| Fresh goji berries have a high nutritional value and good economic benefits,the preservation has become a research hotspot in recent years.Vacuum precooling,as a key technology in the cold chain preservation,has a significant effect on eliminating field heat and reducing respiration rate of fresh goji berries.This technique has been applied to fresh goji berries preservation by our research group,and found that vacuum precooling can significantly affect the activity of antioxidant enzymes in fresh goji berries.At the same time,some tissues and cells exhibited premature necrosis during storage.In addition,we observed that fresh goji berries were compressed and then gradually expanded during the vacuum precooling process.Based on this,we speculated that inappropriate precooling process parameters may cause fruit tissue damage.in this paper,in order to optimize the vacuum precooling process,the goji was taken as studying object,the effects of vacuum precooling(final pressure of precooling is 600-800 Pa,800-1000 Pa,and 1000-1200 Pa)on the its mechanical properties,cell morphology,and physiological and biochemical characteristics were investigated,meanwhile,the inner link between mechanical properties and biochemical characteristics were explored,which aimed to investigate the damage caused by vacuum precooling to fresh goji berries.The research content is as follows:(1)Study on the effects of vacuum precooling on the mechanical properties of fresh goji berries.It was found that the five-element Maxwell model was more effective in fitting the stress relaxation curve of fresh goji berries through stress relaxation and TPA experiments.The vacuum precooling process mainly affects the change of elastic modulus,and the elastic modulus of each group of fruits gradually decreases during storage.particularly,the group with a final pressure of 1000-1200 Pa had the largest decrease in value.By the 10th day of storage,the elastic modulus of the 1000-1200 Pa group had decreased to approximately 1/3 of the initial value,but that of the 800-1000 Pa group,600-800 Pa group,and CK group had decreased to approximately 1/2 of the initial value.In addition,texture data showed that vacuum precooling accelerated hardness and chewiness decline.Compared with the CK group,the fruit hardness and chewiness of the 1000-1200 Pa group decreased significantly,at the end of storage,they had dropped by 39.11%,45.9%,respectively,but the 600-800 Pa and 800-1000 Pa groups were not significantly different from the CK group.(2)Study on the effects of vacuum precooling on the cell morphology of fresh goji berries.Through the analysis of the cell morphology of goji berries during the storage period,it was found that maturation and aging of goji berries were accelerated by vacuum precooling,mainly manifested as cell shrinkage,increased cell gaps,and disordered cell arrangement.In the late storage period,cell structure began to disintegrate,with a large number of free cavities appearing.while the cell wall lost its supporting role and the cell expansion pressure decreased,leading to the aggregation of cell wall into blocks.Compared with the CK group,the cell tissue of goji under 800-1000 Pa was more complete,while the cell changes in the 600-800 Pa and 1000-1200 Pa groups were more obvious in the above aspects,and the tissue damage was serious.(3)Study on the effect of vacuum precooling on the physiological and biochemical characteristics of fresh goji berries.The effects of vacuum precooling on the physiological and biochemical characteristics of goji berries were evaluated by measuring the respiration rate,cell membrane permeability,moisture content,soluble solids(TSS)content,superoxide anion(O2-)content,malondialdehyde(MDA)content,catalase(CAT),peroxidase(POD),and superoxide dismutase(SOD)activity.The results showed that vacuum precooling could inhibit the increase of respiration rate and delay the loss of TSS,but it also increased the loss of moisture.800-1000 Pa group performed better in maintaining fruit quality in the above indicators.However,vacuum precooling increased cell membrane permeability,accelerated the accumulation of O2-and MDA in goji tissues,1000-1200 Pa group showed the highest accumulation level.And the CAT,POD and SOD activities of this group were significantly higher than those of the CK group within 4-10 days,which may be a self-regulatory response of the fruit to alleviate oxidative stress damage.In conclusion,800-1000 Pa is more beneficial to maintain the storage quality of goji berries,while 1000-1200 Pa will cause certain damage to the goji tissue.(4)Exploration of the coupled relationship between fruit texture and physiological characteristics.Firstly,the experimental parameters significantly correlated with fruit hardness were determined by correlation analysis.Based on this,principal component analysis(PCA)was carried out to reduce the various parameters that affect hardness change into two principal components,PC1 and PC2,with a cumulative contribution rate of 72.9%,and a comprehensive evaluation of fruit quality was conducted.Finally,a multiple linear regression equation between hardness and cell morphology parameters as well as physiological and biochemical indicators were established using the multivariate linear regression method.The determination coefficients R2 of the equations for the CK group,600-800 Pa group,800-1000 Pa group,and 1000-1200 Pa group were 0.967,0.978,0.989,and 0.971,respectively,indicating a good predictive effect of the equation. |
PDF Full Text Request