COMPARATIVE ANALYSIS OF VITAMIN C (ASCORBIC ACID) IN PINEAPPLE AND BANANA
CHAPTER ONE
INTRODUCTION
Vitamin C is the most important vitamin for human nutrition that is supplied by fruits and vegetables. L-Ascorbic acid (AA) is the main biologically active form of vitamin C. AA is reversibly oxidised to form L-dehydroascorbic acid (DHA), which also exhibits biological activity. Further oxidation generates diketogulonic acid (Fig. 1), which has no biological function (Davey et al., 2000; Deutsch, 2000). Since DHA can be easily converted into AA in the human body it is important to measure both AA and DHA in fruits and vegetables to know vitamin C activity (Lee & Kader, 2000). AA is widely distributed in plant cells where plays many crucial roles in growth and metabolism. As a potent antioxidant, AA has the capacity to eliminate several different reactive oxygen species, keeps the membrane-bound antioxidant α-tocopherol in the reduced state, acts as a cofactor maintaining the activity of a number of enzymes (by keeping metal ions in the reduced state), appears to be the substrate for oxalate and tartrate biosynthesis and has a role in stress resistance (Arrigoni & De Tullio, 2002; Davey et al., 2000; Klein & Kurilich, 2000).
Since humans cannot synthesise ascorbate, their main source of the vitamin is dietary fruit and vegetables. Fruits (especially citrus and some tropical) are the best sources of this vitamin. An accurate and specific determination of the nutrients content of fruits is extremely important to understand the relationship of dietary intake and human health. Several analytical methods have been reported for the determination of vitamin C using titrimetry (AOAC, 1990; Kabasakalis, Siopidou & Moshatou, 2000), spectrometry (Arya, Mahajan & Jain, 1998) and amperometry (Arya, Mahajan & Jain, 2000). Most of these methods may give overestimes due to the presence of oxidizable species other than AA and/or not to measure DHA. For example, the AOAC’s official method (AOAC, 1990), based on the titration of AA with 2,6-dichloroindophenol in acidic solution, is not applicable in all the matrices. Substances naturally present in fruits such as tannins, betannins, sulfhydril compounds, Cu(II), Fe(II), Mn(II) and Co(II) are oxidised by the dye. Moreover, the method is applicable only when the concentration of DHA is low (Arya, et al., 2000).
The preferred choice for AA determination are separation techniques: capillary electrophoresis (Versari, Mattioli, Parpinello & Galassi, 2004), gas chromatography (Silva, 2005) and liquid chromatography (LC). LC avoids the problems of non-specific interference and ion-pair (Ke, El-Wazir, Cole, Mateos & Kader, 1994), NH2 bonded-phase (Silva, 2005; Zerdin, Rooney & Vermuë, 2003) and reverse phase (Franke, Custer, Arakaki & Murphy, 2004; Gökmen, Kahraman, Demir & Acar, 2000) techniques have been reported. To increase the sensitivity for DHA, derivatisation prior to or after the chromatographic separation is necessary. Usually, DHA is determined as the difference between the total AA after DHA reduction and AA content of the original sample. Various reducing agents, such as homocysteine, dithiothreitol (Gökmen et al., 2000; Silva, 2005) and L-cysteine (Zerdin et al., 2003) have been studied. Moreover, DHA can be determined after LC separation and detection by fluorimetry after a post-column derivatisation with O-phenyldiamine (Kall & Andersen, 1999).
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COMPARATIVE ANALYSIS OF VITAMIN C (ASCORBIC ACID) IN PINEAPPLE AND BANANA