https://orcid.org/0000-0003-2145-2498
ABSTRACT
Pomegranate is a deciduous tree crop cultivated commercially for its fruit and juice in many parts of the world. Minimally processed fruits and vegetables have become one of the fastest-growing sectors of the food industry. Pomegranates have a relatively new minimally processed product comprised of arils packaged in plastic containers. Fruit quality of minimally processed pomegranate arils is largely unknown in the American market as is the quality of imported fruit. These minimally processed products have been reported as having poor quality by schoolchildren panelists participating in sensory analysis. This investigation included physicochemical and color quality assessment of pomegranate whole fresh market fruit and minimally processed arils. Data were collected on the total weight of arils, 100 aril weight, fruit diameter, fruit length, proportions of intact, damaged, and decayed arils in weight and number, total soluble solids (TSS), titratable acidity (TA), total phenolics, and color. There were significant quality differences between minimally processed pomegranate products and between imported versus domestic fruit. Pomegranate fruit imported from Chile did not meet market standards for internal quality in that they had significantly lower TSS and the TA was rather high for public consumption. The results of this investigation demonstrate a need for quality and grading standards in the international pomegranate trade and the potential for better quality control in the minimally processed prepackaged pomegranate aril market.
Introduction
Pomegranate is a deciduous tree crop cultivated commercially for its fruit and juice in many parts of the world (Preece and Moersfelder, Citation2016; Stover and Mercure, Citation2007). Minimally processed fruits and vegetables have become one of the fastest-growing sectors of the food industry (Ali et al., Citation2018). Pomegranates have a relatively new minimally processed product comprised of arils in packaged plastic containers. Fruit quality of minimally processed pomegranate arils is largely unknown in the American market as is the quality of imported fruit.
In preliminary investigations, these minimally processed products have been reported as having poor quality by schoolchildren panelists participating in sensory panels (unpublished data). Investigations into pomegranate fruit and its juice quality have often focused on fruit (Cristofori et al., Citation2011), pressed juice (Miguel et al., Citation2004), and manually extracted arils (Caleb et al., Citation2012), yet limited investigations have centered on mechanically processed arils that are now common in the market. Seasonally available mechanically processed arils are found on the market in the United States and there is limited information in the literature available evaluating the quality of these products. Though a significant proportion of pomegranate sales now comes from value-added products containing processed arils, not enough is known about the postharvest quality of this minimally processed product that is being sold to consumers in the United States and abroad. There have been attempts to analyze the consumer acceptance of manually extracted arils and previous studies have shown a range of components that contribute to the fruit’s quality and overall consumer acceptance, such as color, aril size, taste, acidity, and seed hardness (Chater et al., Citation2018a). Recent reports indicate the largest company selling minimally processed pomegranate arils in the United States has had shipment orders increase by 37% in 2018 compared to the previous year (Nickle, Citation2018). It is important to compare mechanically processed and fresh market pomegranates due to the increased consumption of the processed fruit for its putative health benefits.
Investigations into fruit quality have indicated the Wonderful cultivar is considered to be mature and edible when the total soluble solid content (TSS) of the juice is greater than 15% and the titratable acidity (TA) is less than 1.85% (Fawole and Opara, Citation2013). The ratio of TSS/TA is considered a significant indicator in the determination of fruit maturity (Fawole and Opara, Citation2013). In previous studies, pomegranates were observed to have a high number and quantity of phenolic compounds that have been related to the antioxidant activity (Derakhshan et al., Citation2018; Gil et al., Citation2000). It has been demonstrated that the fruit chemical composition, phenolics, and antioxidant activity can be influenced by the maturity of the fruit during harvest (Chater et al., Citation2018b), and it is important to know-how minimally processed pomegranate arils change in quality during postharvest life.
American importation of fresh market pomegranates has increased significantly over the last 10 years, with fruit coming into the markets from countries in the Southern Hemisphere, especially Chile and Peru. There has been no known characterization of the quality of pomegranates coming into the American market from foreign producers. It is important to know the quality of fruit being imported to determine if controls are needed to maintain an acceptable level of quality for consumers. Little is known about how the effect of postharvest handling and long-term, long-distance transport of fresh fruit from overseas affects quality, although there are known reports on the effects of minimal processing on pomegranate arils (Caleb et al., Citation2012).
