Explain the differences between food thermometers.

Digital Food Thermometers

Thermocouples. Of all food thermometers, thermocouples read and display the final temperature the fastest within 2 to 5 seconds. The temperature appears on a digital display.

A thermocouple measures temperature at the junction of two fine wires located in the tip of the probe. Thermocouples used in scientific laboratories have very thin probes, similar to hypodermic needles, while others may have a thickness of 1/16-inch.

Since thermocouple thermometers respond so rapidly, the temperature can be quickly checked in a number of locations to ensure that the food is thoroughly cooked. This is especially useful for cooking large foods, such as roasts or turkeys, when checking the temperature in more than one place is advised. The thin probe of the thermocouple also enables it to accurately read the temperature of thin foods such as hamburger patties, pork chops, and chicken breasts.

Thermocouples are not designed to remain in the food while it is cooking. They should be used near the end of the estimated cooking time to check for final cooking temperatures. To prevent overcooking, check the temperature before the food is expected to finish cooking.

Thermistors. Thermistor-style food thermometers use a resistor (a ceramic semiconductor bonded in the tip with temperature-sensitive epoxy) to measure temperature. The thickness of the probe is approximately 1/8-inch, and it takes roughly 10 seconds to register the temperature on the digital display. Since the semiconductor is in the tip, thermistors can measure temperature in thin foods, as well as thick foods. Because the center of a food is usually cooler than the outer surface, the tip should be placed in the center of the thickest part of the food.

Thermistors are not designed to remain in the food while it's cooking. They should be used near the end of the estimated cooking time to check for final cooking temperatures. To prevent overcooking, check the temperature before the food is expected to finish cooking.

Oven cord thermometers. Oven cord thermometers allow the cook to check the temperature of food in the oven without opening the oven door. A base unit with a digital screen is attached to a thermistor-type food thermometer probe by a long metal cord. The probe is inserted into the food, and the cord extends from the oven to the base unit. The base can be placed on the counter or attached to the stovetop or oven door by a magnet. The thermometer is programmed for the desired temperature, and it beeps when the temperature is reached. While designed for use in ovens, these thermometers can also be used to check foods cooking on the stove.

Thermometer fork combination. This utensil combines a cooking fork with a food thermometer. A temperature-sensing device is embedded in one of the tines of the fork. There are several different brands and styles of thermometer forks on the market; some using thermocouples and some using thermistors. The food temperature is indicated on a digital display or by indicator lights on the handle within 2 to 10 seconds (depending on the type). These lights will tell if the food has reached rare, medium, well done, etc. Particularly useful for grilling, the thermometer fork will accurately measure the internal temperature of even the thinnest foods. The thermometer fork should be used to check the temperature of a food toward the end of the estimated cooking time. Thermometer forks are not designed remain in a food while in the oven or on the grill. Thermometer forks cannot be calibrated.

Dial Food Thermometers

Bimetallic-coil thermometers. These thermometers contain a coil in the probe made of two different metals that are bonded together. The two metals have different rates of expansion. The coil, which is connected to the temperature indicator, expands when heated. This food thermometer senses temperature from its tip and up the stem for 2 to 2-1/2 inches. The resulting temperature is an average of the temperatures along the sensing area. These food thermometers have a dial display and are available as "oven-safe" and "instant-read."

"Oven-safe" bimetallic-coil thermometers. This food thermometer is designed to remain in the food while it is cooking in the oven, and is generally used for large items such as a roast or turkey. This food thermometer is convenient because it constantly shows the temperature of the food while it is cooking. However, if not left in the food while cooking, these thermometers can take as long as 1 to 2 minutes to register the correct temperature.

The bimetal food thermometer can accurately measure the temperature of relatively thick foods (such as beef roasts) or deep foods (foods in a stockpot). Because the temperature-sensing coil on the stem is 2- to 2-1/2 inches long and the stem is relatively thick, it is not appropriate to measure the temperature of any food less than 3 inches thick.

There is concern that because heat conducts along the stem's metal surface faster than through the food, the area of the food in contact with the thermometer tip will be hotter than the area a short distance to the side (the "potato nail effect"). To remedy this, the temperature should be taken in a second, and even third, area to verify the temperature of the food. Each time the thermometer is inserted into the food, let the thermometer equilibrate (come to temperature) at least 1 minute before reading the temperature. Some models can be calibrated. Check the manufacturer's instructions.

Instant-read bimetallic-coil thermometers. This food thermometer measures the temperature of a food in about 15 to 20% seconds. It is not designed to remain in the food while it is cooking in the oven, but should be used near the end of the estimated cooking time to check for final cooking temperatures. To prevent overcooking, check the temperature before the food is expected to finish cooking.

For accurate temperature measurement, the probe of the bimetallic-coil thermometer must be inserted the full length of the sensing area (usually 2 to 3 inches). If measuring the temperature of a thin food, such as a hamburger patty or boneless chicken breast, the probe should be inserted through the side of the food so that the entire sensing area is positioned through the center of the food. Some models can be calibrated. Check the manufacturer's instructions.

Single-use temperature indicators. One of the most recent developments in the retail food market is the emergence of disposable temperature indicators. Several brands are available, and all make quick work of determining if a food has reached its final temperature. These temperature sensors are designed for specific temperature ranges, for example, 160°F-170°F. These sensors should only be used only with foods for which they are intended. Read the package directions to ensure that the temperature the sensor will reach is consistent with the recommended internal cooking temperature.

