by Jeff Potter
The danger zone rule suggests that bacteria multiply right up until 140°F / 60°C, whereas the real multiplication rates of foodborne illness-related bacteria follow a curve with an ideal breeding range in the middle. Of the foodborne pathogens listed in the FDA’s "Bad Bug Book," Bacillus cereus has the highest survival temperature, at 131°F / 55°C.
"But wait," you might be saying. "What about all that food in the pantry? Why doesn’t it go ‘bad’?" There are other factors that bacteria need in order to multiply. The acronym FAT TOM is commonly used to describe the six factors necessary for multiplication:
F = Food
Bacteria need proteins and carbohydrates to multiply. No food, no multiplication.
A = Acidity
Bacteria can only survive in certain pH ranges. Too acidic or basic, and the proteins in the bacteria denature.
T = Temperature
Too cold, and the bacteria effectively sleep. Too hot, and they die.
T = Time
Bacteria have to have enough time to multiply to a sufficient quantity to overwhelm our bodies.
O = Oxygen
As with pH levels, bacteria will reproduce only if sufficient oxygen is present, or, for anaerobic bacteria (e.g., Clostridium botulinum), if no oxygen is present. Keep in mind that vacuum-packed bags are not necessarily devoid of oxygen.
M = Moisture
Bacteria need water to reproduce. Food scientists use a scale called water activity, which is a measure of the freely available water in a material (from 0 to 1). Bacteria need a water activity value of 0.85 or greater to multiply.
The reason so many pantry goods are "shelf stable" is either low moisture content (crackers, dry goods like beans and grains, oils, even jams and jellies in which the sugar is hygroscopic and "holds on to" the water) or acidity (pickled items, vinegars). Given these six variables, you can see why some foods don’t need refrigeration. When in doubt, though, stick it in the fridge, which you should keep on the chilly side (34–36°F / 1–2°C).
Note
Keep basic physics in mind. Placing a large pot of hot soup into the fridge will warm up all the contents of the fridge until the evaporator has a chance to transfer the heat back out. Refrigerators are made to keep things cool, not to chill things, so when you’re storing a large quantity of hot food, place it in an ice bath first to chill it and then transfer it to the fridge once it’s cooled down.
There’s one variable that deserves special mention, because of the potentially fatal consequences, and that’s the oxygen level needed for bacterial growth. Specifically, anaerobic environments—ones without oxygen—are necessary for some types of bacteria to multiply. Oil creates an anaerobic environment, but by itself doesn’t provide any moisture for bacteria to grow. But with the addition of something like a raw clove of garlic, an anaerobic environment is created, and the garlic provides the food and moisture necessary for anaerobic bacteria to thrive when given sufficient time at the right temperatures.
Note
Oil, even though a liquid, is technically dry because there is no water present.
A garlic clove stored in oil can become the perfect breeding ground for botulism, the illness caused by Clostridium botulinum. And since C. botulinum doesn’t produce any noticeable odor, there’s little to tip you off that the food is teeming with bacteria and their toxins. The toxins generated by C. botulinum are much more heat stable than the bacteria themselves and might remain active through the cooking process. And now to scare you senseless, the botulinum toxin is the most acutely toxic substance known: a dose as small as ~250 nanograms—1/120,000th the weight of a grain of rice—will do you in.
If you’re going to make something like duck confit or homemade jam the traditional way—cooking it and then sealing it for storage at room temperature—make darn sure you get the food hot enough to sterilize it and that you avoid recontaminating it after it’s cooked and before it’s sealed. To be safe, stick it in the fridge and treat it as perishable. Note that sterilization means completely eradicating any bacteria, as opposed to pasteurization, which reduces bacteria to "safe" levels for near-term consumption. Sterilized milk can be left at room temperature indefinitely; pasteurized milk cannot.
