| Lotus root (Nelumbo nucifera) is one of the important hydrophytic perennial and export vegetable in China with rich nutrients and medical value. Lotus root has the characteristics of high water, thin skin, white color and crisp texture, but is easy to suffer water loss, shrinkage and color change during storage period. Although lotus root is suitable for fresh-cut use, the mechanical injury due to cutting process would result in serious browning thereafter quality deterioration both sensory and objective. The previous experiment showed that the maximum storage life of fresh-cut lotus root at4℃is about15days, and the browning is mainly enzymatic. This study was focused on "Meirenhong"(most important cultivar planted in Jiangsu province) and "3735"(most commonly consumered in Nanjing markets) to determine the physiological and biochemical changes after fresh-cut during storage, and further to investigate the browning model, changes of phenols and differentiate of protein expression during browning process at room temperature by taking "3735" as experimental material. The main results are as follows:1. Comparison of quality and physiological characteristics of "Meirenhong" and "3735"Quality and physiological characteristics of "Meirenhong" and "3735" during storage at4℃were measured and compared. The results showed that the higher contents of soluble protein, Vc and total phenol (TP), and the greater peroxidase (POD) and superoxide dismutase (SOD) activities were observed in "Meirenhong", while "3735" had higher respiratory rate and malondialdehyde (MDA) content. Although the change trends of most measured indexes were similar between "Meirenhong" and "3735" during storage, such as becoming more servere browning, enhancing the respiration intensity, increased in reducing sugar, O2·-and MDA, reduced in losses of soluble protein, TP and Vc, decreased in POD activity and increased in SOD activity, etc."3735" was easier to brown and had a greater respiratory rate, a greater increase in reducing sugar and MDA content, and slower O2·-increase. Overall,"Meirenhong" is more appropriate for fresh-cut use. 2. Mathematical Model on browning of fresh-cut lotus root during storageSix treatments of water (CK),0.1%NaHSO3,0.2%citric acid (CA),0.01%ascorbic acid (AA),0.1%L-cysteine (Cys) and composite preservative (CP:0.1%CA,0.005%AA and0.05%Cys) were used to determine the effect of each treatment on quality and physiology of fresh-cut lotus root during storage at low temperature. Compared to all treatments, CP treatment had less browning, lower levels of MDA, respiration, O2·-and H2O2and higher activities of SOD, POD and CAT.The primary factor on the browning of fresh-cut lotus root was mainly related to the quality and physiological indicators such as L*, conductivity, respiration, MDA content, reactive oxygen and POD activity, and the contribution rate of L*was64%. The secondary factor was mainly related to SOD and CAT activities. As the visual reflect of browning degree, L*was used for dependent variable, other nine indexes were used for independent variables, and linear regressive analysis was presented. The simplified regression model was L*=65.784-11.062x3-0.101x5+0.003x6(x3:MDA, x5:O2·-,X6:POD). The regression analysis showed that browning degree had significant negative correlation with MDA and O2·-content, but had significant positive correlation with POD activity.3. Relationship between phenol and browning of fresh-cut lotus root and free radical scavenging capacity in during browningDifferent parts of lotus root were chosen to compare phenol content and free radical scavenging capacity. HPLC method was used to detect the components and quantitative phenol. The results show that the total phenol contents in different parts of lotus root were different:peel7.19mg/g, node7.61mg/g, tip2.54mg/g and root flesh2.10mg/g. The components of phenols in peel were pyrogallic acid, catechol and hydroxyltyramine, and mainly was gallic acid. The main phenol in node was hydroxyltyramine, gallic acid in tip. The flesh of lotus root contained gallic acid, hydroxyltyramine, catechin and caffeic acid. Under the same phenol condition, free radical scavenging capacities in different parts of lotus root were also different:root>tip>peel>node, and this was not in direct proportion to phenol content. There was no significant relationship between phenol contents and free radical scavenging capacity. The phenol content in fresh-cut lotus root present a down trend during storage, while free radical scavenging capacities showed an ascending-descending-ascending trend.There was close relationship between the difference of phenol contents and composition and antioxidant capacity. Phenol composition in fresh-cut lotus root was the same during storage at room temperature, and pyrogallic acid, catechol and caffeic acid were closely related to browning, especially the amount of pyrogallic acid.4. Differential expression of proteins before and after browning in fresh-cut lotus rootTrichloroacetic acid/acetone precipitation and phenol extraction were used to determine their efficiency on extracting total proteins from fresh-cut lotus root. Differential expression of proteins was analyzed by2-DE and MALDI-TOF-TOF. The suitable2-DE analysis conditions for lotus root were:phenol extraction for protein extraction,1500ug sample handling, IPG for17cm with pH4-7,12%gel concentration and G-250staining.By comparing2-DE protein patterns of fresh-cut lotus root before and after the browning,40protein spots were detected which had statistically significant and the difference was greater than2times or less than0.5. Among them,5spots only exited before browning, and5new spots appeared after browning,8spots increased and22spots decreased with the progress of browning.38spots were successfully identified based on all available information, including4storage and34functional proteins, and successful identification rate was95%. There were4proteins (L10, L3, L15and L20) expressed only before browning,2proteins (L5and LI2) expressed only after browning,7proteins (L11, L16, L17, L21, L28, L32and L33) up-expressed after browning, and the last21proteins down-expressed.The34functional proteins were classified into following groups:stress response (7), cell structure (4), material and energy metabolism (12), respiration (4), signal transduction (2), gene expression (2) and unknown protein (3). The group with greatest difference in protein expression was related to material metabolism and regulation, reactive oxygen species metabolism and respiratory control.Differential expression showed that function of distinct proteins was focut on stress response, respiratory, material and energy metabolism, etc. The distinct proteins included USP, SOD, POD, ferrin, ATPase and so on. The down-regulation of these proteins after brown showed that the way and direction of metabolic control and regulation changed, stress response activity declined, catabolism strengthened, energy utilization decreased, expression of Cu/Zn-SOD and TPx decreased, activity of cleaning reactive oxygen system weakened, and destruction of cell structure got intense after brown. |
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