Effects of methionine treatment on storage quality and antioxidant activity of postharvest jujube fruit

LlU Yao,LEl Xing-meng,GUO Yu-xiao,YAO Shi-xiang, ,ZENG Kai-fang, ,

1 College of Food Science,Southwest University,Chongqing 400715,P.R.China

2 Food Storage and Logistics Research Center,Southwest University,Chongqing 400715,P.R.China

3 National Citrus Engineering Research Center,Chongqing 400712,P.R.China

Abstract

Jujube fruits usually suffer from physiological disorders or infectious diseases during storage,leading to quality deterioration,softening,or rotting.Therefore,this study aims to investigate the effect of methionine soaking treatment on the postharvest jujube fruit decay rate and storage quality.Methionine treatment significantly reduced the decay rate of postharvest jujube fruit and effectively maintained fruit color and titratable acid.Methionine treatment effectively delayed the decrease of firmness,soluble solids,ascorbic acid,and lignin contents in jujube fruit.Methionine treatment reduced the content of alanine and phenylalanine,and increased the content of glycine in jujube fruit,but displayed no significant effect on total amino acid content.In addition,jujube fruits in the methionine treatment group had a higher total phenolic and flavonoid content and antioxidant capacity,both in free and bound forms.Compared with the control,methionine treatment also significantly increased the content of individual phenolic acid fractions (gallic acid,vanillic acid,and syringic acid) and flavonoid fractions (catechin,epicatechin,rutin) in free form as well as individual phenolic acid fractions in bound form (gallic acid and ferulic acid) in jujube fruits.Overall,this study suggested that the methionine treatment could be used as a prospective preservative to reduce the postharvest decay of jujube fruit and alleviate its nutritional quality deterioration during cold storage at 4°C.

Keywords: methionine,jujube,decay rate,sensory quality,nutritional value

1.lntroduction

Fresh jujube fruit is high in moisture content,crisp and sweet in taste,and rich in nutrients.Mature jujube fruit is rich in bioactive compounds,such as ascorbic acid (AsA) and phenolic compounds,amino acids,polysaccharides,and minerals,making jujube fruit widely considered to be a fruit that has a high market and health values (Gaoetal.2013).In 2018,China’s jujube production reached 8.5 million tons,accounting for more than 90% of jujube production in the world (Wangetal.2021).However,the peel of jujube fruit is susceptible to mechanical damage during transportation,resulting in storage intolerance after harvest.When stored at room temperature for about a week,jujube fruit could lose water,shrink,soften,and rot,which causes huge economic losses for jujube fruits after harvesting (Dengetal.2021).Therefore,the study of storage technology for postharvest jujube fruit to delay the occurrence of quality deterioration and decay is a current problem that needs to be solved.

Fruit quality is primarily divided into external quality and internal quality.Firmness and color,as the external quality of the fruit,are the main quality attributes by which consumers judge the fruit and are important factors limiting the postharvest life and quality deterioration of the fruit (Kumarihamietal.2022).Soluble solids (TSS) and titratable acid (TA) are the main compounds affecting the sweetness and acidity of fruits,which are significant sensory properties related to the edibility of ripe fruits (Kumarihamietal.2022).Lignin is a source of dietary fiber in the human diet and is beneficial to human health (Sch?feretal.2016).AsA,phenolic acids,and flavonoids are important nutrients in fruits and vegetables as well and can be used as antioxidants to endow fruits and vegetables with functions that promote human health,such as strengthening the immune system and preventing cardiovascular diseases (Kimetal.2008).It was reported that these substances not only positively contribute to the maintenance of nutritional quality but are also closely related to improving disease resistance in fruits and inhibiting decay (Kumarihamietal.2022).Gununuetal.(2019) found that peach fruit with a high content of antioxidant substances had a lower fruit incidence and surmised that high disease resistance was associated with a high content of phenolic compounds.Jujube fruit with postharvest disease caused byAlternariaalternatainfestation was related to a decline in the content of individual phenolic compounds in jujube pericarp tissue,including catechin and epicatechin (Yuanetal.2019).

