Sour milk mystery unravelled

After 60 years of research, scientists have found the reason why milk turns sour when exposed to sunlight.

Scientists at the Norwegian Institute of Food used a sensory panel to identify the relationship between illumination and oxidation in milk and discovered that the presence of chlorophyll is responsible for the off-taste of milk.

The team found milk contains trace amounts of chlorophyll and its breakdown products, which are extremely sensitive to light, and will initiate oxidation. It is thought that these substances end up in milk since cows eat green grass. The products’ incredible sensitivity to violet, orange and red light is thought to increase the off-flavour.

For the past 60 years it was thought that the vitamin B2, also known as riboflavin, in milk initiates the process that leads to the sour taste. However, recent research indicates that riboflavin only initiates these reactions when it is exposed to violet and blue light; orange and red light are ineffective and do not cause any negative effects.

Senior research scientist at Nofima, Dr Jen Petter Wold, said: “Knowing which substances in milk give rise to the off-flavour means that we also know how we can protect against it”.

The scientists believe that the research could help create specially designed packaging, and advice about the type of illumination that should be used in shops to reduce the risk of the sour taste.

The packaging currently used in the UK could change from the plastic bottle to a paperboard carton, which transmits little light and will therefore help the fight against sour tasting milk.

Author: Craig Blanchard