Abstract:
Pineapple is an economically important tropical fruit and has previously been shown to have a cell wall composition quite different from that of most other fruits and vegetables. One of the most interesting differences is the presence of ester-linked hydroxycinnamic acid. The presence of these compounds influences texture of plant foods, and may have potential health effects. However, previously difficulties have been encountered in consistently quantifying the ferulic acid from cell walls of the pineapple cv. Tropical Gold. The approach taken in the present study was to re-design the saponification conditions and include antioxidants, ascorbic acid (5 mM or 10 mM) or gallic acid (5 mM or 10 mM) in the sodium hydroxide solutions. The effects of different concentrations of sodium hydroxide solution (0.1 M, 1 M and 2 M) on the release of hydroxycinnamic acids for different time intervals have been investigated. These optimised hydrolysis conditions, which have been determined ensure consistency in identification and quantification of the monomeric hydroxycinnamic acids or dimeric hydroxycinnamic acid released from the cell walls. The released hydroxycinnamic acids were analysed as trimethylsilyl (TMS) derivatives by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detector (GC-FID). Three types of monomeric hydroxycinnamic acids were detected. E-Ferulic acid was the most abundant one, followed by Ep- coumaric acid and Z-ferulic acid which were present in much smaller amounts. Five cyclodimers of p-coumaric and ferulic acid were tentatively identified, including three PPht (truxillic) type and two FFht (truxinic) type cyclodimers. Nine ferulic acid dehydrodimers were found. These were 8-8' DFA (aryltetralin), 8-8' DFA, 8-5' DFA, 8-8' THF-DFA, 8-O-4' DFA, 8- 5' DFA (benzofuran), 5-5' DFA, 8-5' DFA (decarboxylated) and 4-O-5' DFA. Among the ferulic acid dehydrodimers that were detected, 8-5' DFA (benzofuran) was the most abundant, followed by 8-O-4' DFA.
