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Aloe vera gel coating delays postharvest browning and maintains quality of harvested litchi fruit
Postharvest Biology and Technology 157 (2019) 110960
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Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Aloe vera gel coating delays postharvest browning and maintains quality of harvested litchi fruit
T
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Sajid Alia,b, Ahmad Sattar Khana, , Aamir Nawazb, Muhammad Akbar Anjumb, Safina Nazb, Shaghef Ejazb, Sajjad Hussainb a b
Postharvest Research and Training Centre, Institute of Horticultural Sciences, University of Agriculture, Faisalabad, 38040, Pakistan Department of Horticulture, Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University, Multan, 60800, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: Antioxidant activity Edible coating Oxidative stress Postharvest quality Skin browning
Postharvest surface browning is the leading constraint for extension of shelf life and marketing of litchi fruit. In the present work, litchi fruit were treated with Aloe vera (ALV) gel coating [50% (v/v)] and kept at 20 ± 1 °C for 8 d to investigate its effect on browning and postharvest quality. ALV gel coated fruit showed reduced browning index, weight loss, superoxide anion, relative electrolyte leakage, hydrogen peroxide and malondialdehyde content, compared to control. ALV coated fruit had higher total anthocyanin content and reduced peroxidase and polyphenol oxidase activities. ALV treatment had higher ascorbic acid content and total phenolic concentration, compared to control. In addition, ALV coated fruit maintained higher catalase, superoxide dismutase and ascorbate peroxidase activities along with higher total soluble solids and titratable acidity, than control. In conclusion, ALV gel coating could be considered an ecofriendly non-chemical alternative treatment for postharvest quality management of litchi fruit.
1. Introduction Litchi is a sub-tropical fruit. It possesses pinkish/bright red colored skin and has juicy and sweet edible aril (Liu et al., 2011; Fahima et al., 2019). It is a non-climacteric fruit and does not ripe off the trees after harvest. So, its fruit should be harvested at proper maturity stage to ensure its characteristic quality (Jiang et al., 2006). However, ripe litchi fruit perish rapidly after harvest and lose their attractive color in 1–2 d during ambient conditions due to skin browning (Sivakumar et al., 2010). Browning of litchi fruit reduces visual quality and negatively affects purchase decision of the consumers in the markets eventually leading to significant economic losses (Sivakumar et al., 2010). Several factors have been found associated with browning of litchi fruit. Mechanical injuries, moisture loss, pathogen infection, lipid peroxidation, cellular compartmentalization loss, oxidative stress and mixing of phenolic substrates with oxidative enzymes lead to development of browning in litchi fruit (Jiang et al., 2004). The integrity loss of membrane disrupts cellular structure and phenolic substrates subsequently mix with polyphenol oxidase and peroxidase enzymes. Sulfur dioxide is the most commonly used anti-browning chemical at commercial scale in litchi industry. It primarily inactivates polyphenol oxidase enzyme and decreases browning of litchi fruit (Zhang et al.,
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2018). However, due to potent allergic reactions in the end users and packaging house workers the use of sulphur is not appropriate for litchi fruit. It imparts negative effects and leads to reduced edible quality due to sulphur-induced increased aril acidity (Sivakumar et al., 2010). Thus, environmental friendly, safe and natural alternative treatment is required that not only maintains quality of the treated fruit but is also safe for the consumers and packaging house persons. Coating application could be considered highly suitable because litchi fruit are consumed after peeling. Plant based edible coatings are considered potentially safe and effective in quality conservation and shelf-life extension of fresh produce. The said coatings provide an effective barrier that eventually leads to reduced mass loss, delayed ripening and maintained quality (Mahajan et al., 2018). ALV gel is also a natural and ecofriendly non-chemical plant based edible coating. AVL gel is pulp based gelatinous matrix obtained from leaf tissues of aloe plants (Aloe vera and Aloe arborescens). Its use has been found appropriate in conserving quality of various fruits including plum (Guillén et al., 2013), peach (Guillén et al., 2013), raspberry (Hassanpour, 2015), blueberry (Vieira et al., 2016), papaya (Mendy et al., 2019), cherry (Ozturk et al., 2019), sapodilla (Khaliq et al., 2019) and orange (Rasouli et al., 2019). It has been noted that ALV gel coating suppresses phenolic oxidation and
Corresponding author. E-mail address: [email protected] (A.S. Khan).
https://doi.org/10.1016/j.postharvbio.2019.110960 Received 1 April 2019; Received in revised form 17 July 2019; Accepted 17 July 2019 0925-5214/ © 2019 Elsevier B.V. All rights reserved.
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2.4. MDA content and relative electrolyte leakage
inhibits enzymes activity of peroxidase as well as polyphenol oxidase and reduces browning along with preserved quality of treated commodity (Supapvanich et al., 2016; Ali et al., 2019a). However, at present no information is available regarding the effect of ALV based edible coating on surface discoloration and antioxidant activities of litchi fruit. So, current research was done to evaluate efficacy of ALV gel coating on browning and postharvest quality of litchi fruit.
MDA was assayed with the method of Sun et al. (2011). Peel (1 g) was carefully homogenized in 5 mL trichloroacetic acid (TCA) and centrifuged at 10,000×g for 20 min. Supernatant (2 mL) and thiobarbituric acid (2 mL) was reacted and assay mixture was boiled for 15 min at 100 °C. The assay mixture was cooled and centrifuged at 5000×g for 15 min. Absorbance was noted at 600, 532, and 450 nm on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). MDA was calculated and it was reported as nmol kg−1. MDA= [6.45 (A532 − A600) − 0.56 × A 450] × Vt × Vr/( Vs × m) , where Vt = volume of extract solution, Vr = reaction mixture volume, Vs = total volume of extract solution containing solution of reaction mixture and m = sample mass (Yang et al., 2011). Relative electrolyte leakage (REL) was measured by following the protocol of Chen et al. (2008). Peel discs [10 mm (20)] were added in 20 mL deionized water and placed at 25 °C for 30 min. Initial reading (Lt) was measured with the conductivity meter (HI-98304, Hanna, Mauritius) after 30 min and mixture of each sample was boiled in water bath for 15 min. Final reading (L0) after cooling of the boiled solution was noted and REL was calculated with equation in percent. REL (%) = Lt/Lo × 100.