In this investigation, data on a range of characteristics of different minimally processed pomegranate products and fresh market fruit were collected. The effects of mechanically processing fruit on the quality of value-added pomegranate products were evaluated. The objectives of this investigation were (1) to assess the quality of processed pomegranate products manufactured in California, United States of America; (2) to compare the quality differences of fresh market pomegranates versus processed pomegranate products; and (3) to compare fresh market fruit quality of domestic versus imported pomegranates in the United States market.
Materials and Methods
Plant Materials
Packaged processed pomegranate arils and whole pomegranate fruit were purchased from markets in Riverside, California (). Twelve fruit sourced from California, United States of America, and 10 imported fruit sourced from Chile were selected in Jan and Mar 2018, respectively. Of those fruit, six were selected and processed for data collection. Eight containers of Brand A arils and five containers of Brand B arils were selected to be used for data collection of minimally mechanically processed arils. Arils were processed before their expiration dates.
Table 1. Descriptions of four pomegranate store-bought products used in this study
Physical Fruit Properties
Measurements of the mass of the whole pomegranate fruit were taken using a tared digital scale. Measurements of the height, stem to blossom end length, and equatorial diameter were measured with a digital caliper. To extract the arils, the pomegranates were cut open with a knife and then each aril was manually separated from the locules. The arils collected per pomegranate were weighed for total weight and edible fruit fraction (%) was calculated by dividing the total aril weight by the pomegranate weight then converting the result to a percent. The arils were then randomly separated into a group of 100 arils that were weighed for 100 count weight.
Arils contained in prepackaged minimally processed arils and whole fruit were sorted based on their condition or 'grade' and were separated into three categories. The total numbers and weights of arils in each grade were recorded. Additionally, arils were categorized into percentage of intact arils to determine the grading percentages as a measure of quality for each product. Whole fruit was scored with a knife, cracked open, and the arils were manually removed and processed on the same day. The categorization of each aril was completed by comparing its appearance to set of standards that determined each category:
Arils determined to be 'intact' met the following characteristics: no punctures in the membrane, unblemished, no pitting, firm to the touch, plump, and full of juice.
Arils determined to be 'damaged' met the following characteristics: some punctures in the membrane or visible damage, some pitting, and little to no juice present.
Arils determined to be 'decaying' met the following characteristics: loose or no membrane present, brownish or grayish in color, and little to no juice present.
Juice Properties
Pomegranate juice was extracted from intact and undamaged arils by placing the arils into a plastic bag and manually squeezing the juice into a test tube. A centrifuge was then used to separate the pomegranate juice from solids. The supernatant was transferred into a clean test tube for analysis. The juice was then stored at −20 ⁰C for three weeks until it was used for quantifying total phenolics.
Total Soluble Solids
A Vee Gee Scientific PDX-1 Digital Refractometer (Kirkland, Washington, United States of America) was used to measure the pomegranate juice relative sugar content in percent total soluble solids (TSS). The benchtop refractometer was cleaned between samples by applying deionized (DI) water to the sample reservoir and then was wiped dry with Kimwipe tissues (Kimberly-Clark Professional, Roswell, Georgia, United States of America). The device was calibrated using DI water before beginning the juice data collection. For each sample, 0.5 mL of juice was transferred to the sample reservoir from the collected juice sample obtained earlier and the resulting measurement was recorded. One sample was measured to represent the whole fruit and 5–8 samples were analyzed per product.
Titratable Acidity
Acidity of the pomegranates was measured using a Hanna HI 84532 fruit juice titratable acidity (TA) mini-titrator (Woonsocket, Rhode Island, United States of America). Low range and high range titrants containing sodium hydroxide, product numbers HI84532-51 and HI84532-50 (Woonsocket, Rhode Island, United States of America), were used. Samples were prepared by combining 5 mL of expressed juice with 45 mL of DI water and were auto-titrated to calculate the percent of citric acid equivalents in solution. One sample was titrated per sample.
Maturity Index
Maturity index (MI) was determined by calculating a sugar to acid ratio (TSS:TA). The Wonderful cultivar was selected for this calculation because the Wonderful pomegranate is classified as the industry standard. Calculating this ratio allowed us to estimate the acceptability of the store-bought pomegranates used in this study. The TSS and TA values were used to determine the MI for each sample. Based on the literature, mature pomegranates have a TSS value above 15% and a TA value less 1.85% in citric acid equivalents, resulting in an MI of 8.1 or higher and these standards were used for quality assessment.
