Obsessive Habits to Protect Yourself from Food-Borne Bacteria — Part 2 — Killing Food-Borne Bacteria with Heat
By Kelly Dean
Habits. Habits. Habits.
Doing battle with the little bacteria critters is simply a matter of practice. Being aware and getting into good habits keeps food-borne bacteria at bay. This includes the good sanitation habits I wrote about here. It also includes knowing when and how much heat is required to kill them all through the cooking process. I often call them “bugs” but I’m not talking about the crawling kind of bugs, I’m talking about germs.
A few sanitary cooking tips:
When cooking, take similar precautions: Your meat in a pan or dish will be at varying stages of raw until it cooks thoroughly. Make sure the utensils you’re using for turning and stirring get a good heating or washing as the meal progresses, to keep that utensil from contaminating the very food that you’re cooking.
A pot of boiling water can be a handy friend
At sea level, water boils in a pot at about 212 degrees Fahrenheit. For sanitizing smooth utensils and cutting surfaces, that temperature is more than enough for killing food-borne bacteria on contact up to 99.999997%, according to the USDA’s Food Safety and inspection Service.
Pouring a pot of boiling water into standing hot dishwater kicks the temperature up high enough to get closer to that magic 158 degrees needed to kill bad bacteria, in many cases. But after soaking a spell, cool the water before you stick your hand in to avoid scalding. More on this later.
Blanching salad veggie’s surface bugs
Blanching is a technique used for home canning that involves submerging veggies in boiling water for a short time. It serves a number of stabilization purposes, but another is killing surface microorganisms. It is not meant to kill deep pathogens. That’s what cooking does.
Briefly, it requires dipping cool, raw vegetables in boiling water for a specified amount of time; the timing starts when water returns to boiling. This is generally 2, 3 or 4 minutes for small, medium and semi-large veggies, accordingly. Larger or chunkier veggies, blanch slightly more (7 or 8 minutes or so). Then it’s immediate cooled in cold water.
So how does that apply to non-canning?
For surface germs, a 5 to 10-second dip-blanching in light boiling water with a strainer is a quasi-blanching technique. I sometimes use this on certain raw veggies and herbs meant for veggie dips shakes. These will ultimately be chopped in a processor, like salsa, pico de gallo, spinach, artichoke, peppers and other veggie or fruit dips.
These should not be considered deep-cooked (per internal temperature); however, most surface bacteria are likely eradicated in 5-seconds at 212 degrees Fahrenheit. Unlike blanching, these veggies are not always cooled, but allowed to naturally cool down during prep. The veggies remain crispy enough if not over-dipped. The dip is then refrigerated.
Depending upon how raw vegetables are used in a meal, I’ve boil-dipped tomatoes, potatoes, cucumbers, bell peppers, chilies, fresh spices, and other garden products for mostly sanitation reasons.
I have used this technique with crudités as well when meant as snacks for my undiscriminating children — sometimes guests.
Wilted lettuce and spinach salad blanched or quick-fried in bacon grease also provides some germ-killing heat and an interesting texture twist to a salad served warm.
“Microwave blanching” salad veggies
Microwaves work by stimulating water molecules in food, producing heat. This is not irradiating. It is non-ionizing. It is a fast heat-maker than happens to heat internal water to cook. Microwaves could be thought of as reverse blanching.
I will often microwave 3 or 4 large romaine or iceberg lettuce leaves for 30-seconds (and larger salad veggies slightly longer) immediately before I use them in a salad or wrap. This is not enough to cook or even wilt them, per se, but enough to where they are too hot to hold in my hand — yet still perky.
Here’s an interesting side-bar: As a possible impromptu gauge, human hands can hold things up to 140 degrees Fahrenheit for up to 5-seconds, according to these industrial experts. Also, 140 degrees is high enough to kill bacteria if that sustained temperature is 12 minutes or more, according to data at this USDA site.
So if, say, a microwaved potato is still too hot to hold in your hand after it has cooled down for 12 minutes, it’s potentially free of food borne bacteria, theoretically. As far as a useful gauge in sanitation, your author cannot attest to this with certainty. But the data suggests it could perhaps be a rough, folksy cooking test for checking “doneness,” like testing the temperature of baby’s milk by dripping it on your inner wrist.
Furthermore, if you can hold a hot food article quite easily, it’s likely not hot enough to be killing bacteria effectively or quickly enough. A food thermometer will always be best, of course.
I also mention wilting as alluded to earlier as a flashback to my mother, whose teeth were sensitive to cold salads. She favored “wilted lettuce leaf and greens” and room temperature salads with vinaigrette dressing.
Cooking temperature and time
Cooking is still the best way to kill food borne bacteria. But how much cooking is enough to do the job? Should I nearly burn everything?
