Effect Of Methyl Jasmonate On Storage Quality Of Fresh-cut Cistanche Deserticola Ⅱ

2 Results and analysis

2.1 Effects of different concentrations of MeJA treatment on the total number of bacterial colonies in fresh-cut desert Cistanche deserticola

The total number of colonies is an important indicator for evaluating the level of microbial contamination of fresh-cut fruits and vegetables during storage. It is generally believed that when the total number of colonies of fresh-cut fruits and vegetables reaches 6 lg (CFU/g), they lose their commercial value [16]. It can be seen from Figure 1 that during the 14-day storage period, the total number of bacterial colonies in fresh-cut desert Cistanche deserticola continued to increase. In the early stage of storage (0-4 d), the total number of colonies in each treatment group remained at a low level, and the rate of increase was relatively slow. The rising rate of total bacterial colonies in groups CK, M1 and M2 accelerated after 4 days, while the rising rate in group M3 increased after 6 days. After storage for 14 days, the total number of colonies in the CK group reached 5.36 lg (CFU/g), which was significantly higher than that in the M1, M2, and M3 groups, indicating that MeJA treatment can effectively delay the increase in the number of bacteria during the storage of fresh-cut desert Cistanche deserticola.

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2.2 Effects of different concentrations of MeJA treatment on hardness, weight loss rate and TSS content of fresh-cut desert Cistanche deserticola

Fruits and vegetables will undergo aging as time goes by during storage. The degree of aging of fruits and vegetables can be judged by changes in hardness. Figure 2-A shows that during the 14-day storage period, the hardness of the CK group decreased rapidly, and MeJA treatment could delay the decrease in hardness of fresh-cut desert Cistanche deserticola to varying degrees. The order of hardness decrease rate in each treatment group is CK>M1>M2>M3. After storage for 14 days, the hardness of the CK group decreased to 2.851 kg/cm2, while that of the M3 group was 3.361 kg/cm2, which was 1.18 times that of the CK group, indicating that the M3 group could better maintain the hardness of fresh-cut desert Cistanche deserticola.

During the storage process of fruits and vegetables, the degree of loss of water and nutrients can be judged by the weight loss rate. It can be seen from Figure 2-B that during the 14-day storage period, the weight loss rate of fresh-cut desert Cistanche deserticola continued to increase. At the early stage of storage (2-4 d), the weight loss rate of the CK group was significantly higher than that of the M2 and M3 treatment groups, but was not significantly different from the M1 group. After 6 d of storage, the weight loss rate of the CK group was significantly higher than that of the MeJA-treated group. It shows that MeJA treatment can effectively reduce the increase in weight loss rate and delay the loss of water and nutrients in fresh-cut desert Cistanche deserticola.

Soluble solids (TSS) have a strong correlation with sugar and are usually used as an important indicator to evaluate the nutritional content and storage quality of fruits and vegetables. It can be seen from Figure 2-C that on the second day of storage, the TSS content of each treatment group of fresh-cut desert Cistanche deserticola increased slightly. This may be due to the gradual degradation of macromolecular carbohydrates in Cistanche deserticola into soluble sugars and other organic matter, resulting in an increase in TSS content. However, after the 2nd day, the TSS content of each treatment group began to show a downward trend. This may be because during storage, respiration was enhanced and a large amount of sugar was consumed, resulting in a decrease in TSS content. After 10 days of storage, the TSS content of the CK group was significantly lower than

In the MeJA-treated group, when stored for 14 days, the TSS contents of the MeJA-treated groups with different concentrations were 11.3%, 12.17%, and 12.9% respectively, which were 1.23, 1.32, and 1.4 times that of the CK group. It shows that MeJA treatment can reduce the consumption of nutrients during the storage of fresh-cut desert Cistanche deserticola, thereby maintaining the TSS content of fresh-cut desert Cistanche deserticola at a high level.