Total Phenolics
To determine the total phenolic content of the arils, a spectrophotometer was used to quantify total phenolics using a standard curve derived from the Folin–Ciocalteu method as reported by Tezcan et al. (Citation2009). First, to prepare the juice for phenolic measurements, 100 intact and undamaged arils per cultivar were pressed manually to express the juice, which was centrifuged in a test tube. When preparing the samples in disposable plastic cuvettes, 3 µL of pomegranate juice was combined with 297 µL 6:4 MEOH:H2O and 1.5 mL of Folin-Ciocalteu reagent. The mixture was then left alone for about 3 min before 1.2 mL of sodium carbonate was added to each cuvette. Following 90-min incubation period in the dark at room temperature, the cuvette was placed in the spectrophotometer set at 765 nm to record absorbance. This process was repeated five times per juice extract sample.
Color Analysis
A CR-410 chroma meter (Konica Minolta, Ramsey, New Jersey, United States of America) was used to determine the color of the arils and fruit peel in CIE L*a*b* color space coordinates. The color data were produced by SpectraMagic NX Lite software. The chroma meter was calibrated using a plain white calibration plate each day before recording data from the arils and fruits. Five readings per sample of the three quality categories (intact, damaged, decayed) were taken and the average was reported. Arils that were lighter than L* = 51.5 in CIE L*a*b* color space were considered to be of lower quality compared to arils that were darker. This value was calculated based on Munsell 5R 5/12 reported in Fawole and Opara (Citation2013). For a*, the red/green coordinate, values were compared relative to each product for quality assessment.
Statistical Analysis
All variables were analyzed with Analysis of Variance (ANOVA). When ANOVA indicated significant differences, post hoc comparisons were performed utilizing Tukey Pairwise Comparisons with an experiment wise error rate of α = 0.05. Relationships between variables were analyzed using linear regression (α = 0.05). Analysis was performed with Minitab Software, version 19 (Coventry, UK).
Results
Total Mass of Arils, Mass of 100 Arils, and Aril Grading
For total aril mass, there was evidence for statistically significant differences among the pomegranate products evaluated (P < .001). Domestic fruit had the most aril mass compared to all other pomegranate products (). The minimally processed arils Brand A and Brand B had the second greatest mass and imported fruit had significantly lower aril mass compared to domestic fruit and the aril products. For mass of 100 arils, Brand B had heavier 100 aril count than Brand A and imported fruit (P < .001). Imported fruit had arils with about half of the weight of Brand B arils, with just 17.5 g per 100 arils versus 35.6 g per 100 arils for Brand B’s arils.
Table 2. Mean total aril weight (g), 100 aril weight (g), fruit mass (g), fruit diameter (mm), fruit length (mm) of two minimally processed pomegranate products (Brand A and Brand B) and domestic and imported whole fresh market pomegranate fruit
Domestic whole fruit had more intact arils than imported and prepackaged arils when considering mass (P < .001), but there was no difference in intact arils when considering numbers of arils (). There were no differences between intact aril numbers (data not shown) nor mass for Brand A and Brand B arils. Brand A had significantly more number and mass of damaged arils compared to all other products and Brand A also had approximately three times the weight and number of decaying arils compared to their competitor, Brand B (). Brand A also had hardened off fruit peel fragments in the packaged product.
Fruit Mass, Diameter, and Length
There were significant differences between cultivars for fruit mass (P < .001). Domestic fruit were significantly heavier than imported fruit (). Imported fruit had a mean mass of 241.7 g and Californian fruit had a mean mass of 336.3 g, with Californian fruit being around the mass acceptable for commercial production, which was not the case for imported fruit, which were imported into the United States during the off season in the Northern Hemisphere. This represents over a one-third difference in mass between the two whole fruit products.
The fruit sizes were very similar between the domestic Californian-grown and imported Chilean-grown pomegranates (). Californian fruit had a mean diameter of 87.7 mm and imported fruit had a mean diameter of 81.7 mm, which are typical values for commercial grade fruit in the United States. Californian fruit were longer than imported fruit, with Californian fruit having a mean 77.3 mm fruit length and imported fruit having 70.8 mm fruit length.
