Phenol-Explorer is the most comprehensive electronic database on polyphenol contents in foods and beverages. Content values were retrieved from scientific papers published in peer-review journals. Information on original food description, commercial origin of the sample and number of analysed samples to produce each original content value was registered together with the analytical method used. Data were evaluated and those considered acceptable were used to produce mean content values for the various polyphenols in the main foods and beverages.
The lack of a value for a particular polyphenol in a food does not necessarily imply the absence of this polyphenol, but only that data were not available. Zero value, when reported in original publications, have been included. These zero-values are not shown by default but only upon request using the option “Show zero-value entries”.
Food entries were defined according to the availability of data on their polyphenol content. Some individual foods, e.g. the different varieties of a fruit, were not separated under different entries due to the lack of sufficient data to provide content values which would truly reflects differences in polyphenol contents between each of these foods. Similarly no entry could be made for various cooked foods due to the lack of sufficient reliable data. The user of Phenol-Explorer will find information on the factors influencing polyphenol content (varieties, processing, storage conditions, cooking, etc.) together with appropriate literature references in the reports on the different food categories.
Collection of composition data from literature sources
The literature search for published analytical values of polyphenols in foods and beverages was performed using the ISI Web of KnowledgeSM platform (Thomson Reuters). Different queries were built per food group from a template using the most representative food names in the food group of interest, associated to keywords related to the compounds (names, classes and sub-classes) and to quantitative information. The search was limited to peer-reviewed journal articles published in English. Additional searches were made in books and reviews (1). The comprehensiveness was checked in citations of the collected publications.
Critical evaluation of data
Articles containing quantitative data on polyphenols in foods or beverages were critically analysed and evaluated, before inclusion of the data in the database. Data were evaluated considering sampling, polyphenol extraction and analytical method, disclosure of experimental details. Only those values filling specified minimal requirements were selected for aggregation (production of mean values). In particular, the following exclusion criteria were considered:
* Lack of information on the nature of the samples analysed.
* Analysed samples correspond to non-edible parts of the plant.
* For non-commercial products, samples obtained experimentally using procedures clearly different from regular industry or domestic practice.
- Analytical method:
* Inaccurate identification of the compounds.
* Inappropriate method of polyphenol extraction or lack on information on the method used.
* Inappropriate method of analysis (see below)
* Lack of information on phenolic standards used for quantification.
- Expression of results:
* Results expressed as dry weight without providing moisture content.
* Content values reported in graph.
* Mean content values without a description of the number of samples analysed.
* Content values corresponding to sums of the content of different individual phenolic compounds.
Food definition and classification
A specific food ontology was built, based on the LanguaL system (2). LanguaL is a food description thesaurus that provides a standardized language using facetted classification. Each food is described by a set of controlled terms, such as the botanical origin, food processing including cooking and conservation. The exact identity of the foods described in the original sources was checked and botanical names added using botanical databases and other resources (3, 4, 5).It was sometimes necessary to separate some specific groups of cultivars because of their clearly different polyphenol profiles (e.g. blond orange and blood orange). However, for other foods, cultivars were not differentiated due to the lack of sufficient analytical data.
Phenolic compounds and their classification
The classification for phenolic compounds was principally derived from reference books, on-line chemical databases (6, 7, 8). Classes, sub-classes and families are named according to structural features of the compounds: presence of functional groups such as acid (COOH), alcohol (OH), aldehyde (CHO) or ketone (CO), methoxylation, alkylations, and polymerization. The compound name is based on the most common name used in the literature. A list of synonyms is also provided.
Cis and trans isomers of phenolic compounds (cinnamic acids, stilbenes) are not given separately but as a total. Indeed these isomers can be converted from one form to the other following exposure to UV light or heat treatment (9, 10), factors not necessarily controlled during the preparation of the samples.
Some phenolic compounds can also be present in different enantiomeric forms (flavanols, lignans). For example, (-)-epicatechin is partially epimerized to (-)-catechin during cocoa processing (11). However these enantiomers have very rarely been analysed separately. For this reason, no distinction is made in the database for the different enantiomers.
