10 June 2008

The Myths about Raw-Milk Cheese



By Janet Fletcher

Parmigiano-Reggiano, Roquefort and English Cheddar are among the most sought-after selections in any cheese case. But are they also among the most dangerous?
Along with dozens of other raw-milk treasures—Fontina Val d’Aosta, Swiss Gruyère, Comté, Vermont Shepherd—these classic cheeses bear the weight of the myths and misperceptions surrounding raw milk. Some advocates suggest that raw-milk cheese is as safe as corn flakes—people aren’t dying like flies in Europe, are they? Yet others preach caution. With some physicians advising pregnant women to avoid all raw-milk cheese, consumers are wondering where the danger lies.
Does your cheese department staff know the raw-milk facts? Let’s examine some of the commonly held beliefs about pasteurization and its impact on milk, cheese and health. Anyone in the business of selling cheese needs to be able to tell the myths from the truth.

Myth 1: Pasteurized milk is sterile.

Not true. If pasteurized milk were sterile, an unopened carton wouldn’t spoil. Pasteurization kills a lot of bacteria, including all the pathogens (disease-causing organisms) like Listeria and Salmonella and some but not all of the bacteria that make milk spoil. So pasteurization does make milk safer while it also increases the shelf-life.

There is more than one way to pasteurize. You can heat the milk to a high temperature (161 degrees F.) for a short time (15 seconds). Or you can heat the milk to a lower temperature (145 degrees F.) for a longer time (30 minutes). Most cheesemakers would say that the high-temperature, short-time (HTST) method does less damage to milk quality.
A third heat-treatment procedure, known as thermization or thermalization, stops short of pasteurization. This method preserves the milk enzymes while significantly reducing bacterial counts. The FDA considers thermalized milk as raw milk, so cheeses made with thermalized milk must still be aged at least 60 days.

Myth 2: Raw milk has more nutrient value than pasteurized milk.

Not significantly. According to Moshe Rosenberg, food science professor at the University of California at Davis, vitamin loss from pasteurization is either too small to measure or less than 10 percent. The exception is vitamin C, which drops by about 20 percent.

Many people believe in the health benefits of raw milk’s enzymes. Pasteurization does denature enzymes, but according to Rosenberg, milk enzymes can’t withstand the low pH in the human stomach anyway. They don’t do anything for our digestion or health.

Myth 3: Pregnant women should avoid all raw-milk cheese.

The concern here is Listeria, which can harm a fetus. So do physicians have science behind them when they tell pregnant patients to forego all raw-milk cheese?

Studies suggest not. Although high-moisture, unripened cheeses like cottage cheese and queso fresco and soft cheeses such as Brie and Camembert can support Listeria growth, aged raw-milk cheeses like Parmigiano-Reggiano, English Cheddar, Gruyère and Emmenthal cannot. They’re too dry, too low in pH, too high in salt. When Listeria turns up in cheese, it’s almost always in moist, soft cheese made with milk that was improperly pasteurized or contaminated after pasteurization. There have been several large outbreaks of listeriosis associated with Hispanic-style cheeses prepared under non-commercial conditions.

The FDA’s own risk assessment puts hard cheese last in Listeria potential among 23 common foods, including produce (www.foodsafety.gov/~dms/lmr2-toc.html).

From the standpoint of Listeria risk, it would make more sense to warn pregnant women away from fruits, vegetables or deli meats than to caution them about aged raw-milk cheese. Aged raw-milk cheeses are excellent sources of calcium and protein, needed by pregnant women.
The law requires raw-milk cheese—domestic or imported—to be aged at least 60 days at 35 degrees F. or above. By that point, most cheeses are no longer soft or moist, and are highly unlikely to harbor Listeria. (Some underage raw-milk cheeses do enter this country illegally, and pregnant women should avoid them.)

Myth 4: Raw-milk cheese tastes better than cheese made from pasteurized milk.

That widespread belief comes under fire once you taste a Colston-Bassett Stilton, Spain’s luscious Nevat, a mountain Gorgonzola or the washed-rind Red Hawk from California’s Cowgirl Creamery—all from pasteurized milk. In the hands of a good cheesemaker, pasteurized milk does just fine.

Pasteurization does destroy some of the microflora in milk—the “bad” bacteria as well as the desirable flavor- and aroma-producing enzymes. Without these enzymes, cheese made from pasteurized milk has less potential for flavor development.

In one study, researchers at France’s Institut National de la Recherche Agronomique made the same cheeses from both raw and pasteurized milk. The raw-milk versions developed flavor sooner and the flavor was richer and more complex. The researchers’ conclusion: Pasteurization alters the biochemistry and microbiology of ripening and thus the texture and flavor of the cheese.

All things being equal, raw milk will produce a more complex cheese than pasteurized milk. Nevertheless, most cheesemakers would agree that fresh, high-quality pasteurized milk is better than low-quality raw milk any day.

Myth 5: Raw-milk cheeses aged more than 60 days are risk-free.

Nothing we eat is risk-free. Cheese can be contaminated at any stage from farm to table: in the milking barn, in the dairy, in the aging room or at the retail counter. The 60-day rule, which dates from 1949, derives from the belief that pathogens can’t survive the low-pH, low-moisture environment of an aged cheese.

In fact, scientists have since shown that some pathogens—strains of Salmonella, Listeria and E. coli—can survive in cheese beyond 60 days. In most of these so-called challenge studies, they have inoculated pasteurized milk (not raw milk) with large quantities of pathogens—far more than you would find in quality milk—then made and aged the cheese. Lo and behold, in some cases, pathogens survived beyond 60 days.

Catherine Donnelly, a food microbiologist at the University of Vermont and an international expert on Listeria, believes that pathogens may behave differently in pasteurized-milk cheese than in raw-milk cheese. Says Donnelly, “Having lots of beneficial bacteria present is a good way to combat pathogenic bacteria. Once you eliminate all those good organisms, there’s nothing for the pathogenic bacteria to compete with.”

What do these challenge studies prove about aged raw-milk cheese, which has a remarkable safety record? In more than 50 years of scientific literature, there are virtually no reports of illness outbreaks from aged raw-milk cheese that can be blamed on the raw milk.

Myth 6: Mandatory pasteurization would make cheese safe for all.

It’s true that pasteurization puts all known milk pathogens out of commission, but it doesn’t prevent milk or cheese from being infected downstream. In fact, it may make it easier, as Donnelly suggests. Not only can pathogens get a foothold when there aren’t any “good” bacteria to outcompete them, but dairies may relax their sanitary procedures when they know they’re working with pasteurized milk. One recent study of European washed-rind cheeses found almost twice as much Listeria in the pasteurized samples than in the raw-milk samples.

“The greatest threat posed to the safety of cheese is due to post-process environmental contamination,” writes Donnelly in a scientific paper. In that light, mandatory HACCP plans in dairies would probably do more to safeguard public health than mandatory pasteurization.
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Janet Fletcher is the author of The Cheese Course and a staff food writer for The San Francisco Chronicle.