2011 Volume 80 Issue 1 Pages 103-112
The role of phenylalanine ammonialyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD) in browning of fresh-cut galangal (Alpinia siamense K. Schum, ‘Kha Ta Daeng’) rhizome was investigated by determining changes in colour, total phenolic content (TPC), and the activities of these enzymes in the exterior (subepidermis and cortex) and the interior (vascular) tissues within 24 h under ambient conditions (29 ± 1°C and 86 ± 2% RH) after wounding. Browning of the peeled (exterior) and the cut (interior) surface of the rhizome increased progressively after wounding, but the peeled surface showed more browning than the cut surface. In the exterior tissue, TPC increased slightly from 190 to 220 mg gallic acid equivalent/100 g FW over the time of peeling, while PAL activity significantly increased by 37% of the initial level during the first 6 h after peeling, and declined thereafter. TPC in the interior tissue showed the same trend as the exterior tissue with a 10% lower in quantity, while PAL activity in the interior tissue only slightly increased. Changes in TPC and PAL activity in both exterior and interior tissues showed no significant correlations to any of the browning parameters over time (r = 0.16 to 0.48 and r = 0.01 to 0.50, respectively). PPO activity in the exterior tissue increased by 30% of the initial value during the first 6 h after peeling coupled with a slight increase in surface browning of fresh-cut galangal, while the POD activity remained unchanged. As browning progressed, both PPO and POD activities markedly increased in the exterior tissue and showed significant correlations to browning parameters, except for hue angle (r = 0.66 to 0.73 and r = 0.87 to 0.92, respectively). Histochemical assay for PPO showed that the site of browning began on the peeled surface, then gradually progressed to the cut surface, while that for POD occurred immediately on the entire wounded surface and no browning progression was observed, indicating that browning of fresh-cut galangal was likely mediated by PPO. However, POD may likely be involved in the lignification process in response to wounding because, from a microscopic examination, lignin was deposited on the parenchyma cell walls of the wounded exterior of the galangal rhizome.
