Effect of Modified Atmosphere Packaging (MAP) on the Stability of Anthocyanins and Degradation of Phenolic Compounds during Postharvest Storage of Pomegranate Fruit ()
1. Introduction
The role of phenolic compounds in food quality, their health-promoting and therapeutic effects, along with their mechanism of actions on human health has been the topic of numerous researches in recent years. In addition to their health beneficial properties, phenolic compounds are connected with the quality of plant-derived foods such as appearance, color, taste and aroma. Pomegranate fruit (Punica granatum L.) is quite well-known for its nutritious nature and presenting a broad range of health beneficial activities, in particular antioxidative, anti-inflammatory, anti-carcinogenic, antimicrobial, neuro- and cardio-protective properties [1] - [6].
Most of these biological properties are attributed to the pomegranate’s primary and secondary metabolites in specific phenolic compounds such as anthocyanins, flavanols, ellagi- and gallotannins, proanthocyanidins, and lignans [5] [7] [8] [9] [10]. These phenolic compounds are found in all parts of the fruit, including fruits peel (ellagitannins, phenolic acids, flavonoids), arils (flavonoids, hydrolysable and condensed tannins, and phenolic acids), seeds (phenolic acids, flavonoids, hydrolysable tannins), and membrane walls (mostly ellagitannins) [11] [12] [13] [14] [15]. Pomegranate’s phenolic compounds possess very diverse chemical structures and appear in different quantities depending on the plant cultivar, geographical, agronomic, and other environmental factors including postharvest handling and storage conditions [16] [17] [18] [19].
Scientific findings have provided evidence that the type of technologies and treatments being applied during postharvest handling and storage have significant impact on the total contents of phenolic compounds, their biosynthesis, metabolism, biological activities and the involved enzymatic reactions [16] [20] [21] [22] [23]. These effects could be either favorable or unfavorable towards preserving the desired quality of the pomegranate fruit [16]. Therefore, current studies are also focused to find and develop technologies or a combination of them that allow maintaining the original quality of the fresh fruit, i.e., color, flavor, texture and its nutritious compounds including vitamins, sugars, amino acids, antioxidants, and other health-promoting compounds, while avoiding weight loss, fungal decay, and common physiological disorders such as husk scald, pitting, and chilling injury during cold storage.
Various studies have shown that modified atmosphere packaging (MAP) technology by decreasing the oxygen content and/or increasing the carbon dioxide content of the atmosphere surrounding the fresh produce, can delay quality loss, tissue softening, enzymatic activities, and incidence of various physiological disorders and pathogenic developments during long storage of pomegranate fruit [20] [21] [23] - [31]. MAP can also prevent the fruit weight loss and shriveling by creating a higher relative humidity in the surrounding environment of the fruit. Therefore, MAP could be applied or combined with other storage treatments, for instance with various thermal treatments or refrigeration to inhibit potential development of any decay or physiological disorders [16] [32].
Nonetheless, identifying the applicable MAP that obtains the desired outcome in terms of maintaining the quality and nutritional aspects while avoiding decay and unwanted physiological disorders during long storage requires extensive research studies. The optimum MAP can vary for one pomegranate to another depending on the cultivar, packaging material, and other storage conditions in particular temperature [27]. Within this context, the aims of this study were 1) to determine the effects of MAP technology on the total contents of anthocyanins and degradation of phenolic compounds during cold storage of pomegranate fruit cv. “Malase Torsh Saveh”, and 2) to investigate the effect of pre-storage thermal dipping (TD) as a complementary treatment along with MAP technology on the stability of anthocyanins and degradation of phenolic compounds during cold storage of pomegranates. This research is the first study that investigates the effects of MAP technology on the phenolic compounds of an Iranian pomegranate (Cv. Malas e Torsh Saveh) during long storage.
2. Materials and Methods
2.1. Materials
2.1.1. Plant Material
For this study, “Malas e Torsh Saveh” pomegranates, which is a popular commercial pomegranate cultivar in Iran, were harvested from Saveh Pomegranate Research Station on October 23rd. Harvested fruits were transported to the University of Tehran’s College of Agriculture the same day and stored overnight in one of the research cold rooms at 4˚C - 5˚C. Pomegranates with uniform size, shape, appearance, and sound outer skin were sorted for the study the day after.