Total Soluble Solids
There was evidence for statistically significant differences among the pomegranate products tested for TSS (P < .001). Brand B arils had the highest TSS content among the products, with an average TSS of 17.0% (). California fruit and Brand A had the next highest TSS with 15.6% and 14.5% TSS, respectively. Imported fruit had the lowest TSS compared to all the other products, with a relatively low mean TSS of 12.3%. There was a negative correlation between TSS and TA (R2 = 68.6%). There was a strong positive correlation between weight of 100 arils and TSS (R2 = 88.0%).
Table 3. Mean total soluble solids (TSS), titratable acidity (TA), total phenolics (TP), aril lightness (L*) and aril redness (a*) of two minimally processed pomegranate products (Brand A and Brand B) and domestic and imported whole fresh market pomegranate fruit
Titratable Acidity
There were no differences among Brand A, Brand B, and domestic fruit for TA, but imported fruit did have significantly higher titratable acidity than all other products (P < .001). Imported fruit had an average TA of 2.70% in citric acid equivalents, whereas Brand B arils, Californian fruit, and Brand A arils had 0.87%, 0.93%, and 1.15% TA in citric acid equivalents, respectively (). Imported fruit had almost triple the acidity of Brand B arils and domestic Californian fruit. There was a strong negative correlation between mass of 100 arils and acidity (R2 = 87.6%).
Total Phenolics
Domestic fruit had the highest quantity of total phenolics (TP) in gallic acid equivalents (). Brand B arils had a higher quantity of total phenolics than imported fruit (P < .001). The difference in TP between domestic and imported fruit was about 2 g/L in gallic acid equivalents on average. Brand A was not analyzed for TP content for its minimally processed arils due to the nature of the Brand A samples, which contained excessive liquid in the containers.
Color Analysis
The color of the three of the pomegranate products was assessed for relative lightness (L*) and redness (a*). Imported fruit had significantly lighter arils than California minimally processed arils product Brand A and Brand B (P = .029). There were no differences between Brand A arils and Brand B arils for lightness (). There was evidence for significant differences among products for redness (P = .001). Imported fruit had redder arils than both Brand A and Brand B minimally processed arils. Brand A had redder arils than Brand B, which were rather burgundy or purplish red compared to the other products. Damaged and decaying arils had lighter, grayer arils than intact ones. Domestic whole fresh market fruit arils were not analyzed for color. There were no significant differences between imported and domestic fruit peel color for lightness (L*) (). There was evidence for significant differences for redness (a*) between cultivars, with imported pomegranates having 25% redder peel than Californian fruit (P = .001). Imported and domestic fruit had average a* values of 40.30 and 30.14, respectively.
Figure 1. Color values of the fruit peels of imported Chilean versus Californian pomegranates as measured by a CR-410 Konica Minolta Chroma Meter. Lightness (L*) and redness (a*) were determined with five measurements per sample. Fruit were sourced from local markets in Riverside CA, the United States of America in winter of 2018. Bars labeled with different letters are significantly different (P < .05)
Discussion
There were stark differences in quality among pomegranate arils of the products analyzed in this study. One major problem discovered during this investigation was that Brand A had hardened off fruit peel fragments in the packaged product, which is a hazard to consumers if they are not paying attention to what they are consuming (e.g., watching television while eating with a spoon) and indicates a need for better quality control on the packing and processing lines. Brand A and Brand B arils both had offensive odors, Brand A had a strong alcohol odor and Brand B had a strong acetic acid odor, which indicates postharvest handling problems. Imported pomegranates were likely not mature enough for consumption, yet they made it on the international market.
Total Mass of Arils, Mass of 100 Arils, and Aril Grading
Domestic fruit had the most edible content of arils compared to the other products. Imported fruit had significantly less amount of edible content than all other products, which indicated that the fruit were either picked too early or they are of an inferior cultivar in terms of aril mass and other horticultural traits. It is more likely that the fruit were harvested too early because the fruit were in the market exceptionally earlier for the Southern Hemisphere. Because there were no significant differences in total aril weight between Brand A and Brand B aril products, basing quality on product total weight is not feasible. Values for total aril weight for the fruit were similar to values reported in the literature for Californian fruit (Chater et al., Citation2018b), but not for imported fruit, which had less than 100 g of arils per fruit on average.
