The optimal harvest date of nectarines can no longer be based on the colour since newer cultivars have an intensive blush, even in the unripe stage. Measuring the absorption of the fruit flesh by Time Resolved Spectroscopy (TRS) at 670 nm, provide information on the ripening stage and the variation in ripening stages of batches of fruit. Since each individual fruit is harvested and measured at some arbitrary stage of development, the analysis of the data gathered was based on the system of the biological shift factor (Tijskens et al. 2005) applying non-linear mixed effects regression analysis. The results show that ¿a, and therefore also the fruit flesh colour and chlorophyll content, change according a sigmoidal pattern, which was approximated with a symmetrical logistic function. The explained parts obtained (R2 adj) were about 0.97 for storage at 20ºC and about 0.86 at 10ºC. The rather large variation on apparent behaviour between individual fruit, each with its own stage of development, was nicely taken care of by the system of the biological shift factor. During ripening, the distribution of the biological shift factor was constant and normal, while the distribution of ¿a changed. Also that change in distribution over time is nicely described by the system and models used.