2.1.2. Chemicals and Other Materials
Unperforated low-density polyethylene (LDPE) and polypropylene (PP) packaging bags were purchased from a local provider. Citric acid, sodium citrate, and sodium hydroxide were procured from Merck KGaA (Darmstadt, Germany).
2.2. Methods
2.2.1. Sample Preparation
For the purpose of effect screening, selected pomegranates were divided into different groups with two storage temperatures of 2˚C & 6˚C, two packaging materials of unperforated PP and LDPE, and two modified headspace gas compositions coded as GC1 and GC2 with elevated contents of CO2 and reduced O2 compare to the ambient atmosphere. The two control groups consist of 60 coded pomegranate fruits with no polymeric packaging were evenly placed inside wooden trays and stored at the ambient atmosphere of 2˚C & 6˚C cold rooms with 85% - 90% RH throughout the study (Figure 1).
The one-chamber table-top packaging machine (Henkelman 200A, Denmark) was used to flush and insert the programmed gas mixtures inside the unperforated LDPE and PP bags. The two gas mixture compositions that applied prior the storage phase coded as GC1 with 5% CO2 + 5% O2 and GC2 with 10% CO2 + 5% O2 gas composition respectively. Nitrogen gas made the remaining component of both mixtures. All sealed packages that each contains a pair of pomegranates
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Figure 1. Control and modified atmosphere (MA) packaged pomegranates remained at one of the research cold rooms by the end of study.
were checked, and then transferred and stored in the 2˚C and 6˚C cold rooms for the long storage study.
2.2.2. Thermal Dipping Treatment
To investigate the potential effects of thermal dipping treatment on the total anthocyanins and phenolic compound contents of fruits packaged with MA during long storage, 60 pomegranates were selected and divided into two groups. Half of these fruits were dipped in the 46˚C water bath (Tattnauer, NY, USA) for 2 minutes and left to completely air-dry prior to packaging. After they completely dried, they were coded and placed inside the unperforated LDPE bags in pairs (2 pomegranates in each package). The package headspace of both sampling groups was replaced with the gas composition of 5% CO2 + 5% O2 (GC1) and got sealed immediately. Both groups were stored at the cold room of 6˚C for a duration of 12 weeks.
2.2.3. Quantification of Total Anthocyanins and Phenolic Compounds’ Degradation Products
To measure the total anthocyanin contents and phenolic compound degradation products (or the browning compounds) during storage, first three packages from each sampling group were randomly selected and one pomegranate from each of these packages separated for the study. Also, three pomegranates were randomly selected from each tray of control groups. To obtain the juice, the fruits were half-cut with a clean knife and the juice from the arils was extracted by using a manual stainless-steel pomegranate squeezer. Next the extracted juices were centrifuged (Andreas Hettich centrifuge, Germany) for 20 min at 3000 rpm and the supernatant of the samples were used for the spectrophotometry measurements. For maximum stability of anthocyanins during measurements, the juice supernatants were diluted with citric acid buffer (pH = 3.4) at 1:3 (v/v) ratio. Later, the total anthocyanin contents of the extracts were recorded by measuring the absorbance at 510 nm with the UV/Vis spectrophotometer instrument (Perkin Elmer Lambda EZ 201, MA, USA) and the recorded values were reported as the Absorbance Unit (AU), which is also expressed as the optical density (OD) per unit length (Equation (1)); the cuvette thickness used in this study was 1 cm:
(1)
where:
l = the distance that light travels through the sample (the sample thickness) measured in cm;
Aλ = absorbance at wavelength λ.
Absorbance at wavelength of 446 nm was recorded by the same spectrophotometer as an indication of polymerization and degradation of phenolic compounds or production of browning compound. Citric acid buffer (pH = 3.4) was used as the reference solution for the absorbance measurements at both 510 and 446 nm wavelengths [20] [21] [25] [26]. All measurements were performed in triplicate.
