Food Service Thermometers

One of the critical factors in controlling bacteria is controlling temperature. Pathogenic microorganisms grow very slowly at low temperatures, multiply rapidly in mid-range temperatures, and are killed at high temperatures. For safety, foods must be held at proper cold temperatures in refrigerators or freezers and they must be cooked thoroughly. It is essential to use a thermometer when cooking meat and poultry to prevent undercooking and, consequently, prevent foodborne illness.

Why Use a Thermometer?
Using a thermometer is the only reliable way to ensure safety and to determine the ‘doneness' of most foods. To be safe, a product must be cooked to an internal temperature high enough to destroy any harmful bacteria that may have been in the food.

A thermometer should also be used to ensure that cooked foods are held at a safe temperature until served.

Food thermometers come in several types and styles, and range in level of technology and price.
Liquid-filled Thermometers
Also called ‘spirit-filled' or ‘liquid in glass' thermometers, are the oldest type of thermometers. As the internal temperature of the food increases, the colored liquid inside the thermometer expands and rises to indicate the temperature on a scale. Some liquid-filled thermometers can be calibrated by carefully moving the glass stem within the holder.
Bimetallic-coil Thermometers
These thermometers contain a coil in the probe made of two different metals with different rates of expansion that are bonded together. This coil, which is connected to the temperature indicator, expands when heated. This thermometer senses temperature from its tip up the stem for 2 to 2 ½". The resulting temperature is an average of the food-contact temperatures along the sensing section (in other words, if the temperature at the tip of the probe is 170°F, and the temperature 2 inches above the tip is 180F, the thermometer will register about 175°F). These thermometers come in both oven-safe and ‘instant-read’ forms. Many of the dial thermometers can be calibrated.
Oven-safe Bimetallic-coil Thermometers
Oven-safe thermometers are the traditional ‘meat’ thermometers designed to be placed in food before it goes into the oven. It is generally used for large items, such as roast or turkey.
These thermometers show the temperature with a dial. They can take as long as 1-2 minutes to register the correct temperature. The bimetal stem thermometer can accurately measure the temperature of relatively thick or deep foods such as beef roasts and foods in stockpot. However, since the temperature-sensing coil on the probe is between 2-2 ½" long, and this probe is relatively thick, this instrument is not appropriate to measure temperature of any food less then 3" thick. This thermometer is convenient because a quick glance will show how the food is progressing. There is concern, however, that because heat conducts along the stem’s metal surface faster then through the food, the food in contact with the thermometer tip will be hotter than the food a short distance to the side. This is the ‘potato nail effect.’ To remedy this, the temperature should be taken in a second and even third place to verify the temperature of the food. Each time the thermometer is inserted into a food let the temperature equilibrate, or come to temperature, at least 1 minute before reading the temperature.

Bimetallic-coil "Instant Read" Thermometers are designed to quickly measure the temperature and cannot be left in the oven while the food is cooking. About 15 to 20 seconds are required for the temperature to be accurately displayed on a dial. The thermometer is inserted in the good only while assessing the temperature. Once the temperature is determined, the thermometer must be removed. It is important to wash the probe with hot, soapy water and sanitize after each insertion to prevent cross-contamination.

