Abstract: Background and Objectives: Grapefruit cv. Star Ruby has the ability of exporting and marketing but appears to be very susceptible fungal infection during post-harvest. This study was carried out in order to reduce pathogenic fungi resistant by using biological control to be fungicide alternatives. Materials and Methods: For this, Star Ruby grapefruit were coated with pomegranate peel extracts (PPE) and gelatin after harvest during 2017 and 2018 seasons to control post-harvest diseases and improvement fruit storability during cold storage for 45 days at 8°C±1 with 90-95% RH. Results: Our results indicate that all natural extracts have demonstrated good results for inhibiting the growth of pathogens over untreated. Similarly, all extracts reduced physiological loss in weight, decay inhibit the decay fungi and supply a long protection to treated fruits during storage and handling processes. All the extracts were able to retain post-harvest quality of fruits without any adverse effect on quality parameters such as; TSS, total acidity and total sugar (%). Overall, the uses of pomegranate peel extracts (PPE) and gelatin extracts are two promising examples of treatments that are beginning to be adopted on a commercial scale. Conclusion: Thus, it is evident that, combination of pomegranate peel extracts (PPE) and gelatin can be recommended as a safe method for maintaining fruit quality, control post-harvest diseases and extending storage life at the same time.
INTRODUCTION
Grapefruit is a sub-tropical citrus tree known for its sour fruit with pharmaceutical properties. It is an excellent source of vitamin C, potassium and dietary fiber. The Star Ruby grapefruit is the benchmark standard of grapefruits regarding color, flavor and fragrance1. The extent of post-harvest losses in this crop is comparatively very high. Hence, in order to reduce the post-harvest losses, there is need to enhance shelf life of fruits under ordinary marketing conditions.
Green mold, blue mold and sour rot caused by Penicillium digitatum, P. italicum and Geotrichumcitri-aurantii, respectively, are the main citrus post-harvest diseases2. The most common post-harvest fungal diseases of grapefruit are green mold caused by Penicillium digitatum, which is responsible for approximately 90% of total post-harvest losses3. The fruits are contaminated through skin post-harvest damage during their picking, packaging, storage and transportation4. The use of fungicides has been efficient in decreasing losses due to deterioration of food, but also generates health and environmental concerns.
For many years, synthetic fungicides are currently used as primary means for the control of plant disease. In order to reduce the potential development of pathogenic fungi resistance to applied fungicides some preservatives are suggested to be fungicide alternatives. The uses of plant-derived products as disease control agents have been studied, since they tend to have low toxicity, less environmental effects and wide public acceptance. Thus, there has been a growing tersest on the research of the possible use of plant secondary metabolites for pest and disease control in agriculture5.
In the recent years, the attention has been focused on the natural substances for biological control of plant pathogens such as agro-industrial wastes. Pomegranate peel is considered as a good example for this type of agro-industrial by-products. The peel constitutes about 50% of the total fruit weight6. However, the peel of the pomegranate is an important source of bioactive compounds such as; phenolic compounds including hydrolysable tannins and flavonoids7. These compounds possess significant antioxidant, antibacterial and antifungal activities8.
Gelatin, an important biopolymer obtained by hydrolysis from collagen, is widely used with a broad range of functional properties and applications such as; in the food, pharmaceutical and photographic industries, including its film-forming ability. Gelatin films generally have effective barrier properties against oxygen and carbon dioxide9. The application of gelatin coatings extended the post-harvest shelf life of avocado. The positive effect of coating on shelf life could be due to modifying the atmosphere (MA) and reducing moisture loss and surface wounding as well as reducing a variety of diseases. The MA created can delay the ripening process by delaying ethylene production and reducing the internal oxygen level, consequently extending the shelf life of fruits10.
Therefore, the objectives of the present study were to investigate the antifungal properties of pomegranate peels extract against Penicillium digitatum to control green mold and to evaluate its efficacy with or without gelatin edible coating in maintaining quality and extending shelf life of Star Ruby grapefruit.
MATERIALS AND METHODS
This study was conducted during the two successive seasons of 2017 and 2018 for the evaluation of pomegranate peel extracts (PPE) and gelatin as a safe method on storability of grapefruit cv. Star Ruby under cold storage.