2. Materials and methods 2.1. Plant materials Litchi fruit (cv ‘Gola’) were harvested from an orchard at Haripur, KPK, Pakistan. The color of fruit was 90–100 % red with 22.36 ± 0.43% total soluble solids (TSS) and 0.45 ± 0.01% titratable acidity (TA). The fruit were free from disease and mechanical injuries having uniform size, shape and color. ALV (Aloe vera Miller) leaves were sourced from a local nursery. The age of Aloe vera leaves was 3 years. The leaves were washed with tap water to remove dirt and subsequently dipped in 0.1% sodium hypochlorite for 3 min. After drying, the leaves were manually peeled with stainless steel knife. The obtained mucilaginous gel was collected and blended in a blender. TSS, TA (citric acid) and pH values of raw gel obtained from Aloe vera leaves were 0.95 ± 0.012%, 0.039 ± 0.007% and 4.69 ± 0.090, respectively. After blending, the resultant gel was filtered with sterile muslin cloth to remove any fibrous fraction. The pH of raw gel was adjusted to 3.75 (Navarro et al., 2011) with citric acid, then it was pasteurized at 65 °C for 30 min (Vieira et al., 2016) and cooled for further use. ALV gel solution was diluted with distilled water at 1:1 (v/v) ratio and glycerol (1%) was added as a plasticizer.
2.5. H2O2 and O2–• contents H2O2 content was determined with the method of Velikova and Loreto (2005). Peel sample (1 g) was extracted with 1 mL TCA (0.1%) and centrifuged at 12,000×g for 15 min. Then 0.5 mL supernatant was mixed with phosphate buffer (10 mmol L−1) and 1 mL of 1 M KI. The solution absorbance was read at 390 nm and it was expressed as μmol kg-1. Production of O2–• content was assayed with the methodology of Yang et al. (2011). Peel sample (1 g) was homogenized in 3 mL of 50 mmol L−1 phosphate buffer (pH 7.8) having polyvinylpyrrolidone (1%) at 4 °C. The assay mixture was centrifuged at 10,000×g for 15 min. Production of O2–• content was calculated by monitoring NO2 (after comparing with standard curve) formation from hydroxylamine in the presence of O2–• and was expressed as nmol min−1 kg-1.
2.2. Treatments Fruit were separated into two groups. Total 600 fruit were used in the experiment [treatments (2) × replications (3) × fruit per replication (20) × sampling intervals (5)]. One group was treated with 50% ALV coating solution; whereas, other group was dipped in distilled water. Dipping time was 5 min for both treatments. The concentration was selected based on the preliminary trial in which 25, 50 and 75% (v/ v) ALV coatings were used. After dipping, fruit were left at room temperature until complete drying and stored at 20 ± 1 °C with 90% RH. Each treatment had three biological replicates and each replicate had 20 fruit. A composite sample of 20 fruit was used for every studied parameter. Ascorbic acid content, TSS and TA were assayed on the same d of sampling; whereas, for total phenolic content (TPC), total anthocyanin content, DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity (RSA) and enzyme assays, samples were treated with liquid nitrogen and stored at −80 °C until analyzed. The fruit were sampled on 0 (before coating), 2, 4, 6 and 8 d of storage period.
TPC in peel of fruit was analyzed with Folin–Ciocalteu reagent assay after taking absorbance at 765 nm (Ainsworth and Gillespie, 2007). Standard curve of gallic acid was plotted and concentration of TPC was expressed as mg kg−1. DPPH-RSA in peel of fruit was measured as described previously (Brand-Williams et al., 1995). In brief, 50 μL peel extract in methanol was mixed with 0.1 mmol L−1 concentrated DPPH (3 mL). The solution was placed in dark at 25 °C for 30 min and absorbance was read at 517 nm. Finally, DPPH-RSA was expressed in percent.
2.3. Browning index, weight loss and total anthocyanin content
2.7. Assays of enzyme activities
Browning index was measured visually on a scale that comprised of no browning = 1; 1–2 brown spots = 2, few spots of browning = 3; 50% browning = 4 and 75% to complete browning of fruit surface = 5 and expressed as score (Sivakumar and Korsten, 2010). For the measurement of weight loss, fruit were weighed on electronic balance after dipping in respective treatments (after drying of fruit) before storage and at the end of each sampling interval. Loss in weight was expressed in percent. For total anthocyanin content, Zheng and Tian (2006) method was used. Peel (10 g) was extracted in HClmethanol solution and absorbance of obtained extract was read at 530, 620 and 650 nm on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). Total anthocyanins content was calculated as ΔA g−1 = (A530 − A620) − 0.1(A650 − A620).
Activities of polyphenol oxidase (PPO), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD) enzymes were assayed in peel tissues of fruit. Peel (1 g) was homogenized in 2 mL citrate (pH 4) buffer and centrifuged at 10,000×g for 10 min (Ali et al., 2016a). All steps of sample homogenization, crude enzymes extraction and centrifugation were carried out at 4 °C. Supernatant was collected and was used for assays of the said enzymes.
2.6. TPC and DPPH-RSA
2.7.1. PPO and POD enzymes activities Activities of PPO and POD enzymes were assayed with the method of Ali et al. (2016a). For PPO enzyme, activity was assayed by monitoring absorbance change at 412 nm. “One PPO activity unit (U) was equal to enzyme quantity that causes absorbance change in 0.001 units 2
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per min”. Activity of POD enzyme was analyzed by observing tetraguiacol formation from the guaiacol at 470 nm. “One POD activity unit (U) was equal to enzyme quantity that leads to absorbance change in 0.01 units per min”. 2.7.2. APX, CAT and SOD enzymes activities Activity of APX was assayed by monitoring ascorbic acid oxidation at 290 nm in presence of H2O2 (Nakano and Asada, 1987). “One APX activity unit (U) was defined as enzyme quantity that oxidizes one μmol ascorbat per min”. CAT and SOD activities were assayed with the protocol of Ali et al. (2016a). For activity of CAT enzyme, H2O2 decomposition was measured at 240 nm; while SOD enzyme activity was determined with nitroblue-tetrazolium (NBT) method at 560 nm after illumination of assays solutions under UV-light. “One CAT activity unit (U) was equal to enzyme quantity that causes decomposition of H2O2 per min” whereas “One SOD activity unit (U) was defined as enzyme quantity that inhibited 50% photochemical reduction of NBT”. 2.7.3. Assay of protein content Protein content was determined with Bradford method (Bradford, 1976) and enzymes activities were expressed as U mg−1 protein. Absorbance for all enzymes was read on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). 2.8. TSS and TA TSS and TA were determined in pulp of fruit (Ali et al., 2016a). TSS of fruit was estimated with digital refrectometer (Atago Pal-1, Japan) and was expressed as percent. TA was determined with titration method by using 0.1 N NaOH and expressed as percent. 2.9. Ascorbic acid content Ascorbic acid content in pulp of fruit was assayed with 2, 6-dichlorophenol-indophenol method and was expressed as mg kg−1 (Ali et al., 2016a).
