Thermometers and Food Safety

Keeping Food Safe, with Calibration

I’m a foodie, and I’ve told foodie friends of mine for years that they should never trust their food thermometer unless it’s been calibrated. While doing my weekly shopping the other day I spotted this item. “Smoky Pecan and Bourbon Seasoned Beef Flat Iron Steak.”

It sounded wonderful for the grill on warm fall evening. I didn’t notice the rest of the label at that point, so I put it in the cart with the rest of the loot, paid for my weekly bounty, and headed home. While unpacking the groceries I noticed the diagram showing how to slice a flat iron, and then I saw it.
After all my years of preaching about thermometer calibration someone finally listened.

“Always use acalibrated meat thermometer.” We finally have convinced everyone to use meat thermometers at home, now we need to convince them that they need to be calibrated.

How much is this going to cost?

If you were to send your kitchen thermometer off to a calibration lab to be calibrated, then they are going to charge you anywhere from \$50 to \$100, or sometimes more. For that price you don’t just receive calibration, you also receive traceability. Traceability can be expensive. I send my platinum resistance thermometers off to be calibrated to a lab that uses special cells that use the melting and boiling points of various materials to establish stable temperatures.

The measurement of temperature is not as simple as it seems. Sure, we can understand that temperature has something to do with molecular motion. The faster the molecules of a substance vibrate the warmer it feels to us when we touch it. The problem with this is that different substances have different amounts of molecular motion at the same temperature. So when we define temperature we have to use a temperature scale to assign a number to how hot something is.

ITS-90

“The international Temperature Scale of 1990” lays out our system for a temperature scale. It started with absolute zero, 0 Kelvin (-273.15°C) and the triple point of water, 273.16 Kelvin (0.01°C), and then a Kelvin or degree Celsius was just the difference between these two measurements divided by 273.16. This seems to work in theory, but in practicality we need more points along the scale to be able to use it. So numerous other reference points like the boiling point of helium, the triple point of argon, triple point of mercury, the freezing point of tin, and the freezing point of aluminum are all included on the ITS-90 scale. These points are used because they all involve phase transitions of a material. During a phase change the substance will remain at the same temperature while the change is taking place, and as long as you have an identical substance the phase change will occur at the same temperature every time.

I Just Wanted a Steak

Getting back to our kitchen thermometer, we can borrow a page from ITS-90 and calibrate our thermometer at home. There are two phase transitions we deal with all of the time in our kitchens and both of them are included in ITS-90. The boiling point of water and the ice-point of water. Bring some water to a rolling boil and measure its temperature using your thermometer. (Be careful, the steam above that water is HOT!!) Your thermometer should read somewhere around 212°F. Now fill a container with either small ice cubes or crushed ice. I have some trays that make gum drop size ice cubes and they work great for this experiment. You can also take regular sized cubes and put them in a bag and break them up a bit will a rolling pin or a meat mallet. Once your container is full of ice add water to about ¾ of the level of the ice. Insert your thermometer and wait for a stable reading, and you should see 32°F. With both of these measurements make sure you’re not touching the sides or the bottom of the container and that the tip of your thermometer is sufficiently submersed in the liquid.

How accurate can this method be?

That depends on how pure your water is. If you are using regular tap water you should be easily within ±0.5°F of the boiling and ice-point of water. If you are willing to use distilled water and correct for atmospheric pressure you can achieve and an accuracy of ±0.005°F.

My Thermometer is Off

So after all this well-meaning effort you discovered your thermometer is reading off by 5°F, what do you do? If it’s your typical bi-metal dial thermometer those typically have a hex nut on the back side of the dial that will let you adjust the temperature up or down as needed. Most of the higher quality digital thermometers today have some form of calibration function built in. I hope you kept the manual from when you purchased your thermometer because the instructions should be in there. If you purchased a cheap digital thermometer, then you might be out of luck if you are reading incorrectly.

I Still Want My Steak

That was a long way to go to make sure your steak is done to your liking, but think about undercooked poultry that can make people sick and you’ll understand why thermometer calibration is so important to the food industry. Ovens, freezers, refrigerators, and all types of thermometers all need to be calibrated in the food industry for safety. I hope I’ve convinced you to take a moment to consider your own food safety.

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