Isolation and identification of the pathogen: Penicillium digitatum (green mold) was isolated from naturally infected grapefruit cv. Star Ruby fruits after storage of several weeks. These isolates were the most aggressive one in our collection and produced the largest lesions on inoculated fruits. These fungi were purified and maintained on Potato Dextrose Agar (PDA) and stored at 4oC with periodic transfers through citrus fruits to maintain its aggressiveness. Fruits were ready for examination under a stereoscopic binocular microscope (6-50X) for the presence of fungi and to study their habit characters. When necessary the compound microscope was used for confirming the identification after having examined the morphology of conidia and conidiophores11.
Effect on linear growth and dry weight of fungi isolated from grapefruit cv. Star Ruby
Linear growth: Pomegranate peel extracts were tested in vitro on the linear growth of the pathogenic fungi. Different concentrations were added to 10 mL of sterilized PDA before solidification and then poured in sterile Petri dishes. After solidification, the plates were inoculated with fungal disc (5 mm) in the center of the plate and incubated at 27±1°C. Three plates for each treatment for each fungus were used as replicates, three plates were prepared to serve as control for each fungus. Linear growth was observed daily and diameter of fungal colonies were recorded when plates of any treatment were filled with the fungal growth. Scanning electron microscopy micrographs were used for showing the microscopic structural changes of Penicillium digitatum hyphae in response to all treatments applied (Fig. 1).
Dry weight: One hundred milliliter of liquid PD medium in 250 mL Erlenmeyer flasks were amended with different concentrations of the tested compounds after autoclaving. Each flask was inoculated using two discs of 0.6 mm in diameter of fungal culture, then incubated at 20°C±2 for 7 days. Control flasks contain no concentrations of these compounds. Three replicates were used for each concentration. At the end of incubation period, the mycelium was filtered off and washed several times with distilled water, then dried in an oven at 80°C for 48 h till constant weight12.
Disease infection: It was determined according to the following equation:
Field study: Grapefruit trees were cultivated on sandy clay soil under drip irrigation system, planted at 3×3 meters grown in private orchard at EL-Nubaria region (80 km Alexandria Cairo desert road) Beheira Governorate. The healthy fruits were harvested in last week of November at both seasons, which is the optimum maturity stage (more than 2/3 of fruits surface showing yellow color and minimum TSS/Acid ratio1 of 6.5-7.5. Selected fruits were directly transported to the laboratory faculty of agriculture, Mansoura University, Egypt. Defective fruits were almost equal in size and apparently insect and pathogen injury free. The freshly harvested fruits of grapefruit cv. Star Ruby were washed in clean water with wet foam, followed by dip in chlorinated water (0.01%) for one minute, then air-dried and a quick sorting were done to research fruit for any defects. At the beginning of the experiment, samples of 15 fruits were taken to determine the initial fruits properties and the other fruits received the following treatments:
| • | Dipping fruits into pomegranate peel extracts (PPE) 5%+ coating with gelatin 5% |
| • | Dipping fruits into pomegranate peel extracts (PPE) 10%+ coating with gelatin 5% |
| • | Dipping fruits into pomegranate peel extracts (PPE)15%+ coating with gelatin 5% |
| • | Dipping fruits into pomegranate peel extracts (PPE) 20% + coating with gelatin 5% |
| • | Coating fruits with gelatin 5% |
| • | Control (dipping fruit in tap water) |
The fruits were left to dry and then packed in one layer inside Corrugated Fiber Board boxes of standard size 45×23×18 cm each consists of 15 fruits. Nine boxes served for each treatment. The total numbers of boxes were 54 for all treatments and then storage at 8°C±1 with 90-95% Relative Humidity (RH) then 3 carton boxes for each treatment was taken 15 days intervals to evaluate the storability.
Preparation of Pomegranate Peel Extracts (PPE) solutions: Pomegranate (Punica granatum) were purchased from the local market and peeled manually. Pomegranate Peels Extracts (PPE) were dried at 50°C until a constant weight and ground to powder. Powder was dissolved in ethanol (1:20 w/v) and then extracted in water bath with shaker at 40°C for 4 h. The extract was filtered and concentrated in a water bath to get crude extract.
Gelatin-based coating is prepared as follows: Fifteen gram of gelatin is dissolved in 1 L of CH3COOH 1%, oleic acid 3%, 30 g, Tween 80: 1 g. Adding gelatin into stainless steel containing CH3COOH 1% mixing thoroughly in 30 min at 40°C, heating in 10 min, cooling and filtering to remove insoluble particles, adding above materials and mixing, adjusting solution to pH 5.6 by NaOH 0.1N. This solution is ready for coating.