In recent years,many compounds have been investigated,including 1-methylclopropene (Ozturketal.2021),propyl gallate (Wangetal.2022),melatonin (Wangetal.2021),Prunusmumepolyphenols (Yuetal.2021),etc,to effectively reduce jujube fruit quality deterioration and decay during postharvest storage.Methionine is an essential amino acid that is safe,inexpensive,and simple to use.Additionally,as a nutrient essential for the vital activities of organisms,humans and other mammals require small amounts of methionine in their diets to maintain normal growth and body functions (Willke 2014).In addition,studies have reported that methionine also has a wide range of functions in plants,such as participating in plant systemic defenses and reducing the incidence of plant-infectious diseases (Boubakrietal.2013; Hasabietal.2014).However,little research has been reported on the control of postharvest fruit and vegetable diseases by exogenous methionine treatment.Early laboratory studies found that 0.1 g L–1methionine treatment could effectively control jujube fruit black spot disease by inducing fruit disease resistance (Liuetal.2022).Studies have also reported that methionine treatment is beneficial in maintaining the biochemical and sensory quality of postharvest broccoli and litchi fruit (Alietal.2018; Sohailetal.2021).Nevertheless,there are no reports about the effect of methionine treatment on the decay and overall quality of postharvest jujube fruit.Therefore,the effect of methionine soaking treatment on postharvest jujube fruit quality was studied by measuring the physiological and biochemical indices of fruits during low-temperature storage,which can provide a theoretical basis for the postharvest storage and preservation of jujube fruits.

2.Materials and methods

2.1.Fruit treatment and sampling

Green mature jujube fruits were harvested from an orchard in Linfen City,Shanxi Province,China.Before use,the surface of the jujube fruit was disinfected with 2% sodium hypochlorite solution and then rinsed with water.Jujube fruits were then dipped in pure water (control) and 0.1 g L–1methionine (Macklin Biochemical Co.,Ltd.,Shanghai,China) solution for 10 min at room temperature,dried and stored at 4°C.The jujube fruits of both treatment groups were sampled,and the relevant indices were measured every 10 d.Fresh jujube tissue with uniform thickness(5 mm) was cut,quickly chopped,and then stored at –80°C for measuring other indices.Each sampling included 45 jujube fruits and contained 3 replicates.

2.2.Measurement of decay rate

Jujube fruit decay rate (%)=[(The number of decayed fruit/The number of total fruit)]×100.Each treatment group had 300 jujube fruits,containing 3 replicates.

2.3.Measurement of color and firmness

The color of the peel of the jujube fruit was measured using a chromameter (Minolta,Osaka,Japan) according to the method of Maetal.(2021).TheLvalue (Lightness),avalue (Red/Green value),andbvalue (Blue/Yellow value) were measured to indicate the color of the fruit.The firmness of the jujube fruit was measured using a Fruit Texture Analyzer (GS-15,South Africa) for two symmetrical points at the equator of the fruit,and the average of the two points measured the firmness of the fruit,which is expressed in g.

2.4.Determination of TSS,TA,AsA,and lignin content

Jujube fruits were homogeneously ground and filtered,and the TSS content of the juice was recorded using a digital refractometer and expressed as a percent.The TA content was determined by acid-base titration (Fanetal.2022) and was expressed as a percent of tartaric acid.A colorimetric method was used to determine the AsA concentration (Jiangetal.2018) and the units are expressed in mg g–1fresh weight (FW).For the determination of lignin content,we refer to the method of Dengetal.(2015),and the units are expressed in OD280g–1FW.

2.5.Determination of free amino acid content

To measure the content of free amino acid content,the sample powder (2.0 g) was mixed with 2 mL of 6% sulfosalicylic acid in a centrifuge tube with a vortex mixer and ultrasonicated at 20–25°C for 1 h.Subsequently,the filtrate was filtered through a 0.22-μm aqueous membrane after centrifugation at 4°C for 20 min and analyzed in an amino acid analyzer (L-8800,Hitachi,Japan) (Jinetal.2019).The units are expressed in μg g–1FW.

