Effect of both homogenisation and storage on the spectra of red grapes and on the measurement of total anthocyanins, total soluble solids and pH by visual near infrared spectroscopy

The effect of homogenisation and storage on both the visible (Vis) and near infrared (NIR) spectra as well as on the calibration performance for the measurement of total anthocyanins (colour), total soluble solids (TSS) and pH in red grapes were investigated. Homogenates obtained from fresh and previously frozen red grapes (overnight freezing for 24 hours, and after one, three, six and 12 months frozen storage) were scanned in reflectance mode (400-2500 nm) using a monochromator instrument (Foss NIRSystems 6500). Principal component analysis was used to examine the influence of homogeniser type and frozen storage on the spectra. Neither the homogeniser type used nor overnight freezing had visual effects on the spectra. Spectra and analytical data were used to develop partial least squares calibrations to quantify the influence of storage on the measurement of total anthocyanins (colour), TSS and pH. When cross-predicting between fresh and frozen samples, the standard error of prediction was slightly increased when compared with calibrations developed on fresh samples alone. On the other, hand frozen samples were predicted well with the fresh sample calibration for the chemical parameters evaluated. However, although there may be some increases in error, these results suggest that it might be possible to use Vis-NIR calibrations developed on frozen samples alone to measure the concentration of total anthocyanins, TSS and pH in either fresh or frozen samples after appropriate slope and bias correction.