Frequently Asked Questions (FAQ)
Content values- How should I compare polyphenol content values for different foods ?
- How to interpret differences in content values obtained by different analytical methods?
- Some values appear surprisingly low
- What are aglycone equivalents?
- What are the different modes of expression of polyphenol content values?
Database- How comprehensive is the database?
- How reliable are the data?
Exportation- Can I export the data?
Foods- Why cannot I find some foods in Phenol-Explorer?
Methods- Why are there different options for analytical methods?
Polyphenols- Are all food polyphenols included in the database?
- Which names can be used for polyphenol searches?
Publications- I do not see data from a familiar publication
Reports- What can I find in the text reports?
Content values for two different foods can only be compared using similar analytical methods. For example, content values for tea obtained by chromatography should not be compared to content values for coffee obtained by chromatography after hydrolysis.
Phenol-Explorer contains content values for individual polyphenols obtained by chromatography, as well as content values for total polyphenols or total anthocyanins obtained by global colorimetric methods.
Results obtained by the different methods should be critically compared:
- Due to the large variability of polyphenol content in a given food, differences between analytical methods might first be explained by the limited representativity of the mean content value when too few samples have been analysed.
- Differences between methods can also be explained by the intrinsic quality of the analytical methods. For example, contents in total polyphenols obtained by the Folin assay are usually higher than the sum of the contents of individual polyphenols determined by chromatography. This is explained by the eventual presence in the food of interfering compounds such as vitamin C, which also reduce the Folin reagent.
Some compounds may not be extractable (e.g. phenolic acids bound to the cell wall in cereals) and only analysed after hydrolysis and solubilization.
By default, polyphenol content values are displayed as determined by chromatography techniques without hydrolysis. More polyphenols or higher concentrations can eventually be seen by “chromatography after hydrolysis” (click on the corresponding option).
Certain polyphenols appear in foods as glycosides or as esters and the term aglycone describes the core flavonoid, phenolic acid, lignan or stilbene, without the linked sugars or polyols. Contents expressed as aglycone equivalents correspond to the content of the core polyphenol (e.g. if a given food contains 1.80 mg/100g quercetin 3-O-glucoside, the content in aglycone equivalent (quercetin) will be 1.80/M(quercetin 3-O-glucoside)*M(quercetin) or 1.17 mg/100 g.
By default, content values are displayed in mg/100 g for solid foods and oils and mg/100 mL for beverages and certain other foods, such as vinegar or soy sauce. Upon request, values can be displayed in molar units or as aglycone equivalents (option available only for the method “chromatography”, see the FAQs “What are aglycone equivalents?”).
A systematic literature search was performed on the ISI Web of KnowledgeSM platform (Thomson Reuters) between 2005 and 2009, depending on the food category. Different queries were built per food group using the most representative food names and polyphenol names. The search was limited to peer-reviewed journal articles published in English. Comprehensiveness was checked in citations of the collected publications.
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 to produce mean content values (see “Methods used to build the database” for criteria). A key parameter is the number of samples analysed and the number of data collected from the original publications. A mean content value calculated from 50 individual content values is more reliable than one derived from a single content value.
Search results are easily exported as Excel file using the “Export” function.
Food entries in the database 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 (see the corresponding page on the website).
There is not a single valid method to determine polyphenol contents in foods. Analytical methods were classified into five categories, each one providing different information:
- Chromatography. The results obtained by this method are shown by default. It is the most common technique used to estimate polyphenols in food.
- Chromatography after hydrolysis. Acid or alkaline hydrolysis is used to cleave polyphenol glycosides or esters. Phenolic aglycones are then quantified.
- Folin assay. It is the spectrophotometric assay most commonly used to estimate total polyphenol content.
- Normal phase HPLC. This method is used to estimate proanthocyanidin oligomers according to their degree of polymerization (dimers to decamers), which cannot be easily quantified individually.
- pH differential method. It is the spectrophotometric method most commonly used to determine total anthocyanin content.
All these methods have their limits and drawbacks. See “Methods used to build the database” for more information.
The database includes contents for about 500 phenolic compounds. Actual total polyphenol contents may be in some cases higher than the values given in Phenol-Explorer due to the eventual lack of data for some polyphenols in a given food, or to the lack of reliable analytical methods for complex polyphenols such as proanthocyanidins (particularly those of high polymerization degree) and oxidized polyphenols (e.g. thearubigins in tea). See “Methods used to build the database” for more information.
Phenol-Explorer database contains a number of synonyms for each polyphenol. Synonyms were collected in the various publications as well as in the ChEBI, PubChem and CAS databases. The list of synonyms for a given polyphenol can be seen by clicking on its name. Any synonym can be used for search. Search can also be made on parts of the polyphenol chemical structure. For example, a search on caffeic acid, will retrieve all polyphenols containing caffeic acid, either esterified or glycosylated.
This may be due to the following reasons:
- The publication did not fit the quality criteria considered to build the database (see “methods used to build the database” for more information).
- The collection of data on those foods included in that publication was completed before the publication appeared. The database will be updated periodically.
The Phenol-Explorer reports provide information on the factors influencing polyphenol content (varieties, processing, storage conditions, cooking, etc.), complementing the database. For example, information on the content of polyphenols in several apple varieties or in cooked foods is not directly found in the database, but many useful literature references can be found in the reports.