2.6.Measurement of polyphenols

Extraction of free phenolicsA powdered sample of 1.0 g was added to 25 mL of 80% precooled acetone and homogenized in an ice bath.The homogenate was centrifuged at 3 500 r min–1at 4°C for 10 min and the supernatant was collected.The above residue extraction was repeated twice.All supernatants were collected and filtered,and then the filtrate was evaporated to dryness using a rotary vacuum evaporator (RV 10,IKA,Germany) at 45°C.Finally,the residue was dissolved in pure water,fixed to 25 mL for further determination (Okarteretal.2009; Viacavaetal.2020).

Extraction of bound phenolicsThe residue obtained after the extraction of free phenolics was used to extract bound phenolics.After mixing 25 mL NaOH solution (2 mol L–1) with the residue,the reaction was carried out for 1.5 h under light-proof conditions.Then,25 mL of hexane was added,stirred for 10 min under light-proof conditions,and centrifuged.The above operation was repeated once.We then added 25 mL of ethyl acetate and centrifuged the mixture after 10 min of reaction,collected the supernatant,and repeated the extraction three times.All collected ethyl acetate extracts were evaporated to dryness at 45°C.Finally,the residue was dissolved in pure water,fixed to 10 mL for further determination.

Determination of total phenolic contentThe total phenolic content of jujube fruits was determined using the Folin-Ciocalteu method (Lietal.2022).First,200 μL of free and bound phenolic extracts were added separately to the centrifuge tubes,and 800 μL of pure water and 200 μL of Folin-Ciocalteu reagent were subsequently added.A total of 2 mL of 7% Na2CO3were added after 6 min and finally fixed to 5 mL with pure water.After thorough mixing,the mixture was placed for 90 min in the dark and measured at 760 nm.The units are expressed as gallic acid equivalents per gram of fresh weight sample.

Determination of total flavonoid contentThe total flavonoid content was determined according to a method described by Lietal.(2022).Aliquots of 0.5 mL extracts were reacted with 0.15 mL 5% NaNO2and 0.15 mL 10% Al(NO3)3for 5 min,then 2 mL 4% NaOH and 2.2 mL 60% ethanol were added into the centrifuge tube.The mixture was reacted at room temperature for 15 min,and the absorbance of the solution was measured at 510 nm.The flavonoid content is expressed as equivalent to rutin standard per gram of fresh weight sample.

Determination of polyphenol compositionFree and bound phenolic components were determined following the method of Chen Oetal.(2021) with appropriate modifications.Mobile phase A: 0.2% formic acid aqueous solution; mobile phase B: 100% acetonitrile.The linear gradient procedure for the separation of phenolic compounds was applied: 0–5 min,90% A; 5–50 min,90–60% A; 50–55 min,60–10% A; 55–62 min,10% A; 62–65 min,10–90% A; 65–75 min,90% A.The elution rate was 0.7 mL min–1and the column temperature was kept at 30°C.The sample solution (20 μL) was injected each time.The detection wavelength was 278 nm for epicatechin,259 nm for vanillic acid,275 nm for syringic acid,262 nm for gallic acid,280 nm for catechin and ferulic acid,and 266 nm for rutin.

2.7.Determination of antioxidant activity

2,2-Azino-bis-3ethylbenzothiazoline-6-sulfonic acid (ABTS) radical scavenging assayThe ABTS assay was performed using a previous method with minor modifications (Chen Z Cetal.2021).The reaction solution for the free phenolic ABTS radical scavenging rate assay (10 μL free phenolic extract,190 μL pure water,and 1 mL ABTS+solution) and the reaction solution for the bound phenolic ABTS radical scavenging activity assay (200 μL bound phenolic extract,1 mL ABTS+solution) were reacted for 6 min under light-proof conditions,and the absorbance ODjwas measured at 734 nm.OD0was obtained by using pure water as a control.ABTS radical scavenging rate/(%)=(1–ODj/OD0)×100.