2.2.4. Browning Index (BI) Calculation
Browning Index of the extracted juice, which reflects the ratio of the total anthocyanin to the development of browning compound or phenolic degradation products was calculated for all sampling groups from Equation (2) [33].
(2)
2.2.5. Determination of pH, Titratable Acidity (TA), & Total Soluble Solids (TSS)
4 grams of the extracted juice was diluted with 20 mL of distilled water, and values of pH were measured by the pH meter (744 HannaTM porable pH meter, Portugal) according to Hess-Pierce & Kader, 2003 [26]. Titratable acidity was determined by titrating the diluted juice with 0.1 N NaOH to pH = 8.1 - 8.3 and results were reported as (%) or gr Citric acid per 100 gr of juice. The TSS of the extracted juice was measured by using a refractometer (Bellingham + Stanley, U.K.) at 20˚C.
2.2.6. Statistical Analysis
Storage temperature with 2 levels (2˚C and 6˚C), Packaging type with 3 levels (LDPE, PP, and no packaging or None), head space gas composition with 3 levels (air or ambient atmosphere, GC1, & GC2), and storage time with 4 levels (3 weeks intervals) were applied as the experimental design factors. The effect of hot-water dipping pretreatment on the MA packaged pomegranate fruits stored at 6 ̊C was studied on 2 groups for 12 weeks and analyzed separately. All measurements were conducted in 3 replicates and the mean value was reported. JMP Pro software was used to perform the statistical analysis and data visualization. Analysis of variance (ANOVA) at p < 0.05 and Student’s t Test (for mean comparison) were applied when appropriate.
3. Results
3.1. Changes in the Total Content of Anthocyanins
Among the multiple factors being studied, only the single effect of storage time was found to have a significant effect on the total content of anthocyanins at p < 0.01 during this study. The results showed a decline from the average absorbance value of 0.917 ± 0.054 (Mean ± SE) at the beginning of the study through the 7th week of the storage followed by a slight increase towards the end of the study (0.49 ± 0.054). The total content of anthocyanins was also affected by the 3-way interaction effects of the storage duration, storage temperature, and the surrounding gas composition. Overall, samples with MA packaging displayed lower values of total anthocyanins around the seventh week of their storage compared to the control samples with no packaging (Figure 2). Among the MA packaged treatments, storage temperature at 2˚C, LDPE packaging, and the initial gas mixture of 5% CO2 + 5% O2 (GC1) were associated with higher amounts of total anthocyanin contents by the end of study. As illustrated in Figure 3, the interaction profile also indicates an interaction between the storage temperature and the type of packaging although it was not found to be statistically significant. Besides, higher amounts of anthocyanins were observed on average in the extracts of samples that were stored at 2˚C storage temperature compared to samples that were stored at 6˚C during long storage regardless of their packaging type and their surrounding atmospheric composition.
3.2. Changes in the Degradation of Phenolic Compounds
The results of this study showed that the single effects of storage time and temperature have had significant effects on the degradation of phenolic compounds and formation of browning pigments at p < 0.05. On average higher amount of phenolic compounds degradation products was observed by the end of study (0.332 ± 0.024) compared to the beginning of the study (0.278 ± 0.024) throughout the 10-week storage time. Likewise, degradation of phenolic compounds was enhanced at 2˚C storage temperature with the average absorbance value of 0.323 ± 0.017 at 446 nm compared to the corresponding values of 0.264 ± 0.017 when samples were stored at 6˚C temperature (Figure 4). None of the 2 & 3-way interaction effects was found to be significant. Yet, the interaction profile (Figure 5) displayed an interaction between the storage temperature and the type of packaging that may not be statistically significant. As shown in Figure 5, storage temperature of 2˚C, UPP packaging, and the initial gas mixture of 5% CO2 + 5% O2 (GC1) induced higher amounts of degradation of phenolic compounds and subsequent formation of browning pigments among pomegranate fruits with MA packaging. Extracts of pomegranates that were stored with no packaging and surrounded by the air or ambient atmosphere of the cold room
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Figure 2. Effects of packaging type (top) and gas compositions (bottom) on the total anthocyanins content during the 10 weeks of storage. Each error bar is constructed using 1 standard error from the mean. Packaging type: PP, polypropylene; LDPE, low-density polyethylene; NONE, no packaging is used. Gas composition: NRM, air or ambient atmosphere; GC1, initial MAP with 5% CO2 and 5% O2; GC2, initial MAP with 10% CO2 and 5% O2.
showed the least amounts of browning pigments on average at the end of study (Figure 4, bottom).
