http://well.blogs.nytimes.com/2016/1...butter-yellow/

Since Milk Is White, Why Is Butter Yellow?
By Sophie Egan October 14, 2016 5:30 am October 14, 2016 5:30 am

The difference in color is primarily due to the higher fat content of
butter. Cows that eat grass and flowers store the yellow pigment beta
carotene, found naturally in those plants, in their fat. The pigment
gets carried over into the fat in their milk. Milk consists mostly of
water, with just over 3 percent fat in whole milk; cream is usually
about 30 to 40 percent fat; and butter contains at least 80 percent fat.

The fat globules suspended in milk or cream are surrounded by a thin
membrane that, in essence, ends up hiding the beta carotene pigment.
This structure reflects light in such a way that the milk looks white.

Making butter requires churning cream, and during that agitation process
€œyou break the membrane apart, and the fat globules cluster together,€�
said Elaine Khosrova, the former editor of the publication culture,
about cheese making, and author of the new book €œButter: A Rich
History.€� €œThats the goal of butter making: to break that membrane.€� In
doing so, you expose the beta carotene, she said. When you separate out
the buttermilk after churning, what remains is mostly butterfat, which
is the most yellow of all.

You may notice, however, that butter from sheeps milk, goats milk or
water buffalos milk is white. Those animals dont store beta carotene
the way cows do. Instead, they convert it to vitamin A, which is colorless.

If cows are raised on pasture, their butter is more yellow when the milk
is collected in late spring or summer, when the cows have more beta
carotene-rich forage to chew on. In wintertime, even cows raised on
pasture are usually brought inside and fed grain, which doesnt have
much beta carotene. Some dairies freeze butter so they can sell the
yellow-tinged kind year-round.

But, of course, many industrial dairy producers raise cows without ever
putting them out on pasture, in which case seasonality makes no
difference. Those butters, which are quite common in grocery stores,
arent very yellow at any point in the year.

€œIn agricultural communities, every spring their butter was prettier:
more yellow,€� Ms. Khosrova said. €œIts interesting that weve gotten
used to pale butter. Now with the rise of artisanal butters that are
more golden and yellow, chefs want that on the table, so I really wonder
if companies will start sneaking in more color.€�

Some commercial dairy producers do add color, usually annatto, which is
also sometimes added to cheeses to give them a yellow-orange hue.
Annatto is a derivative of seeds from the achiote tree, which is native
to Central and South America and grows in tropical regions.

€œSeeing the color could affect how our brains perceive the taste,€� said
J. Kenji López-Alt, the managing culinary director of Serious Eats and
the author of "The Food Lab: Better Home Cooking Through Science."

€œSo if you have that association, it could make your enjoyment of the
butter higher.€