Studies On The Color Regulation Mechanism And Flavor Changes Of Freeze-dried Carrot Slices

Freeze drying（FD）is an effective method for preserving the structure and bioactive components of fruits and vegetables.However,yellow fruits and vegetables often experience discoloration and flavor deterioration during FD processing and storage,which can negatively impact consumer acceptance and market satisfaction.Despite this issue,there is a lack of understanding regarding the discoloration mechanism as well as a shortage of effective quality control technology.To address these gaps in knowledge,this study focused on carrots as a representative research object due to their high carotenoid content that contributes to color and flavor quality.The study dynamically monitored changes in carrot state during FD from both physical（including optical）and chemical perspectives to reveal its discoloration mechanism.Additionally,the positive effects of ultrasound-assisted osmosis dehydration treatment on color,flavor,and carotenoid content were investigated during FDC storage while clarifying the underlying protection mechanisms.Furthermore,this study explored how carotenoids and their metabolites affect FDC’s flavor quality over time.The specific results are as follows:（1）Compared to traditional hot air-dried carrots（HADC）,this study examined the changes in color quality of carrot cylinders during freeze-drying（FD）from optical,physical structure,and chemical composition perspectives.The low-field nuclear magnetic resonance and scanning electron microscopy results showed that water sublimation in carrot tissue retained the intact porous structure of FD carrots,resulting in less than 30%volume shrinkage.This prevented accumulation of colored components and increased reflectivity on the sample surface and brightness value of the product.Carotenoids were identified as the main color substances,however,lutein and caffeic acid also had significant effects on a*value（r=0.917）and b*value（r=0.836）respectively according to correlation and PCA-X model analysis.Additionally,lipoxygenase indirectly brightened FDC by participating in carotenoid degradation.Overall,changes in physical structures and colored chemical components significantly affected discoloration of FDC.（2）A color protection method suitable for FDC was identified for the first time in this study.The results indicated that ultrasound-assisted osmosis dehydration with ascorbic acid（2%,w/v）and Ca Cl₂（1%,w/v）at 40°C and 100 W for 10 minutes（UAA-Ca Cl₂）can effectively improve the physico-chemical quality of FDC during storage for up to 120 days.Compared to the control group,UAA-Ca Cl₂pre-treatments increased the a*value,chroma,and total carotenoid content（TCC）by 29.66%,16.59%,and 3.40%,respectively.Storage at a temperature of 25°C was found to be conducive to maintaining FDC quality over time.Furthermore,correlation analysis revealed that non-enzymatic browning in FDC was significantly positively correlated with water content（r=0.63）,water activity（r=0.84）,and TCC（r=0.62）.Terpenoids and sulfur-containing organic compounds were found to be the most sensitive volatile components in FDC during storage with p<0.05 significance level.（3）The color enhancement mechanisms of UAA-Ca Cl₂pre-treatments on FDC were comprehensively investigated from the physical microstructure and the colored chemical components（carotenoids）profiles.Firstly,the results of scanning electron microscopy as well as laser confocal scanning microscopy showed that the sample treated by UAA-Ca Cl₂had an intact cell structure and fewer pores,which also compactly rearranged the section microstructure and showed strong carotenoid autofluorescence.On the other hand,UAA-Ca Cl₂not only significantly increased the TCC（36.38%）andβ-carotene retention ratio（51.73%）in FDC,maintained a high Raman intensity of C=C plane expansion（9986A.U）,but also induced the formation of other colored carotenoid derivatives.Additionally,the results of correlation analysis and PCA-X model analysis showed that the fresh carrot samples had the highest extractable color value（78.46）,and this indicator was linearly positively correlated with the content of 2-n-pentylfuran（p<0.01）,while the surface color of FDC was mainly affected by theβ-carotene content.These results provide a practical analysis and theoretical basis for the color improvement of FDC foods.（4）The volatile components of FDC samples during storage were recognized and identified by E-nose and HS-SPME/GC-MS,respectively,and the effects of carotenoids and their metabolites on the flavor quality of FDC were investigated.A total of 144 volatile compounds were detected from six groups of FDC samples through electronic nose and HS-SPME/GC-MS assays.Caryophyllene was found to be the most important volatile component in FDC,with a content range of 70.8～275.74μg/g（dry basis）.Additionally,β-carotene was enzymatically reactive or oxidized to produce various volatile components such asβ-ionone（22.85～117.26μg/g）,β-cyclocitral（0～113.84μg/g）,and dihydroactinidiolide（4.04～128.37μg/g）.The results showed that UAA-Ca Cl₂pre-treatments significantly reduced bad flavor components likeβ-cyclocitral released from samples at the end of storage,indicating that these treatments could help retain carotenoids in FDC while improving its flavor quality.