Phenolic acids can be present in foods in different forms: some are soluble (free phenolic acids, esters or glycosides) and are usually directly analysed by chromatography. Others are insoluble and esterified to polysaccharides in the cell wall as in cereals. Cell-wall bound phenolic acid esters are generally estimated by hydrolysis of the extractive-free cell-wall material. In Phenol-Explorer, phenolic acid content estimated by chromatography after hydrolysis is the calculated sum of the contents of all esters (soluble and insoluble) and free phenolic acids.
Proanthocyanidins are commonly analysed by two different methods: reverse phase HPLC allows to estimate dimers and trimers. Proanthocyanidins of high polymerization degree can only be separated by normal phase HPLC (12). Contents measured by this last method are given per individual degree of polymerization. In Phenol-Explorer, five categories of proanthocyanidins were made: dimers, trimers, 4-6mers, 7-10mers and polymers as used in the USDA database (13).
Analytical methods used to determine polyphenol content in foods were grouped in five categories and data aggregated to produce mean content values per category:
- Chromatography. HPLC, the most common technique, followed by GC and CE, are used to estimate phenolic compounds as present in the food. Polyphenol glycosides, phenolic acid esters together with aglycones and free phenolic acids are simultaneously quantified.
- Chromatography after hydrolysis. Acid or alkaline hydrolysis is used to cleave glycosylated or esterified polyphenols. It is used with two different purposes: a) to release certain compounds such as insoluble phenolic acids esterified to the cell wall in cereals that cannot be solubilized without hydrolysis; and b) to simplify the analysis of certain flavonoids that are present in the food matrix as several glycosides and that, in this way, can be determined as aglycone equivalents. No distinction was made between acid and alkaline hydrolysis. Lignans are commonly analysed after liberation of the aglycone by a combination of chemical and enzymatic hydrolyses.
- Folin assay. It is a spectrophotometric assay commonly used to determine simultaneously all phenolic compounds. It is based on the chemical reduction of the Folin reagent into a coloured product by all phenolic groups. It is not a specific assay for phenolic compounds as many other food constituents such as ascorbic acid may also reduce the Folin reagent. However, it is largely used as a global assay to give a crude estimate of the total concentration of antioxidants. It can be particularly useful in samples containing complex polyphenols such as proanthocyanidins or oxidized polyphenols.
- Normal phase HPLC (proanthocyanidins) (14). This method has been used in a few laboratories to estimate proanthocyanidin oligomers according to their degree of polymerization (dimers to decamers), which cannot be easily separated by reverse phase HPLC. Only proanthocyanidin dimers and trimers are individually analysed by reverse phase HPLC (content values reported under “Chromatography” category). A separate category is made here with normal phase HPLC to avoid the user of Phenol-Explorer to sum total proanthocyanidin dimers and trimers as analysed by this method together with the contents of the various individual dimers and trimers as analysed by reverse phase HPLC.
- pH differential method (anthocyanins) (15, 16). It is an spectrophotometric method commonly used to determine total anthocyanin content.
Content values obtained by other analytical methods are not included in the database. Some of these methods are potentially useful but they have so far been used by a too limited number of authors, e.g. thiolysis for proanthocyanidin estimation (17), or total polyphenols estimated after removal of interfering compounds with solid phase extraction (18). Other methods were not considered reliable due to poor stoechiometry (UV spectrophotometry or Prussian blue method for total polyphenols, vanillin-HCl, DMACA-HCl or butanol-HCl assays for proanthocyanidins) (19).
Production of mean content values and content units
Phenol-Explorer gives weighted mean content values and standard deviations which take into account the different number of samples used to generate each original data collected from the publications. The data displayed are expressed in standard units: mg/100 g fresh weight for solid foods and oils and mg/100 mL for beverages and other liquid foods such as vinegar or soy sauce. Various options are propose to either retrieve content values in molar units or to calculate contents in aglycone equivalents when glycosides and esters are analysed by chromatography.
Content values for a given polyphenol are sometimes expressed in the original publication in equivalents of a related compound used as a standard for quantification. They were recalculated on the basis of the molecular weight of the considered polyphenol.
For some cereal products of low moisture content, samples are eventually dried before analysis and the content values expressed per dry wt units without indicating the initial moisture content. Content values have then been converted per fresh weight units using standard moisture contents (20).