There were differences among products for weight of 100 arils and perhaps this variable is one that is more likely to be associated with quality. Because there were differences for 100-count aril weight between both Brand A and Brand B, it provides some evidence that the two companies are possibly harvesting the ‘Wonderful’ fruit for their products on different dates. It is most likely that both brands are sourcing their arils from ‘Wonderful’ trees, which is the industry standard in the United States and comprises 90-95% of the industry. The 100 aril weight for the two whole fruits has about a 50% difference in mass, which provides evidence that the imported fruit were likely harvested too early. This is not the only evidence of immature fruit in this study, but having arils weigh so little is indicative of immature fruit, perhaps picked about a month early when compared to previous research reported by this group and others (Chater et al., Citation2018b; Wetzstein et al., Citation2011). Wetzstein et al. (Citation2011) reported similar values for 100 aril weight for Wonderful (35.7 g) for all pomegranate products except for the imported fruit. It is possible that this imported fruit is of another cultivar that is a sour, low sugar cultivar with low aril weight, but considering these traits would be not desirable for a commercial-grade pomegranate, it is more likely the fruit were immature.
Aril grading indicated quality issues with mechanically processed arils as compared to manually extract arils (). Fresh whole fruit that was manually deseeded had typically less compromised arils than mechanically processed, but it can depend on if weight or number is considered in the statistical model. This demonstrated the efficiency and protective compartmentalization of the pomegranate fruit in transport. Brand A had significantly more damaged arils and decaying arils than all other products when considering the number of arils damaged or decayed. This provides evidence that there are problems with significant damage to arils in American pomegranate products, which is likely to play into the off-odors and flavors of these minimally processed products due to increased respiration thus causing anaerobic environment (Kader, Citation1980; Rolle and Chism, Citation1987). Because the mechanical injury is one of the most important problems in minimally processed products because of its effects on plant metabolism and sensory quality, it is important for companies to minimize tissue damage of their commodities (Rolle and Chism, Citation1987). Because pomegranate seeds are self-contained units, it is easier to mitigate damage in this commodity as compared with one that undergoes cutting, such as cut pineapples, apples, watermelon, and carrots.
Figure 2. Aril grading in weight (g) of two minimally processed arils products (Brand A and Brand B) and fresh market fruit from domestic (California, United States of America) and imported (Chile) sources. Samples are ordered by the proportion of decaying to intact product, from highest (left) to lowest (right). Fruit and aril products were sourced from local markets in Riverside CA, the United States of America in winter of 2018
Total Soluble Solids
Perhaps the most important indicator of whether a pomegranate is ready is the juice’s TSS. As the season progresses in the Northern Hemisphere, pomegranate fruit juice TSS increases steadily in the months of September, October, and November, depending on cultivar, so the higher the TSS the more mature the fruit is likely to be (Chater et al., Citation2018b). It has been determined that a TSS of 15-17% is the minimum TSS of maturity for ‘Wonderful’ pomegranate (Fawole and Opara, Citation2013). The maturity index of sugar to acid ratio is generally used to determine fruit maturity in ‘Wonderful’ and an MI of 18.5 has been proposed. Based on these data in the present study, only one product (Brand B minimally processed arils) would have met the maturity standard set by Fawole and Opara (Citation2013). Additionally, research on United States of America pomegranate germplasm did not see ‘Wonderful’ reach the desired maturity index of 18.5 TSS:TA for germplasm and commercially available ‘Wonderful’ pomegranate (Beaulieu et al., Citation2015; Chater et al., Citation2018b), which indicates a significant pomegranate quality problem in the American food system. Another indicator of a quality problem in American pomegranate products is that Brand A had a mean TSS of less than 15%, which is below the minimum TSS content for fruit maturity. Because mean TSS for Brand A was about 14.5%, there is evidence that this company possibly harvested their ‘Wonderful’ fruit too early for public consumption, which is the major contributing factor affecting the taste and thus flavor of the product. There is a possibility that the fruit were harvested for Brand A at or above 15% TSS and sugar content decreased during postharvest life, but more research would be needed to determine if that is a possibility.