3.3. Changes in the Browning Index (BI)
The results of the data analysis indicate that the Browning Index was affected by the single effect of the storage time, its 2-way interaction effects with the type of packaging, and its 3-way interaction effects with the storage temperature and the surrounding atmospheric composition at p < 0.01. The average BI value was increased from 0.31 ± 0.02 at the beginning of the study to 0.69 ± 0.04 by the end
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Figure 3. Interaction profile of temperature, packaging type, and gas composition effects on the total anthocyanins content of the MA packaged pomegranates. Packaging type: PP, polypropylene; LDPE, low-density polyethylene. Gas composition: GC1, initial MAP with 5% CO2 and 5% O2; GC2, initial MAP with 10% CO2 and 5% O2.
of study. The increase occurred during the first 4 weeks of the storage and remained around the same value during the rest of the 10-week study. Extracts of pomegranates with no packaging that were surrounded by the ambient atmosphere during storage displayed the lowest values of BI (0.53 ± 0.06) at the end of the storage compared to the treatments with MA packaging. Additionally, the extracts of pomegranates with PP packaging showed the highest BI value (0.63 ± 0.06) at the end of study (Figure 6, top). Overall higher BI values were associated with storage temperature at 2˚C. Control samples with no packaging that were surrounded by the ambient atmosphere showed higher BI values when stored at 6˚C. In contrast to air-stored samples, the extracts of MA packaged pomegranates displayed higher BI values when stored at 2˚C cold room (Figure 6, bottom).
3.4. Changes in pH, TA, & Total Soluble Solids (TSS)
Storage time showed to have significant effects (at p < 0.01) on the pH values of
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Figure 4. Effects of Storage temperature (top) and type of packaging (bottom) on development of browning compounds over 10 weeks of storage. Each error bar is constructed using 1 standard error from the mean. Packaging type: PP, polypropylene; LDPE, low-density polyethylene; NONE, no packaging is used.
the treatments over the course of study. The pH values displayed an increase from 3.45 ± 0.05 at the harvest to 3.67 ± 0.17 during the first month of storage that remained around the pH value of 3.64 ± 0.19 till the end of 10 weeks of storage. pH was not influenced significantly by the storage temperature; however, it was affected by the 2- & 3-way interaction effects of storage time with packaging type & the surrounding gas composition (p < 0.01). Overall, the control samples that were surrounded by the ambient atmosphere and had no packaging showed pH values closer to the average pH value at the harvest (pH = 3.53 ± 0.03). Whereas extracts from pomegranates that had unperforated PP packaging and
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Figure 5. Interaction profile of temperature, packaging type, gas composition effects on the development of phenolic compounds degradation products of MA packaged pomegranates. Packaging type: PP, polypropylene; LDPE, low-density polyethylene. Gas composition: GC1, initial MAP with 5% CO2 and 5% O2; GC2, initial MAP with 10% CO2 and 5% O2.
packaged with the initial gas mixture of 5% CO2 + 5% O2 (GC1) were associated with higher values of pH at the end of 10 weeks of storage with the average pH values of 3.66 ± 0.03 and 3.63 ± 0.02 respectively (Figure 7, top).
