Polyphenols are omnipresent in plants, for example as anthocyanes in black grapes and red fruit, citroflavonoids in citrus fruit, and quercetin in apples. They are also used as additives – colouring agents, flavourings, preservatives – by the agrifood industry.
There are different classes of polyphenols, too, especially flavonoids, lignans, phenolic acids and stilbenes. Flavonoids consist of eight subclasses, including anthocyanins. Caffeic acid is a phenolic acid in coffee. Resveratrol, from grapes and wine, is the most widely studied stilbene. Flavonoids and phenolic acids are the two main groups present in food. Flavonoids only occur in vegetables, whereas phenolic acids can also be formed from other polyphenols within the organism.
Polyphenols have much better antioxidant properties than vitamins, and have been the object of growing interest on the part of nutritionists, epidemiologists, agrifood firms and consumers over the past decade or so. Their main advantage is that they protect against numerous diseases such as cancer or cardiovascular disease. In particular, they help fight the formation of free radicals in the human body and thus slow cell ageing. They are found in many common consumer products, primarily in fruit and vegetables but also in processed products such as chocolate, tea, or wine).
However, polyphenols are hard to study because of their very high structural diversity. There are many thousands of molecules in higher plants. Several hundred of these affect the human diet, but most vegetables and fruits only contain a few.
So food composition tables would make a big impact on their study. Until recently, only a few scattered studies have been made of the many thousands of individual pieces of research quoted in the literature.
With this in mind, the French Agricultural Research Centre for International Development (CIRAD) and its partners have developed a global method for the determination of total phenolic content in plant-derived products.
The Paris-based organisation has just produced such a table in an effort to provide and answer to some of these questions.
The table was produced under the French Research Ministry's Nutrialis programme. Researchers studied 162 samples from 24 vegetables and 71 samples from
28 fruits. The total polyphenol content of 85 tea samples was also analysed.
One of the main findings is that it is not always the fruits and vegetables with the highest polyphenol contents that are the most consumed. Rather, it is strawberries, lychees and grapes that have the highest polyphenol contents, but vegetables are not far behind, particularly artichokes, parsley and brussels sprouts.
Moreover, the total amount consumed plays a considerable role. As Pierre Brat, a CIRAD biochemist, points out: "If we look at total polyphenol content in apples, they rank fifth compared to other fruits, but the extent of their consumption places them first." Likewise, in terms of vegetables, potatoes rank just 19th, but their massive consumption means that they account for almost 60percent of the polyphenols obtained from vegetables.
This was the aim of the table compiled by CIRAD and its partners: to set product composition against consumption. The working method used to do this is in itself a tangible result of the study. In effect, the researchers had to select a range of fresh fruits and vegetables that was representative of consumption in France.
To this end, they took account of the different varieties eaten, the different production sites and countries, and where those fruits and vegetables are purchased.
Total polyphenol content was then analysed using a technique adapted from a chemical colorimetric assay method: the Folin Ciocalteu method. The researchers then established a relation between the result obtained and consumption levels. In this last stage, the team’s links with the Agence française de sécurité sanitaire des aliments (AFSSA) proved crucial, since the researchers were able to use two AFSSA databases: one on food consumption – Suvimax – and the other – Secodip – on fresh fruit and vegetable purchases. Moreover, AFSSA will be making the table available to researchers and the general public on its website shortly.
This research is now continuing under a new project, Phenobase, coordinated by the Centre technique de la conservation des produits agricoles in Avignon, in which CIRAD is also involved. The aim is to supplement the composition table, this time by looking at the so-called ‘processed’ products included in the daily diet in France. Known as project ACTIA, the aim is develop specific analysis methods that can be standardised and validated so that the final table will be of use to both industry and in ongoing epidemiological studies in France.
Both these projects have called upon CIRAD researchers’ expertise, notably in terms of citrus fruit treatments. The method developed under the Nutrialis programme, for its part, is currently being disseminated to CIRAD's partners in developing countries.