1. Shahidi, F. & Naczk, M. (2004) Phenolics in food and nutraceuticals.
2. Ireland, J. D. & Moller, A. (2000) Review of international food classification and description, Journal of Food Composition and Analysis. 13, 529-538.
3. ARS USDA. National Germplasm Resources Laboratory, http://www.ars.usda.gov/main/site_main.htm?modecode=12-75-15-00.
4. Mansfield's World Database of Agricultural and Horticultural Crops, http://mansfeld.ipk-gatersleben.de/pls/htmldb_pgrc/f?p=185:6:2469611851434185.
5. The Cook's Thesaurus, www.foodsubs.com.
6. ChEBI database, http://www.ebi.ac.uk/chebi/.
7. PubChem Database, http://pubchem.ncbi.nlm.nih.gov/.
8. Harborne, J. B. e. (1993) The flavonoids. Advanced in research since 1986, Chapman & Hall.
9. Kolouchova-Hanzlikova, I., Melzoch, K., Filip, V. & Smidrkal, J. (2004) Rapid method for resveratrol determination by HPLC with electrochemical and UV detections in wines, Food Chemistry. 87, 151-158.
10. Romero-Perez, A. I., Ibern-Gomez, M., Lamuela-Raventos, R. M. & de la Torre-Boronat, M. C. (1999) Piceid, the major resveratrol derivative in grape juices, Journal of Agricultural and Food Chemistry. 47, 1533-1536.
11. Cooper, K. A., Campos-Gimenez, E., Jimenez-Alvarez, D., Nagy, K., Donovan, J. L. & Williamson, G. (2007) Rapid reversed phase ultra-performance liquid chromatography analysis of the major cocoa polyphenols and inter-relationships of their concentrations in chocolate, Journal of Agricultural and Food Chemistry. 55, 2841-2847.
12. Gu, L., Kelm, M., Hammerstone, J. F., Beecher, G., Cunningham, D., Vannozzi, S. & Prior, R. L. (2002) Fractionation of polymeric procyanidins from lowbush blueberry and quantification of procyanidins in selected foods with an optimized normal-phase HPLC-MS fluorescent detection method, Journal of Agricultural and Food Chemistry. 50, 4852-4860.
13. Nutrient Data Laboratory. (2004) USDA Database for the Proanthocyanidin Content of Selected Foods. http://www.ars.usda.gov/nutrientdata.
14. Gu, L., Kelm, M. A., Hammerstone, J. F., Beecher, G., Holden, J., Haytowitz, D. & Prior, R. L. (2003) Screening of foods containing proanthocyanidins and their structural characterization using LC-MS/MS and thiolytic degradation, Journal of Agricultural and Food Chemistry. 51, 7513-7521.
15. Wrolstad, R. E., Acree, T. E., An, H., Decker, E. A., Penner, M. H., Reid, D. S., Schwartz, S. J., Schoemarker, C. K. & Sporns, P. E. (2001) Current protocols in Food Analytical Chemistry, Wiley.
16. Cheng, G. W. & Breen, P. J. (1991) Activity of phenylalaline ammonialyase (PAL) and concentrations of anthocyanins and phenolics in developping strawberry fruit, Journal of the American Society for Horticultural Science. 116, 865-869.
17. Guyot, S., Marnet, N., Laraba, D., Sanoner, P. & Drilleau, J. F. (1998) Reversed-phase HPLC following thiolysis for quantitative estimation and characterization of the four main classes of phenolic compounds in different tissue zones of a french cider apple variety (Malus domestica Var. Kermerrien), Journal of Agricultural and Food Chemistry. 46, 1698-1705.
18. George, S., Brat, P., Alter, P. & Amiot, M. J. (2005) Rapid determination of polyphenols and vitamin C in plant-derived products, Journal of Agricultural and Food Chemistry. 53, 1370-1373.
19. Scalbert, A. (1992) Quantitative methods for the estimation of tannins in plant tissues in Plant Polyphenols, Synthesis, Properties, Significance (Hemingway, R. W. & Laks, P. E., eds) pp. 259-280, Plenum Press: New York.
20. Souci, S. W., Fachman, W., Kraut, H., Scherz, H., Kloos, G. & Senser, F. (1986) Food composition and nutrition tables 1986/87, Food composition and nutrition tables 1986/87., Ed. 3, xxvi + 1032.