Quality of Star Ruby grapefruit:
• Loss in fruit weight: It was determined according to the following equation:
• Decay: It was determined according to the following equation:
• Fruit juice (%): It was obtained from the following formula:
• Vitamin C (mg 100 g–1 fresh weight): Ascorbic acid (mg/100 g fresh weight): Ascorbic acid (vitamin C) was measured by the oxidation of ascorbic acid with using 2, 6-dichlorophenolindophenol and 2% oxalic acid as a substrate then the results were expressed as mg/100 g fresh weight according to Ranganna13
• Total Soluble Solids (TSS%): Soluble solids content in fruit juice was measured using a Carl-Zeiss hand refractometer according to AOAC14
• Titratable acidity (TA%): It was determined in 10 mL of fruit juice by titrating with 0.1N sodium hydroxide in the presence of phenolphthalein as indicator and the results were expressed as a percentage of citric acid according to AOAC14
• Total soluble solids (TSS/acid ratio %): This ratio was calculated from the results recorded for fruit juice TSS and titratable acidity
• Lycopene assay: Lycopene content of juice extract was determined by using a colorimetric method according to Rao et al.15. The samples were extracted with hexane, methanol and acetone (2:1:1) for 1 h. Absorbance of the extract at 502 nm was measured using spectrophotometer against the blank extract solvent.
Preparation of the methanol extract of fruit peel: About 2 g of fruit peel (randomly collected from 5 fruit/replicate) were extracted by shaking at 150 rpm for 1 2 h with 20 mL methanol (80%) and filtered through filter paper No. 1. The filtrate designated as methanol extract that was used for estimations of total phenols, total flavonoids and antioxidant activity.
Total phenolic compounds (mg g–1 FW): Total phenols concentration was measured according to Chun et al.16. About 50 μL of the methanol extract was mixed with 100 μL Folin-Ciocalteu reagent, 850 μL of methanol and allowed to stand for 5 min at ambient temperature. A 500 μL of 20% sodium carbonate was added and allowed to react for 30 min. Absorbance was measured at 750 nm. Total phenols were quantified from a calibration curve obtained by measuring the absorbance of known concentrations of gallic acid and the results expressed as mg g–1 FW gallic acid equivalent.
Antioxidant activity (%): Antioxidant capacity (DPPH radical scavenging assay of fruit peel): The DPPH free radical scavenging activity of methanol extract of fruit peel was determined by using the 1, 1-diphenyl-2-picrylhydrazyl (DPPH) according to the method of Aoet al.17. A methanol extract (0.1 mL) was added to 0.9 mL of freshly prepared DPPH methanol solution (0.1 mM). An equal amount of methanol was used as a control. After incubation for 30 min at room temperature in the dark, the absorbance (Abs) was measured at 517 nm using a spectrophotometer. Activity of scavenging (%) was calculated by using the following formula: Absorbance of control-DPPH radical Absorbance of sample = 100 scavenging (%) Absorbance of control×The inhibition concentration (IC50) was defined as μg phenolics of the test sample that decreases 50% of initial radical. The IC50 values were calculated from the dose responses curves.
Statistical analysis: Data of both seasons of the study were designed by using analysis of variance (ANOVA) with two factors; time and temperature. Differences between the conducted treatments means were compared using Duncan’s multiple tests at p<0.05 and means separation using the CoStat program.
RESULTS AND DISCUSSION
The result in this experimental indicated that all treatments of Pomegranate Peel Extracts (PPE) and gelatin extended the storage period of Star Ruby grapefruit with preserve their qualities.
Linear growth and dry weight of Penicillium digitatum isolated from Star Ruby grapefruit: Data in Table 1 showed the effect of pomegranate peels extract on linear growth and dry weight of Penicillium digitatum isolated from Star Ruby grapefruit. Pomegranate peels extract at 20% followed by pomegranate peels extract at 15% treatments completely inhibited the linear growth and dry weight of Penicillium digitatum. This result agreed with the finding of Latifa et al.18 on citrus who reported complete inhibition of mycelia growth of Penicillium itelicum which was generally associated with complete inhibition of sporulation by organic acids and salts.