Fig. 1. Effect of Aloe vera gel coating on browning index (A), weight loss (B) and total anthocyanin content (C) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates. Weight loss was assessed from whole fruit.
2.10. Data analysis
dioxide (it is used commercially in litchi industry) and relatively easy to use. Also, storage temperature was 20 °C and this temperature can be easily maintained at major super markets of the world. Hence, ALV application could be considered as a commercial treatment of browning reduction in litchi fruit. In the current study, non-coated control fruit had significantly higher weight loss (WL), than ALV coated samples (Fig. 1B). The increase in WL was considerably lower in ALV coated fruit, than noncoated control. After 8 d of storage, WL in ALV coated fruit was 2.56fold less, than non-coated control (Fig. 1B). Litchi fruit storage under an environment with reduced relative humidity leads to significantly higher water and mass loss. Water loss from the surface of fruit negatively affects consumer acceptability and market value of litchi fruit (Sivakumar et al., 2010). ALV coating reduces WL by inhibiting desiccation and prolongs shelf life of the coated produce as observed in sapodilla (Khaliq et al., 2019), cherry (Ozturk et al., 2019) and papaya (Mendy et al., 2019) fruits. So, ALV coating acted as an effective barrier between environment and surface eventually resulted in reduced WL of litchi fruit. Total anthocyanin content showed progressive reduction in both treatments at all sampling periods (Fig. 1C). However, total anthocyanin content was decreased at higher rate in non-coated control; whereas, ALV coated fruit showed significantly higher total anthocyanin content throughout the storage time. Overall on d-8, total anthocyanin content was about 2-fold higher in ALV coated fruit, than
Data were analyzed by analysis of variance technique. Different parameters were evaluated at 5% significance level by using least significance difference test with Statistix-8.1® software (Tallahassee, USA). The study was conducted under factorial layout of completely randomized design. The factors were sampling days and coating treatments. 3. Results and discussion 3.1. Browning index, weight loss and total anthocyanin content The browning index (BI) was increased at higher rate in non-coated control; whereas, ALV coated fruit showed significantly reduced BI throughout the storage time (Fig. 1A). ALV coated fruit had no BI until d-2; but, a slight increase in BI was observed on d-4. On the other hand, BI rapidly increased in non-coated fruit and the fruit underwent complete browning on d-8 (Fig. 1A). Postharvest browning negatively affects visual quality and causes significant economic losses due to reduced or lack of market potential (Sivakumar et al., 2010). Postharvest browning of litchi fruit occurs due to desiccation-induced degradation of anthocyanin pigments (Zhang et al., 2015). ALV coating reduces moisture loss by keeping the fruit peel structure intact (Supapvanich et al., 2016) and delays loss of anthocyanins (Hassanpour, 2015; Sridevi et al., 2018). So, ALV coated fruit showed reduced browning probably due to higher conservation of anthocyanins. Furthermore, it is important to mention that ALV is a non-chemical alternative to sulphur 3
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Fig. 2. Effect of Aloe vera gel coating on malondialdehyde content (A) and relative electrolyte leakage (B) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 3. Effect of Aloe vera gel coating on H2O2 (A) and O2–• (B) contents in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
non-coated control (Fig. 1C). The color of litchi fruit is red due to anthocyanins. However, anthocyanins degrade rapidly after harvest. The characteristic red color is considered important for visual quality and market potential of litchi fruit (Sivakumar et al., 2010). Degradation of anthocyanins occurs due to breakdown of vacuoles, which leads to loss of cellular compartmentation, eventually resulting in enzymes-induced degradation of anthocyanins (Jiang et al., 2018). ALV coating conserves cellular compartmentation due to delayed senescence, leading to suppressed pro-oxidant enzymes activities (Supapvanich et al., 2016). So, ALV coating possibly conserved cellular compartmentation of litchi peel and maintained higher anthocyanin pigments.
integrity and higher antioxidant activities. 3.3. H2O2 and O2–• contents The concentration of H2O2 and O2–• significantly increased during storage (Fig. 3A and B). However, postharvest ALV coating treatment significantly suppressed the production of H2O2 and O2–• contents throughout the storage time, compared with non-coated fruit. On an average, H2O2 and O2–• contents in ALV treated fruit were 1.50-fold and 1.35-fold less on d-8, in contrast with non-coated control (Fig. 3A and B). H2O2 and O2–• mediated oxidative damage is the key factor in starting browning of litchi fruit (Zhang et al., 2015; Li et al., 2019). Use of appropriate edible coatings can protect the treated produce against oxidative damage (Jiang et al., 2018). ALV coating application suppresses senescence and membrane peroxidation eventually leads to alleviation of oxidative damage in treated fruit (Hassanpour, 2015). So, in our investigation ALV coating possibly reduced H2O2 and O2–• production due to reduction in membrane peroxidation and senescence.
3.2. MDA content and REL Postharvest application of ALV coating significantly suppressed production of MDA content, than non-coated control (Fig. 2A). The difference in MDA content between ALV coated and non-coated control fruit was significantly higher from d-2 to d-8. Overall, it was noted that non-coated control had 1.52-fold higher production of MDA content on d-8, compared with ALV coated fruit (Fig. 2A). REL was also significantly influenced in response to the storage intervals and treatments. Overall, both ALV coated and non-coated control fruit showed increasing trend of REL throughout the storage time (Fig. 2B). However, ALV coated fruit maintained significantly higher membrane integrity and exhibited lower REL, than non-coated control on d-8. On an average, ALV treatment showed 1.42-fold reduced REL after 8 d storage, in contrast with non-coated control fruit (Fig. 2B). Oxidative stress causes cellular membrane damage (Sun et al., 2011) leading to increased MDA content and REL in litchi fruit (Zhang et al., 2015; Li et al., 2019). Application of ALV coating can reduce increase in REL and MDA content due to protection of cellular membranes against lipid peroxidation and membrane disruption by conserving antioxidant activity and reducing postharvest senescence of the treated fruits (Hassanpour, 2015; Rasouli et al., 2019). So, lower REL and MDA content in ALV gel coated litchi fruit was probably due to reduced oxidative damage and senescence as well as conserved membrane
3.4. TPC and DPPH-RSA TPC decreased with increasing time of storage from d-2 to d-8 (Fig. 4A). However, decreasing trend was significantly reduced with the application of ALV coating. The non-coated control fruit showed substantial reduction in TPC. On d-8, TPC in ALV treated litchi fruit was 1.66-fold higher, than those of the non-coated control (Fig. 4A). Reduction of TPC probably takes place due to oxidation and TPC concentration is important because its oxidation ultimately leads to browning of litchi fruit (Shah et al., 2017). ALV coating reduces the reduction of TPC by protecting against oxidation due to restricted availability of O2 in the treated fruit (Khaliq et al., 2019). So, ALV coated litchi fruit had higher TPC possibly due to its reduced oxidation during storage. DPPH-RSA progressively and substantially decreased in ALV coated and non-coated control during entire period of 8 d storage (Fig. 4B). However, ALV coating application efficiently suppressed the reduction 4
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Fig. 4. Effect of Aloe vera gel coating on total phenolic content (A) and DPPH radical scavenging activity (B) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 5. Effect of Aloe vera gel coating on PPO (A) and POD (B) enzymes activities in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
of DPPH-RSA, compared with non-coated fruit. After 8 d storage period, ALV coated litchi fruit had 1.74-fold higher DPPH-RSA, than noncoated control (Fig. 4B). Higher DPPH-RSA of litchi peel is considered suitable to alleviate oxidative stress (Ali et al., 2016b). However, DPPHRSA of litchi peel tissues generally declines due to increased production of free radicals during prolonged postharvest storage (Shah et al., 2017). ALV gel coating has good potential to conserve higher DPPHRSA due to reduced production of free radicals (Hassanpour, 2015; Khaliq et al., 2019). So, ALV coated litchi fruit showed higher DPPHRSA probably due to less senescence and lower production of H2O2 and O2–• contents.