2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assayIn this stage,the method of Shamilietal.(2021) was used.The reaction solution for the free phenolic DPPH radical scavenging rate assay (20 μL of free phenolic extract,180 μL of pure water,and 1 mL of 0.1 mmol L–1DPPH solution) and the reaction solution for the bound phenolic DPPH radical scavenging activity assay (200 μL of free phenolic extract and 1 mL of 0.1 mmol L–1DPPH solution) were reacted for 30 min in the dark,and the absorbance ODjwas measured at 517 nm.OD0was obtained by using pure water as a control.DPPH radical scavenging rate/(%)=(1–ODj/OD0)×100.

2.8.Statistical analysis

All data in this study were analyzed for significant differences using SPSS 26.0 at different levels ofP＜0.05.

3.Results

3.1.Effect of methionine treatment on decay rate of jujube fruits

The decay rate of jujube fruits for both the control and methionine-treated groups gradually increased with storage time (Fig.1).Compared to the control group,jujube fruits in the methionine-treated group showed lower decay rates during storage (P＜0.05).

Fig.1 Effect of methionine soaking treatment on the decay rate of postharvest jujube fruit.Data are mean±SD (n=3).＊ indicates a significant difference between the control and methioninetreated groups (P＜0.05).

3.2.Effect of methionine treatment on color and firmness of jujube fruits

The color change of jujube peel during storage is presented in Fig.2-A–C.TheLvalue (Fig.2-A) andbvalue (Fig.2-C) showed an overall decreasing trend,and theavalue (Fig.2-B) gradually increased.However,methionine treatment significantly delayed the decrease in theLandbvalues and inhibited the increase in theavalue in jujube fruits compared with those of the control group.

Fig.2 Effect of methionine soaking treatment on the L value (A),a value (B),b value (C),and firmness (D) of postharvest jujube fruit.E,Visual appearance of jujube fruits in control group and methionine treatment group at 40 d.Data are mean±SD (n=3).＊ indicates a significant difference between the control and methionine-treated groups (P＜0.05).

The variation pattern of the firmness of jujube fruits during storage is displayed in Fig.2-D.The firmness of jujube fruits in the control and methionine-treated groups tended to decrease gradually with increasing storage time.At the beginning of storage,the fruit firmness was 1.84 g.After 40 d of storage,the fruit firmness of the control group and methionine-treated group was 1.78 and 1.81 g,respectively.Methionine treatment effectively delayed the decrease in fruit firmness during storage.The results showed that methionine treatment could inhibit fruit softening and maintain fruit firmness.

3.3.Effect of methionine treatment on the TSS,TA,AsA,and lignin content of jujube fruits

The TSS content of jujube fruits in the control and methionine treatment groups began to increase and then decreased during storage (Fig.3-A).At the 10th d of storage,the methionine treatment group had a lower content than the control,but at the later stage of storage,the methionine treatment group had a higher content,and there was a significant difference at the 20th and 30th d of storage.The trend of the TA content of jujube fruits in the control and methionine-treated groups was the same,decreasing first and then increasing (Fig.3-B).On the 10th d of storage,there was no significant difference between the two groups.However,after the 10th d of storage,the TA content in the methionine-treated group was significantly higher than that in the control group.The AsA content of postharvest jujube fruits decreased continuously during storage (Fig.3-C).However,compared with the control group,the decrease in AsA content of jujube fruit was slowed down after methionine treatment,and the difference between the two groups was significant throughout the storage period.The lignin content of jujube fruits increased first and then decreased gradually (Fig.3-D).The methionine treatment was significantly higher at the 10th,20th,and 30th d of storage compared to that of the control.

Fig.3 Effect of methionine soaking treatment on the total soluble solids (TSS) (A),titratable acid (TA) (B),ascorbic acid (AsA) (C),and lignin (D) contents of postharvest jujube fruit.Data are mean±SD (n=3).＊ indicates a significant difference between the control and methionine-treated groups (P＜0.05).