Effect on disease infection (%) of Star Ruby grapefruit: Data in Table 2 showed the effect of dipping into pomegranate peels extract and coating with gelatin and its combinations on disease infection of Star Ruby grapefruit during 45 days in cold storage period compared with the untreated fruits (control) during both seasons. In both seasons, all treatments resulted decrease in disease infection. On the other hand, the most effective treatment dipping into pomegranate peels extract at 20%+coating with gelatin at 5% followed by dipping into pomegranate peels extract at 15%+coating with gelatin at 5% treatments gave reduction in disease infection caused by Penicillium digitatum as shown in Fig. 1.
| Fig. 1(a-f): | Scanning Electron Microscopy (SEM) Micrographs of Penicillium digitatum hyphae in response to all treatments applied, (a) Mycelium hyphae in control (untreated fruits) showed a normal morphology and linearly shape of P. digitatum hyphae and the apical were tapered with a smooth surface, (b) Treatment of gelatin 5% on P. digitatum revealed alterations in the morphology of the hyphae, which appeared severely collapsed and squashed due to the lack of cytoplasm, (c) Treatment of pomegranate peel extracts PPE 5%+coating with gelatin 5% on the P. digitatum caused irregular branching of hyphae in the apical part and the loss of linearity, (d) Treatment of pomegranate peel extracts PPE 10%+coating with gelatin 5% on P. digitatum showed warty surface of mycelium, (e) Treatment of pomegranate peel extracts PPE 15%+coating with gelatin 5% on P. digitatum showed a few small vesicles on the apical surface and a layer of extruded material completely covered the older part of the mycelium and (f) Treatment of pomegranate peel extracts PPE 20%+coating with gelatin 5% on P. digitatum revealed a massive increase in fibrillary material in the innermost part of the mycelium |
In fruits, grapefruit among them are usually stored after harvest. During cold storage, losses of economic importance are produced by several decays due to fungal rot. Penicillium expansum and Botrytis cinerea are well-known post-harvest pathogens which produce blue and gray rots19.
Quality of Star Ruby grapefruit
Weight loss (%): It is known that the weight loss% is in side by side with storage period prolonged. Results in Table 3 indicated that all treatments reduced percentage of weight loss during the cold storage period compared with the untreated fruits (control) during both seasons.
| Table 1: | Effect of pomegranate peels extract on linear growth (cm) and dry weight (g) of Penicillium digitatum isolated from Star Ruby grapefruit |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
| Table 2: | Effect of PPE and gelatin on disease infection percentage of Star Ruby grapefruit during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
Coating fruits with gelatin 5% achieved the highest values in reducing the percentage of weight loss followed by PPE 15%+coating with gelatin 5% insignificantly during the two seasons.
The time required for water loss or evaporation relies upon the thickness of the natural product skin, the temperature and the time of storage period. Fruits need a longer period for water loss if they have a thick peel and storage in low temperature. The basic method of weight loss from fresh fruit and vegetables is by vapor pressure at different locations, addition to that, respiration also causes a weight reduction20. Decrease in weight loss was likely due to the impact of the coating materials (gelatin) as a semi-permeable barrier against O2, CO2, moisture and solute movement, along these lines, reducing of respiration, water loss and oxidation reaction rates. Gelatin is an essential functional biopolymer widely used in foods to improve elasticity, consistency and stability21.
Decay (%): From Table 3 information exhibited the impact of diverse applied treatments on decay percentage of Star Ruby grapefruit in the two seasons. Decay percentage expanded slowly with the prolongation of cold storage period. All treatments demonstrated lower significant percentage of decayed fruits compared with the untreated ones. In this respect, PPE 10%, PPE 15% and PPE 20%+coating with gelatin 5% did not have any decayed fruits after 15 days of cold storage in the two seasons. While, PPE at 15%+coating with gelatin 5% was the most effective treatment that it did not give any decayed fruits until 30 days of cold storage. Also, it maintains the largest amount of fruits without any disease at the end of the experiment during the two seasons of the study.
In this respect, micrographs in Fig. 1a cleared that, through the scanning electron microscopy the mycelium hyphae of P. digitatum in control (untreated fruits) showed a normal morphology with linearly shape and the apical were tapered with a smooth surface.
