Optical techniques to estimate the ripeness of red pigmented fruits

During fruit ripening, chlorophyll degradation is responsible for the degreening of the ground color, which is a well-established ripeness indicator for several species. In completely red-pigmented cultivars of fruits such as apples and peaches, this process is not visible, being masked by anthocyanins in the skin. Two different optical systems were developed to non-destructively assess the chlorophyll content in these fruits, to estimate ripeness, and to optimize harvesting and postharvest management. A fluorescence imaging system equipped with a UV-blue actinic light was used to obtain fluorescence images of fruit in which the gray level of pixels correlated (R2 = 0.81) with the firmness of fresh apples (Malus domestica cv. Red Delicious). With this technique it was possible to estimate changes in the firmness and soluble solids sugar content of stored Red Delicious apples undergoing no detectable hue change in the skin. Using the same system with a red actinic light, fluorescence correlated fairly well with firmness for fresh peaches and nectarines (Prunus persica cv. Elegant Lady, Summer Rich, and Morsiani 90), even though the detected fluorescence signal was low in intensity. A laser-diode based, dual-band reflectance probe was developed and tested on fresh peaches (cv. Summer Rich) and stored apples (cv. Royal Gala). The R/IR index, defined as the ratio of the signal measured in red and near-infrared bands, was found to correlate with the chlorophyll content of the fruits (R2 = 0.66), regardless of fruit species and anthocyanin presence. The R/IR index was used to track the postharvest ripening process for fresh peaches harvested at different maturity stages.