throughout the storage time (Fig. 6A). Similarly, APX activity in ALV coated fruit also decreased from d-2 to d-8 of storage but its reduction rate was markedly lower, than non-coated fruit (Fig. 6A). On the other side, CAT enzyme activity showed progressive reduction at all sampling periods (Fig. 6B). However, the reduction rate of CAT enzyme activity was higher in non-coated control at all sampling periods, compared with ALV coated fruit (Fig. 6B). Activity of SOD enzyme was progressively decreased throughout the storage time (Fig. 6C). However, SOD activity in ALV treated fruit was maintained higher throughout the storage period than control (Fig. 6C). ALV coating delays senescence and maintains higher antioxidant enzymes activities (Hassanpour, 2015; Mirshekari et al., 2019). Antioxidative enzymes for instance SOD, CAT and APX detoxify different free radicals and reduce browning of litchi fruit by alleviating oxidative damage. So, higher activities of the said enzymes are imperative to reduce incidence of browning in litchi fruit (Ali et al., 2018).
3.5. PPO and POD enzymes activities Activities of PPO and POD enzymes revealed progressive increase in both ALV coated and non-coated fruit during storage (Fig. 5A and B). Nevertheless, treatment of litchi fruit with ALV coating was effective in delaying the increase of the said enzymes activities throughout the storage period of 8 d (Fig. 5A and B). After 8 d of storage, ALV coated litchi fruit revealed 1.37-fold and 1.36-fold reduced PPO and POD activities, respectively than control (Fig. 5A and B). PPO and POD are prooxidant (that catalyze oxidative reactions) enzymes and are present (separate from phenolics) in organelles that oxidize phenolics and lead to browning of litchi fruit (Ali et al., 2018). So, activities of the said enzymes should be suppressed to reduce browning of the harvested litchi fruit (Zhang et al., 2015). Coatings protect membrane integrity and reduce PPO and POD activities by acting as a barrier against membrane disruption (conserved membrane integrity eventually leads to reduced disruption of membrane) between the said enzymes and phenolics leading to reduced phenolics oxidation (Saba and Sogvar, 2016; Lo’ay and Taher, 2018). Hence, ALV coating possibly reduced activities of PPO and POD enzymes due to conserved compartmentation.
3.7. TSS and TA TSS constantly decreased over the period of 8 d storage. However, ALV coated litchi fruit maintained its higher concentration, compared with non-coated fruit (Fig. 7A). In contrast, non-coated control fruit showed reduction in TSS at significantly higher rate, than ALV coated litchi fruit (Fig. 7A). After d-8 of storage, ALV coated fruit had 1.41-fold higher TSS, than non-coated control (Fig. 7A). Concentration of TSS is important for flavor quality of litchi fruit (Jiang et al., 2018). Postharvest senescence leads to reduction of TSS eventually resulting in reduction of its eating quality (Ali et al., 2019b). It has been observed that application of ALV coating reduces its degradation due to delay in senescence of fruit (Hassanpour, 2015; Qamar et al., 2018). So, ALV coating application suppressed senescence and conserved higher concentration of TSS of coated litchi fruit. TA was gradually reduced with increase in storage period (Fig. 7B). The reduction was substantially lower in ALV coated fruit, compared with non-coated control. However, on d-4 difference between the two treatments regarding TA was non-significant; but, at all other sampling
3.6. APX, CAT and SOD enzymes activities Activity of APX enzyme in non-coated fruit showed steady decline 5
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Fig. 7. Effect of Aloe vera gel coating on total soluble solids (A), titratable acidity (B) and ascorbic acid content (C) in pulp of litchi fruit. Single overlapped letter on zero d time period represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 6. Effect of Aloe vera gel coating on APX (A), CAT (B) and SOD (C) enzymes activities in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
2009; Sogvar et al., 2016; Khaliq et al., 2019).
periods the difference was statistically significant (Fig. 7B). After d-8 of storage period, TA was 1.66-fold higher in ALV coated fruit, than noncoated control (Fig. 7B). TA of litchi fruit normally reduces with increased period of storage (Shah et al., 2017). Its reduction occurs due to oxidation during postharvest fruit senescence (Khaliq et al., 2019; Mendy et al., 2019). ALV coating slows down its reduction due to inhibition of senescence and oxidation of organic acids in treated produce (Song et al., 2013). In our investigation, ALV coating application also probably reduced oxidation and fruit senescence that in turn maintained higher TA concentration in treated litchi fruit.
4. Conclusion In conclusion, ALV has been found an effective coating in conserving postharvest quality of litchi fruit. Application of ALV coating as a postharvest dip treatment reduced browning index and retained significantly higher total anthocyanins. ALV coating prevented water loss and cell damage consequently the increase in free radicals, PPO and POD as well decrease in SOD, APX and CAT activities was prevented. So, ALV coating could be considered an ecofriendly non-chemical treatment for surface color conservation and quality management of litchi fruit during storage at 20 °C conditions.
