Using Thermometers in the Kitchen
Essential tools for food safety
Thermometers are some of the most essential food safety tools in the kitchen. Food thermometers should be used when cooking meat, poultry, fish and egg products. Refrigerator and freezer thermometers are food safety tools that help us to insure that risky foods are being stored at temperatures that keep food safe.
Using a food thermometer is the only reliable way to ensure the safety of cooked meat, poultry, fish and eggs and foods containing these ingredients. To be safe, these foods must be cooked to a safe minimum internal temperature to destroy any pathogens (microorganisms that make us sick) that may be in the food.
Color is Not a Reliable Indicator
Many food handlers believe that visible indicators, such as color changes, can be used to determine if foods are cooked to a point where pathogens are killed. However, recent research has shown that color and texture indicators are unreliable. For example, ground beef may turn brown before it reaches a temperature where pathogens are destroyed. A consumer preparing hamburger patties and using the brown color as an indicator of “doneness” is taking a chance that pathogenic microorganisms may survive. A hamburger cooked to 160 °F, regardless of color, is safe.
Safety Versus Doneness
The temperature at which different pathogens are destroyed varies, as does the “doneness” temperature for different meat and poultry. A beef, lamb, or veal roast, steak, or chop that is not pierced in any way during processing or preparation and reaches an internal temperature of 145 °F is safe to eat. Cook steaks, roasts, or chops that have been tenderized, boned, rolled, etc., to a safe minimum internal temperature of 160 °F. A pork chop or roast cooked to 160 °F is safe to eat. A consumer looking for a visual sign of “doneness” might continue cooking it until it is overcooked and dry. However, a consumer using a food thermometer to check for “doneness” can feel assured the food has reached a safe temperature and is not overcooked.
Likewise, all poultry should reach a safe minimum internal temperature of 165 °F. For reasons of personal preference, consumers may choose to cook poultry to higher temperatures for acceptability and palatability.
A food thermometer should also be used to ensure that cooked food is held at safe temperatures until served. Cold food should be held at 40 °F or below. Hot food should be kept hot, at 140 °F or above.
Types of Food Thermometers
Food thermometers come in several types and styles, and vary in level of technology and price.
DIGITAL FOOD THERMOMETERS
Thermistor-style food thermometers are made so that they can measure the temperature of thin foods as well as thick foods. The thickness of the probe is about 1/ 8 of an inch and it takes about 10 seconds to register the temperature on the display. 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.
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.
Not all thermistors can be calibrated**. Check the manufacturer’s instructions.
Oven Cord Thermometers:
This food thermometer allows 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 beeps when it is reached. While designed for use in ovens, these thermometers can also be used to check foods cooking on the stove. Oven cord thermometers cannot be calibrated**.
Thermometer Fork Combination:
This utensil combines a cooking fork with a food thermometer. A temperature-sensing device is placed in one of the tines of the fork. 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 towards the end of the estimated cooking time.
Thermometer forks are not designed to remain in a food while in the oven or on the grill. Thermometer forks cannot be calibrated**.
DIAL FOOD THERMOMETERS
These thermometers contain a coil in the probe made of two different metals that are bonded together. Because this food thermometer senses temperature from its tip and up the stem for 2 to 2 1/ 2 inches, these thermometers must be inserted at least 3” into the food. Often there is an indentation on the probe that tells the cook how far to insert the probe. The indicated temperature is an average of the temperatures along the sensing area. These food thermometers have a dial display and are available as “oven-safe” (usually with an oven-glass face) and “instant-read.”
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 between 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. These thermometers can be easily calibrated—and should be at least weekly.
“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, they can take as long as 1 to 2 minutes to register the correct temperature.
A food thermometer is only as good as its’ ability to give you an accurate reading. Some thermometers cannot be calibrated (most thermistors), while often bimetallic thermometers can be. First, check with the manufacturer. If your thermometer cannot be calibrated, then find out how long you are likely to be able to use the thermometer before you need to replace it.
You can still check to see if the thermometer is accurate by placing the thermometer probe into a container of ice and water slush. The temperature should read 32°F. If the thermometer is only a few degrees off, just remember to make the adjustment when reading the temperature. If it is more than a few degrees off, buy a new one.
Bimetallic probe thermometers may have an adjustment nut under the dial. You can calibrate these by inserting the probe into icy, slushy water (32°F) or boiling water (212°F at sea level—adjust for altitude). Adjust the nut under the dial so that the indicator shows the correct temperature on the dial.
Unfortunately, home refrigerators, freezers and combinations are not manufactured with any type of thermometer attachment. While there are dials for adjusting the temperature to be colder or warmer (for example, you may need to make it colder in the summer), there is no way to determine the actual temperature in the box—unless you buy a refrigerator or freezer thermometer and place it in your appliance.
To slow the processes of spoilage and the growth of bacteria in refrigerated foods, you need to keep the temperature in the range of 38-40°F. The temperature in the freezer should be at 0°F or lower. At this temperature, bacterial growth will be stopped. However, freezing does not kill most bacteria. Though food will be safe indefinitely at 0°F, quality will decrease the longer the food is in the freezer. Freezing does not destroy enzymes that can cause flavor changes or changes in texture or quality over time
The only way to know the temperature in your refrigerator or freezer is to place a thermometer in the appliances. If your power should go out or the appliance breaks down, the only way to know if the food is safe to eat is to know the temperature in the box.
It is best to place your thermometer in the warmest part of the refrigerator or freezer—towards the front and top of the box. Check the temperature weekly.
Most refrigerator/freezer thermometers are either liquid-filled or bimetallic-coil thermometers. The United States Department of Agriculture describes these thermometers as follows:
Liquid-filled thermometers are the oldest types of thermometers used in home kitchens. As the temperature increases, the colored liquid (usually an alcohol solution) inside the thermometer expands and rises to indicate the temperature on a scale.
Bimetallic-coil thermometers contain a coil made of two different metals with different rates of expansion that are bonded together. The bimetal element is coiled, fixed at one end, and attached to a pointer stem at the other end. As the temperature increases, the pointer will be rotated by the coiled bimetal element to indicate the temperature.
Purchase food and refrigerator/freezer thermometers in the housewares section of department, appliance, culinary and grocery stores. Be sure to buy two refrigerator/freezer thermometers--one for your refrigerator and one for your freezer.
NOTE: If the freezer compartment isn't a separate freezer compartment, but a compartment inside the refrigerator, it may be impossible to obtain a 0°F temperature. One sign of this will be soft ice cream. Plan to use food within a few weeks.
Information adapted from:
Kitchen Thermometers, USDA/FSIS and Use a Refrigerator AND a Freezer Thermometer, by Alice Henneman, MS, RD, University of Nebraska-Lincoln Extension in Lancaster County and Joyce Jensen, REHS, CFSP, Lincoln-Lancaster County Health Department