Milk can exhibit a noticeable glow when exposed to a black light. This phenomenon is not due to artificial additives but a naturally occurring compound within the milk. Its interaction with ultraviolet light causes the liquid to emit visible light, creating the observed luminescence.
The Role of Riboflavin
The primary reason milk glows under a black light is the presence of riboflavin, also known as Vitamin B2. This water-soluble vitamin possesses a property called fluorescence, meaning it can absorb invisible ultraviolet (UV) light and then re-emit it as visible light.
When a black light, which emits UV radiation, shines on milk, riboflavin molecules absorb this energy. The absorbed energy excites electrons within the riboflavin structure to a higher energy state. As these excited electrons return to their original, lower energy state, they release excess energy as photons, which our eyes perceive as a greenish-yellow glow.
Why Not All Milk Glows Equally
The intensity of milk’s glow under a black light can vary due to several factors, largely related to its riboflavin content and how it has been handled. The concentration of riboflavin naturally differs, with some studies showing raw goat milk having a higher average content (2.3 µg/mL) compared to raw cow milk (1.66 µg/mL). Factors like the cow’s breed or diet can also influence the riboflavin levels in dairy milk.
Processing methods like pasteurization, while essential for safety, can lead to some reduction in riboflavin content. Although pasteurized milk remains an excellent source of the vitamin, a significant decrease in riboflavin has been observed in some cases. However, other studies suggest riboflavin levels are largely unaffected by standard pasteurization or ultra-high temperature (UHT) processing. This variation implies that the processing specifics can play a role in the eventual glow.
How milk is packaged and stored greatly influences its riboflavin preservation. Riboflavin is highly sensitive to light, and exposure can lead to its degradation. Milk stored in clear containers, such as polyethylene terephthalate (PET) bottles, can lose a significant portion of its riboflavin when exposed to light. Opaque containers like high-density polyethylene (HDPE) bottles or coated paperboard cartons offer better protection against light-induced degradation, preserving the riboflavin and the potential for a stronger glow.
The type of milk also contributes to differing glow intensities. Plant-based alternatives like soy, almond, or oat milk can have varying riboflavin levels. Many plant-based milks are fortified with riboflavin to match the nutrient profile of cow’s milk, and the level of this fortification will directly impact how brightly they fluoresce.