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Fruit Juice Authenticity Analysis
Composition of Pomegranate Juice
(J. AOAC International, 2012, 95(1) 163-168)
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Introduction
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The recent popularity and high price of pomegranate juice have led to concerns regarding the authenticity of some of the products in the marketplace. The available literature on pomegranate juice composition is very thin. There is, thus, a need for additional information regarding the typical composition of commercial pomegranate juice.
In the last three years, we have analyzed over 500 samples of commercial pomegranate juice for purposes of assessing its authenticity. Evaluation of the data indicates a widespread problem of adulteration of a variety of types.
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In order to produce a better compositional guideline for assessing pomegranate juice purity, we did some selective paring of this dataset in the hopes of producing a large statistical data subset which would include at most only a few adulterated samples, and which would exclude no more than a few outlying authentic samples. The parameter means and distributions of such a subset should then be a fairly good model of the behavior of an authentic pomegranate composition. |
Data Sorting Technique
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The data set obviously contained a large number of adulterated or suspect samples. We assumed that the number of authentic samples was much larger than any particular type of adulterated sample. We further assumed that the distribution of values for each parameter in pure pomegranate juice was unimodal and approximately Gaussian.
For each parameter in the data set, a mean and standard deviation was calculated. Any sample whose result was more than 3 standard deviations from the mean was then removed from the set. The means and standard deviations of the new data set were then calculated and the same procedure applied. This process was repeated until a stable data set was obtained. At the end, about a dozen samples were also excluded due to their having anthocyanin profiles which clearly indicated an identifiable added color. The results from the final reduced data set are listed in the table below. |
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The complete process was repeated several times, varying the order of parameters used for data exclusion. While different selection runs resulted in a slight difference in the number of samples excluded or included, the final statistical results for the samples in the final dataset were always very similar.
This approach appears to produce a good estimate of the population mean and standard deviation for pure pomegranate juice. The statistical approach necessarily excludes any minor bimodalities in the data that may occur. If any minor varieties of pure pomegranate juice occur that are compositionally distinct, this approach may have excluded them from the authentic set. |
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Conclusions
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1. The composition of pomegranate juice is very consistent, almost regardless of variety or geographic origin.
2. The only consistent compositional variability is the level of total acidity and citric acid. Most juices have citric acid levels near to or more than 1, but low acid varieties exist with citric acid levels as low as 0.1%. Most of the Indian samples analyzed were of this low acid type.
3. There is a fairly good correlation between citric acid and isocitric acid levels. |
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4. The anthocyanin profile is highly consistent across varieties and geographic origins. The profile consists of six components, the 3-O-glucosides and 3,5-O-diglucosides of Delphinidin, Cyanidin and Pelargonidin. The overall anthocyanin concentration varies considerably, but the profile is consistently similar to the figure.
5. Close examination of the data suggests the possibility that some Chinese varieties of intermediate acidity (0.7-1.0%) may have slightly higher malic acid levels (possibly as high as 0.2%) than other varieties.
6. There has been a serious problem of adulteration in the pomegranate juice market the last three years |
Composition of Pomegranate Juice*
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| Parameter |
Units |
Mean |
Std. Dev. |
| Carbon Stable Isotope Ratio |
‰ PDB |
-26.5 |
0.8 |
| Fructose |
g/100g |
6.63 |
0.85 |
| Glucose** |
g/100g |
6.62 |
0.83 |
| Sucrose |
g/100g |
0.00 |
0.00 |
| Sorbitol |
g/100g |
0.00 |
0.00 |
| Acidity (as Citric Acid) |
g/100g |
1.30 |
0.33 |
| Citric Acid |
g/100g |
1.24 |
0.33 |
| Malic Acid |
g/100g |
0.053 |
0.017 |
| Tartaric Acid |
g/100g |
0.00 |
0.00 |
| Isocitric Acid |
mg/kg |
68 |
21 |
| Potassium |
mg/kg |
2476 |
304 |
| Formol Value |
meq/100mL |
1.04 |
0.24 |
| Proline |
mg/kg |
5 |
3 |
| Citric/Isocitric Ratio |
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190 |
63 |
*Based on 263 samples of presumptively authentic commercial pomegranate juice (data normalized to 16 Brix)
**Includes mannitol, which is not separated from glucose and which is a normal pomegranate component at levels of a few tenths of a percent |
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Patterns of Pomegranate Adulteration Observed
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1. Addition of sorbitol containing fruit juices, such as apple, pear, cherry or aronia, detectable by elevated levels of sorbitol, malic acid, sucrose and often added anthocyanins of various types.
2. Addition of grape juice and grapeskin color, detectable by elevated levels of malic acid, proline and tartaric acid, as well as grape or other anthocyanin pigments.
3. Addition of cane sugar or corn syrup sweetener, detectable by elevated carbon stable isotope ratio and sucrose or maltose.
4. Addition of blackberry juice, detectable by elevated isocitric acid and high cyanidin-3-glucoside level in the anthocyanin profile. |
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5. Addition of citric acid, detectable by low isocitric acid and high citric/isocitric ratios.
6. Addition of anthocyanin colorants such as aronia, roselle, grapeskin, elderberry, blackcurrant, black carrot. Detectable by recognizable patterns of added peaks, adding to or substituting for the normal pomegranate anthocyanin profile.
7. Addition of artificial colors, especially carmoisine, amaranth, FD&C red 40 and ponceau red 4R. |
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Method References
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Carbon Stable Isotope Ratio. Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 978.17
Fructose, Glucose, Sucrose. Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 932.14C (10)
Sorbitol. RSK Values - The Complete Manual, 1st ed (1987) Flussiges Obst GmbH (11), p. 142-144
Acidity (as Citric Acid). Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 942.15B, reported as citric acid.
Citric Acid, Malic Acid, Tartaric Acid. Official Methods of Analysis of the AOAC, 15th ed.,(1990), Method 986.13. |
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Isocitric Acid. Methods of Enzymatic Food Analysis Using Test Combinations, 1986 Boehringer Mannheim GmbH, p.60-61 (12). Samples are pretreated by the alkaline hydrolysis procedure described in the method appendix.
Potassium. Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 965.30
Formol Value. Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 965.31B
Proline. Official Methods of Analysis of the AOAC, 15th ed. (1990), Method 979.20
Anthocyanin Pigments. Ø. M. Andersen, Chromatographic Separation of Anthocyanins in Cowberry (Lingonberry) Vaccinium vites-idaea L., J. Food Science. 50, 1230 (1985) |
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