Physico-chemical properties
At harvest, total soluble solids (TSS) ranged from 11.80 ± 0.75% to 12.50 ± 0.70% without significant variation, while acidity was approximately 0.65% (Table 1). After 15 days of cold storage, TSS values significantly decreased in control treatment, while it significantly increased in treatments 2 (strawberry treated with laser radiation for 6 min), 3 (strawberry treated with laser radiation for 6 min and coated with chitosan), 4 (strawberry treated with laser radiation and coated with pomegranate peel extract (PPE), and 5 (strawberry treated with laser radiation and coated with chitosan and PPE). These results are in agreement with Tanada-Palmu et al. [23] who reported that the rise of TSS may be due to excess water loss of strawberries. Also, Hernández-Muñoz et al. [24] attributed the increase in TSS to the solubilization of the cell wall polyuronides and hemicelluloses in mature strawberry. Regarding titratable acidity (TA) of strawberry, there was a significant increase in all treatments except treatments 3 and 4 where TA values were not affected by storage period. The increase in TA value may be attributed to the loss of water content where TA is estimated as a proportion of citric acid of fresh strawberry. Our results are in agreement with those of Ali et al. [25].
Table 1
Total soluble solids (TSS) and acidity (%) of strawberry treated with laser irradiation and coating with chitosan and pomegranate peel extract.
Treatments
|
TSS (Means ± SD)
|
Storage period (Day)
|
0
|
5
|
10
|
15
|
Treatment 1
|
12.5±0.70cde
|
8.50±0.40h
|
9.20±0.32gh
|
10.1±0.28fg
|
Treatment 2
|
11.8±0.75de
|
9.60±1.31gh
|
13.3±0.50bc
|
15.0±0.90a
|
Treatment 3
|
12.4±0.64cde
|
9.90±0.70fg
|
11.2±0.33ef
|
14.0±0.80ab
|
Treatment 4
|
12.3±0.65cde
|
11.5±0.51e
|
11.7±0.57de
|
13.2±0.62bc
|
Treatment 5
|
12.1±0.50cde
|
11.9±0.95de
|
12.8±0.966bcd
|
12.1±0.22cde
|
Treatments
|
Acidity (Means ± SD)
|
Storage period (Day)
|
0
|
5
|
10
|
15
|
Treatment 1
|
0.62±0.03e
|
0.95±0.03de
|
1.74±0.05c
|
3.25±0.56a
|
Treatment 2
|
0.65±0.01e
|
0.86±0.04de
|
0.92±0.05de
|
2.17±0.20b
|
Treatment 3
|
0.66±0.02e
|
0.83±0.02de
|
0.90±0.01de
|
1.10±0.1d
|
Treatment 4
|
0.65±0.01e
|
0.71±0.01de
|
0.80±0.01de
|
0.87±0.02de
|
Treatment 5
|
0.64±0.005e
|
0.68±0.005de
|
0.72±0.005de
|
0.77±0.02de
|
Means with different superscript letters differ significantly.
Treatment 1: untreated strawberry; treatment 2: strawberry irradiated with laser; treatment 3: strawberry irradiated with laser and coated with chitosan; treatment 4: strawberry irradiated with laser and coated with pomegranate peel extract, and treatment 5: strawberry irradiated with laser and coated with chitosan + pomegranate peel extract.
Weight loss and firmness
As shown in Fig. 1, uncoated strawberry (control treatment) had significantly the highest weight loss of 77.67% after 15 days of storage. The strawberry samples exposed to laser irradiation for 6 min then coated with chitosan and PPE significantly exhibited the lowest weight loss value (14.85%). The laser irradiation for 6 min and coating with pomegranate peel extract was more effective in delaying weight loss and decreased significantly weight loss by 27.56% compared to that of strawberry coating with chitosan. As expected, the firmness of strawberry samples had the opposite trend. During storage period, the firmness of all treated samples was significantly higher than the control samples (Fig. 1). At the end of storage period, the strawberry samples of control treatment lost around 67.53% of their firmness compared to 19.68% for strawberry exposed to laser irradiation for 6 min then coated with chitosan and PPE. Meanwhile, the loss of firmness in strawberry samples treated by laser irradiation then coated with PPE was lowest then loss in strawberry samples treated by laser irradiation then coated with chitosan which recorded 33.31 and 41.55%, respectively. Ali et al. [25] found that treated strawberry samples with 6 min of laser had a significant reduction in weight loss compared to untreated strawberries. Also, they observed that treated samples of strawberries with laser light retained firmness until the fifth day of storage period, whilst control strawberries became softer and fully matured. Saeed et al. [26] found similar results through coating of tomatoes with pomegranate peel extract and chitosan where they observed that the coated samples had lower weight loss and more firmness than uncoated samples. Our results may be attributed that the coating reduced the moisture loss and respiration process. Kumar et al. [2] revealed that the combination of higher concentration of pomegranate peel extract in chitosan film retains significant amount of moisture, and that may be attributed to molecular interactions and modifications in the hygroscopic characteristic of the chitosan combine.
