Milk is naturally white, but the butter made from it often has a yellow colour. While many people may have noticed this difference, few know the simple scientific reason behind it.
The answer lies primarily in what cows eat.
Grass and other fodder consumed by cows contain natural pigments known as carotenoids, particularly beta-carotene, which are fat-soluble compounds.
These pigments are absorbed by the animal's body and eventually make their way into milk fat.
Carotenoids are the same compounds that give carrots, sweet potatoes, and pumpkins their characteristic orange colour.
Although grass contains much lower concentrations of these pigments, the amount is still sufficient to influence the colour of butter.
So why doesn't milk appear yellow as well?
The reason lies in the fat content. Milk contains only about 3 to 4 percent fat, and the fat molecules are dispersed throughout the liquid.
As a result, the carotenoid pigments are too diluted to noticeably affect the milk's white appearance.
Butter, on the other hand, contains approximately 80 to 85 percent fat. During the butter-making process, fat is concentrated, and so are the carotenoids.
In fact, the concentration of these pigments in butter can be many times higher than in milk.
This increased concentration gives butter its characteristic yellow hue and richer texture.
The more carotenoids present in the cow's diet, the deeper the yellow colour of the butter is likely to be.
Scientists say that variations in butter colour can also depend on factors such as the cow's breed, diet, season, and farming practices.
Cows that graze on fresh pasture generally produce more yellow butter than those fed mainly on grain-based diets.
In short, butter's yellow colour is a natural result of carotenoid pigments becoming concentrated in milk fat during the butter-making process—a small but fascinating example of how animal diet influences the foods we eat.