3.8. Ascorbic acid content
References
No significant difference in ALV coating treated and non-treated control was noted in case of ascorbic acid (AA) content on d-2 of storage period (Fig. 7C). However, as storage time progressed further the decline in AA content was significantly increased; but, ALV coated litchi fruit had higher concentration of AA content, than non-coated control. At d-8 of storage, ALV coated fruit showed 1.46-fold higher AA content, in contrast with non-coated control (Fig. 7C). AA content typically declines during storage due to its oxidative breakdown (Mditshwa et al., 2017). ALV coating reduces decline of AA content by inhibiting its oxidation during postharvest storage (Khaliq et al., 2019). ALV coating reduces its degradation due to restricted availability of O2 for its oxidative breakdown and reduces fruit senescence (Ahmed et al.,
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Aloe vera gel coating delays postharvest browning and maintains quality of harvested litchi fruit
T
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Sajid Alia,b, Ahmad Sattar Khana, , Aamir Nawazb, Muhammad Akbar Anjumb, Safina Nazb, Shaghef Ejazb, Sajjad Hussainb a b
Postharvest Research and Training Centre, Institute of Horticultural Sciences, University of Agriculture, Faisalabad, 38040, Pakistan Department of Horticulture, Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University, Multan, 60800, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: Antioxidant activity Edible coating Oxidative stress Postharvest quality Skin browning
Postharvest surface browning is the leading constraint for extension of shelf life and marketing of litchi fruit. In the present work, litchi fruit were treated with Aloe vera (ALV) gel coating [50% (v/v)] and kept at 20 ± 1 °C for 8 d to investigate its effect on browning and postharvest quality. ALV gel coated fruit showed reduced browning index, weight loss, superoxide anion, relative electrolyte leakage, hydrogen peroxide and malondialdehyde content, compared to control. ALV coated fruit had higher total anthocyanin content and reduced peroxidase and polyphenol oxidase activities. ALV treatment had higher ascorbic acid content and total phenolic concentration, compared to control. In addition, ALV coated fruit maintained higher catalase, superoxide dismutase and ascorbate peroxidase activities along with higher total soluble solids and titratable acidity, than control. In conclusion, ALV gel coating could be considered an ecofriendly non-chemical alternative treatment for postharvest quality management of litchi fruit.
1. Introduction Litchi is a sub-tropical fruit. It possesses pinkish/bright red colored skin and has juicy and sweet edible aril (Liu et al., 2011; Fahima et al., 2019). It is a non-climacteric fruit and does not ripe off the trees after harvest. So, its fruit should be harvested at proper maturity stage to ensure its characteristic quality (Jiang et al., 2006). However, ripe litchi fruit perish rapidly after harvest and lose their attractive color in 1–2 d during ambient conditions due to skin browning (Sivakumar et al., 2010). Browning of litchi fruit reduces visual quality and negatively affects purchase decision of the consumers in the markets eventually leading to significant economic losses (Sivakumar et al., 2010). Several factors have been found associated with browning of litchi fruit. Mechanical injuries, moisture loss, pathogen infection, lipid peroxidation, cellular compartmentalization loss, oxidative stress and mixing of phenolic substrates with oxidative enzymes lead to development of browning in litchi fruit (Jiang et al., 2004). The integrity loss of membrane disrupts cellular structure and phenolic substrates subsequently mix with polyphenol oxidase and peroxidase enzymes. Sulfur dioxide is the most commonly used anti-browning chemical at commercial scale in litchi industry. It primarily inactivates polyphenol oxidase enzyme and decreases browning of litchi fruit (Zhang et al.,
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2018). However, due to potent allergic reactions in the end users and packaging house workers the use of sulphur is not appropriate for litchi fruit. It imparts negative effects and leads to reduced edible quality due to sulphur-induced increased aril acidity (Sivakumar et al., 2010). Thus, environmental friendly, safe and natural alternative treatment is required that not only maintains quality of the treated fruit but is also safe for the consumers and packaging house persons. Coating application could be considered highly suitable because litchi fruit are consumed after peeling. Plant based edible coatings are considered potentially safe and effective in quality conservation and shelf-life extension of fresh produce. The said coatings provide an effective barrier that eventually leads to reduced mass loss, delayed ripening and maintained quality (Mahajan et al., 2018). ALV gel is also a natural and ecofriendly non-chemical plant based edible coating. AVL gel is pulp based gelatinous matrix obtained from leaf tissues of aloe plants (Aloe vera and Aloe arborescens). Its use has been found appropriate in conserving quality of various fruits including plum (Guillén et al., 2013), peach (Guillén et al., 2013), raspberry (Hassanpour, 2015), blueberry (Vieira et al., 2016), papaya (Mendy et al., 2019), cherry (Ozturk et al., 2019), sapodilla (Khaliq et al., 2019) and orange (Rasouli et al., 2019). It has been noted that ALV gel coating suppresses phenolic oxidation and
Corresponding author. E-mail address: [email protected] (A.S. Khan).
https://doi.org/10.1016/j.postharvbio.2019.110960 Received 1 April 2019; Received in revised form 17 July 2019; Accepted 17 July 2019 0925-5214/ © 2019 Elsevier B.V. All rights reserved.
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2.4. MDA content and relative electrolyte leakage
inhibits enzymes activity of peroxidase as well as polyphenol oxidase and reduces browning along with preserved quality of treated commodity (Supapvanich et al., 2016; Ali et al., 2019a). However, at present no information is available regarding the effect of ALV based edible coating on surface discoloration and antioxidant activities of litchi fruit. So, current research was done to evaluate efficacy of ALV gel coating on browning and postharvest quality of litchi fruit.
MDA was assayed with the method of Sun et al. (2011). Peel (1 g) was carefully homogenized in 5 mL trichloroacetic acid (TCA) and centrifuged at 10,000×g for 20 min. Supernatant (2 mL) and thiobarbituric acid (2 mL) was reacted and assay mixture was boiled for 15 min at 100 °C. The assay mixture was cooled and centrifuged at 5000×g for 15 min. Absorbance was noted at 600, 532, and 450 nm on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). MDA was calculated and it was reported as nmol kg−1. MDA= [6.45 (A532 − A600) − 0.56 × A 450] × Vt × Vr/( Vs × m) , where Vt = volume of extract solution, Vr = reaction mixture volume, Vs = total volume of extract solution containing solution of reaction mixture and m = sample mass (Yang et al., 2011). Relative electrolyte leakage (REL) was measured by following the protocol of Chen et al. (2008). Peel discs [10 mm (20)] were added in 20 mL deionized water and placed at 25 °C for 30 min. Initial reading (Lt) was measured with the conductivity meter (HI-98304, Hanna, Mauritius) after 30 min and mixture of each sample was boiled in water bath for 15 min. Final reading (L0) after cooling of the boiled solution was noted and REL was calculated with equation in percent. REL (%) = Lt/Lo × 100.
