Effects of Different Treatments on White-blush of Fresh-cut Carrots During Storage

: In order to screen out the preservatives that had better effects on inhibiting the white-blush of fresh-cut carrots, prolong its shelf life. Using fresh carrots as the test material, the effects of different treatments on the white-blush of fresh-cut carrots during storage were compared, and the changes in whiteness value of fresh-cut carrots in each group were analyzed. Other relevant physical and chemical indicators were measured. The results showed that compared with other treatment groups, the onion treatment group had the lowest whiteness index, the strongest antioxidant capacity, and the best quality of fresh-cut carrots. This indicated that onions were suitable for processing fresh-cut carrots, which cold better inhibit the activities of POD, PAL, and other enzymes, inhibit the white-blush of fresh-cut carrots, and delay their aging.


INTRODUCTION
Fresh-cut carrots, due to dehydration and lignification during storage, lead to whiteness on the surface, losing the characteristic bright yellow, limiting carrot development. Therefore, it is necessary to inhibit the white-blush of fresh-cut carrots and extend their shelf life.
There have also been some reports on the preservation of fresh-cut carrots and the correlation of their whitening control. Ren, L.F's (2014) research showed that 0.6% citric acid and 0.4% ascorbic acid had the best effect on reducing the whiteness value of fresh cut carrots. Chen Chen et al. (2018) reported in their research that treating fresh cut carrots with H2S could significantly inhibit their whitening, reduce the activity of lignin synthesis related enzymes (PAL, PPO, POD), and improve the antioxidant ability of fresh cut carrots. However, the application of biological preservatives such as natural plant extract and bioantagonistic bacteria in fresh-cut carrots has not been reported. Onions contain a large number of sulfur compounds, mainly disulfides and trictrulfides. A research [9] that showed that disulfides and triculfides could inhibit the enzyme activities of PPO and POD in fruits and vegetables, which provides a theoretical basis for onion juice to inhibit the white-blush of fresh-cut carrots.
In this experiment, fresh-cut carrots were treated with citric acid, ascorbic acid, hydrogen sulfide (donor: sodium hydrosulfide), and onion juice, with water cleaning group as a control group. The changes in whiteness values during storage in each group were analyzed, and relevant physiological and biochemical indicators were measured to explore the inhibitory effect of onion juice on the whitea 495501694@qq.com b 631126501@qq.com c *Corresponding author 543842249@qq.com blush of fresh-cut carrots and select the best preservative for the blanching inhibition of fresh-cut carrots.

Main Materials
Carrots, buy on market; onion, buy on market.

The Processing of the Samples
(1) Select fresh, pest-free, uniform size and quality carrots, peel and cut them into 2 cm thick pieces, clean with water, drain and set aside.
(2) Preparation of onion juice: In a sterile environment, peel and cut onions into pieces, place them in a juicer to extract juice, use sterilized four layers of gauze to aseptically filter on an ultra clean workbench to obtain a 100% concentration of onion stock solution, then dilute the stock solution with deionized water to a concentration of 75%, store at 4℃.
(3) Fresh-cut carrots were treated with citric acid, ascorbic acid, hydrogen sulfide (donor: sodium hydrosulfide), and onion juice. The water washing group was used as a control group, and stored at 4℃. Their physiological and biochemical indexes and whiteness values were measured on 1, 3, 5, 7 and 9 day, respectively, to analyze the changes in their whiteness value and physiological and biochemical indexes.

Determination of the Whiteness Value
Refer to the method of Bolin et al. (1991) for measurement. Whiteness index (Wi) was calculated as the follows:

Measurement of Phenylalanine Lylyase (PAL)
Refer to the method of Sun (2002) for measurement.

Measurement of Peroxidase (POD)
The guaiacol method (Li, 2002) was used for determination .

Determination of the Carotenoids
Refer to the method of Fan (2011) for determination.

The DPPH Free Radical Clearance Rate
Refer to the method of Chen et al (2016) for measurement.

