High-performance liquid chromatography with diode-array detection for the determination of phenolic compounds in peel and pulp from different apple varieties
Introduction
Phenolic compounds are found in plants and thus are part of the human diet [1]. Dietary phenolic compounds, and especially the flavonoids, consist mainly of anthocyanidins, flavonols and catechins [2]. They are absorbed from the gastrointestinal tracts of humans and are excreted in faeces and urine. Accurate data on population-wide intakes of flavonoids are not available but a recent review [1]points out that the most important dietary sources are vegetables, fruits and beverages. It is estimated that tea, onions and apples are the main dietary sources of flavonoids [3]. However, current estimates of daily consumption of flavonoids differ considerably [4]and, consequently, more quantitative data are necessary [1].
The phenolic composition of fruits has been studied by HPLC–diode array detection (DAD) in pulps and juices with regard to their contribution to the color and flavor [5], their qualitative and quantitative differences appearing as a function of the species, degree of ripening and storage 6, 7as well as of their presence in commercial juices and jams 8, 9.
However, few studies of extracts from fresh peel matrices have been reported 10, 11, and comparative studies between peel and pulp are especially limited 12, 13. This comparison is important since apples are consumed both peeled and unpeeled. Taking into account all these considerations, more quantitative data of phenolic compounds from apples (peel and pulp) are needed.
The aim of this work was to carry out a comparative systematic study on the quantitative composition of phenolic compounds in fresh pulp and peel from the apple varieties, Golden Delicious, Red Delicious, Granny Smith and Reineta Green (Spanish apple). These varieties are usually consumed as fresh fruits in the Mediterranean diet [14].
Section snippets
Reagents and standards
The standards (+)-catechin and (−)-epicatechin as well as gallic, caffeic and chlorogenic acids, phlorizdin and rutin were acquired from Sigma (St. Louis, MO, USA). Methanol of HPLC grade was acquired from Sharlau and all other chemicals of analytical-reagent grade were purchased from Merck. In all cases, the water used was of HPLC quality, purified in a Milli-Q system (Millipore, Bedford, MA, USA). All the samples (solutions and extracts) were filtered through 0.45-μm membranes (Millipore) and
Separation and identification
Fig. 1 shows the chromatograms of the extracts from peel and pulp. Table 1 lists the retention times including standard deviation (S.D.) for six replicates of each variety studied, the selectivity factor (α), as well as UV absortion maxima of each peak obtained by DAD.
The gradient elution method used allowed a good separation of the phenolic compounds present with values of α above 1.00 in all cases. This method enabled the identification of the phenolic compounds in all samples studied.
Conclusion
The methodology developed in this work enabled the quantitative determination of the phenolic compounds present in peel and pulp from different varieties of apple. Significant quantitatively differences were observed between the varieties studied. Golden Delicious was the variety with the lowest phenolic content whereas Reineta was the variety with the highest levels of phenolic compounds. In all cases, apple peels showed higher phenolic content than pulp extracts. These quantitatives
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