TA values were significantly affected by the single effect of gas composition and its interaction with the storage duration (p < 0.01). The average TA values displayed slight decrease from the average TA value at the harvest (1.02% ± 0.032%) until the 7th week of storage (0.918% ± 0. 032%) and then started to slowly increase toward the end of the study (0.974% ± 0.032%). As shown in Figure 7 (bottom), the control samples that were surrounded by the ambient atmosphere and had no packaging showed higher amounts of TA, while the samples with PP packaging and the initial gas mixture of 5% CO2 + 5% O2 (GC1) showed the lowest values for TA by the end of study. These results are in agreement with the study findings on pH changes throughout the storage. Also, pomegranates that were packaged with the initial gas mixture of 10% CO2 + 5% O2 (GC2) exhibited the lowest pH values (pH = 3.5) and the highest TA (TA =
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Figure 6. Effects of (top) packaging type and storage temperature, and (bottom) storage temperature, time, and gas composition on the values of Browning Index (BI) during 10 weeks of storage. Each error bar is constructed using 1 standard error from the mean. Packaging type: PP, polypropylene; LDPE, low-density polyethylene; NONE, no packaging is used. Gas composition: NRM, air or ambient atmosphere; GC1, initial MAP with 5% CO2 and 5% O2; GC2, initial MAP with 10% CO2 and 5% O2.
1.19%) values by the end of study. These findings suggest the potential slow penetration of CO2 gas within the pomegranate’s tissues and arils during the long storage that might have led to the formation of carbonic acid and the subsequent observation of pH reduction and %TA increase of the pomegranate extracts. These changes were particularity noticeable from the seventh week of storage.
The values of the extracts’ total soluble solids (TSS) were affected only by the storage duration at p < 0.05 and none of the other factors i.e., storage temperature, surrounding gas composition, and type of packaging and their 2- and 3-way interactions was found to be significant. The average TSS values showed slight decrease from the average value of 18.17 ± 0.13 at the harvest to 17.57 ± 0.13 by the end of storage, which indicates minor loss of saugers due to the low
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Figure 7. Effects of packaging type and surrounding gas composition on the (top) pH, and TA (bottom) of pomegranates during 10 weeks of storage. Each error bar is constructed using 1 standard error from the mean. Packaging type: PP, polypropylene; LDPE, low-density polyethylene; NONE, no packaging is used. Gas composition: NRM, air or ambient atmosphere; GC1, initial MAP with 5% CO2 and 5% O2; GC2, initial MAP with 10% CO2 and 5% O2.
respiratory activities of pomegranate fruits during the long storage.
3.5. Effects of Thermal Dipping (TD) Treatment
The results show that the single effect of thermal dipping (TD) pretreatment was not significant on the total content of anthocyanins, degradation of phenolic compounds, and Browning Index during the long storage. However, as shown in Table 1. The 2-way interaction effect of thermal dipping treatment with the storage time had significant effect at p < 0.01 on the values of the Browning Index. The extracted juice obtained from pomegranates that were subjected to the TD treatment prior to the storage had higher BI values (mean BI = 0.95) compared to the control group (mean BI = 0.66) after 12 weeks of study (Figure 8).
The values of pH, TSS, and TA also have not been affected by the single effect of TD treatment. Still, the 2-way interaction effect of thermal dipping treatment with storage time was found to have significant effects on the pH values at p < 0.01, TSS at p < 0.05, and TA at p < 0.05. By the end of 12 weeks of storage, the extracted juice of samples that received TD treatment showed higher pH & lower TA and TSS values compared to the control samples that have not received thermal dipping treatment prior to the storage.
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Table 1. Effect of thermal dipping treatment, storage time and their two-way interaction on the contents of total anthocyanins, degradation of phenolic compounds, and browning index (BI) of the juice extracts of the stored MA packaged pomegranates.
Data are presented as the Mean ± ST.DEV. Comparisons of Means and LSD between pairs were determined by Student’s t test (α = 0.05). For each source of effect, levels or means that are not connected by the same letter are significantly different (p < 0.05).
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Figure 8. Effects of thermal dipping treatment on the values of Browning Index (BI) during 12 weeks of storage. Each error bar is constructed using 1 standard error from the mean. Dark gray bars represent the sample group with no thermal dipping treatment, and bars in lighter gray represent the sample group that were subjected to the thermal dipping treatment.