Functional properties
In this study, the effect of laser irradiation and coating with chitosan or PPE on the total phenolic, ascorbic acid, anthocyanin, and antioxidant activity of strawberry samples during cold storage was investigated and the results are shown in Fig. 2. Compared to control treatment, laser radiation for 6 min had a significant positive effect on the total phenolic, ascorbic acid, and antioxidant properties of strawberry in treatment 2, particularly after 15 days of storage. The level of anthocyanin significantly increased in all treatments during the storage period; pointing out that the strawberry became duskier as maturation advancement. Our results are in correspondence to those of Taha et al. [27] who found that the exposure of Sequoia sempervirens shoots to laser radiation for 5 min led to increase the total phenolic, tannins, and DPPH radical scavenging capacity compared to untreated shoots. Ali et al. [25] reported that laser radiation could have a secondary impact on the anthocyanin cumulation in strawberry fruits, whilst the major effect was the storage time and temperature. They also found that the ascorbic acid level of the strawberry treated by laser light for 3 min was significantly greater than both strawberry samples treated by laser for 6 and 12 min. Cordenunsi et al. [28] observed that laser radiation did not expose the strawberry to heat through process and did not influence on the ascorbic acid content. In another study, Pirvu et al. [29] observed that the exposure of the aqueous extract of Plantago lanceolata L. leaves to laser radiation enhanced the antioxidant activity comparing with control. Salyaev et al. [30] attributed the effect of laser to stimulate morphogenetic processes in plant fractions or tissues inducing metabolic changes and these changes lead to generate various antioxidant components. Maraei et al. [31] found that the antioxidant capacity of gamma treated and untreated strawberry samples enhanced during the cold storage (9 days). They attributed these results to the destructive behavior of gamma radiation and oxidation that may fracture the chemical bonds of polyphenol components, liberating low molecular weight soluble phenol components with antioxidant properties.
Regarding the effect of coating materials on strawberry, it was noted that there were no significant differences in the total phenolic, anthocyanin, ascorbic acid, and antioxidant activity values among all strawberry treatments at day 1 of cold storage (Fig. 2). A decline trend in total phenolic, ascorbic acid, and antioxidant activity was observed in all treatments during cold storage period, but anthocyanin had opposite trend. After 15 days of storage, the strawberry of treatment 5 significantly showed the highest values of total phenolic, ascorbic acid, and antioxidant activity compared to other treatments. However, the strawberry of treatment 5 significantly exhibited the lowest anthocyanin content. Our results revealed a good positive correlation between the ascorbic acid (r = 0.99) or total phenols (r = 0.89) and the antioxidant activity. These results attributed to the coating with pomegranate peel that contains high content of phenolic compounds, particularly ellagitannins. Tannins are known as high molecular weight phenolic components that exhibit noteworthy antioxidant capacity [32]. Furthermore, Kaderides et al. [33] reported that the major phenolic component in pomegranate peel is punicalagin, which rates about 70% of the ellagitannins and approximately 76.5% of total phenolics. Cruz-Valenzuela et al. [34] reported that pomegranate peel extract had high DPPH radical scavenging activity (86.12%). Opara et al. [35] observed that the Egyptian pomegranate peel had higher vitamin C content (80 mg 100 g− 1) than that of pomegranate aril (58 mg/100 g). The greater vitamin C level in fruit peels is in corresponding with the studies of other researchers, who approved higher antioxidant activity in various fruit peels than other fruit fractions [36, 37]. Pomegranate is a rich source of anthocyanins that act as antioxidants through radical scavenging, metal chelating, hydrogen donors, and singlet O2 quenching [38, 39]. In this respect, Kumar et al. [2] made chitosan-based edible film enriched with pomegranate peel extract and found that the total phenolic and antioxidant activity ranged between 5.75–32.41 mg/g and 23.13–76.54%, respectively depending on the volume fraction of pomegranate peel extract. Moreover, our findings revealed a positive correlation between weight loss and anthocyanin level and this result is in agreement with the result of Darwish et al. [40].