2. Materials and methods 2.1. Plant materials Litchi fruit (cv ‘Gola’) were harvested from an orchard at Haripur, KPK, Pakistan. The color of fruit was 90–100 % red with 22.36 ± 0.43% total soluble solids (TSS) and 0.45 ± 0.01% titratable acidity (TA). The fruit were free from disease and mechanical injuries having uniform size, shape and color. ALV (Aloe vera Miller) leaves were sourced from a local nursery. The age of Aloe vera leaves was 3 years. The leaves were washed with tap water to remove dirt and subsequently dipped in 0.1% sodium hypochlorite for 3 min. After drying, the leaves were manually peeled with stainless steel knife. The obtained mucilaginous gel was collected and blended in a blender. TSS, TA (citric acid) and pH values of raw gel obtained from Aloe vera leaves were 0.95 ± 0.012%, 0.039 ± 0.007% and 4.69 ± 0.090, respectively. After blending, the resultant gel was filtered with sterile muslin cloth to remove any fibrous fraction. The pH of raw gel was adjusted to 3.75 (Navarro et al., 2011) with citric acid, then it was pasteurized at 65 °C for 30 min (Vieira et al., 2016) and cooled for further use. ALV gel solution was diluted with distilled water at 1:1 (v/v) ratio and glycerol (1%) was added as a plasticizer.
2.5. H2O2 and O2–• contents H2O2 content was determined with the method of Velikova and Loreto (2005). Peel sample (1 g) was extracted with 1 mL TCA (0.1%) and centrifuged at 12,000×g for 15 min. Then 0.5 mL supernatant was mixed with phosphate buffer (10 mmol L−1) and 1 mL of 1 M KI. The solution absorbance was read at 390 nm and it was expressed as μmol kg-1. Production of O2–• content was assayed with the methodology of Yang et al. (2011). Peel sample (1 g) was homogenized in 3 mL of 50 mmol L−1 phosphate buffer (pH 7.8) having polyvinylpyrrolidone (1%) at 4 °C. The assay mixture was centrifuged at 10,000×g for 15 min. Production of O2–• content was calculated by monitoring NO2 (after comparing with standard curve) formation from hydroxylamine in the presence of O2–• and was expressed as nmol min−1 kg-1.
2.2. Treatments Fruit were separated into two groups. Total 600 fruit were used in the experiment [treatments (2) × replications (3) × fruit per replication (20) × sampling intervals (5)]. One group was treated with 50% ALV coating solution; whereas, other group was dipped in distilled water. Dipping time was 5 min for both treatments. The concentration was selected based on the preliminary trial in which 25, 50 and 75% (v/ v) ALV coatings were used. After dipping, fruit were left at room temperature until complete drying and stored at 20 ± 1 °C with 90% RH. Each treatment had three biological replicates and each replicate had 20 fruit. A composite sample of 20 fruit was used for every studied parameter. Ascorbic acid content, TSS and TA were assayed on the same d of sampling; whereas, for total phenolic content (TPC), total anthocyanin content, DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity (RSA) and enzyme assays, samples were treated with liquid nitrogen and stored at −80 °C until analyzed. The fruit were sampled on 0 (before coating), 2, 4, 6 and 8 d of storage period.
TPC in peel of fruit was analyzed with Folin–Ciocalteu reagent assay after taking absorbance at 765 nm (Ainsworth and Gillespie, 2007). Standard curve of gallic acid was plotted and concentration of TPC was expressed as mg kg−1. DPPH-RSA in peel of fruit was measured as described previously (Brand-Williams et al., 1995). In brief, 50 μL peel extract in methanol was mixed with 0.1 mmol L−1 concentrated DPPH (3 mL). The solution was placed in dark at 25 °C for 30 min and absorbance was read at 517 nm. Finally, DPPH-RSA was expressed in percent.
2.3. Browning index, weight loss and total anthocyanin content
2.7. Assays of enzyme activities
Browning index was measured visually on a scale that comprised of no browning = 1; 1–2 brown spots = 2, few spots of browning = 3; 50% browning = 4 and 75% to complete browning of fruit surface = 5 and expressed as score (Sivakumar and Korsten, 2010). For the measurement of weight loss, fruit were weighed on electronic balance after dipping in respective treatments (after drying of fruit) before storage and at the end of each sampling interval. Loss in weight was expressed in percent. For total anthocyanin content, Zheng and Tian (2006) method was used. Peel (10 g) was extracted in HClmethanol solution and absorbance of obtained extract was read at 530, 620 and 650 nm on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). Total anthocyanins content was calculated as ΔA g−1 = (A530 − A620) − 0.1(A650 − A620).
Activities of polyphenol oxidase (PPO), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD) enzymes were assayed in peel tissues of fruit. Peel (1 g) was homogenized in 2 mL citrate (pH 4) buffer and centrifuged at 10,000×g for 10 min (Ali et al., 2016a). All steps of sample homogenization, crude enzymes extraction and centrifugation were carried out at 4 °C. Supernatant was collected and was used for assays of the said enzymes.
2.6. TPC and DPPH-RSA
2.7.1. PPO and POD enzymes activities Activities of PPO and POD enzymes were assayed with the method of Ali et al. (2016a). For PPO enzyme, activity was assayed by monitoring absorbance change at 412 nm. “One PPO activity unit (U) was equal to enzyme quantity that causes absorbance change in 0.001 units 2
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per min”. Activity of POD enzyme was analyzed by observing tetraguiacol formation from the guaiacol at 470 nm. “One POD activity unit (U) was equal to enzyme quantity that leads to absorbance change in 0.01 units per min”. 2.7.2. APX, CAT and SOD enzymes activities Activity of APX was assayed by monitoring ascorbic acid oxidation at 290 nm in presence of H2O2 (Nakano and Asada, 1987). “One APX activity unit (U) was defined as enzyme quantity that oxidizes one μmol ascorbat per min”. CAT and SOD activities were assayed with the protocol of Ali et al. (2016a). For activity of CAT enzyme, H2O2 decomposition was measured at 240 nm; while SOD enzyme activity was determined with nitroblue-tetrazolium (NBT) method at 560 nm after illumination of assays solutions under UV-light. “One CAT activity unit (U) was equal to enzyme quantity that causes decomposition of H2O2 per min” whereas “One SOD activity unit (U) was defined as enzyme quantity that inhibited 50% photochemical reduction of NBT”. 2.7.3. Assay of protein content Protein content was determined with Bradford method (Bradford, 1976) and enzymes activities were expressed as U mg−1 protein. Absorbance for all enzymes was read on UV-1800 UV-VIS spectrophotometer (Shimadzu, Japan). 2.8. TSS and TA TSS and TA were determined in pulp of fruit (Ali et al., 2016a). TSS of fruit was estimated with digital refrectometer (Atago Pal-1, Japan) and was expressed as percent. TA was determined with titration method by using 0.1 N NaOH and expressed as percent. 2.9. Ascorbic acid content Ascorbic acid content in pulp of fruit was assayed with 2, 6-dichlorophenol-indophenol method and was expressed as mg kg−1 (Ali et al., 2016a).