3.4.Effect of methionine treatment on the free amino acid content of jujube fruits

A total of 13 free amino acids were detected in jujube fruits (Table 1).Compared with the control group,the content of alanine and phenylalanine decreased and the content of glycine increased in the methionine-treated jujube fruits (P＜0.05),while the content of other amino acids did not change significantly.There was no significant difference in the total amino acid content between the two groups at the 10th d of storage.

Table 1 Effect of methionine soaking treatment on the free amino acid contents of postharvest jujube fruit

3.5.Effect of methionine treatment on the total phenolic content and phenolic acid components of jujube fruits

Total phenolic contentTotal phenols were primarily divided into free phenols and bound phenols (Ahmadetal.2021).Table 2 shows that the total phenolic content of jujube fruits was primarily free phenolics,and the content of bound phenolics was very low.Compared with the initial storage period,the free phenolic and bound phenolic contents of jujube fruits in the control and methioninetreated groups showed an overall decreasing trend after 40 d of storage.Methionine significantly delayed the reduction of total phenol content in jujube fruits.

Table 2 Effect of methionine soaking treatment on the total phenolic content of postharvest jujube fruit

Phenolic acid componentsThe phenolic compositions of free phenol and bound phenol in jujube fruits were determined using HPLC.The phenolic acids in the free phenols of jujube fruit were primarily gallic acid (Fig.4-A),vanillic acid (Fig.4-B),and syringic acid (Fig.4-C).The methionine treatment resulted in a higher gallic acid content than the control at the 20th and 40th d of storage but was lower than the control at the 30th d of storage.In comparison to the control group,the content of vanillic acid and syringic acid in the methionine-treated group was significantly increased during storage.

The phenolic acids in the bound phenols of jujube fruit were primarily gallic acid (Fig.4-D) and ferulic acid (Fig.4-E).At the 10th,30th and 40th d of storage,the content of gallic acid in the methionine-treated jujube fruits was significantly higher than that in the control group.The content of ferulic acid was not detected in the jujube fruits at the 40th d of storage.During storage,the ferulic acid content was significantly higher in the methioninetreated group than in the control group.

3.6.Effect of methionine treatment on the total flavonoid content and flavonoid components of jujube fruits

Data are mean±SD (n=3).Different letters represent significant differences between the control and methionine-treated groups (P＜0.05),as determined by the independent samplest-test.

Total flavonoid contentOnly the free flavonoid content was detected in the jujube fruits (Fig.5-A),and the bound flavonoid content was not quantified.The free flavonoid content of the control group showed a trend of decreasing and then increasing during storage.When compared to the control,methionine treatment increased the free flavonoid content of jujube fruit throughout storage.

Fig.5 Effect of methionine soaking treatment on the free flavonoid content (A) and flavonoid components (B–D) of postharvest jujube fruit.Data are mean±SD (n=3).Different letters represent significant differences between the control and methionine-treated groups at different storage time (P＜0.05).

Flavonoid componentsThe flavonoid fractions in the free phenols of jujube fruits were mainly identified as catechin (Fig.5-B),epicatechin (Fig.5-C),and rutin (Fig.5-D).No flavonoid components were identified in the bound phenolic fraction of jujube fruits.In comparison with d 0,the content of catechin and rutin in the control group decreased after a period of storage,but the content in the methionine-treated jujube fruit remained at a higher level,both significantly higher than the control group,except for the rutin content at 30th d of storage.In addition,epicatechin content increased throughout storage,but the increase was more significant in the methionine-treated group than in the control group.

3.7.Effect of methionine treatment on the antioxidant capacity of polyphenols in jujube fruits

The results of ABTS radical scavenging of free and bound phenols showed that the ABTS radical scavenging of polyphenols in the control jujube fruits showed a gradual decreasing trend,while methionine treatment effectively delayed the decrease of this trend (Fig.6-A and B).Similarly,the results of the DPPH radical scavenging assay showed that the methionine-treated group had a higher DPPH radical scavenging rate than the control group (Fig.6-C and D).

