Introduction: Fruits and vegetables are highly perishable products due to their high moisture content. Various technologies have been developed and used to prevent decay and increase the shelf life of agricultural products. However, many methods are not appropriate for fresh food products due to quality deterioration. Irradiation technology effectively reduces postharvest losses by prevent of insects and acts on the stored products. This technology does not create toxic or radioactive compounds in food. Gamma irradiation eliminates most microorganisms on the fruit and the package surface but does not guarantee the re-contamination. Consequently, the type of product packaging is critical to prevents re-contamination of the product during postharvest stage. Few studies examined the simultaneous effects of these two factors on the shelf life of perishable products such as strawberry, which is one of the most perishable fruits, and is susceptible to destruction by microorganisms, especially Botrytis cinerea, which causes extensive losses (40%) in the postharvest stage. Selecting appropriate packaging is the initial step in reducing the waste of this product. Strawberries come in various packaging, consist of perforated polyethylene packages and perforated packages designed for precooling operations. The type of packaging can play an essential role in irradiation and re-contamination after this operation. Consequently, both gamma irradiation and the precooling of fruits are cost-effective methods for controlling pathogens and preserving quality. This study evaluated the simultaneous effect of various packaging types, including commercial non-perforated polyethylene package with a top polymer film, non-perforated polyethylene package without a top polymer film, and specific precooling package, long with the effects of gamma irradiation at different doses on qualitative characteristics, contamination reduction, shelf life extension, and the subsequent contaminants throughout storage and decay control.
Material and methods: In this study, three types of packages were used, including perforated polyethylene package designed for the strawberry precooling operation, non-perforated polyethylene package with polymer film on top of that and non-perforated polyethylene package without a lid and film. Strawberry cv. Gavita was acquired from a greenhouse. Fruits with a uniform shape and no damage were selected and packed. Packages were situated in trays and then placed in five-layer thick cartons for irradiation in accordance with the suggestion of the Food and Agricultural Product Radiation Center. The prepared samples were immediately transferred to the irradiation center. The cartons were placed in the irradiation pallet and irradiated by a cobalt-60 source (Gamasel) with doses of 1, 2, and 3 kGy. Then, the irradiated samples were stored with the control samples at 1 ± 3 °C and relative humidity of 90 %. Qualitative changes in strawberry were evaluated in five phases, including days 1, 4, 8, 12, and 16 after process. In each step, the three packages of each treatment were removed from the cold storage, and quality characteristics were measured, such as fungal decay, fruit tissue firmness, ascorbic acid content, total soluble solid content (TSS), titratable acidity (TA), flavor index and the pH (acidity) of the juice. The results were analyzed using a randomized complete block design (as factorial experiments) in three replications. The mean comparison was performed by the Duncan test.
Results and discussion: Contamination and decay indices serve as the key indicators for assessing the shelf life and quality of fruits in the postharvest stage. The results indicated that the type of packaging and storage time had a significant effect on the decay index at a probability level of 1%.
The lowest decay was witnessed in fruits packed in precooling and polyethylene package with a film (1.44 and 1.42, respectively). In polyethylene package with a film, the fruit’s surface area exposed to the outside is nonexistent, leading to no chance of secondary contamination. Nonetheless, within these packages rising humidity and the condensation of moisture on the product create a suitable environment for the development of microorganisms present in the strawberry achenes and exacerbating their deterioration. In precooling packages, the storage conditions of the fruit were superior to those of the polyethylene packages with a film as moisture can exit these packages, leading to a reduced spoilage rate for the product. The irradiation dose has a non-significant effect, based on the results, the decay and its rate in treated samples were lower than in the control sample. After being stored for 12 days, the decay index of the treated strawberry was approximately 1.66 whereas 4.33 for the control sample. The type of Packaging type, the dose of irradiation, and duration of storage significantly influenced the firmness of strawberry texture. The fruits kept in the precooling package exhibited the greatest firmness (0.97 N). There was no significant effect between the precooling package and polyethylene package with a top film. The evaluation of the firmness between irradiated and control fruits showed that control samples were firmer than irradiated samples on the 4th and 8th days. In addition, maximum and minimum firmness were recorded at doses of 2 and 3 kGy with values of 0.966 and 0.918 N. Based on these findings, the use of 3 kGy is not advisable. Irradiation at doses of 1 and 2 kGy may preserve fruit firmness throughout storage. The firmness of fruit diminished the storage period, reaching its minimum after 16 days. Nonetheless, there was no difference between fruit firmness at the 8 and 12 day of storage. As a results, a storage period of 12 days was advised. The packaging type and storage duration influenced the ascorbic acid content of strawberry. The level of ascorbic acid was greater in the fruits kept in precooling and polyethylene package with a film, possibly because this package created optimal storage conditions that preserve the fruit quality. The greatest level of ascorbic acid was noted on the initial day of the storage (71.11 mg/100 g) which declines as the fruit is stored because this vitamin is utilized throughout the storage duration. Irradiation doses did not affect the ascorbic acid content. Nonetheless, the control samples contained more ascorbic acid than the treated samples. Although the amount of ascorbic acid was equal in all samples at the end of storage, the control samples shoed higher ascorbic acid levels on the 8th and 12th days of storage. The maximum pH (3.22) was observed at irradiation doses of 1 and 2 kGy and throughout the storage period, the pH showed a declining trend. Comparing control specimens with other samples demonstrated that the fruit in the control package exhibited a higher pH until the 8th day. However, no difference was noted between the control sample and the other samples following this duration. The polyethylene package with the film and the precooling package showed a more effective preservation of TSS, with no significant differences noted between the two. Thus, the type of packaging influenced the quality attributes of the fruit by regulating the rate of respiration and transpiration of the product. The irradiation dose also had a significant effect on the TSS in the samples. Raising the irradiation dose from 1 to 3 kGy led to increase TSS (from 6.29 to 6.59%). On the 1st, 4th, and 8th days of storage, the TSS was reduced, it rose over the storage period. The maximum TSS of 7.32% was associated with a 3 kGy treatment on the 16th day post-irradiation. The irradiation dose and storage duration influenced the TA of strawberry. It rose with increasing the irradiation dose and showed a downward trend over 16 days of storage. The maximum and minimum TA percentages of 1.51 % and 1.02 % were associated with the 2 kGy dose treatment on the first and 16th days following irradiation, respectively. the flavor index of the fruit increased over the storage period and there was no significant difference between the irradiation levels.
Conclusion: The finding indicated that the firmness, decay index, and ascorbic acid content of fruits stored in precooling packages and polyethylene packages with a protective top film were better preserved during storage. The irradiated fruits with a dose of 2 kGy exhibited increased firmness and pH alongside a reduced decay index, which was significantly lower than that of non-irradiated control samples. In general, irradiating strawberry with gamma at a dose of 2 kGy and packing them in precooling packaging or polyethylene packaging with a top film can serves as an effective method in strawberry postharvest technology. These processes delay the re-contamination of the fruit and help to preserve the quality index of product at a high level for up 12 days, which is adequate for a perishable fruit like strawberry.