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| Mr. Bowman poses beside a pasteurization vat. Karl Kazaks |
For kids: Milk's trip from cow to you
On their dairy farm, two brothers oversee the journey of milk from their cows into cartons and finally to corner markets.
from the November 13, 2007 edition
Page 2 of 3
Its method is called vat pasteurization. And it involves the slow heating of milk in large vats to 150 or 155 degrees F. over a period of about 40 minutes.
The vats are round, stainless-steel tanks whose walls are designed to allow hot steam to pass through them. The steam heats the inside walls of the tanks, and that warms the milk.
In the center of each tank is a big mixing paddle that stirs the milk to make sure it all comes in contact with the outer walls and gets evenly heated.
From the pasteurizing vats, milk is pumped to a homogenizer. That's a large, squat machine that blends cream particles into the rest of the milk.
Normally, whole milk contains between 4 and 5 percent cream. Without homogenization, that cream would rise to the top of a container of milk because it's less dense.
"The homogenizer breaks down the fat molecules in the cream," Chris explains.
It does this through a motor that drives a set of pistons that pump back and forth like the pistons in a car engine.
The pistons help pump the milk in high-pressure streams of 2,000 pounds per square inch through small tubes. This causes the larger fat molecules in the cream to break into smaller molecules. They become heavier and then are no longer able to float to the top.
If the Bowmans aren't processing whole milk, however, they don't want that cream in the milk in the first place. To make low-fat or skim milk, they separate the cream from the milk before pasteurization using in a machine called a cream separator.
"It's a giant centrifuge," Chris says. "Whole milk goes in, and out comes 60 percent skim milk and 40 percent cream."
The separated cream is pasteurized, but not homogenized. Then it is bottled individually for sale as cream. Or it may be mixed with milk to make half-and-half. Another option is to keep the cream to churn it into butter, or use it to make ice cream.
After the cream is removed, skim milk is processed just as whole milk is.
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