There’s one last aspect I should mention in this brief primer on food safety, and that’s the risk/consequences equation. While following food safety rules reduces the risk of illness, it doesn’t completely eliminate risk. For most of us, the consequence of contracting most foodborne illnesses is gastrointestinal distress—diarrhea, vomiting, muscle spasms, and the like. However, for those who are in an at-risk group—anyone for whom having foodborne illness can lead to further complications—the consequences of a bout of foodborne illness can be much, much greater, so the acceptable risks are accordingly much lower. If you are cooking for someone who is elderly, extremely young, pregnant, or immunocompromised, be extra vigilant with regard to food safety issues and skip dishes that have higher risks (e.g., raw egg in Caesar salads, unpasteurized cheeses, suspect meats that might be past their expiration dates).
For additional information on food safety and bacteria, check out the FDA’s "Bad Bug Book," available online at http://www.fda.gov/Food/FoodSafety/FoodborneIllness/. Also, Texas A&M University’s Department of Horticultural Sciences maintains a good overview page at http://aggie-horticulture.tamu.edu/extension/poison.html.
While bacteria are the most common and easiest-to-manage cause of foodborne illnesses, they’re not alone: viruses, molds, toxins, and contaminants are also of concern. Toxins and contaminants are primarily issues for producers of foods, so as a consumer you’re (mostly) off the hook for those. (If you grow your own veggies, test your soil for contaminants.)
Besides proper cooking, the best way to combat bacteria, mold, and viruses is good washing and avoiding cross-contamination:
Wash your hands! And don’t double-dip with the same spoon when tasting a dish.
Nuke your sponges (rinse them and give them two minutes on high) or run them through the dishwasher weekly. Better yet, use towels and allow them to dry completely between uses.
When working with raw meats, be careful not to use the same towel for wiping your hands that you then use to wipe with post-cooking.
Wash can lids before opening them. Wash the can opener, too. The blade picks up food while cutting through the lid.
How to Prevent Foodborne Illness Caused by Bacteria
Salmonella: it’s the poster child of foodborne illness. And for good reason. Salmonella accounts for a full 30% of the roughly 1,800 deaths due to foodborne illnesses per year in the U.S.—more than any other cause in this category. Salmonella’s ideal breeding temperature? Around 100°F / 38°C—close to body temperature. Clearly it likes us. And according to some reports, the most likely food that’ll harbor the bacteria in our modern food supply isn’t chicken or meats, but produce. Wash your veggies!
The odds of dying from foodborne illness are actually surprisingly low, especially considering the media attention given to it. But the media attention isn’t unmerited: 1 in 8 of us will contract illnesses from foods in any given year, and about 1% of those cases will require hospitalization, according to the U.S. Centers for Disease Control and Prevention (CDC).
Contracting a foodborne illness is a game of probabilities: a single bacterium of salmonella isn’t likely to cause a problem, but given a few dozen cells, the odds change. E. coli is similar: only a few bacteria are necessary for the possibility of infection. A few strains are decidedly nasty, O157:H7 being the most talked about.
It’s not always "just a handful" of cells, though. Contracting listeriosis requires ingesting somewhere around 1,000 organisms of Listeria monocytogenes, which tends to be present in animal products and multiplies at temperatures as low as 34°F / 1°C. Luckily, listeriosis isn’t an issue for many of us, but it can cause complications for at-risk groups—especially pregnant women, where the baby is at risk. This is why pregnant women are told to avoid foods such as soft and
surface-ripened cheeses, deli salads, raw milk, hot dogs, and shrimp; to ensure that chicken is thoroughly cooked; and to be careful with previously cooked ready-to-eat foods.
Bacteria can be grouped into three broad categories, based on the temperature at which they are most active. There are bacteria that remain active in food above 122°F / 50°C, but these are only beneficial (e.g., Bacillus coagulans) or spoilage bacteria, and not related to foodborne illness. From a taste perspective, we’re extremely lucky that no thermophilic bacteria cause foodborne illness; otherwise, we’d have to cook foods to higher temperatures to kill them.