Microbiological examination
Results in Table 2 present that the exposure to laser radiation led to decline the count of fungi in strawberry of treatment 2 at day 1 and after 15 days of cold storage. Strawberry samples in treatments 3, 4, and 5 were free from fungi on day 1 of storage. A growing trend in fungi count of strawberry was noted during storage period. The increase in fungi count was recorded in uncoated strawberry samples, and the growth rate of fungi was not great in coated strawberry samples. At the beginning of storage, psychotropic bacteria did not appear in the strawberries of all treatments. However, the count of psychotropic bacteria increased gradually in all treatments except treatment 5. After 15 days of storage, the strawberry of treatment 5 significantly recorded the lowest count of psychotropic bacteria compared to other treatments. In this regard, Saeed et al. [26] observed that the pomegranate peel extract and chitosan were successfully utilized for coating of tomato as antifungal agents. Azam et al. [41] attributed these results to the bioactive components in pomegranate peel oil that inhibit the growth of fungi and bacteria. Hence, the coated strawberries contained lower bacteria and fungi counts than uncoated strawberries. Nazeam et al. [42] reported that phloretin and coutaric acid gained from pomegranate peels showed strong antimicrobial activity, particularly against Staphylococcus epidermidis, and punigratane had the most essential antibacterial impact on Micrococcus kristinae. Nazeam et al. [42] and Charalampia and Koutelidakis [43] pointed out that pomegranate extracts are powerful inhibitors of Bacillus cereus, Bacillus subtilis, Bacillus coagulans, Listeria monocytogenes, Enterobacter aerogenes, Pseudomonas aeruginosa, Escherichia coli, Salmonella typhimurium, Staphylococcus aureus, Trichoderma reesei, Aspergillus niger, Rhizopus oryzae, Penicillium citrinum, and Mucor indicus. They attributed that to the high content of pomegranate of tannins, flavonoids, and phenolic components.
Table 2 Psychrotrophic bacteria and fungi counts (log CFU/g) of strawberry treated with laser irradiation and coating with chitosan and pomegranate peel extract.
Treatments
|
Psychrotrophic bacteria (Means ± SD)
|
Storage period (Day)
|
0
|
5
|
10
|
15
|
Treatment 1
|
ND
|
3.71±0.49c
|
4.14±0.32b
|
5.05±0.06a
|
Treatment 2
|
ND
|
2.48±0.38e
|
2.99±0.37d
|
3.83±0.50bc
|
Treatment 3
|
ND
|
ND
|
2.17±0.27ef
|
2.89±0.22d
|
Treatment 4
|
ND
|
ND
|
1.83±0.05fg
|
1.98±0.01f
|
Treatment 5
|
ND
|
ND
|
ND
|
1.49±0.02g
|
Treatments
|
Fungi (Means ± SD)
|
Storage period (Day)
|
0
|
5
|
10
|
15
|
Treatment 1
|
2.26±0.46cd
|
2.61±0.46c
|
3.38±0.39b
|
4.38±0.48a
|
Treatment 2
|
1.70±0.32efg
|
1.84±0.09ef
|
2.39±0.30cd
|
3.68±0.18b
|
Treatment 3
|
ND
|
1.56±0.27fg
|
1.85±0.20ef
|
2.07±0.40de
|
Treatment 4
|
ND
|
1.33±0.18gh
|
1.58±0.09fg
|
1.85±0.08ef
|
Treatment 5
|
ND
|
ND
|
ND
|
1.09±0.23h
|
Means with different superscript letters differ significantly.
Treatment 1: untreated strawberry; treatment 2: strawberry irradiated with laser; treatment 3: strawberry irradiated with laser and coated with chitosan; treatment 4: strawberry irradiated with laser and coated with pomegranate peel extract, and treatment 5: strawberry irradiated with laser and coated with chitosan + pomegranate peel extract.
Color attributes
The effect of laser irradiation, and coating with chitosan and PPE on strawberry color properties during storage period is shown in Fig. 3. The L* values indicate to strawberry shine, while positive + a* values denote to redness. Hue (h°) values point out the visible spectrum color, whilst Chroma (C*) values refer to the purity or intensity of hue related to neutral gray. There were no significant differences among all treatments in all color characteristics at the initial of storage period. During storage period, the L*, +a*, and h° values of all treatments significantly decreased, but C* values had the opposite trend. After 15 days of storage period, strawberry in treatment 5 significantly recorded the highest values of L*, +a*, and h°, and the lowest value of C* compared to other treatments. Low values of L* refer to more intense color and storage effect, but high values indicate less tincture accumulation and less storage effect [44]. Positive a* values are related to the anthocyanin content of strawberry [45]. Ali et al. [25] reported that low exposure period of laser irradiation declined darkness of strawberry, while chroma remained the same.
Sensory evaluation
Using laser irradiation and coating with fruit or vegetable waste-based materials for preservation of fruits or vegetables quality may change their sensory attributes that might lead to a decline in the acceptability of consumers. Hence, it was important to follow the changes in taste, odor, texture, and overall acceptability of strawberry as a result of laser irradiation and coating with chitosan and PPE (Fig. 4). Storage period had negative effects on the sensory attributes of strawberry fruits, particularly in the control treatment. Laser irradiation for 6 min protected the strawberry fruits somewhat from the effect of storage period. Furthermore, the coating with PPE or chitosan and PPE greatly evanesced the negative effect of storage period on the sensory characteristics of strawberry fruits. These findings are in consistency with the above strawberry properties, especially the weight loss and color. In this regard, Jiang et al. [46] reported that the coating of blue berries with chitosan did not affect negatively on the flavor attribute. Alqahtani et al. [47] found that the dipping of Barhi date fruits in PPE maintained sensory characteristics during storage period.