Thermometers are one of the most important kitchen tools, and perhaps they are the most overlooked by new cooks. Thermometers are used in many kitchen applications, but the two most important are to measure: the temperature inside equipment (ovens, freezers, walk-ins, pizza ovens, display cases, etc.); and the internal food temperature to see if it is fully cooked.

Let’s start with measuring the temperature inside equipment. Most commercial kitchen equipment has a means of measuring the inside temperature from the outside. Refrigerators, walk-ins, freezers, etc. usually have digital or needle displays. Newer ovens frequently display the temperature or have a light that goes off (or sometimes on) on the control panel when the proper temperature is reached. For older equipment, or residential style equipment, there are two different approaches. The first is to place a special purpose thermometer inside the unit. These are mounted in the warmest place (for freezers and refrigerators) and the coldest place for ovens. Generally, this is usually by the door.

The first major problem with these thermometers is that in order to read them you need to open the door. However, opening the door of an oven, refrigerator or freezer changes the temperature quite rapidly. Thus, they tend to be mounted in the front so they can be read quickly to minimize the time the door is open. The second major problem with these thermometers is they have a relatively short life in a commercial kitchen. They tend to get knocked onto the floor since they are mounted by the front of the unit.

Next, let’s look at thermometers which are used to measure whether food is cooked. There are three basic types: instant read thermometers, probe thermometers, and infrared sensors.

The most commonly used in commercial kitchens are instant read. The term “instant” is a bit misleading. They often take 15 to 30 seconds to get a reading. There are two types: digital and the traditional needle on the dial. Both units are relatively inexpensive. The digital thermometers are usually easier to read, can switch with the push of a button from Centigrade to Fahrenheit, and many don’t need to be calibrated. The needle on the dial style tends to be a tad cheaper and never needs battery replacement.

Most new cooks make several common mistakes when using either type of instant read thermometer.

• Most people don’t wait long enough. Basically, they use the thermometer until it stops going up.
• Most students do not put the thermometer in far enough. For all needle types, and for most digitals, there is a dimple on the stem. In order to get an accurate reading, the stem, from the point to the dimple, must be inserted in the product. If the product is something like a steak or piece of chicken that is not thick, you have to hold the product with tongs and put the thermometer in from the side.
• If the product has a bone, the thermometer cannot touch the bone. The bone will not be the same temperature as the flesh. Touching the bone will give a false reading.

As mentioned, most digital thermometers do not need to be calibrated (check the package) but needle thermometers need to be calibrated after they are dropped (which seems to happen frequently) and at least every couple of days.

To calibrate an instant read needle thermometer, make an ice bath in a cup or kitchen container using about three-fourths ice and one-fourth cold water. Wait a minute and put the thermometer through the opening in the side of its holder (see the instructions that came with the thermometer) and put the stem from point to the dimple into the ice bath. Make sure the thermometer does not touch the side. It should read 32 degrees Fahrenheit or 0 degrees Celsius. If it doesn’t, then use the holder as a wrench (see the instructions that came with the thermometer) and adjust the nut below the dial until it reads 32 degrees.

Speaking of the holder, one of the most common types of cross-contamination is from sticking a thermometer back into its holder without cleaning and sanitizing it, and then using it to check another food.

For example, let’s say your student checks the temperature of a roasting chicken to find out it is only 135 degrees. She or he then places the thermometer back into the holder without sanitizing it and goes to check a pork roast. Your student is then putting undercooked chicken pathogens into a pork roast, which will never be cooked to a high enough temperature to kill them. An instant read thermometer should be washed--either by hand in a wash sink-- or in a properly mixed cleaning bucket on the work station. The thermometer then needs to be sanitized by placing the probe in a sanitizer solution or using a thermometer wipe. 

Never submerge either type of thermometer. Never put them into the dishwasher. In a perfect kitchen world, the thermometer would also be sanitized when it is removed from the holder before it is used. If a thermometer has been placed into its holder after use without first being cleaned and sanitized the holder must be discarded since there is no way to wash it out. This applies even if the product was fully cooked. You wouldn’t want chicken juices from the thermometer (even if the chicken was fully cooked) to sit in the case forever. The chicken juices would basically be in the danger zone forever.

Probe thermometers are somewhat popular at home but, except in expensive commercial ovens, usually aren’t found in a commercial kitchen. You stick the probe into the food and connect the other end to a display or to a computer-based oven that will tell you when the food is at the correct temperature. Thus, you don’t have to open the oven as often—so it cooks the food more consistently and faster. In programming a probe thermometer remember that carry-over heat will cause the food to continue cooking outside of the oven, so set the alarm on the probe thermometer lower than the final desired temperature.

Finally, infrared thermometers are great for chefs and senior cooks, and the health department, to check the temperatures of fryers, coolers, walk-in’s, freezers, food being held on the line, food on display, food in salad bars, etc. However, your students need to understand that these thermometers only measure the surface temperature of the food (or the equipment.) For example, if you use an infrared thermometer on a whole turkey it will only tell you the temperature of the skin and not what the internal temperature is. Thus, you will have to use an instant read or probe thermometer to know if the food is cooked to a safe temperature. You can’t use an infrared thermometer to see if the food is cooked to a safe temperature.

This month I wrote about measuring heat. Next month, I will write about measuring liquids and solids. 

Chef Adam Weiner, JD, CFSE, has been a culinary instructor in the San Francisco Bay Area for more than 16 years.