From Scientific American. Thanks to Karen Selick for pointing this out.
Editor’s note: The following is an edited excerpt from a chapter in Modernist Cuisine: The Art and Science of Cooking(The Cooking Lab, 2011), a six-volume set consisting of 2,348 pages of text and photography.
Scientific research on foodborne pathogens provides the foundation for all food safety rules. Generally speaking, two kinds of research inform us about issues of food safety. The first is laboratory experimentation: for example, testing how much heat will kill a pathogen or render it harmless. Data from these experiments tell us the fundamental facts about pathogens of interest. The second kind of research is investigation of specific outbreaks of foodborne illness.
You might think that scientific evidence would constitute the “last word” when food safety rules are made, but in fact it’s only the beginning. Policy makers take many other factors into consideration, including tradition, cultural trends, political expediency, and pressure from industry.To some extent, it’s reasonable to apply these modifiers because public health, not scientific purity, is the ultimate goal of food safety regulations. But this approach sometimes imposes arbitrary and scientifically indefensible restrictions that limit food choices, confuse the public, and prevent cooks from preparing the highest-quality meals.
To complicate matters, some guesswork and compromise are inevitable in setting safety standards. Take, for example, the way in which health officials decide how much the pathogen count should be reduced when heating food. In the preceding chapter, we reviewed the terminology used to describe these reductions.Killing 90% of the pathogens within a specific food, for example, is called a 1D reduction (where D stands for “decimal,” or factor of 10). Killing 99 percent of the pathogens is referred to as a 2D reduction, killing 99.99 percent is termed a 4D reduction, and so forth.
Cooks achieve these reductions by maintaining food at a given temperature for a corresponding length of time. The practical impact of an elevated D level is a longer cooking time at a particular temperature. If a 1D reduction requires 18 minutes at 54.4 degrees C / 130 degrees F , then a 5D reduction would take five times as long, or 90 minutes, and a 6.5D reduction would take 6.5 times as long, or 117 minutes. Clearly, the D levels targeted for food can have a profound effect on the manner and quality of cooking.
What D level should regulators choose to ensure food safety? If the food contains no pathogens to begin with, then it’s not necessary to kill pathogens to any D level! Highly contaminated food, on the other hand, might need processing to a very high D level. Right away, you can see that decisions about pathogen-reduction levels are inherently arbitrary because they require guessing the initial level of contamination. That guess can be supported by the results of scientific studies measuring the number of foodborne pathogens present under the various conditions that cooks encounter. But it’s still a guess.
Many people don’t realize that authorities rely on guesswork to develop these standards. Chefs, cookbook authors, and public health officials often make dogmatic statements that food cooked to
a standard is “safe,” but food cooked less than the standard is “unsafe.” That can never be literally true. No matter what the standard is, if the food is highly contaminated, it might still be unsafe (especially owing to cross-contamination). And on the other hand, if the food is not contaminated, then eating it raw won’t hurt you.
All food safety standards deal in probabilities. Reaching a higher standard (i.e., cooking food longer or at a higher temperature) will make the food less likely to be unsafe, and targeting a lower standard will make it a bit more likely. But there are no guarantees and no absolutes….”