GRAPH BASED ON E. ANDERSEN, M. JUL, AND H. RIEMANN (1965), "INDUSTRIEL LEVNEDSMIDDEL-KONSERVERING," COL. 2, KULDEKONSERVERING, COPENHAGEN: TEKNISK FORLAG)
Salmonella gets most of the limelight in the media for a couple of reasons, though: it’s hardy—that is, able to survive in the environment for longer periods of time and at temperatures above what most other common-to-food bacteria can tolerate—and it’s surprisingly prevalent, affecting 1.4 million Americans a year on average.
Caliciviruses—a family of viruses, norovirus being the best known—are also getting more attention these days, and deservedly so; these are typically spread by a sick individual preparing food for others. If you’ve spent a night "praying to the porcelain god"—diarrhea, vomiting, chills, headache—you can probably thank salmonella or norovirus for the experience.
Note
If you’re sick, don’t cook for others. If you’re around someone sick, wash your hands. Often.
Now, pay attention, because this is important. Salmonella is killed at 136°F / 58°C only when held for a sufficient length of time. Seeing your thermometer register an even hotter temperature—say, 140°F / 60°C—does not guarantee that the food will be free of salmonella. Think of it like being in a hot desert: you can survive 136°F / 58°C heat for a while, but if you’re exposed to it for too long, eventually you will die. The same is true for bacteria like salmonella: given a short amount of time at a particular temperature, the bacteria might survive, but given a longer exposure, they will eventually die.
Salmonella actually lives in a temperature range of 35–117°F / 2–47°C according to the FDA’s "Bad Bug Book." The 136°F / 58°C temperature is based on what the FDA Food Code gives as the lower bound for pasteurization.
Back to the desert analogy. Let’s say an average human can survive for four hours in 136°F / 58°C heat. Given 100 people in a desert, though, this doesn’t mean all 100 people will be alive at 3 hours, 59 minutes and all suddenly drop dead one minute later. The same is true for bacteria that might be hitching a ride on that chicken you’re about to cook: the proteins in the bacteria don’t all spontaneously denature at a specific temperature. It’s a probability thing: as the temperature goes up, the probability of the molecular structure of each kind of protein denaturing increases. There’s not an exact temperature at which this occurs, like there is when a solid melts into a liquid.
Note
When talking about reducing the number of bacteria in food, scientists use the term log10 reductions. A single log10 reduction is simply the reduction of the number of bacteria present by a factor of 10; a 7 log10 reduction is a 10,000,000-fold reduction. The USDA’s Food Safety and Inspection Service (FSIS) division is responsible for providing guidelines relating to the number of log reductions necessary to achieve an acceptable quantity of bacteria. Given that different kinds of meats have different properties—different amounts of fats, water, etc.—the number of log reductions necessary to reduce the bacterial count from a potential starting amount to an acceptable number differs. Hold time for sufficient pasteurization is also affected by variables such as how smooth the surface of the food is and its chemical composition (e.g., nitrite levels).
One important caveat about pasteurization: sometimes it’s not the bacteria themselves that are the issue, but the toxins they produce. While appropriate cooking might safely reduce the bacterial count, the toxins themselves, such as those produced by B. cereus, can be heat-stable. Refrigeration of meats is therefore critical to prevent the multiplication of bacteria in the meat tissue. Remember the simple food safety rule mentioned earlier: avoid holding foods at temperatures between 40°F / 4°C and 140°F / 60°C for more than two hours. This includes the amount of time it takes to bring the food from fridge temperature to a safe hot temperature! While it is true that the 40-to-140°F / 4-to-60°C rule for two hours is a vast simplification of the real multiplication rates of bacteria, it’s a simple rule accepted by the food industry, and rarely is there any need to skirt it.
Note
Who said scientists don’t have a sense of humor? Try saying B. cereus out loud.
