The objective of this work was to better understand the photosensitizing effect of riboflavin versus naturally occurring tetrapyrroles in cow's milk. This was done by exposure of milk samples to blue light (400-500 nm), which is absorbed by riboflavin and tetrapyrroles, orange light (575-750 nm), which is absorbed by tetrapyrroles but not riboflavin, and white light, which contains the entire visible region. The milk was exposed to about 1.6 W/m(2) in 20 h, and two different light sources were tested: HMI lamp and fluorescent light tubes used for commercial display. Sensory analysis showed that wavelengths longer than 575 nm induced significantly more off-flavors than wavelengths shorter than 500 nm. By fluorescence spectroscopy it was observed that tetrapyrroles, in particular, chlorophyllic compounds, were degraded more by orange light than by blue and that the degree of degradation correlated closely with the formation of sensory off-flavors. The fluorescent agent Singlet Oxygen Sensor Green (SOSG) was used to monitor the formation of singlet oxygen under the different light exposure conditions, and the method verified that singlet oxygen was formed in large proportions in milk exposed to wavelengths longer than 575 nm, presumably with minor or no involvement of riboflavin. The results suggest that cholorophyllic compounds are responsible for a major part of photooxidation in milk. It is also suggested that β-carotene protects against photooxidation under blue light because it absorbs a major portion of the light below 500 nm and thereby reduces reactions with photosensitizers.