Vis/NIR estimation at harvest of pre- and post-storage quality indices for 'Royal Gala' apple

Visible/Near Infrared (Vis/NIR) spectrometric measurements, made with a fast low cost polychromatic spectrometer operating over the range 500-1100 nm, have been made on 'Royal Gala' apples (Malus domestica Borkh.). The spectral measurements were taken on fruit from eight orchards in two growing regions and in each of two seasons through a 7-week period centred around the commercial harvesting period. Data analysis involved creating predictive models, by chemometric modelling, for various quality indices measured both immediately after harvest and after 6 weeks cool storage. The quality indices included background colour (BC), starch pattern index (SPI), soluble solids content (SSC), penetrometer firmness, quantitative starch (QS) and titratable acidity (TA). The predictive models were significant in regression terms, typically explaining between 50 and 80% of the data set variance, but they were, nonetheless, very poor in prediction terms. For each quality index, the ratio of data standard deviation to the root mean square error of prediction (RMSEP) was always well less than 3; a ratio value considered the minimum for only coarse screening/grading of fruit. Most of the prediction models appear to be primarily dependent on changes in the chlorophyll absorbance peak, which dramatically reduces in intensity during the progression of the fruit through the harvest period, rather than on the constituent or property of direct interest. Exceptions were the SSC prediction models that are dependent, to varying degrees, on the relevant constituents of interest (soluble carbohydrates). The SSC predictions were much better post-storage than at harvest time (RMSEP approximately 0.50 and 0.72%, respectively) indicating, we theorise, the confounding influence of starch, present in the fruit at harvest time, on the SSC model.