What is whey and what is casein? What are the differences between whey and casein in cow’s milk and breast milk?

Whey and casein are proteins found in milk from mammals like cow’s and humans.

One way of describing these two proteins is by using the children’s nursery rhyme about Little Miss Muffet where “Little Miss Muffet sat on a tuffet eating her curds and whey.” You may want to picture Little Miss Muffet eating cottage cheese! The casein portion of the cottage cheese can be pictured like the “curds,” and the whey portion of the cottage cheese can be pictured as the “liquid” portion.

When milk – from cows or humans - is mixed with an acid (like those in our stomach’s digestive juices!), the milk protein casein develops small curds while the whey largely remains a liquid.

In cow’s milk, approximately 82% of the protein is casein and 18% is whey and this is often referred to as the whey to casein ratio of 18:82, or approximately 20:80.

In breast milk, the percentage of these two proteins varies by stage of lactation. Research has documented that the whey to casein ratio in early lactation is about 90:10. In mature human milk, the ratio is approximately 60:40, then it changes just slightly to 50:50 in late lactation.(1)

Formulas made from cow’s milk can be casein-predominant when only cow’s milk (and no added whey protein ingredients) are used. In formulas with just cow’s milk as the protein source, the whey to casein ratio is approximately 20:80. Alternatively, formulas can be whey predominant. In these scenarios, formulas often include added whey protein to achieve a higher whey to casein ratio similar to the 60:40 ratio of whey to casein found in mature breast milk.

Both casein-predominant formulas and whey-predominant formulas have been used for decades with demonstrated support for the growth and development of children.

Modeled after breastmilk, Baby’s Only® Organic Gentle Infant Formula has a whey-to-casein ratio of 60:40.


  1. Kunz C, Lönnerdal B. Re-evaluation of the whey protein/casein ratio of human milk. Acta Paediatr. 1992;81(2):107-112. doi:10.1111/j.1651-2227.1992.tb12184.x
Have more questions? Submit a request