Fig.2 Changes in hardness, weight loss rate, and TSS content of fresh-cut Cistanche deserticola during storage A-hardness; B-weight loss rate; C-TSS content

2.3 Effects of different concentrations of MeJA treatment on the color of fresh-cut desert Cistanche deserticola

Color is the most intuitive indicator for consumers to evaluate food quality and directly affects food sales. L

The * value indicates the brightness of the color, and a positive number indicates a whiter color. It can be seen from Figure 3-A that within 14 days of storage, the L* value of each group showed an overall downward trend, indicating that the browning of fresh-cut desert Cistanche deserticola gradually intensified, with the M3 treatment group declining the slowest. In the first 4 days of storage, there was no obvious difference in the L* value of each treatment group. After 6-12 days of storage, the L* value of the M3 group was significantly higher than that of the CK group and M1 group, and the difference with the M2 group was not significant. After storage for 14 days, the L* value of the M3 group was 17.28% higher than that of the CK group. a * represents the red-green value, and positive numbers represent reddish. It can be seen from Figure 3-B that during storage, the a* value of each group showed a gradually increasing trend. At the early stage of storage (0-4 d), the difference between each group was not significant. After 6 d of storage, the a* value of the M3 group was significantly lower than that of the CK group. After storage for 14 d, the a* value of the M3 group was significantly lower than that of the CK group. , M1 and M2 groups decreased by 26.28%, 16.95% and 7.75%. The b* value represents the yellow-blue value, and a positive number represents a yellowish color. It can be seen from Figure 3-C that the b* value of each group gradually increased during the storage process, and the CK group increased the fastest. After 4 days of storage, the b* value of the M3 group was significantly lower than that of the CK group. After storage for 14 days, the b* value of the CK group was 1.19 times that of the M3 group. It shows that the M3 group treatment can effectively delay the decrease of L* value and the increase of a* and b* values, and slow down the color change of fresh-cut desert Cistanche deserticola.

Fig.3 Changes in L * value, a * value, and b * value of fresh-cut Cistanche deserticola during storage

2.4 Effects of different concentrations of MeJA treatment on browning degree of fresh-cut desert Cistanche deserticola

Fresh-cut desert cistanche deserticola is prone to browning during storage, which affects its storage value. It can be seen from Figure 4 that during the storage period, the browning degree of fresh-cut desert Cistanche deserticola gradually increased. Compared with the CK group, the browning degree of the MeJA treatment group increased more slowly. Among them, the M3 group had the slowest increase rate and the degree of color change was slower. Small, which is consistent with the conclusion of color changes in this article. After 14 days of storage, the browning degree of the CK group reached 0.971, which was significantly higher than that of the MeJA-treated group, indicating that MeJA treatment could effectively inhibit the increase in browning degree of fresh-cut desert Cistanche deserticola during storage.

Fig.4 Changes in browning degree content of fresh-cut Cistanche deserticola during storage

2.5 Effects of different concentrations of MeJA treatment on the total phenolic content of fresh-cut desert Cistanche deserticola

Phenolic substances are key substrates for the browning of fruits and vegetables. Because their molecules contain a large number of phenolic hydroxyl groups, they have antioxidant and free radical scavenging functions. After Cistanche deserticola is cut into pieces, mechanical damage induces the synthesis of a large amount of phenolic substances, resulting in an increase in the total phenolic content, and then the phenolic substances are consumed by oxidation, resulting in a decrease in the content [13]. As shown in Figure 5, the total phenolic content of fresh-cut desert Cistanche deserticola showed a trend of first increasing and then decreasing during storage. It may be the reason for the more serious browning of the CK group. The total phenolic content of the CK group was lower during the entire storage process. On the 8th day of storage, the total phenolic content of each group reached the peak, among which the M3 group was 8.58 mg/100 g, which was significant. Higher than the CK group and M1 group. After 8 days of storage, the total phenolic content of each group began to decrease, and the CK group was significantly lower than the MeJA-treated group after the 12th day, indicating that MeJA treatment can effectively inhibit the browning of fresh-cut Cistanche deserticola.

Fig.6 Changes in PPO and POD activities of fresh-cut Cistanche deserticola during storage

2.6 Effects of different concentrations of MeJA treatment on PPO and POD activities of fresh-cut desert Cistanche deserticola

