INFLUENCE OF GROUND COLOUR AND SKIN DAMAGE ON FLUORESCENCE EMITTED BY BRAEBURN APPLE FRUIT

Abstract

In apples, as in the leaves of higher plants, blue, green and red fluorescence is emitted by skin regions excited by UV light. In the blue wavelength range fluorescence is emitted by phenolic compounds of cell walls in peripheral skin regions which is independent of the ground colour of the site excited. Red fluorescence emitted corresponds to the chlorophyll-a fluorescence of leaves, with a maximum near 690 nm and a shoulder near 730 nm. Fluorescence in the green spectral range is greatly dependent on the ground colour of the skin region excited. Green skin (shade), depending on the excitation wavelength, produces a maximum between 524 and 540 nm while red skin (sunny side) emits fluorescence in the red spectral range with a maximum between 600 and 613 nm. Fluorescence of red skin, compared to green, is usually less intense. In the transitional skin range between shade and sunny side both maxima have frequently been found to appear. When skin damaged by microbial infestation, browning or mechanical stress is excited by any of the three excitation wavelengths a drastic increase of blue fluorescence has been observed which is probably caused by polyphenol compounds in cell walls. The location of maxima is not specific of the nature of damage; the intensity of signals, furthermore, usually exceeds the measuring range, and fluorescence is emitted over a wide spectrum.