| Table 3: | Effect of PPE and gelatin on loss weight (%) and decay (%) of grapefruits cv. Star Ruby during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
Moreover, the mycelium hyphae of P. digitatum (Fig. 1b) treated with gelatin 5% revealed alterations in the morphology, which appeared severely collapsed and squashed due to the lack of cytoplasm. The mycelium hyphae of P. digitatum (Fig. 1c) treated with Pomegranate Peel Extracts (PPE) at 5%+coating with gelatin 5% caused irregular branching in the apical part and the loss of linearity. Since, the mycelium hyphae of P. digitatum (Fig. 1d) treated with Pomegranate Peel Extracts (PPE) at 10%+coating with gelatin 5% showed warty surface. While, the hyphae treated with Pomegranate Peel Extracts (PPE) at 15%+coating with gelatin 5% on P. digitatum (Fig. 1e) showed a few small vesicles on the apical surface and a layer of extruded material completely covered the older part of the mycelium. Also, the hyphae treated with Pomegranate Peel Extracts (PPE) at 20%+coating with gelatin 5% on P. digitatum (Fig. 1f) revealed a massive increase in fibrillary material in the innermost part of the mycelium.
Pomegranate peel extract demonstrated different degrees of hindrance against the development of explored micro-organisms22. Furthermore, the pomegranate extracts act as natural inhibitors of pathogens, bacteria and fungi23. These outcomes give proof to the nearness of antimicrobial phenolic compounds in PPE. These compounds can debase the cell wall, disturb the cytoplasmic membrane, harm membrane proteins and meddle with membrane incorporated enzymes, which may in the end lead to cell death24.
Fruit juice (%): Concerning juice content (%) in Table 4, it was diminished within storage period expanded for all treated fruits. In any case, fruits coating can save juice content (%) for longer period during the storage. The obtained data showed that there were significant differences among treated fruits during storage period. Whereas, coating Star Ruby grapefruit with gelatin 5% only gave the largest percentage of fruit juice during the two seasons.
There is a relation between weight loss and juice content. Increasing in weight loss is indicator for loss of juice content in the fruits during storage. Most of water loss from the peel tissue as opposed from pulp. The time required for water loss or evaporation depends on the thickness of the fruit skin, the temperature and the length of storage period. Fruits need a longer period for water loss if they have a thick peel and storage in low temperature20. Decrease in weight loss was presumably because of the impact of the coating with gelatin a semi-permeable barrier against O2,CO2, moisture and solute development thereby, reducing of respiration, water loss and oxidation reaction rates. Our results are in line with Sakhale and Kapse25.
Vitamin C (mg100 g–1 fresh weight): Data in Table 4 reported that, vitamin C decreased with prolonged the period of storage of the fruits. All treatments maintain the loss of VC in Star Ruby grapefruit compared with uncoated ones. Data showed significant values between treatments in all storage periods during the two seasons. Since, PPE at 15% + coating with gelatin at 5% keep the higher value of vitamin C after 45 days of cold storage compared with other treatments or the control, respectively during the seasons of the study. In a previous study, ascorbic acid levels in Star Ruby grapefruit decreased with increase in storage period26. These results agreed with the observation that fruits and vegetables showed a gradual reduction in ascorbic acid content as the temperature or time at cold storage increments. Ascorbic acid is one of the most important antioxidants in plants, as it works to combat biotic and abiotic stress by detoxifying the types of reactive oxygen produced under pressure, with the help of the ascorbate-glutathione cycle27. In this respect, the observed effect of temperature conditioning and/or storage time on ascorbic acid degradation could be clarified due to direct oxidative destruction of ascorbinase activity or by indirect degradation through polyphenol oxidase, cytochrome oxidase and peroxidase activity. The decreasing in vitamin C content due mainly to oxidation of ascorbic acid to dehydroascorbic acid, but ascorbic acid was probably protected by the ascorbate-sparing effect of the polyphenols that may be attributed to their higher redox potential as compared to ascorbic acid28. The maintenance of ascorbic acid in the treated fruits could be because of the diminishing of respiration process and lessening of oxidation of ascorbic acid content while diminishing degree of ascorbic acid in control may be because of increment respiration process. The results were in line with El-Anany et al.29.
Total Soluble Solids (TSS): Recognizable that, total soluble solid was progressed with prolonging cold storage. The results in Table 5 presented that coating pomegranate fruits with gelatin at 5% gave significantly the higher value of TSS. In the first season, while the same treatment and the control represented significantly the higher values in the second season. Slightly insignificant decrease of TSS was found in the two seasons, respectively by the end of cold storage. Measure of total soluble solids is important for quality attributes of most fruits that will influence the flavor and market ability of the fruit represents as the amount of soluble sugars30.
