The fruits were dipped in a solution containing 5% w/v of citric, ascorbic or oxalic acid for 5 min. Immediately after dipping, the fruits were fumigated with ozone for 60 min compared with SO2 or ozone treatment alone. Thereafter all treatments were packed into plastic boxes and over-wrapped with plastic film, and then stored at 5 °C for 3 weeks. Fruit eating quality was assessed by a trained
panel of six researchers, on a hedonic scale of 1–9 based on fruit favour and sweetness. At each withdrawal, 30 fruits were randomly selected and rated for quality on the scale of
1 = poor, 5 = acceptable and 9 = excellent. PPO was extracted by the method of Huang et al. (1990).
Longan pericarp (10 g) was homogenized in 40 ml of 0.05M potassiun phosphate buffer (pH 6.2) containing 1M KCl and 2% polyvinylpyroritidone and then centrifuged for 5 min at 13,500 rpm (Hermel model Z383K) and 4 °C. The supernatant was collected as the enzyme extract. PPO activity was assayed by a modification based on the method of Jiang and Fu (1998) using the reaction mixture of 0.05M potassium phosphate buffer (pH 7.5) containing 0.2M catechol (0.2 ml) and crude enzyme (0.5 ml). Tubes
were incubated for 5 min at 30 1C, the absorbance was measured at 420nm by visible spectrophotometer (model Thermo Spectronic). The unit of enzyme activity was defined as the amount of enzyme that caused a change of 0.01 in absorbance per minute.