Polyphenol oxidase is widely distributed in higher plants and fungal cells and plays an important catalytic role in the browning process of fruits and vegetables. Figure 6-A reflects the changes in PPO activity of fresh-cut desert Cistanche deserticola during storage. The PPO activity of different treatment groups continued to increase during the first 8 days of storage and reached the highest peak on the 8th day. The peak value of the CK group was 63.33 U/g FW, which was significantly higher than that of the MeJA-treated group. After 8 days, the PPO activity of each group began to decrease. In the late storage period (12 d-14 d), the PPO activity of the M3 group was significantly lower than that of other treatment groups, indicating that the M3 treatment could effectively inhibit the increase in PPO activity and reduce the browning of fresh-cut desert Cistanche deserticola. After fruits and vegetables are injured, POD activity will increase. POD is mainly responsible for removing H2O2 produced by SOD in plants, so H2O2 induces an increase in POD activity. Figure 6-B reflects the changes in POD activity of fresh-cut desert Cistanche deserticola during storage. During storage, the POD activity of Cistanche deserticola in each treatment group showed a trend of first increasing and then decreasing. Fresh-cut desert Cistanche deserticola had its highest peak on the 6th day of storage, and the peak value in the CK group was 139.33 U/g FW, which was significantly higher than that in the MeJA treatment group. After 6 days of storage, the POD activity of each group began to decrease. When stored for 14 days, the POD activity of the CK group was significantly higher than that of the MeJA-treated group, indicating that MeJA treatment can more effectively inhibit the POD activity of fresh-cut desert Cistanche deserticola during storage.

2.7 Effects of different concentrations of MeJA treatment on MDA of fresh-cut desert Cistanche deserticola

Malondialdehyde is one of the products of peroxidation of fruits and vegetables. It is usually used to evaluate the degree of cell membrane lipid peroxidation and oxidative stress, and has toxic effects on tissue cells. As can be seen from Figure 7, the MDA content of fresh-cut desert Cistanche deserticola shows an increasing trend during storage. This is because antioxidant substances are massively oxidized during storage, resulting in an increase in MDA content. At the beginning of storage, there was no significant difference in MDA content between CK and each treatment group. In the middle and late stages of storage (6-14 d), the MDA content of the fresh-cut desert cistanche deserticola CK group was significantly higher than that of the other groups. When stored to 14 d, the M3 group had less MDA accumulation, which delayed the process of membrane lipid peroxidation. Compared with the CK group, the MDA content of the M3 group decreased by 21.32%, indicating that the M3 group better maintained the stability of the cell membrane structure and was more conducive to the storage of fresh-cut desert Cistanche deserticola.

2.8 Comprehensive analysis of quality indicators of fresh-cut desert Cistanche deserticola

2.8.1 Correlation analysis of MeJA treatment on the storage quality of fresh-cut desert Cistanche deserticola

Fresh-cut fruits and vegetables are easily contaminated by microorganisms during processing and handling, which is an important reason for the deterioration of the quality of fruits and vegetables. The results of the correlation analysis between microbial indicators and other indicators of fresh-cut desert Cistanche deserticola during storage are shown in Table 1. The total number of bacterial colonies was extremely significantly or significantly positively correlated with the weight loss rate, a* value, b* value, browning degree, total phenol and MDA content (P<0.01, P<0.05), indicating that as the total number of bacterial colonies increased, It accelerated the browning and quality loss of fresh-cut desert cistanche; it was extremely significantly negatively correlated with hardness, TSS, and L* values (P<0.01), indicating that when the microbial content of fresh-cut desert cistanche is low, its hardness, nutrition Substances as well as L* values are maintained at a high level. It can be seen that MeJA treatment can better maintain the storage quality of fresh-cut desert Cistanche deserticola by inhibiting the growth of microorganisms.

Table 1 Correlation analysis of various indicators of fresh-cut Cistanche deserticola during storage

2.8.2 Principal component analysis of the impact of MeJA treatment on relevant indicators during storage of fresh-cut desert Cistanche deserticola

Table 2 shows the variance contribution rate obtained from factor analysis of fresh-cut desert Cistanche deserticola. SPSS 25.0 was used to perform principal component analysis (PCA) on the quality indicators of fresh-cut desert cistanche. Two principal components were extracted, with eigenvalues both greater than 1, and the cumulative variance contribution rate reached 92.019%, indicating that most of the information in the original data was absorbed by these two The principal components are covered, so the number of subsequent principal component analyzes is determined to be two. It can be seen from Table 3 that hardness, weight loss rate, L* value, a* value, b* value, MDA, browning degree, total bacterial colonies, TSS and total phenols have a greater contribution to the first principal component; while PPO and POD The contribution to the second principal component is larger. The absolute values of the loadings of hardness, weight loss rate, L* value, a* value, b* value, MDA, browning degree and total number of bacterial colonies were all greater than 0.96, indicating that these indicators have a greater impact on the storage quality of fresh-cut desert Cistanche deserticola.