There is an outstanding article about proper cooking temperatures which is written by a cook who is also a doctor, surgeon and researcher. His name is Terry Simpson, MD and the article can be found here. Avoiding bacteria is more about careful preparation than high cooking temperature. Temperature is important, but not in the way you’d think.
Temperature and rest time
The USDA’s inspection goal is designed to kill 99.9997% of bacteria or more for most cooking, or 6.5 log10 on the chart in the linked USDA articles above and below. Another level, 7.0 log10 requires a few more minutes to kill even more bacteria, but generally more than required, per Dr. Simpson. This is internal temperature, of course.
So purchase a good cooking thermometer for checking internal temperatures.
An easy to remember benchmark number is 130 degrees Fahrenheit. That’s enough internal temperature to halt reproduction of the bacteria that harm people. They will die. Unfortunately, 130 degrees requires holding that temperature (rest time) for 112 minutes to do that 99.9997% job – fine for slow, low temp crock pot cooking while at work. Here is the USDA’s original guidelines which consider holding temperature, or rest time to kill 99.9997% of food-borne bacteria at progressively ascending temperatures and rest periods. This is my preferred chart.
But with consumers recently, the USDA has become more cautious in their published recommendations for fear of promoting under-cooking. Instead, they favor simplicity over taste by simplifying and virtually eliminating the rest time factor. These updated recommendation numbers suggest we should simply target 165 degrees Fahrenheit, leave it at that and largely forget about rest time.
Holding temperature or rest time is a key consideration in home cooking because it’s more realistic. Cooks might or might not instantly serve a roast that’s internal temperature is suddenly 160-165 degrees, which is what would be needed to hit the magic kill number and serve instantly. The exterior temperature is likely to be much higher if the dish was cooked at an overall ambient 350 degrees or more. There’s going to be a cooling down and still bug-killing rest time, regardless.
Conversely, cooks don’t crank the heat just to get the internal temperature high enough to kill pathogens immediately as their primary task. Cooking to that immediate-kill internal heat temp too quickly will adversely affect the quality of that dish in most cases. It’s called burning, or shoe-leather, or simply over-cooking in polite conversation. Lower heat and longer cooking times can make the roast better than just cranking the heat while still killing the germs.
It’s a balance. That’s cooking.
Again, rest or holding time is expressed as holding a given temperature long enough to get the job done on a variety of roasted meats. 150 degrees internal temperature needs just 67 seconds rest time to be lethal to bacteria. 158 degrees internal temperature requires no additional time at all (no rest time); therefore, heat kills to 99.9997% right away at that temperature. 160 degrees is easier to remember, however. Anything higher, of course, will kill bacteria even more thoroughly.
Note: there are higher-temp-tolerant bacteria you might read about in the media, but they are not a threat to humans, according to Dr. Simpson.
In our kitchen
Regardless, most cooking, of course, uses much higher heat from the outset anyway — getting heat to the interior quite reasonably. Water lightly boils at 212 degrees Fahrenheit on a stove at sea level. And the lowest setting on most home ovens is 170 degrees Fahrenheit. So given the proper cooking time, you have more than enough heat at your fingertips to kill the nastiest bugs.
Even your Keurig coffee/tea maker heats water to 165 degrees in seconds. That’s sufficient to kill surface bacteria. Use it as a quick splash when needed.
Note: Remember, if you add cold veggies to a nearly done roast, that will temporarily lower ambient temperature and therefore, internal meat temperature as well. So keep an eye on that thermometer if you’re not experienced.
The water heater
Your water heater would have to produce 158 degrees Fahrenheit or hotter at the kitchen tap to kill most bad bacteria on immediate contact, like for dishwashing. This would be hotter than most home water heaters are set (140 degrees) and potentially scalding in your bathtub as well.
Again, 140 degrees needs held for 12 minutes to kill 99.9997% of bacteria. So the water heater itself is okay, do to its consistent hold time. And the dishwasher is likely good, due to its hold time, if it re-heats its own water.
I use the hot water kitchen tap almost exclusively — for everything. I know that water has recently been held at a hot enough temperature to kill bacteria for a protracted time. The cold tap has not.
But the hottest sink water in your home at 140 degrees would need to soak items 12 or more minutes – perhaps up to 20 minutes (depending on cool-down time) to do the job.
So when hand washing, the soap is doing more of the work than the water temperature. For that reason, I recommend pouring in a pan of boiling water in your dishwater to end the debate, as touched upon earlier.
Finally, all of this is meant to limit using harsh chemicals as much as possible. Heat is a more natural way to address pathogens. So it’s perhaps the best way, so long as it does the job well.
More information about sanitation techniques can be found here in part 1 of this article.