For accurate temperature measurement the probe of the bimetallic-coil thermometer must be inserted the full length of the sensing device (usually 2-3"). If measuring the temperature of a thin food, such as hamburger pattys or boneless chicken breasts, the probe should be inserted sideways with the sensing device in the very center of the meat. 
Thermistors (digital) Thermometers
These use a resistor (a ceramic semiconductor bonded in the tip with temperature-sensitive epoxy) to measure temperature. The probe diameter is approximately 1/8" thick and takes roughly 10 seconds to register a temperature on a digital display. Since the semiconductor is in the tip, thermistors can measure temperature in thin foods. As with the bimetal instant read thermometers, thermistors should be placed in foods towards the end of cooking time to check for final cooking temperatures. Because the center of a food is usually cooler than the outer surface, place the tip in the center of the thickest part of the food.
Thermocouple Thermometers
Thermocouple thermometers are the fastest reading of all thermometers. They can show a final temperature in seconds on a digital display. They have very small tips and can accurately measure the temperature of very thin foods, depending on the size of the probes. Thermocouples measure the temperature at the junction of two fine wires located in the tip of the probes. Since thermocouple thermometers respond so rapidly, the temperature can be easily read in a number of locations to ensure that the food is thoroughly cooked. Thermocouples can be calibrated for accuracy. It is important to wash the probe after each use with hot, soapy water before reinserting it into a food.
Refrigerator/Freezer Thermometers
Refrigerator/Freezer Thermometers are used to verify the temperature of refrigerators and freezers. An appliance thermometer can be kept in the refrigerator and freezer to monitor the temperature.
Oven Thermometers
Oven Thermometers can be left in the oven to verify that the oven is heating to the desired temperatures. These bimetallic-coil thermometers can measure temperatures from 100°F to 600°F.
Pop-up Timers
Pop-up Timers are commonly used in turkeys and roasting chickens, the ‘pop-up’ temperature device is constructed from a food-approved nylon. Inside is a firing material, and a stainless steel spring. The firing material may be an organic salt compound or an alloy of metals commonly used in other thermo-sensing devices. The tip of the stem is embedded in this hardened material until it melts, releasing the stem, which is then ‘popped up’ by means of the spring. This indicates that the food has reached the final temperature for safety and doneness. Pop-up thermometers are reliable to within 1-2°F if accurately placed in the product.

T-Stick Disposable Thermometers
T-Stick Disposable Thermometers are disposable, single use, cardboard thermometers which indicate 160°F(+/- 1 F). At 160°F or higher, a white material inside the plastic coated tip becomes clear. As a result, the tip changes from a white to black indicating a safe temperature has been reached. The T-Stick is made from materials accepted by the FDA for contact with food.

Using a Thermometer
Most thermometers will give an accurate reading within 2-4°F. The reading will only be helpful, however, if the thermometer is placed in the proper location in the product. If not inserted correctly, or if the thermometer is placed in the wrong area, the reading will not accurately reflect the internal temperature of the product. In general, the thermometer should be placed in the thickest part of the food, away from the bone, fat or gristle.

Check the Manufacturer's Instructions First
Before using a food thermometer, read the manufacturer’s instructions. The instructions should tell how far the thermometer must be inserted in a food to give an accurate reading. Most thermometers also come with instructions on how to recalibrate the thermometer (see below for more information on calibrating thermometers). If instructions are not available, check the stem on the thermometer for an indication, or ‘dimple’ that shows one end of the location of the sensing device. Most digital thermometers will read the temperature in a small area of the tip. Dial types must penetrate about 2-3" into the food.

Calibrating a Thermometer
There are two ways to check the accuracy of a food thermometer. One method uses ice water, the other uses boiling water. Many thermometers have a calibration nut under the dial that can be adjusted. Check the package for instructions.
Ice Water
To use the ice water method, fill a large glass with finely crushed ice, and clean tap water to the top of the ice, and stir well. Immerse the thermometer stem a minimum of 2" into the mixture, touching neither the sides nor the bottom of the glass. (For ease in handling, the stem of the thermometer can be placed through the clip section of the stem sheath and, holding the sheath horizontally, lowered into the water.) Without removing the stem from the ice, hold the adjusting nut under the head of the thermometer with a suitable tool and turn head so pointer reads 32°F. Allow a minimum of 30 seconds before adjusting.
Boiling Water
To use the boiling water method, bring a deep pan of clean tap water to a full, rolling boil. Carefully immerse the stem into the boiling water a minimum of 2" and wait at least 30 seconds. Without removing the stem from the pan, hold the adjusting nut under the head of the thermometer with a suitable tool and turn head so the thermometer reads 212°F.