4.Discussion

During storage,fresh jujube fruits turn red and soften soon after harvest.Moreover,jujube fruit peel is thin and easily damaged,making it easy for pathogenic microorganisms to infect,which leads to mold and the decay of the fruit (Zhaoetal.2021).This greatly affects the quality of postharvest storage and transportation and hinders the sales and development of fresh jujube fruit.The present study indicated that methionine soaking treatment was effective in reducing the decay rate of postharvest jujube fruits during storage.Pericarp color is a core indicator of fruit appearance quality (Kouetal.2018).In this study,the color change of jujube fruit during storage was consistent with the results of Chenetal.(2019),which showed an increasing trend in theavalue and a decreasing trend in theLvalue andbvalue.This result indicated that the postharvest jujube fruit treated with methionine effectively delayed the trend of fruit color change during storage.For fresh jujube,these changes in color values are important indicators reflecting pericarp brightness and turning red.Remarkably,the postharvest jujube fruit treated with methionine effectively delayed the trend of fruit color change under storage.Firmness is an important indicator to measure the texture quality of fruit and a visual presentation of the degree of fruit softening.The present study showed that methionine treatment significantly inhibited the decrease in firmness of jujube fruits.

The TSS and TA contents contribute to the sweetness and acidity of the fruit and are important indicators of fruit taste and flavor during ripening and postharvest storage (Kumarihamietal.2021).The results of our study showed that the TSS content of jujube fruit first increased and gradually decreased during storage,and methionine treatment delayed the decrease in TSS content,which was highly similar to the published reports (Lvetal.2022).This may be due to the fact that there are large molecular substances,such as starch,in jujube fruit at the early stage of shelf life,and the degradation of these substances could increase the TSS of jujube fruit.At the later stage of storage,due to nutrient consumption by respiration,the TSS decreased.The TA content of jujube frut decreased initially and then increased,which was similar to the results of Wangetal.(2014).Additionally,methionine treatment maintained the TA content of jujube fruit at a high level.Organic acids,which are generally expressed as TA,are important respiratory substrates to maintain their metabolism and are produced along with the tricarboxylic acid cycle in fruit and vegetable (Zhang et al.2019).We speculate that the increase in TA content at a later stage may be related to the respiratory metabolism and tricarboxylic acid cycle pathway (Zhang et al.2019) of jujube fruit during storage or to the maturity of the fruit (Zhang R N et al.2022).Kou et al.(2018) reported that AsA,as an important nutrient and antioxidant substance in fresh fruits,is easily consumed,oxidized,and decomposed,leading to a decrease in AsA content.This agrees with our results,and methionine treatment effectively delayed the decrease in AsA content.The results also showed that methionine treatment contributes to a higher lignin content in jujube fruits.

Jujube fruits are a good source of amino acids.Amino acids not only enhance the flavor but are also precursors to many secondary metabolites in the fruit (Zhang Betal.2022).The results of our study showed that methionine treatment reduced the content of alanine and phenylalanine,and increased the content of glycine in jujube fruit,but did not significantly change the contents of other amino acids.It has been reported in the literature that protein degradation to produce free amino acids and secondary metabolism may be the cause of some free amino acid content changes in jujube fruit (Huangetal.2022).Studies have reported that phenylalanine,as a substrate of the phenylpropanoid metabolic pathway,undergoes a series of reactions catalyzed by enzymes and eventually produces phenolic secondary metabolites (Adamsetal.2019).The results of previous experiments in our laboratory showed that methionine inoculation treatment activated the phenylpropanoid metabolic pathway in jujube fruits (Liuetal.2022).However,alanine and glycine are few reports about its transport mechanism in plants,so it is not clear how it is utilized in jujube fruit.It is worth noting that methionine treatment displayed no significant effect on total amino acid content of jujube fruit.

