Does Light Affect a Ripe Apple?

Ethylene Reaction

• Ethylene is a gas given off by a plant that relates to flowering, ripening, and leaf drop in a plant. Ethylene synthesis is increased by exposure to light for most plants, which causes an apple tree to flower or its fruit to mature. An apple must be at a certain level of maturity before it can produce a color, like red or yellow, which indicate that the apple is ripe. Without light, an apple never reaches full maturity.

Pigment Formation

• Light is important in the coloration of apples. It promotes a chemical called anthocyanin, which is responsible for the red, purple and blue colors in fruits. Without light, mature apples become yellow as they ripen, even if the apple is red under normal circumstances. Furthermore, not all of the spectrum is influential, only light in the regions of 3,600 to 4,500 Angstrom units (a kind of light measurement) of the color spectrum seem to work. 3,600 to 4,500 falls within the near-ultraviolet range of invisible light and the violet, blue and green colors of visible light.

Sunburn

• Sunburn can affect apples just the same as it affects humans. The sun can damage the skin of an apple, causing brown spots or dead areas. The damage can be caused by either the light radiation or by the heat that light produces. The damaged caused by browning is seldom lethal even if it looks uninviting. The dead areas (necrosis) are lethal and eventually destroy the entire apple.

Maturity Detection

• Light can affect an apple by allowing the grower to know how mature the apple is, so it can be harvested. There is an experimental process called fluorescence spectroscopy, and it induces the apple to fluoresce, or glow, under ultraviolet light. This fluorescence gives feedback about the measurement of the apple's skin thickness and flesh color. It also provides information about the apple's firmness, sugar content and acid content, but this was slightly less reliable. It offers an alternative and non-destructive way of testing maturity.