In pasteurization, fresh milk is briefly heated to a temperature just high enough to kill off those pathogenic bacteria; by default, the process also kills any other bacteria that might be living in the milk. (Homogenization, meanwhile, keeps the cream, or fat, from separating from the milk.) Without it, raw milk producers have to take extra care to prevent contamination of their milk, Bishop says. Sanitary conditions, attention to diet, milk testing and cattle health screening can help prevent contamination episodes, and researchers in Europe -- where raw milk is widely used in cheese production -- have pioneered protocols to help ensure that raw milk is pathogen-free, he says.
"They can have that [raw] milk coming off the farm with minimal bacteria -- but it takes a lot more effort," Bishop says.
In the U.S., raw milk is regulated differently from one state to the next; in many states, it's simply illegal. In California, raw milk producers must meet the same safety standards as producers of pasteurized milk -- without the aid of pasteurization. (Advocates supported a bill last fall that would have established separate standards for raw milk, but it was vetoed by Gov. Arnold Schwarzenegger.) The extra effort it takes to meet those standards accounts for the high price of raw milk -- as much as $10 for half a gallon in California markets.
To the tens of thousands of California consumers purchasing raw milk, that price is worth it. To others, it's not.
"If you want beneficial bacteria," Metzger says, "you'd be better off just eating yogurt."