4. Discussion
The findings of this study showed an overall decrease in the total contents of anthocyanins during 10 weeks of storage, in which multiple factors might have been involved. Some of the factors that affect anthocyanins stability or their susceptibility to degradation are temperature, increased sugar, pH, acidity level, metal chelation, and copigmentation with other flavonols and tannins [19] [34]. Likewise, regulation and activity of phenylalanine ammonia-lyase (PAL), which is the key enzyme in biosynthesis of flavonoids can be another influential factor [16]. Therefore, the overall decline in the contents of anthocyanins during the storage could be to some extent as the result of the effects of cold storage temperature on the enzymatic activities of PAL that inversely has affected the biosynthesis of anthocyanins. Similarly, the pomegranate extracts that obtained from MA packaged samples showed less absorbance at 510 nm compared to those obtained from air-stored pomegranates, which became even more evident by the seventh week of storage. These findings are consistent with the findings of previous studies on MA packaging of “Mollar de Elche” and “Wonderful” pomegranate cultivars, suggesting elevated amounts of CO2 in the surrounding atmosphere of stored pomegranates might have a role in disrupting the PAL activity that subsequently can lead to the suppression of anthocyanins biosynthesis and pigmentation [25] [26] [34]. Additionally, pH reduction and TA increase as the result of gradual penetration of CO2 within the tissues and arils of MA packaged pomegranates and potential formation of carbonic acid could be another contributing factor to the anthocyanins’ stability and their noticeable content increase from week 7th of storage towards the end of the 10-week study. So, despite the lack of significant difference between the anthocyanin contents of control and MA packaged pomegranates by the end of study, yet different underlying factors might have been participated throughout the storage.
Nonetheless, a different pattern in the contents of anthocyanins during the cold storage of MA packaged “Hicaznar” and “Hicrannar” cultivars was reported. Selcuk & Erkan (2014) reported an increase in the total anthocyanin contents of “Hicrannar” pomegranates during the first 100 days of storage at 6˚C that followed by a decrease over the last 20 days of the study in both MAP and control treatments through 120 days of storage [28]. Total anthocyanin contents in “Assaria” pomegranates that were subjected to several storage treatments also showed an increase during the first month of storage with continued decrease towards the end of 4 months of cold storage at 5˚C [22]. “Ruby” pomegranates when air-stored at 5 and 7˚C for 16 weeks displayed an increase in their anthocyanin concentration through 8 and 12 weeks of storage that followed by a slight decrease and no change in their content correspondingly till the end of cold storage [35]. Selcuk & Erkan (2015) and Çandir et al. (2018 & 2019) also reported an overall increase in the anthocyanin content of “Hicaznar” pomegranates during the first 4 months of cold storage and a gradual decrease in the anthocyanin content toward the end of storage in both air-stored & MA packaged pomegranates when stored at 6˚C for 6 months [23] [30] [31]. Yet, Çandir et al. (2018) observed a delayed anthocyanins synthesis and lower anthocyanin concentrations in MA packaged “Hicaznar” pomegranates [30]. The difference in the observed anthocyanins contents of these cultivars during long storage can be attributed to the inherent differences among various cultivars given that the same trend has been recorded for both control and MA packaged pomegranates during long storage that indicates the potential continued postharvest metabolic activities including biosynthesis of anthocyanins in the fruit despite of being refrigerated and/or subjected to the MA packaging.
The current study results also showed higher contents of browning pigments due to the degradation of phenolic compounds when pomegranates stored at 2˚C compared to 6˚C storage temperature. These findings can be attributed to the enzymatic activities of polyphenol oxidases (PPO) and peroxidases (POD) that catalyzed enzymatic browning in fruit tissues as the results of environmental stresses such as low storage temperatures and mechanical injuries [16]. Likewise, elevated CO2 contents in the surrounding atmosphere of MA packaged pomegranates can be another contributing factor that enhanced polymerization of phenolic compounds in the tissues and arils of MA packages pomegranates compared to the control (air-stored) samples. These observations are consistent with the findings reported for “Mollar de Elche” pomegranates when packaged with CO2-enriched MA and stored at 5˚C for 12 weeks [25]. The results of this study also recorded higher amounts of browning pigments over the cold storage that can be an indication of PPO and POD enzymatic activities as the results of minor incident of chilling injury in the tissues that often led to development of browning pigments in pomegranates when stored at low temperatures for a long time.
