Listeria (Listeria monocytogenes) is a pathogen. If you've heard of it, it was probably in connection with being pregnant and the dangers of certain foods, including soft cheeses, that could cause harm to a developing fetus. Or, you remember a newspaper article about one of our periodic listeriosis outbreaks. Almost always these articles open with the canonical list (probably cut-and-pasted from some CDC Website) of possible culprits (raw milk cheeses, processed meats, soft cheeses, coleslaw). If you persevere for a few paragraphs (or maybe for a few days, until the true contaminated culprit is unmasked), the pathogen-bearing food almost always turns out to be processed meat or coleslaw or sometimes, soft cheeses made from pasteurized milk. Raw milk cheeses are never the culprit. Nonetheless, the FDA has been trying to get raw-milk cheeses banned, with listeria leading their list of worrisome pathogens.
I can understand the FDA's desperation. When they try to go after the real big baddies, they get their hand slapped and their budget cut. The multi-billion dollar supplements industry, big-Pharma, big-Farm--these are all, by-and-large, off-limits. So, raw-milk cheese, lacking friends in high places, has acquired enemies at the FDA.
But raw-milk cheese has friends in low places. The American Cheese Society, Slow Food and others have mounted a campaign to save raw-milk cheeses, and I'm on their side. Raw-milk cheese tastes better, its production is a craft, a piece of cultural history worth saving, and science is actually on our side. The story has two parts: the pathogen and pasteurization.
Listeria is ubiquitous, present in soil, in many cattle and, surprisingly, in the gut of many healthy humans. It causes disease in sheep, cattle and goats. It also causes disease in humans, with particularly bad consequences in pregnant women and the immuno-compromised. Given its ubiquity as an organism, it seems that an infectious dose, for a healthy adult, must be large. Coleslaw is often a culprit because cabbage is grown in listeria-laden soil and coleslaw is a hospitable environment for culturing listeria.
Which brings up one of listeria's nastier properties: While many pathogens don't grow (i.e., reproduce) at low temperatures, listeria is perfectly happy to grow, albeit slowly, at refrigerator temperatures. This can turn a non-infectious contamination into an infectious one, given enough time. This unhappy feature of listeria is abetted by another: salt-tolerance. It's happy at a 2 percent salt concentration, but will grow at 10 percent and survive at higher. Ten percent saline is saltier than you would want to eat. This accounts for its role in lunch-meat outbreaks. Packaged processed meats will, from a very small inoculant, grow out lots of listeria while sitting on supermarket shelves. The growth is slow, which explains why short shelf-life products (fresh milk and raw meat) aren't listeria-risky.
Listeria does like an alkaline environment and is unhappy when things get acidic; pH levels below 4.8 make it very unhappy. Thus a traditional, though subtle method of control is competition. Lacto-bacilli, present in sourdough breads and many (naturally-produced) cheeses make for a hostile environment for listeria.
But, you say, why not just use the enemy of all pathogens and kill listeria with heat and we'll all be safe? As I've implied above, the big, unsubtle reason why is that the vast majority of listeria outbreaks involve post-pasteurization contamination. And this point is about more than just this one pathogen. Pasteurization knocks back a whole spectrum of entities: the bad, the good, the tasty, the healthful. And, it only knocks them back; you get logarithmic reductions, but if you start with hugely contaminated material, some beasts will survive.
And if a few bad beasts survive, and the good beasts (lacto-bacilli, for example) are knocked out, and you're dealing with long shelflife lunch-meats, then you get an outbreak. Or, if some time after pasteurization, in the large, complicated, messy factory some pathogen, happily growing in some moist corner, inoculates the product, then you get an outbreak.
Moreover, as with many hyper-hygienic ploys, pasteurization encourages sloppiness on the pre-pasteurization side. (Same with irradiation.) Why be scrupulous when downstream technology will correct your errors.
Indeed, pasteurization makes the modern industrial dairy industry possible, just as the modern industrial dairy industry makes pasteurization necessary. When you handle milk on a huge scale, trucking it over large distance, mixing the product of a multitude of far-away factory farms of uncertain hygiene, you'd be nuts not to pasteurize. In fact, pasteurization is a rather modest description of what happens to milk in a typical dairy factory.
Nonetheless, you might have thought, given the fuss that the FDA is making over raw-milk cheeses, and the listeria danger from raw-milk products every pregnant woman is warned about, that raw milk has been a major public health menace.
I've already indicated that it isn't. I haven't been able to find any cases of professionally produced raw-milk cheeses, in the United States or Western Europe, causing a listeria outbreak. The outbreaks in Europe always involve factory (pasteurized) cheeses. In the United States, we've had just a few outbreaks from fresh Mexican cheese.
So, why is raw-milk cheese safe? Historically, a typical raw-milk cheese would have been made something like this: Take the still warm evening milk, add some reserved whey and let clabber. In the morning, add the morning milk, let the curds form, drain the whey, and proceed with the rest of the cheese-making process. The protective elements here are: healthy cows, naturally occurring lacto-bacilli, acidic whey. (The mandated, in the U.S., 60-day aging period is also protective.) Modern versions of this process chill the milk, combine it, heat to culturing temperatures, add culture (good bacteria), etc.
How did pasteurization enter the picture? Milk didn't use to be consumed in great quantities as a beverage. Milk was consumed as clabbered milk, as (cultured) butter, and as cheese. Cheese was how you safely preserved milk and made an adult food. It took urbanization, the resultant decrease in breast-feeding, and the promotion of milk as a beverage for urban infants and children to make pasteurization a necessity. What grew up to serve this manufactured demand were urban, inner-city dairies, crowded into the middle of 19th century cities; the cows were fed the detritus of urban breweries and disease (listeria being the least of it) was rampant. The mortality rate for urban children was stunning. The invention of refrigeration helped stretch the supply-line into the cleaner countryside, as did the invention of "certified" (i.e., inspected) milk.
But these measures didn't suffice during the rapid industrialization of production, and decades after Pasteur promoted pasteurization for wine, milk pasteurization was instituted and mandated to solve the new problem of urban consumption of mass-produced liquid milk.
None of which had anything to do with cheese. What works for cheese is what always has worked: Good silage, a clean farm, careful cheese-making. And this is nicely compatible with the current gold standard of food safety-HACCP procedures. HACCP (Hazard Analysis and Critical Control Point) is what governments are now mandating in many arenas. Essentially it identifies critical points for contamination in food processing and institutes appropriate controls and documentation at those points. The European Union uses HACCP to certify raw-milk production as perfectly compatible with public safety. And it works.
The cheese industry, Big Cheese, is all in favor of pasteurization because industrial production demands it. This way they can pass the buck on safety questions and eliminate invidious taste comparisons with artisanal cheeses.
So saving raw-milk cheese needs to be a consumer-driven fight.
Remember: Cheeses loves you.