Guru Kate: The mysteries of milk

How do they make “low fat” or “skim” milk?

When the milk comes out of the cow, 3.5% of that liquid is fat. Then it is pasteurized (a specialized heating process that zaps bacteria) and sold as whole milk. If the dairy wants to sell 2%, 1%, or skim milk, it has two options to slim the original product down.

According to Kimberlee J. Burrington of the Wisconsin Center for Dairy Research, one of the techniques is centrifugation of the milk. And have you tasted Wisconsin cheese? I’d totally trust her opinion.

A centrifuge works by spinning the liquid at rocket-ship speeds. This process separates the differently dense components within the milk. Because fat is less dense than water, it will move towards the top of the centrifuge while the rest of the milk will move its way to the bottom.

The processing plant makes 1% different from 2% by just spinning the centrifuge at a different speed and filtering out the finished product at a different time. The dairy can then sell the fatty milk at the top of the centrifuge as cream.

The other option is to go old school on the milk: let it sit. I’m sure you’ve heard old-timers tell you stories about getting the milk delivered to their doorstep from the milkman. This milk had a layer of cream on the top from natural separation.

Milk separates naturally for the same reason it does in the centrifuge; because fat is hydrophobic (water fearing) and it has a lighter density than water, so it floats to the top of the milk bottle. It can be scraped away and used to make cream, butter, mayonnaise or cheese.

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However, most milk is homogenized (squeezing all the milk particles through a sieve to make the fat so tiny that it won’t settle out of solution), and therefore no cream layer will form on top of the milk. If not for homogenization, Lucky Charms would never be the same. Some spoonfuls would be super creamy, while others would be watery.


Is low fat milk really better for you than whole milk?

There is a two-pronged answer for this question. First, it’s important to think about the fat. Second, there are always the vitamins to think about.

The fat in milk contains cholesterol and a lot of calories. So if you’re on a diet restricting cholesterol intake, or just trying to watch your weight, I’d suggest either the skim or 1% milk.

However, Vitamins A and D are fat soluble, and a great reason to drink milk. When you take the fat out of milk, you’re losing those vital nutrients.

The upside is that most low-fat milks are fortified with vitamins A and D, meaning that those vitamins are added back in after processing. So I’d still recommend low-fat milks.

The situation is different for little kids – because they’re growing, they need a slightly more fatty diet than do their adult counterparts. They also need more vitamins in their diet. Therefore, I’d recommend 2% or whole milk. Unless of course the child is obese, and even then, milk probably isn’t the problem.