Table 2 Principal component eigenvalues and contribution rates

Put the factor component matrix coefficients and corresponding eigenvalues (λ) into the following formula to calculate the corresponding eigenvector:

F = PCA/√λ (2) Using the eigenvector as the coefficient, the linear equation of the two principal components is obtained as follows:

Y1=0.320X1-0.329X2+0.326X3-0.297X4-0.327X5+0.327X6+0.324X7+0.322X8+0.197X9+0.160X10+0.049X11+0.326X12 (3)Y2=-0.159X1+0.040X2- 0.019 X3+0.173X4+0.080X5

-0.055X6+0.053X7-0.076X8+0.355X9+0.592X10+0.665X11-0.076X12 (4) Substitute the standardized data into the above formula to calculate the score of each main component. Then, the variance contribution rate of the two principal components is used as a coefficient to calculate the comprehensive score. The comprehensive score calculation formula is as follows:

Y=(Y1×75.912+Y2×16.108)/92.019 (5)

From this, the comprehensive score of each treatment during storage was calculated. The lower the a* value, b* value, weight loss rate, browning degree, PPO activity, POD activity, MDA content, and total number of bacterial colonies, the better the quality of fresh-cut desert Cistanche deserticola, and these 8 indicators are in the main component 1 and the main component. The sum of the coefficients in component 2 is higher than the sum of the other four indicators, so the lower the score, the better the storage quality of fresh-cut desert Cistanche deserticola is with this processing method; vice versa, the worse it is. This is consistent with the PCA analysis results of Li Shanshan et al. [17] on the storage quality of plum fruits at different harvest periods.

Put the factor component matrix coefficients and corresponding eigenvalues (λ) into the following formula to calculate the corresponding eigenvector:

F = PCA/√λ (2) Using the eigenvector as the coefficient, the linear equation of the two principal components is obtained as follows:

Y1=0.320X1-0.329X2+0.326X3-0.297X4-0.327X5+0.327X6+0.324X7+0.322X8+0.197X9+0.160X10+0.049X11+0.326X12 (3)Y2=-0.159X1+0.040X2- 0.019 X3+0.173X4+0.080X5

-0.055X6+0.053X7-0.076X8+0.355X9+0.592X10+0.665X11-0.076X12 (4) Substitute the standardized data into the above formula to calculate the score of each main component. Then, the variance contribution rate of the two principal components is used as a coefficient to calculate the comprehensive score. The comprehensive score calculation formula is as follows:

Y=(Y1×75.912+Y2×16.108)/92.019 (5)

From this, the comprehensive score of each treatment during storage was calculated. The lower the a* value, b* value, weight loss rate, browning degree, PPO activity, POD activity, MDA content, and total number of bacterial colonies, the better the quality of fresh-cut desert Cistanche deserticola, and these 8 indicators are in the main component 1 and the main component. The sum of the coefficients in component 2 is higher than the sum of the other four indicators, so the lower the score, the better the storage quality of fresh-cut desert Cistanche deserticola is with this processing method; vice versa, the worse it is. This is consistent with the PCA analysis results of Li Shanshan et al. [17] on the storage quality of plum fruits at different harvest periods.

Table 3 Composition matrix of fresh-cut Cistanche deserticola

It can be concluded from Figure 8 that from the score rankings of different concentrations and different days, as the storage time prolongs, the quality of each treatment group continues to decline, and the score of the CK group is significantly different from that of the MeJA treatment group, indicating that MeJA can Better to maintain the quality of fresh-cut desert cistanche deserticola. The comprehensive quality scores of different treatment groups from low to high are 200 µmol/L, 150 µmol/L, 100 µmol/L, and CK. In the first 4 days of storage, the scores of each group were relatively close to each other and all were lower than the critical value 0, indicating that the storage quality of fresh-cut desert Cistanche deserticola was better; on the 6th day, only the scores of the M3 group and M2 group were less than 0, and M3 The group has the lowest score, indicating that 200 µmol/L MeJA treatment can better maintain the storage quality of fresh-cut desert Cistanche deserticola.

Fig.8 Comprehensive scores of fresh-cut Cistanche deserticola during storage

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