Titratable Acidity
Acidity is an important trait in sensory perception and consumer acceptance of pomegranate fruit (Chater et al., Citation2018a). Some have reported that fruit acidity, namely citric acid, plays the most significant role in the taste of pomegranate (Hasnaoui et al., Citation2011). Because the imported fruit had very high TA, the fruit were perceived as sour. Additionally, the mean TA of the imported fruit was 2.69% in citric acid equivalents, which is almost 1% higher than the maximum level of acidity for ‘Wonderful’ pomegranate, which is 1.85% maximum (Fawole and Opara, Citation2013). It could be a climatic difference (e.g., coastal influence) causing this high level of acidity, but it is more likely to be caused by immaturity than cultivar or climate considering all factors. The acidity of the other pomegranate products indicated that the trees were grown in inland California in a climate that experiences hot summers. ‘Wonderful’ grown on the coast, with cool summer temperatures, can have very high TA content (Chater et al., Citation2018a).
Acidity is an indicator of whether a pomegranate cultivar is ready to harvest. Because the imported pomegranates had such a high TA, it is important that in standards and grading, TA is included in the quality control of the commodity being tested. High TA imparts a sour and bitter flavor in ‘Wonderful’ pomegranate, and bitterness is a negative trait for this industry standard (Beaulieu et al., Citation2015)
Total Phenolics
The results for total phenolics (TP) reported herein are in general agreement with the literature for Wonderful pomegranate (Chater et al., Citation2018b; Gil et al., Citation2000). The TP of pomegranate fruit is what gives it its high antioxidant activity. Antioxidant activity is a major marketing trend in the United States, but the health benefits of antioxidant activity remain to be seen on a clinical level. Brand B arils had darker arils than the imported fruit, and this could be reflected by the different quantities of TP (e.g., anthocyanins or other plant pigments) in these different products. It is not recommended to use total phenolics as a measure of fruit maturity because of the wide range of values reported in the literature.
Color Analysis
There were significant differences in lightness and redness among the pomegranate products, demonstrating visual quality differences among the arils in the products analyzed in this study. Values for lightness (L*) were similar to those reported in Beaulieu et al. (Citation2015) for juice, but redness (a*) values were higher in arils. There was no significant difference for lightness between domestic and imported fruit peels, but there were differences between the two groups for redness. Interestingly, the imported fruit peel was redder than the domestic fruit peel, which is troublesome for fruit quality assessment because of the poor quality of the imported fruit. Color is not a useful indicator of pomegranate maturity because these data indicated that imported fruit were redder, yet other data reported herein indicate that the imported fruit were not mature.
These findings indicate that evaluating lightness or redness of a fruit is not useful to determine maturity or edibility and that these data can be misleading for these purposes. Considering the TSS and TA values for these products, these data demonstrate the problem with using color for pomegranate maturity, at least for ‘Wonderful.’
Conclusions
We evaluated the quality of minimally processed pomegranate arils and domestic and imported fruit and found that there are major quality issues with both American pomegranate-prepackaged arils and imported fruit from Chile. We also determined that some fruit quality traits may be more useful than others in determining quality for the aril and fruit market. For example, mass of 100 arils and TSS can provide evidence whether arils are mature and perhaps ready to consume, whereas fruit diameter and aril color can be deceptive. We discovered that imported pomegranates may need to be quality controlled. Domestic pomegranates were of higher quality than imported pomegranates, but minimally processed arils had many damaged and decaying arils in the containers. The aril grading of whole fresh market fruit was significantly higher than mechanically processed arils, which provides evidence that the peel and compartmentalization of arils in the fruit are efficient at keeping aril grading quality in conventional postharvest transport protocols, be them from domestic groves or sourced from abroad. Poor product and fruit quality can disadvantage the pomegranate domestic market and reduce consumption. Low-quality specialty crop products are also damaging to the growers of nutrient-dense fruits. Further research is needed to determine methods of increasing product quality of minimally processed arils and sensory panels should be used to determine consumer acceptability of these products in their current form.
Declarations of interest
The authors declare there exists no conflict of interest that would affect the objectivity of this manuscript.
Acknowledgments
The authors wish to acknowledge support from the undergraduate researchers who participated in this investigation, including Adriana Maze, Alyssa Canova, Sam Suzuki, Yiru Qiao, Hung Pang, Jasmin Wong, Jessica Tran, and David Wang.
Additional information
Funding
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