To compensate for this inherent uncertainty, food safety officials often base their policies on the so-called worst-case scenario. They reason that if you assume the absolute worst contamination levels and act to address that threat, then the public will always be safe. Setting relatively high D levels to account for a worst-case scenario establishes such a formidable barrier for pathogens that even highly contaminated food will be rendered safe. High D levels also offer a measure of insurance against an imperfect thermometer, an unevenly heated oven, an inaccurate timer, or an impatient chef. If real-world conditions miss the mark, slightly lower reductions will still suffice.
Not surprisingly, some food safety experts challenge this conservative approach. The required pathogen reductions or “drops” explicitly cited in U.S. federal regulations, for example, range from
a 4D drop for some extended-shelf-life refrigerated foods, such as cooked, uncured meat and poultry products, to a 12D drop for canned food, which must last for years on the shelf. General FDA cooking recommendations for fresh food are set to reach a reduction level of 6.5D, which corresponds to killing 99.99997 percent of the pathogens present. Many nongovernmental food safety experts believe this level is too conservative and instead consider 5D to 6D pathogen reduction for fresh foods sufficient for real-world scenarios.
An expert advisory panel charged with reviewing the scientific basis of food safety regulations in the United States made just this point about standards developed by the U.S. Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS). In a 2003 report, the panel, assembled by the U.S. Institute of Medicine and National Research Council, questioned the FSIS Salmonella reduction standards for ready-to-eat poultry and beef products. In devising its standards, the FSIS had established a worst-case Salmonella population for the precooked meat of each animal species, then calculated the probability that the pathogen would survive in 100 grams / 3.5 ounces of the final ready-to-eat product.
In the case of poultry, for example, the FSIS calculated a worst-case scenario of 37,500 Salmonella bacteria per gram of raw meat. For the 143 grams / 5 ounces of starting product necessary to yield 100 grams / 3.5 ounces of the final, ready-to-eat product, that works out to nearly 5.4 million Salmonella bacteria before cooking. To protect consumers adequately, the FSIS recommended a 7D drop in bacterial levels, equivalent to a reduction from 10 million pathogens to one.
The review committee, however, found fault with several FSIS estimates that, it said, resulted in an “excessively conservative performance standard.”Even “using the highly improbable FSIS worst-case figure,” the committee concluded that the ready-to-eat regulation should instead require only a 4.5D reduction.
The irony is that, although experts debate these matters, their rigorous analyses can be undermined by confounding factors such as cross-contamination. Imagine, for example, that a highly contaminated bunch of spinach really does require a 6.5D reduction in pathogens to be safe. Even if that spinach is properly cooked, it could have contaminated other food or utensils in the kitchen while it was still raw, rendering moot even an extreme 12D reduction during the cooking process. A chain is only as strong as the weakest link, and in food safety, cross-contamination is often the weakest link. One powerful criticism of food safety standards is that they protect against unlikely worst-case scenarios yet do not address the more likely event of cross-contamination.
Another conservative tactic used by health officials is to artificially raise the low end of
a recommended temperature range. Most food pathogens can be killed at temperatures above 50 degrees C / 120 degrees F , yet food safety rules tend to require temperatures much higher than that. Experts may worry that relying on the low end of the range may be dangerous for the same reasons that moderate D levels cannot be trusted: vacillating oven temperatures, varying chef temperaments, and so on. Still, their solution belies the facts.