Fig. 1. Effect of Aloe vera gel coating on browning index (A), weight loss (B) and total anthocyanin content (C) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates. Weight loss was assessed from whole fruit.
2.10. Data analysis
dioxide (it is used commercially in litchi industry) and relatively easy to use. Also, storage temperature was 20 °C and this temperature can be easily maintained at major super markets of the world. Hence, ALV application could be considered as a commercial treatment of browning reduction in litchi fruit. In the current study, non-coated control fruit had significantly higher weight loss (WL), than ALV coated samples (Fig. 1B). The increase in WL was considerably lower in ALV coated fruit, than noncoated control. After 8 d of storage, WL in ALV coated fruit was 2.56fold less, than non-coated control (Fig. 1B). Litchi fruit storage under an environment with reduced relative humidity leads to significantly higher water and mass loss. Water loss from the surface of fruit negatively affects consumer acceptability and market value of litchi fruit (Sivakumar et al., 2010). ALV coating reduces WL by inhibiting desiccation and prolongs shelf life of the coated produce as observed in sapodilla (Khaliq et al., 2019), cherry (Ozturk et al., 2019) and papaya (Mendy et al., 2019) fruits. So, ALV coating acted as an effective barrier between environment and surface eventually resulted in reduced WL of litchi fruit. Total anthocyanin content showed progressive reduction in both treatments at all sampling periods (Fig. 1C). However, total anthocyanin content was decreased at higher rate in non-coated control; whereas, ALV coated fruit showed significantly higher total anthocyanin content throughout the storage time. Overall on d-8, total anthocyanin content was about 2-fold higher in ALV coated fruit, than
Data were analyzed by analysis of variance technique. Different parameters were evaluated at 5% significance level by using least significance difference test with Statistix-8.1® software (Tallahassee, USA). The study was conducted under factorial layout of completely randomized design. The factors were sampling days and coating treatments. 3. Results and discussion 3.1. Browning index, weight loss and total anthocyanin content The browning index (BI) was increased at higher rate in non-coated control; whereas, ALV coated fruit showed significantly reduced BI throughout the storage time (Fig. 1A). ALV coated fruit had no BI until d-2; but, a slight increase in BI was observed on d-4. On the other hand, BI rapidly increased in non-coated fruit and the fruit underwent complete browning on d-8 (Fig. 1A). Postharvest browning negatively affects visual quality and causes significant economic losses due to reduced or lack of market potential (Sivakumar et al., 2010). Postharvest browning of litchi fruit occurs due to desiccation-induced degradation of anthocyanin pigments (Zhang et al., 2015). ALV coating reduces moisture loss by keeping the fruit peel structure intact (Supapvanich et al., 2016) and delays loss of anthocyanins (Hassanpour, 2015; Sridevi et al., 2018). So, ALV coated fruit showed reduced browning probably due to higher conservation of anthocyanins. Furthermore, it is important to mention that ALV is a non-chemical alternative to sulphur 3
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Fig. 2. Effect of Aloe vera gel coating on malondialdehyde content (A) and relative electrolyte leakage (B) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 3. Effect of Aloe vera gel coating on H2O2 (A) and O2–• (B) contents in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
non-coated control (Fig. 1C). The color of litchi fruit is red due to anthocyanins. However, anthocyanins degrade rapidly after harvest. The characteristic red color is considered important for visual quality and market potential of litchi fruit (Sivakumar et al., 2010). Degradation of anthocyanins occurs due to breakdown of vacuoles, which leads to loss of cellular compartmentation, eventually resulting in enzymes-induced degradation of anthocyanins (Jiang et al., 2018). ALV coating conserves cellular compartmentation due to delayed senescence, leading to suppressed pro-oxidant enzymes activities (Supapvanich et al., 2016). So, ALV coating possibly conserved cellular compartmentation of litchi peel and maintained higher anthocyanin pigments.
integrity and higher antioxidant activities. 3.3. H2O2 and O2–• contents The concentration of H2O2 and O2–• significantly increased during storage (Fig. 3A and B). However, postharvest ALV coating treatment significantly suppressed the production of H2O2 and O2–• contents throughout the storage time, compared with non-coated fruit. On an average, H2O2 and O2–• contents in ALV treated fruit were 1.50-fold and 1.35-fold less on d-8, in contrast with non-coated control (Fig. 3A and B). H2O2 and O2–• mediated oxidative damage is the key factor in starting browning of litchi fruit (Zhang et al., 2015; Li et al., 2019). Use of appropriate edible coatings can protect the treated produce against oxidative damage (Jiang et al., 2018). ALV coating application suppresses senescence and membrane peroxidation eventually leads to alleviation of oxidative damage in treated fruit (Hassanpour, 2015). So, in our investigation ALV coating possibly reduced H2O2 and O2–• production due to reduction in membrane peroxidation and senescence.
3.2. MDA content and REL Postharvest application of ALV coating significantly suppressed production of MDA content, than non-coated control (Fig. 2A). The difference in MDA content between ALV coated and non-coated control fruit was significantly higher from d-2 to d-8. Overall, it was noted that non-coated control had 1.52-fold higher production of MDA content on d-8, compared with ALV coated fruit (Fig. 2A). REL was also significantly influenced in response to the storage intervals and treatments. Overall, both ALV coated and non-coated control fruit showed increasing trend of REL throughout the storage time (Fig. 2B). However, ALV coated fruit maintained significantly higher membrane integrity and exhibited lower REL, than non-coated control on d-8. On an average, ALV treatment showed 1.42-fold reduced REL after 8 d storage, in contrast with non-coated control fruit (Fig. 2B). Oxidative stress causes cellular membrane damage (Sun et al., 2011) leading to increased MDA content and REL in litchi fruit (Zhang et al., 2015; Li et al., 2019). Application of ALV coating can reduce increase in REL and MDA content due to protection of cellular membranes against lipid peroxidation and membrane disruption by conserving antioxidant activity and reducing postharvest senescence of the treated fruits (Hassanpour, 2015; Rasouli et al., 2019). So, lower REL and MDA content in ALV gel coated litchi fruit was probably due to reduced oxidative damage and senescence as well as conserved membrane
3.4. TPC and DPPH-RSA TPC decreased with increasing time of storage from d-2 to d-8 (Fig. 4A). However, decreasing trend was significantly reduced with the application of ALV coating. The non-coated control fruit showed substantial reduction in TPC. On d-8, TPC in ALV treated litchi fruit was 1.66-fold higher, than those of the non-coated control (Fig. 4A). Reduction of TPC probably takes place due to oxidation and TPC concentration is important because its oxidation ultimately leads to browning of litchi fruit (Shah et al., 2017). ALV coating reduces the reduction of TPC by protecting against oxidation due to restricted availability of O2 in the treated fruit (Khaliq et al., 2019). So, ALV coated litchi fruit had higher TPC possibly due to its reduced oxidation during storage. DPPH-RSA progressively and substantially decreased in ALV coated and non-coated control during entire period of 8 d storage (Fig. 4B). However, ALV coating application efficiently suppressed the reduction 4
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Fig. 4. Effect of Aloe vera gel coating on total phenolic content (A) and DPPH radical scavenging activity (B) in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 5. Effect of Aloe vera gel coating on PPO (A) and POD (B) enzymes activities in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
of DPPH-RSA, compared with non-coated fruit. After 8 d storage period, ALV coated litchi fruit had 1.74-fold higher DPPH-RSA, than noncoated control (Fig. 4B). Higher DPPH-RSA of litchi peel is considered suitable to alleviate oxidative stress (Ali et al., 2016b). However, DPPHRSA of litchi peel tissues generally declines due to increased production of free radicals during prolonged postharvest storage (Shah et al., 2017). ALV gel coating has good potential to conserve higher DPPHRSA due to reduced production of free radicals (Hassanpour, 2015; Khaliq et al., 2019). So, ALV coated litchi fruit showed higher DPPHRSA probably due to less senescence and lower production of H2O2 and O2–• contents.
