Evaluation of biochemical markers associated with the development of husk scald and the use of diffuse reflectance NIR spectroscopy to predict husk scald in pomegranate fruit

Husk scald is a physiological rind disorder that manifests during storage and commercial shipping of pomegranate fruit. Scald affects the external quality thereby reducing its marketability, causing major economic loss to the industry worldwide. The lack of understanding of the underlying physiological and biochemical mechanisms of this disorder hinders the development of cost-effective solutions to minimise losses and assure consistent supply of quality fruit. Therefore, this study evaluated several biochemical markers associated with the development of husk scald and to discriminate between healthy and scalded fruit non-destructively. The effects of storage temperature and duration on fruit weight loss, colour attributes (L*, a*, C*, h°), browning index, total phenolics, total tannins, total anthocyanin content and enzymatic activity (polyphenol oxidase; PPO and peroxidase; POD) were investigated. The results showed that enzymatic browning was the main cause of peel browning with tannin and phenolic compounds acting as substrates for PPO and POD activity. Fruit stored at 10 °C for five months exhibited a higher incidence of scald compared to fruit at 5 °C. Fourier transform near-infrared reflectance spectroscopy was used to qualitatively discriminate between healthy and scalded fruit using orthogonal partial least squares discriminant analysis (OPLS-DA). The fruit were classified into three categories, namely, healthy, moderate scald and severe scald. A classification accuracy of 100% healthy, 92.6% moderate scald and 93% severe scald was achieved. Furthermore, the variable importance of projection (VIP) plot revealed that wavelength regions responsible for discrimination between classes were at 1350-1450, 1830-1950 and 2150-2250 nm, the wavebands attributed to stretching of CO, OH and CC bonds related to phytochemical compounds such as phenolics and anthocyanins. This study demonstrated that PPO activity, total phenolic and total tannins content were associated with the development of husk scald and these biomarkers were used to discriminate pomegranate fruit into different quality tiers.