Thesis-Final 03 June 2011 pdf - Jacobs University

Background Chapter 2
Two different approaches are used to estimate polyphenols: a) specific compounds
such as chlorogenic acid in potato or coffee, quercetin in onions or catechins in tea are
estimated individually by chromatographic techniques or b) total phenols are
estimated by reduction of the Folin-Ciocalteu reagent. 51 Values obtained by the first
method are usually lower than those estimated by the Folin assay. One reason is that
some polyphenols in a given food source may escape determination by
chromatography. These can be unknown compounds, compounds present as traces
that were not considered in the characterization of food sources, or compounds that
are not resolved by chromatography, such as proanthocyanidin polymers and oxidized
polyphenols 52 as in apple, wine, tea or beer. A second reason is that other reducing
agents may be present in food. Ascorbic acid also reduces the Folin reagent. 53 For
example, the ascorbic acid content of potato, tomato, onion, apple and orange juice
(17, 24, 8, 12 and 54 mg/100 g fresh weight, respectively) 54 would account for 40 and
46% of the estimated total phenols in potato and tomato but for only 6 and 4% in
polyphenol-rich onion and apple.
Environmental factors have a major effect on polyphenol content. These factors may
be pedoclimatic (soil type, sun exposure, rainfall) or agronomic (culture in
greenhouses or fields, biological culture, hydroponic culture, fruit yield per tree, etc).
Exposure to light has a considerable effect on most flavonoids. The degree of ripeness
considerably affects the concentrations and proportions of the various polyphenols. In
general, phenolic acid concentrations decrease during ripening, whereas anthocyanin
concentrations increase. Many polyphenols, especially phenolic acids, are directly
involved in the response of plants to different types of stress: they contribute to
healing by lignification of damaged areas, they possess antimicrobial properties, and
their concentrations may increase after infection. 55
Storage may also affect the content of polyphenols that are easily oxidized. Oxidation
reactions result in the formation of more or less polymerized substances, which lead
to changes in the quality of foods, particularly in color and organoleptic
characteristics. Such changes may be beneficial (as is the case with black tea) or
undesirable (browning of fruit) to consumer acceptability. Therefore, storage may
cause loss of phenolic acids. 56
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