwu Jujube during storage and transportation,this paper used different concentrations of CaCl2 impregnation method to deal with postharvest Lingwu Long-Jujube,to explore the effect of CaCl2 treatment on postharvest Lingwu Long-Jujube's texture quality and physical and chemical indicators.The optimal concentration was determined,and the effects on the changes of cell wall polysaccharide composition and cell wall-related degradation enzyme activities of Lingwu Long-Jujube during storage were studied.The results showed that different concentrations of calcium treatment groups have effects on the weight loss rate,hardness,SSC and TA of Lingwuchang jujube,of which 2%CaCl2 treatment had the best effect on the decrease of Lingwu Long-Jujube fruit hardness and the increase of weightlessness,and it could inhibit the decrease of SSC content at the end of storage,delay the decrease rate of TA content during storage,and maintain the good quality of Lingwu Long-Jujube.According to the correlation analysis between the texture quality of Lingwu Jujube and polysaccharide components and cell wall degrading enzymes,it was found that the fruit hardness,cell wall components and cell wall degrading enzymes of Lingwu Jujube were closely correlated,and the correlation between various indexes is more significant after treated with 2%CaCl2,indicating that 2%CaCl2 treatment can be used as an effective means of postharvest storage of Lingwu Long-Jujube.Life activities of fruits and vegetables were still going on after harvest,and the changes of texture characteristics of fruits and vegetables after harvest were the result of the synergistic effect of various physiological and biochemical reactions,among which the changes of cell wall polysaccharide biomolecularity structure and morphology were the important factors causing texture softening.The physiological changes of Postharvest Jujube Lingwu was studied at the molecular level by using Atomic Force Microscopy(AFM),the quantitative parameters of the microstructures of polysaccharides were obtained,the cell wall polysaccharide degradation process analyzed qualitatively,and the characteristic absorption peaks of polysaccharides were distinguished by Fourier transform infrared spectroscopy(FTIR)to characterize the molecular structure of polysaccharides.The results showed that the CSP chains in the CK group and the 2%CaCl2 treatment group were mostly long chains at the beginning of storage,and the chain widths were distributed between 23-78nm and 15-93nm with many branches.At the end of storage,the long chains,aggregates and branches of CSP pectin were significantly reduced in the two groups,most pectin chains were shortened and significantly degraded,and the number of short chains increased with the appearance of cleavage.The width of CSP chains decreased to varying degrees,among them,the chain width of the CK group and the 2%CaCl2 treatment group decreased to 15-70nm and 19-82nm respectively,indicating that the 2%CaCl2 treatment can inhibit the degradation of the CSP chain from large to small width during storage and delay CSP Chain width degradation.This is consistent with the change in CSP pectin content.For the chain height(V),it can be found that the V value of the 2%CaCl2 treatment group was higher than that of the CK group in the late storage period,but there was no significant difference between the two groups before and after the storage period(P>0.05).The FTIR characteristic absorption peak analysis results of Lingwu Long-Jujube showed that:WSP,CSP,SSP all contain-OH,C-H,C-O-O,C=O and pyranoside bonds,and the degradation of the three pectins were closely related to the pyranoside bonds which were important components involved in the conversion and degradation of pectin components.All three pectins contained xylose,and 2%CaCl2 calcium impregnation treatment can inhibit the decomposition of xylose in Lingwu Long-Jujube WSP.The above results provided a theoretical basis for further research on the physical and chemical properties,three-dimensional structure,storage,freshness,storage and transportation of Lingwu Long-Jujube polysaccharides. |
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