Furthermore,the phenolic compound contents of jujube fruits were determined in this study.Phenolic compounds are important bioactive substances that play an important role in improving the nutritional value of fruits and benefiting human health,such as the prevention of cancer,cell aging,and cardiovascular diseases (Wangetal.2021).Phenolic compounds of fruits are present mainly in free and bound forms.The results showed that phenolic compounds of jujube fruits were mainly in the free form,which is consistent with a previous report (Kouetal.2015).The free phenolic content was much greater than the bound phenolic content in the jujube fruit.Additionally,higher values of both phenolics and flavonoids were detected in jujube fruits of the methioninetreated group during storage.

Moreover,the phenolic acid and flavonoid fractions in the free and bound phenols differed.The main individual phenolic acid fractions detected in the free phenols were gallic acid,vanillic acid,and syringic acid,while the main individual phenolic acid fractions detected in the bound phenols were gallic acid and ferulic acid.Among them,an increase followed by a decrease in ferulic acid content was observed in the jujube fruits of the control and methionine-treated groups.The change of content of ferulic acid,an important metabolite of the phenylpropanoid pathway,may be caused by the stimulation of key enzymes and genes owing to the phenylpropanoid pathway,which is related to the biosynthesis of phenolic acids and flavonoid content in fruit (Duanetal.2023).It has been reported that the sharp decline in phenolic content may be due to the destruction of cell structure or microbial activity (Eshghietal.2022).Also,three flavonoid fractions,catechin,epicatechin,and rutin,were detected in the free phenols,while no flavonoid fraction was detected in the bound phenols.This is similar to previous studies on jujube fruits (Hudinaetal.2008; Ozturketal.2021).Of interest,the results showed that the flavonoid content of the control group decreased and then increased,while the methionine treatment group showed the contrary result,which may be related to the activation of the phenylpropanoid pathway in the jujube fruit by methionine treatment (Liuetal.2022).In general,the phenolic acid and flavonoid fractions of jujube fruit in the methionine treatment group were at high levels during storage.

Phenolics can scavenge free radicals and delay the lipid oxidation process by suppressing the initiation or diffusion of oxidizing chain reactions.In addition,the antioxidant capacity of fruits is highly correlated with phenolic content (Xuetal.2021).The results of previous laboratory studies showed that methionine treatment reduced malondialdehyde content and inhibit membrane lipid oxidation in jujube fruits (Liuetal.2022).Our study showed that the ABTS and DPPH free radical scavenging rates were higher in the free phenol than in the bound phenol.Overall,the antioxidant capacity of ABTS and DPPH radicals for both free and bound phenols showed a similar trend as that of the phenolic content results.This indicates a positive correlation between phenolic content and antioxidant capacity,which is consistent with previous findings (Bagheri and Esna-Ashari 2022).Moreover,the ABTS free radical scavenging rate (free and bound forms) of jujube fruits was higher in the methionine-treated group.Similar results were shown in the DPPH assay.

5.Conclusion

This study showed that methionine soaking treatment was effective in reducing the decay rate and maintaining postharvest jujube fruit appearance quality,including firmness and color,and nutritional value,including TSS,TA,AsA,and lignin content,during storage.Methionine treatment reduced the content of alanine and phenylalanine,and increased the content of glycine in jujube fruit,but displayed no effects on total amino acid content.In addition,jujube fruits in the methionine treatment group had a higher total phenolic and flavonoid content and antioxidant capacity,both in free and bound forms.Compared with the control,methionine treatment also increased the content of individual phenolic acid fractions (gallic acid,vanillic acid,syringic acid) and flavonoid fractions (catechin,epicatechin,rutin) in free form as well as individual phenolic acid fractions in bound form (gallic acid,ferulic acid) in jujube fruits.Overall,methionine treatment not only reduced the decay rate but also effectively maintained the sensory quality and nutritional value of postharvest jujube fruits during storage.The application of methionine is expected to be a promising method to delay the deterioration of jujube fruit quality during storage in the future.

Acknowledgements

This research was supported by the National Key R&D Program of China (2019YFD1002300).

Declaration of competing interest

The authors declare that they have no conflict of interest.