At 140°F / 60°C, a hold time of 35 minutes is necessary for chicken with 12% fat to achieve a 7-log10 reduction. The time drops in leaner chickens; chicken meat with 1% fat requires 25.2 minutes at 140°F / 60°C. Longer hold times are okay; these times are minimum times. Chicken meats can be infected with salmonella throughout the tissue. While sick birds are supposed to be culled, it’s still possible for them to go unnoticed. (Data from http://www.fsis.usda.gov/OPPDE/rdad/FSISNotices/RTE_Poultry_Tables.pdf.)
Minimum amount of time in minutes required to cook chicken safely (assuming 7-log10 reduction in chicken with 12% fat).
"Why then," I bet you’re thinking, "do ‘they’ say to cook chicken to a temperature of 165°F / 74°C?" "They" happen to be the fine folks at the CDC, and what they say specifically is:
All poultry should be cooked to reach a minimum internal temperature of 165°F [74°C].
--http://www.cdc.gov/nczved/divisions/dfbmd/diseases/campylobacter/
Why 165°F / 74°C? One reason is that this is the temperature at which salmonella dies a quick death. From a "keeping it simple" perspective, seeing 165°F / 74°C on the thermometer is an easy guideline. Even if your thermometer is miscalibrated or you misprobe the meat and it’s only reached a temperature of 155°F / 68°C, the pasteurization time for chicken at this temperature is less than a minute, which you’re likely to exceed. The 165°F / 74°C guideline effectively removes the variable of time, making it an easier to follow (harder to screw up) rule.
Since none of the bacteria related to foodborne illness can survive, let alone reproduce, at moderate temperatures, holding food above 140°F / 60°C indefinitely is safe. This is why the soup at your local lunch counter can be kept hot all day long in a heat-controlled container and why hot buffets use steam baths to keep the foods warm. While you might be perplexed by the idea of storing foods hot, from a bacterial control perspective, it’s actually safer than storing them in the fridge: bacteria are unable to survive in the hot environment, while storing them in the fridge generally only slows their reproduction.
Note
The serving spoons, by the way, are supposed to stay in the food, so that they too stay above 140°F / 60°C. Otherwise, that mashed potato clinging to the serving spoon at room temperature will be a potential hangout spot for bacteria.
In the U.S., the FSIS and the FDA run testing programs to monitor the food supply. Both agencies have the ability to hold foods at processing plants, to request voluntary recalls, and to outright seize product through court order if it comes to that. Still, there’s a lot of food going through the system, and lapses in protocol happen (probably more than we want to know about). A lot of work is done in identifying hazard points in the food system (HACCP—Hazard Analysis & Critical Control Points), but still, errors happen. What’s a nervous food geek to do?
The most common vector for foodborne illness is surface contamination, either from contaminated water sprayed on vegetables during farming or from fecal contamination in meats during slaughter and processing. How does this affect you when cooking? Since it’s the surface of most products that becomes contaminated, it’s the surface that needs to be pasteurized. Pan searing a steak heats the outer portion well beyond any temperature that bacteria can survive. Likewise, steaming vegetables thorough
ly heats their surface.
Note
When cooking vegetables in the microwave, use a container with the lid mostly closed and with a small amount of water inside: the microwave will boil the water, and the container will keep the steam in contact with the vegetables.
What about hamburgers? Well, they’re all outside, in the sense that surface contamination will have been ground throughout the meat. Industry calls things like steak whole-muscle intact meat, as opposed to ground meat. When looking at consumer cooking guidelines, the temperatures given are lower for whole-muscle intact than ground meats, presumably because the outside of a whole muscle cut will be well beyond pasteurized by the time the middle comes to temperature.
Simple Cheeseburger
In a clean bowl, work together using your fingers:
1 pound (500g) ground beef or hamburger
1 teaspoon (6g) Worcestershire sauce (optional)