The observed changes in TSS may be due to the hydrolytic conversion of polysaccharides into soluble sugar during the ripening process that increased TSS of the fruits. The increase in total soluble solids in treated fruits is legitimately corresponded to the diminish of ethylene production and that may be resulted in decreasing sucrose-phosphate syntheses and enzyme activity prompting decline in sucrose synthesis. On the other hand, cell walls contain high amounts of polysaccharides, mainly pectin and cellulose and are digested due to the activity of the cell wall degrading enzymes leading to a significant increase in TSS content. These results are in accordance with El-Anany et al.29 and Sabir et al.31.
Titratable acidity: It is obvious that, acidity content was gradually decreased with prolonged the storage period during the examined seasons.
| Table 4: | Effect of PPE and gelatin on fruit juice (%) and vitamin C (mg100 g–1 fresh weight) of grapefruits cv. Star Ruby during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
Data in Table 5 showed that, the treated fruits had the high values of acidity compared with untreated ones during the storage period in the two seasons. There were significant differences values between all coating fruits and the control. On the other side there were insignificant differences values between treatments until the end of storage. While dipping Star Ruby grapefruit in PPE 15%+coating with gelatin 5% was the most effective treatments for maintaining acidity% after 45 days of cold storage. In general, citrus fruits contain considerable amounts of organic acids. The main organic acids in the juice are oxalic, tartaric, malic, lactic, citric and ascorbic of these six acids, citric acid considered the most abundant acid of the total acid constituents of the juice followed by malic acid32.
| Table 5: | Effect of PPE and gelatin on Total Soluble Solids (TSS) (%) and titratable acidity (%) of grapefruits cv. Star Ruby during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
The decrease of acid contents during fruits storage was referred to the use of acids in the fruit as a source of energy and respiration consequently, the conversion of organic acids to form of sugar. Coating fruits reduced respiration rates hence, slow down the utilization of organic acids20.
The obtained results showed that coatings slowed the changes on titratable acidity and effectively delaying fruit senescence. This was probably because the film formed by materials used on the surface of the fruit might have modified the internal atmosphere, the endogenous CO2 and O2 concentration of the fruit, thus retarding ripening33.
TSS/acid ratio: The data in Table 6 showed that, the rate of TSS/acid ratio increased with increasing storage period.
| Table 6: | Effect of PPE and gelatin on TSS/acid ratio (%) and total phenolic (mg g–1 FW) of grapefruits cv. Star Ruby during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
All treated fruits had lowest values of TSS/acid compared with untreated ones after 45 days of cold storage in the two seasons. At the end of storage there were no significant differences between treated fruits in the two seasons. On the other hand data found significant values between control and all coated treatments by the end of the study. Also, coating with gelatin 5% was the lowest value of TSS/Acid ratio in the two seasons comparing with other treatments. The obtained values are because of the use of citric acid in the process of respiration of fruits. Then with the progression of time degradation of citric acid lead to more TSS as structural formula of citric acid is similar to glucose therefore decrease in citric acid is correlated with increase in TSS/acid ratio so sugar contents had gotten higher than acids34.
Total phenolic: Table 6 showed that, total phenol content was increased by prolonging the storage period. Coating Star Ruby pomegranate with 5% gelatin were relatively higher in total phenol compared with all other treatments. Also, the Table 6 indicated that untreated fruits gave the highest significant value of total phenol during the two seasons of study.
Phenolic compounds in grapefruit mainly include flavanone glucosides, p-hydroxybenzoic acids and hydroxycinnamic acids, which are affected by different abiotic and biotic stresses35. Moreover, Star Ruby grapefruit also showed an increase in total phenolic during storage36. In agreement with previous reports it was clear that, ellagitannins are the predominant class of phenolic compounds in pomegranate peel, since they represent over the 99% of the total content of pomegranate phenolics23.
Lycopene assay: Results in Table 7 showed that during storage, the treated fruits had a slight height value of lycopene compared with control in both two investigation seasons. Lycopene increased when storage period progressed even a certain period then began decreased until the end of storage. Lycopene expanded when storage period advanced even a specific period at that point started diminished until the end of storage. There were insignificant values between all treatment and control found in this investigation during the two seasons. Lycopene content relied upon the ripeness stage and the improvement conditions of the fruits there for, ripening process is related with increment in lycopene content along these lines, postponing right now be brought about lower contents of this compound. Besides, modification in the atmosphere can meddle in the synthesis of this pigment since it reduces the fruit metabolic activities bringing about lower ethylene production and diminished physiological changes. The outcomes were in a similar line with Javanmardi and Kubota37 and Tadesse et al.38.