The sensors on disposable thermometers are made from special temperature-sensitive materials. The sensor is inserted into a food. When the food reaches the proper temperature, the sensor changes color. They are designed to be used only once. However, if the desired temperature has not been reached, they can be reinserted until the temperature is reached. These sensors cannot be left in a food while it is cooking. They should be used near the end of the estimated cooking time. To prevent overcooking, check the temperature before the food is expected to finish cooking. Disposable temperature indicators are made from materials approved by the FDA for contact with food.

Pop-up timers. Commonly used in turkeys and roasting chickens since 1965, the "pop-up" temperature device is constructed from a food-approved nylon. The inside contains a stainless steel spring and firing material. The firing material is made of an organic salt compound or an alloy of metals commonly used in other thermo-sensing devices. The tip of the stem is imbedded in the firing 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 timers are reliable within 1°F to 2°F if accurately placed in a food; however, checking the temperature of other parts of the food with a conventional food thermometer is recommended.

Other Types of Food Thermometers

Liquid-filled thermometers. Also called "spirit-filled" or "liquid in glass" thermometers, these thermometers are the oldest kind of food thermometer used in home kitchens. They have either metal or glass stems. As the internal temperature of the food increases, the colored liquid inside the stem expands and rises to indicate the temperature on a scale. Heat conduction in the metal stems can cause false high readings. They are designed to remain in the food while it is cooking. They should be inserted at least 2 inches deep in the thickest part of the food; therefore, they are not appropriate for thin foods. Some liquid-filled thermometers can be calibrated by carefully moving the glass stem within the holder.

Candy/jelly/deep fry thermometers. These thermometers will measure temperatures ranging from 100°F to 400°F. They are used to measure the extra-high temperatures required of candy and jelly making, as well as frying with hot oil.

Appliance Thermometers

Refrigerator/freezer thermometers. For safety, it is important to verify the temperature of refrigerators and freezers. Refrigerators should maintain at a temperature no higher than 40°F. Frozen food will hold its top quality for the longest possible time when the freezer maintains 0°F. An appliance thermometer can be kept in the refrigerator and freezer to monitor temperature. This can be critical in the event of a power outage. When power returns after an outage, the food is safe if the refrigerator is 40°F or colder and the freezer is still colder than 40°F. These bimetallic-coil thermometers are specially designed to provide accuracy at cold temperatures.

Oven thermometers. An oven thermometer 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.

Summary of Food Thermometers

Type

Speed

Placement

Usage Considerations

Digital Thermometers

Thermocouple	2-5 seconds	1/4" or deeper in the food, as needed	Gives fastest reading Good for measuring temperatures of thick and thin foods Not designed to remain in food while cooking Check internal temperature of food near the end of cooking time Can be calibrated More costly; may be difficult for consumers to find in stores
Thermistor	10 seconds	At least 1/ 2" deep in the food	Gives fastest reading Can measure temperature in thin and thick foods Not designed to remain in food while cooking Check internal temperature of food near the end of cooking time Some models can be calibrated; check manufacturer's instructions Available in "kitchen" stores
Oven cord thermometer	2-5 seconds	At least 1/ 2" deep in the food	Can be used in most foods Can also be used outside of oven Designed to remain in the food while cooking in oven or covered pot Base unit sits on stovetop or counter Cannot be calibrated
Oven cord thermometer	10 seconds	At least 1/ 2" deep in the food	Can be used in most foods Can also be used outside of oven Designed to remain in the food while cooking in oven or covered pot Base unit sits on stovetop or counter Cannot be calibrated
Thermometer fork combination	2-10 seconds	At least 1/ 4" in the thickest part of food	Can be used in most foods Not designed to remain in food while cooking Sensor in tine of fork must be fully inserted Check internal temperature of food near the end of cooking time Cannot be calibrated Convenient for grilling

Dial Thermometers

Oven-safe, bimetal	1- 2 minutes	2-2 1/ 2" deep in the thickest part of the food	Can be used in roasts, casseroles, and soups Not appropriate for thin foods Can remain in food while cooking Heat conduction of metal stem can cause false high reading Some models can be calibrated; check manufacturer's instructions
Instant-read, bimetal	15- 20% seconds	2-2 1/ 2" deep in the thickest part of the food	Can be used in roasts, casseroles, and soups Temperature is averaged along probe, from tip to 2"-3" up the stem Cannot measure thin foods unless inserted sideways Not designed to remain in food while cooking Use to check the internal temperature of a food at the end of cooking time Some models can be calibrated; check manufacturer's instructions Readily available in stores

Other

Single-use temperature indicators	5-10 seconds	Approx. 1/ 2" deep (follow manufacturer's instructions)	Designed to be used only once Designed for specific temperature ranges Should only be used with foods for which they are intended Temperature-sensitive material changes color when the desired temperature is reached
Liquid-filled (glass or metal stem)	1-2 minutes	At least 2" deep in the thickest part of the food	Used in roasts, casseroles, and soups Can remain in food while cooking Cannot measure thin foods Some can be calibrated; check manufacturer's instructions Possible breakage of glass stem while in food Heat conduction of metal stem can cause false high reading