As shown in Figure 9, the variations in the browning index (BI) values during long storage is more influenced by the variation in the total contents of anthocyanins rather than degradation of other phenolic compounds, which is an interesting finding and implies that anthocyanins are the main phenolic compounds in the arils. Likewise, these results indicate minor development of browning pigments as the result of degradation of phenolic compounds other than anthocyanins during 10 weeks of storage that is even more noticeable at 2˚C compared to 6˚C storage temperature. Storage temperature of 2˚C is associated with higher contents of both total anthocyanins and development of browning pigments that imply the potential effect of low temperatures on the gene regulations, expression and physiological activity of the enzymes that are involved in the biosynthesis of phenolic compound such as anthocyanins and those that are participating in the enzymatic browning as the results of the incidence of physiological disorders (e.g. chilling injury) due to environmental stresses such as long storage at low temperatures or elevated CO2 contents in the surrounding atmosphere of the fruit.
As presented in Figure 10, the values of the BI of the extracts change quadratically with the variations in the contents of total anthocyanins but linearly
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Figure 9. Profiles of changes in the (top) total anthocyanins, (middle) browning index, and (bottom) development of browning pigments during 10 weeks of storage at 2˚C (solid line) and 6˚C (dashed line).
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Figure 10. (a) Response surface 3D showing changes in the Browning Index (BI) of the extracted juice in relation to the total anthocyanins content (absorbance at 510 nm) and phenolic compound degradation products (absorbance at 446 nm) during storage, and (b) prediction profiler for the anticipated Browning Index.
with the variations in the contents of the browning compounds. Consequently, the prediction profiles can be used to determine the optimum anthocyanin contents and storage conditions based on the desired BI value for the anticipated applications such as fresh consumption, processed food products for instance juices, jams, jellies, or as a value-added ingredient e.g., natural colorant or a bioactive compound.
The study results also revealed that thermal dipping (TD) treatment can affect the BI value of the pomegranate extracts by enhancing degradation of anthocyanins, their polymerization, and subsequent formation of browning compounds throughout the storage. The increase in the BI value of the samples that received TD treatment prior their packaging is more visible since the ninth week of the study. Similarly, the significant 2-way interaction effect of TD with the storage time on the pH, TA, and TSS changes during long storage of MA packaged pomegranates indicates the higher metabolisms of soluble solids, mainly sugars (such as glucose, fructose, arabinose) and organic acids (e.g., citric acid) by the pomegranates that have been subjected to the thermal treatment compared to the control group. Nonetheless, the TSS/TA ratio of pomegranates was not significantly affected (data not shown here) by TD treatment after 12-weeks of storage at 6˚C.
5. Conclusion
In summary, the total contents of anthocyanins in “Malase Torsh Saveh” pomegranates declined through 10 weeks of storage in all treatments. Among samples with modified atmosphere (MA) packaging, pomegranates with LDPE packaging and the initial gas composition of 5% CO2 + 5% O2 in their package headspace maintained higher contents of total anthocyanins when stored at 2˚C. Nonetheless, higher incidence of browning pigments was also recorded at 2˚C, possibly as the results of occurrence of minor chilling injury throughout the cold storage. Lesser browning pigments were developed among samples with MA packaging through the first seven weeks of storage, however, their contents started to increase towards the end of study. The browning index (BI) values of the extracts showed an overall increase throughout the storage and they were more influenced by the variations in the content of total anthocyanins in the extracts rather than the content of browning pigments, which indicates minor development of browning pigments as the result of phenolic compounds degradation during long storage. Pre-storage thermal dipping (TD) as a complementary treatment along with MAP showed no significant effects on the total content of anthocyanins, degradation of phenolic compounds, or development of browning compounds during the cold storage. However, BI values of MA packaged pomegranates that had received TD prior to the storage started to significantly increase after 2 months of storage at 6˚C.
Acknowledgements
This research was conducted at the Department of Food Science & Technology, and it was funded by University of Tehran, College of Agriculture and Natural Resources, Karaj, Tehran. The author also thanks Dr. Banafshe Lashkari, PhD, from University of Waterloo for her assistance in article retrieval.
Ethics Statement
The author declares that this research (a) is the author’s own original work, (b) has not been published previously, and (c) is not under consideration for publication elsewhere.