| Dried goji berry made by freeze drying technology is popular as a product of new formats.However,freeze dried goji berry often discolors and whitens during its shelf life when it is stored at room temperature,which seriously affects its sensory appearance and custom acceptance.To improve the color stability and drying efficiency of lyophilized goji berry,this thesis was aiming to investigate the effects of freeze-thaw pretreatment on the characteristics and color of lyophilized goji berry firstly.It analyzed the protective effect of freeze-thaw pretreatment on the color and functional quality of freeze dried goji berry in terms of changes in carotenoids,volatile components and other antioxidant components.Moreover,the improving mechanism of color stability of lyophilized goji berry subjected to freeze-thaw pretreatment was clarified through chemical and spectroscopic analysis.The possibility of freeze-thaw pretreatment coupled with nitrogen-filled package in solving the whitening problem of lyophilized goji berry during room temperature storage was observed,and a sets of kinetic equations targeting carotenoid degradation were constructed to predict the color changes and shelf life of lyophilized goji berry.The following are main results.(1)Researches on the drying characteristics of freeze dried goji berry showed that freeze-thaw treatment could significantly improve the drying efficiency and thus decrease energy consumption,and retard the loss of key components such as carotenoids,polysaccharides,total phenols,and total flavonoids during drying.The color of the freeze dried berry treated with freeze-thaw was well kept,and their hardness and shrinkage increased,while their rehydration and related contents of carotenoids decreased after storage.After comprehensive evaluation,2 times of freeze-thaw treatment was the best option.The freeze-thaw treatment lead the cell structure of goji berry be destroyed,and the tissue water state be changed,the specific surface area of ice crystals formed before freeze drying increased,which could promote the sublimation of ice crystals during drying and shorten the water removal time of goji berry.Compared with the control group,the freeze drying time of goji berry was shortened by 18.75%,and the freeze drying energy consumption was reduced by 13.24%after 2 times of freeze-thaw pretreatment.(2)The changes of carotenoids,related volatile components and antioxidant activity in freeze dried goji berry during storage showed that freeze-thaw pretreatment could effectively mitigate the degradation of carotenoids and the deterioration of their quality in freeze dried goji berry.The carotenoids in freeze dried goji berry contained 34 monomers,mainly composed of 3 monomers:β-cryptoxanthin palmitate,rubixanthin palmitate,and zeaxanthin palmitate.The carotenoid content in freeze dried goji berry treated with freeze-thaw was significantly higher than the control goji berry after 360 days of storage at both25°C and 40°C.The retention of total carotenoids in freeze dried berry treated with freeze-thaw was increased by 1.19-fold compared to the control at 25°C,and by 1.68-fold at 40°C;and the retention of the three main carotenoid monomers in freeze dried fruits was increased by more than 1-fold in both storage methods.The freeze-thaw pretreatment reduced the volatile substances associated with carotenoid degradation and total phenolic degradation generated in the storage of freeze dried goji berry,maintaining the antioxidant activity of freeze dried goji berriy and effectively reducing the deterioration of goji berry quality.(3)Based on imaging of tissue microstructure and main functional components,spectroscopy and chemical data,freeze-thaw treatment could cause the collapse of freeze dried berry cells,contraction of berry volume and reduction of oxygen mobility in berry tissues,which lead to carotenoid degradation mitigate.Meanwhile,this pretreatment could lead to the leaching of polysaccharides,proteins and carotenoids from the cells and produce an intercalation.The increase of polysaccharides or proteins not only hindered the contact of carotenoids with oxygen,but also might alleviate the degradation of carotenoids because antioxidant substances such as polysaccharides could play an antioxidant protective role after intercalation.Further spectroscopic analysis showed that the freeze-thaw treatment did not cause covalent binding of carotenoids to other components and thus did not affect the nutritional and functional properties of carotenoids and other components.(4)Freeze-thaw pretreatment coupled with nitrogen-filled packaging could increase the half-life of the carotenoids in freeze dried goji berry to 533 days under room temperature storage,and was found to be helpful for effectively solving the problem of color stability of freeze dried goji berry.The degradation of carotenoids was consistent with a first-order degradation kinetic reaction with a reaction coefficient of determination R~2 ranging from 0.9654 to 0.9962.The degradation of total carotenoids and their monomers was an irreversible process with heat-absorbing and non-spontaneous reaction.The degradation in the total carotenoid content of lyophilized goji berry under fixed storage conditions could accurately predicted by its thermodynamic parameters.Moreover,it was seen that there was a good linear relationship between the total visual color differenceΔE and total carotenoid content of lyophilized goji berry(R~2 0.9247-0.9828),and thus the shelf life of freeze dried goji berry color could be presented by the evaluation of visual color difference and total carotenoid degradation.In a word,these results obtained provided theoretical basis not only for predicting the color change of freeze dried goji berry which are subjected to a commercial storage but a technical means to solve the color fading problem as well.In summary,freeze-thaw pretreatment promotes the color stability of freeze dried goji berry during drying and storage by reducing the contact between carotenoids and oxygen and alleviating the degradation of carotenoids.This pretreatment should be promising in improving the drying efficiency and reducing energy consumption of goji berry subjected to freeze drying process,eventually raising the overall quality of the special berry.Additionally,a linear relationship between carotenoid degradation and total color value change could be constructed to accurately predict the color change,and thus determine the“real”storage period of freeze dried goji berry practically. |
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