Pan et al.39 suggested that the antioxidant capability of lycopene likely coordinates with the activation of the antioxidant enzymatic system in the flesh of citrus fruits. In agreement with this finding, the red peel of Star Ruby grapefruit, which accumulated large concentrations of lycopene showed a higher tolerance to chilling injury that the yellow areas of the fruit containing negligible amounts of carotenoids. On the other hand, the increment in lycopene content was associated with a significant delay in the initiation and a reduction of chilling damage in the chilling-sensitive of grapefruit. So, the accumulation of lycopene may be a key factor in the induction of tolerance to chilling40. In citrus fruit, cold stress has been associated with an oxidative burst and carotenoids, specially lycopene, display a powerful antioxidant activity41. It is reasonable to assume that the tolerance to CI injury observed in the red areas of grapefruit could be due to an increased antioxidant capacity induced by the high lycopene concentration in that tissue.
Antioxidant activity (%): Data in Table 7 indicated that antioxidant activity in Star Ruby grapefruits peels were slightly increased as storage period prolonged during cold storage. Moreover, antioxidant activity of all treatments increased at the end of cold storage.
Since, the untreated fruits produced higher significant values of antioxidant activity after 45 days of cold storage in both seasons under the study. The data also disclosed that, dipping fruits in PPE 15%+coating with gelatin 5% presented significantly less percentage values of antioxidant activity compare with applied treatments after 45 days of cold storage in both seasons, respectively. Ascorbic acid and phenolics are the main contributors to antioxidant activity in citrus fruits, with ascorbic acid contributing more than 65% of total antioxidant activity. In addition, a synergistic effect of phenolics and ascorbic acid can influence antioxidant activity. Naz et al.42 introduced the peel of pomegranate as a rich source of antioxidants and phenolic materials. Undoubtedly, the antioxidant capacity of pomegranate is related to the presence of phenolic materials, especially ellagic acid and punicalagin. Pomegranate peel, also, had been shown to be rich in polyphenols.
Noda et al.43 found linear relationship between total phenol content, antioxidant capacity and antibacterial activity against many micro-organisms. In this respect, pomegranate peel tissues usually contain a greater amount of phenol, anthocyanins and flavones. Moreover, pomegranate peel extract has a potential antioxidant activity and the total phenol content are approximately 249.4 mg g–1 which roughly corresponds to this finding as described by Li et al.44. Antioxidant activity of pomegranate peel extract inhibited the stable DPPH radical superoxide anion formation in a dose-dependent manner23. Thus, the utilization of natural preservatives plant extracts such as Pomegranate Peel Extracts (PPE) and gelatin will likely assist commercial producers and retailers in inhibiting the growth of pathogens of green mold and extending the shelf life of products over a broader range in the future.
| Table 7: | Effect of PPE and gelatin on lycopene assay and antioxidant activity (%) of grapefruits cv. Star Ruby during cold storage seasons 2017 and 2018 |
Means followed by the same letters are not significantly different by Duncan multiple range test at 0.05 levels | |
CONCLUSION
Based on the results of this study, it can be concluded that, Pomegranate Peel Extracts (PPE) at 20% followed by PPE at 15% completely inhibited the linear growth and dry weight of Penicillium digitatum. Also, coating fruits with 15% Pomegranate Peel Extracts (PPE) and gelatin 5% as a natural substances had more pronounced effect on prolonging marketing (reducing loss weight and decay) and maintaining most fruit quality parameter (vitamin C, titratable acidity (%)), antioxidant activity (%), lycopene assay and total phenol) compared with all treatments or uncoated fruits. Therefore, it can recommend with Pomegranate Peel Extracts (PPE) and gelatinable to improve quality and enhanced storage life of Star Ruby grapefruit during cold storage.
SIGNIFICANCE STATEMENT
This study discovered the possible effect of natural preservatives plant extracts such as Pomegranate Peel Extracts (PPE) and gelatin that can be beneficial for enhancing the storability of Star Ruby grapefruit. This study will help the researchers to uncover the critical areas of inhibiting the growth of pathogens of green mold caused by Penicillium digitatum natural preservatives that many researchers were not able to explore. Thus, a new theory of Pomegranate Peel Extracts (PPE) and gelatin is two promising examples that are beginning to be adopted on a commercial scale which may be arrived at retain post-harvest quality of grapefruit without any adverse effect on quality parameters.