Data Processing
Each group of samples was measured three times in parallel, and the data obtained were analyzed and processed by Origin8.5 software, and the Duncan method was used for multiple comparisons of difference significance. Those marked with different letters indicated significant differences between groups(P<0.05), while those marked with the same lowercase letters indicated insignificant differences between groups(P>0.05).   Table 1 was the difference between the group the first day and 9 day, from table 1, the difference between the onion juice treatment group and each group was significant (P <0.05), compared with the other group during the storage, indicating that the onion juice treatment was best, and could better suppress the whitening of fresh-cut carrots.   Table 2 showed the significant analysis of the difference between the first day and 9 day of the groups. From table 2, it showed that the difference between the onion treatment group and the other groups was significant (P <0.05). As could be seen from Fig 2, the PAL activity of onion treated group was low during storage, indicating that onion treated fresh-cut carrots could better inhibit its PAL enzyme activity. Therefore, onion juice was more suitable for processing fresh-cut carrots from the perspective of reducing PAL activity.  From Fig 3, the PPO enzyme activity of fresh-cut carrots increased first and then decreased during the whole storage period. Table 3 was an analysis of the difference between the first day and 9 day of each group. From table 3, it showed that the difference between the onion treatment group and the other groups was significant (P <0.05). As could been seen from Fig 3, the onion treatment group had low POD activity during storage, and with POD activity as the reference indicator, onion juice was more suitable for treating the fresh-cut carrots.  According to Fig 4, the carotenoid content of fresh-cut carrots showed a downward trend during storage. Table 4 was the significant analysis of the difference between the first day and 9 day, from table 4, there were significant differences(P <0.05) between the control group and the ascorbic acid treatment group and other groups. From Fig  4, it could been seen that the carotenoid content of freshcut carrots treated with onions decreased slowly during storage. Taking carotenoids as a reference index, onion treated fresh cut carrots had better effect, more suitable for processing fresh cut carrots.

Effect of different treatments on DPPH during storage of fresh-cut carrots
Fig5: The effect of different treatments on DPPH of fresh-cut carrots  Table 5 was an analysis of the significance of the difference between the first day and 9 day of each group. Table 5 showed there were significant(P <0.05) differences between the onion treatment group and other groups. As could be seen from Fig 5, DPPH in onion juice treated group was consistently higher, indicating that onion juice was more suitable for treating fresh-cut carrots and the effect was optimal.

CONCLUSION
Onion is a vegetable belonging to the genus Allium in the lily family, known as the "Queen of Vegetables" (Sun, et al. 2012), Because of its high nutritional composition. The active substances in onion include sulfur compounds [10] , Kaempferol, caffeic acid, lauric acid, mustard acid, quercetin and other substances [5] . The volatile oil is rich in ingredients, mainly alliin, thiol, dimethyl disulfide, trichiulfide, etc. Onions also have some other functions, such as anti-inflammatory and bacteriostatic, preventing hypertension, improving immunity, and reducing blood sugar [11] . In this experiment, citric acid, ascorbic acid, H 2 S, and onion juice were used as treatment groups to treat freshcut carrots, while those washed with clean water were used as control groups. After storage at 4℃ , compare the change of physiological and biochemical indicators, whiteness value as the main index, other indicators for reference, found that the quality of fresh-cut carrots treated with onion was best, the effect of inhibiting the white-blush of fresh-cut carrots was the best, and the antioxidant capacity was the strongest. This indicates that onion juice is more suitable for processing fresh-cut carrots, better inhibiting the activities of POD, PAL, and other enzymes, delaying their aging, and extending their shelf life. Onion juice is also a natural extract with nontoxic, harmless, and natural characteristics, which are unparalleled advantages of chemical preservatives such as citric acid, ascorbic acid, and H2S. Compared with chemical reagents, onion juice has the characteristics of natural, non-toxic and harmless, which opens up a new way to control the white-blush of fresh-cut carrots, and lays a foundation for the control of the white-blush of fresh-cut carrots by natural extracts in the future.