throughout the storage time (Fig. 6A). Similarly, APX activity in ALV coated fruit also decreased from d-2 to d-8 of storage but its reduction rate was markedly lower, than non-coated fruit (Fig. 6A). On the other side, CAT enzyme activity showed progressive reduction at all sampling periods (Fig. 6B). However, the reduction rate of CAT enzyme activity was higher in non-coated control at all sampling periods, compared with ALV coated fruit (Fig. 6B). Activity of SOD enzyme was progressively decreased throughout the storage time (Fig. 6C). However, SOD activity in ALV treated fruit was maintained higher throughout the storage period than control (Fig. 6C). ALV coating delays senescence and maintains higher antioxidant enzymes activities (Hassanpour, 2015; Mirshekari et al., 2019). Antioxidative enzymes for instance SOD, CAT and APX detoxify different free radicals and reduce browning of litchi fruit by alleviating oxidative damage. So, higher activities of the said enzymes are imperative to reduce incidence of browning in litchi fruit (Ali et al., 2018).
3.5. PPO and POD enzymes activities Activities of PPO and POD enzymes revealed progressive increase in both ALV coated and non-coated fruit during storage (Fig. 5A and B). Nevertheless, treatment of litchi fruit with ALV coating was effective in delaying the increase of the said enzymes activities throughout the storage period of 8 d (Fig. 5A and B). After 8 d of storage, ALV coated litchi fruit revealed 1.37-fold and 1.36-fold reduced PPO and POD activities, respectively than control (Fig. 5A and B). PPO and POD are prooxidant (that catalyze oxidative reactions) enzymes and are present (separate from phenolics) in organelles that oxidize phenolics and lead to browning of litchi fruit (Ali et al., 2018). So, activities of the said enzymes should be suppressed to reduce browning of the harvested litchi fruit (Zhang et al., 2015). Coatings protect membrane integrity and reduce PPO and POD activities by acting as a barrier against membrane disruption (conserved membrane integrity eventually leads to reduced disruption of membrane) between the said enzymes and phenolics leading to reduced phenolics oxidation (Saba and Sogvar, 2016; Lo’ay and Taher, 2018). Hence, ALV coating possibly reduced activities of PPO and POD enzymes due to conserved compartmentation.
3.7. TSS and TA TSS constantly decreased over the period of 8 d storage. However, ALV coated litchi fruit maintained its higher concentration, compared with non-coated fruit (Fig. 7A). In contrast, non-coated control fruit showed reduction in TSS at significantly higher rate, than ALV coated litchi fruit (Fig. 7A). After d-8 of storage, ALV coated fruit had 1.41-fold higher TSS, than non-coated control (Fig. 7A). Concentration of TSS is important for flavor quality of litchi fruit (Jiang et al., 2018). Postharvest senescence leads to reduction of TSS eventually resulting in reduction of its eating quality (Ali et al., 2019b). It has been observed that application of ALV coating reduces its degradation due to delay in senescence of fruit (Hassanpour, 2015; Qamar et al., 2018). So, ALV coating application suppressed senescence and conserved higher concentration of TSS of coated litchi fruit. TA was gradually reduced with increase in storage period (Fig. 7B). The reduction was substantially lower in ALV coated fruit, compared with non-coated control. However, on d-4 difference between the two treatments regarding TA was non-significant; but, at all other sampling
3.6. APX, CAT and SOD enzymes activities Activity of APX enzyme in non-coated fruit showed steady decline 5
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Fig. 7. Effect of Aloe vera gel coating on total soluble solids (A), titratable acidity (B) and ascorbic acid content (C) in pulp of litchi fruit. Single overlapped letter on zero d time period represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
Fig. 6. Effect of Aloe vera gel coating on APX (A), CAT (B) and SOD (C) enzymes activities in peel of litchi fruit. Single overlapped letter on zero day time periods represents equal means. Vertical bars show standard error of means and data are mean of three replicates.
2009; Sogvar et al., 2016; Khaliq et al., 2019).
periods the difference was statistically significant (Fig. 7B). After d-8 of storage period, TA was 1.66-fold higher in ALV coated fruit, than noncoated control (Fig. 7B). TA of litchi fruit normally reduces with increased period of storage (Shah et al., 2017). Its reduction occurs due to oxidation during postharvest fruit senescence (Khaliq et al., 2019; Mendy et al., 2019). ALV coating slows down its reduction due to inhibition of senescence and oxidation of organic acids in treated produce (Song et al., 2013). In our investigation, ALV coating application also probably reduced oxidation and fruit senescence that in turn maintained higher TA concentration in treated litchi fruit.
4. Conclusion In conclusion, ALV has been found an effective coating in conserving postharvest quality of litchi fruit. Application of ALV coating as a postharvest dip treatment reduced browning index and retained significantly higher total anthocyanins. ALV coating prevented water loss and cell damage consequently the increase in free radicals, PPO and POD as well decrease in SOD, APX and CAT activities was prevented. So, ALV coating could be considered an ecofriendly non-chemical treatment for surface color conservation and quality management of litchi fruit during storage at 20 °C conditions.
3.8. Ascorbic acid content
References
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