Determination of physical, biochemical and microstructural changes in impact-bruise damaged pomegranate fruit

This study investigated the physical, biochemical and cellular microstructural changes of ‘Wonderful’ pomegranate fruit induced by impact bruising at cold (5 °C) and ambient (20 °C) temperature. Pomegranate fruit were bruised by dropping at various drop heights, from low (20 cm), medium (40 cm) to high (60 cm) impacts onto a rigid impact surface. Physical and biochemical changes such as colour browning, peel electrolyte leakage and polyphenol oxidase (PPO) enzyme activity of bruised and non-dropped (control) fruit peels were measured. Reaction oxygen species (ROS) of fluorescent probe 2,7-dichlorodihydrofluorescein diacetate (H₂DCF-DA) treated fruit peels were measured by confocal laser-scanning microscopy. Qualitative assessment of microstructures between control and bruised tissues of pomegranate fruit peels was performed using scanning electron microscope (SEM). Micrographs of SEM showed cellular microstructural differences between control and bruised fruit tissues were visible after 4 and 48 h of drop impact. Medium and high impact-bruised fruit were characterized by higher ROS than control fruit. Bruise damage to pomegranate fruit lead to increased peel electrolyte leakage (PEL) both at cold and ambient temperatures. Bruising had more effect on increasing PPO activity than did the incubation temperature and time. Activity of PPO enzyme was higher in bruised fruit than control fruit. Browning score (BS) and the total colour difference (TCD) both indicated highest values corresponding to medium and high drop impact bruising. Pearson’s correlation showed strong to moderate relationship between PEL, PPO activity, BS, TCD and ROS. Increase in fluorescent units of ROS showed a strong significant correlation (p < 0.05; r = 0.70) with BS and moderately correlated with TCD (p < 0.05; r = 0.67) and PPO activity (p < 0.05; r = 0.61). This study has confirmed that pomegranate fruit bruising induce the physical and biochemical changes in addition to underlying cellular microstructural alterations.