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WHY THERMOMETERS ARE ESSENTIAL IN THE KITCHEN
Thermometers provide a reliable way to ensure that the food we consume is safe and to
determine the "doneness" of meat, poultry, and egg products. By 'safety' we mean food-borne
diseases caused by bacteria and other harmful microorganisms found in uncooked food, while "doneness"
refers to food cooked to a desired state and indicates the sensory
aspects of foods such as texture, appearance, and juiciness. Unlike the temperatures
required for safety, these sensory aspects are subjective.
To be
safe, foods must be cooked to an internal temperature high enough to destroy any harmful
microorganisms that may be in the food; hence, the critical factor in controlling
disease-causing microorganisms in food is controlling temperature. Pathogenic microorganisms
grow very slowly at temperatures below 40 °F, multiply rapidly between 40 and 140 °F, and
are destroyed at temperatures above 140 °F. For safety, foods must be held at proper cold
temperatures in refrigerators or freezers and they must be cooked thoroughly.
According to the USDA, temperature is the
only way to gauge whether food is sufficiently cooked. Its research reveals that the "color test" can give consumers
misleading information about the safety of the foods they are preparing, since cooked color varies
considerably. For example, freezing and thawing may influence a meat's tendency to brown
prematurely.
However, to be effective, thermometers must be
used properly and calibrated correctly. If the thermometer is inserted incorrectly, or
placed in the wrong area, the reading may 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 bone, fat or gristle. Read the manufacturer's instructions on how to calibrate
(check the accuracy of) the thermometer
Clearly the
availability of a thermometer, in every kitchen, and a knowledge of temperature in relation
to food and cooking is vital, in preventing undercooking, and
consequently, food-borne illnesses.
TEMPERATURE
Temperature is a measure of the amount of
heat
energy possessed by an object. Because temperature is a relative measurement, scales based on
reference points must be used to accurately measure temperature. There are three main scales
commonly used in the world today to measure temperature: the Fahrenheit (°F) scale, the
Celsius (°C) scale, and the Kelvin (K) scale. Each of these scales uses a different set of
divisions based on different reference points as described in detail below.
Fahrenheit..
Daniel Gabriel Fahrenheit (1686-1736) was a German physicist who is credited with the
invention of the
alcohol thermometer in 1709 and the mercury thermometer in 1714. The Fahrenheit
temperature scale was developed in 1724. Fahrenheit originally established a scale in which
the temperature of an ice-water-salt mixture was set at 0 degrees. The temperature of an
ice-water (no
salt) mixture was set at 30 degrees and the temperature of the human body was set at 96
degrees. Using this scale, Fahrenheit measured the temperature of boiling water as 212°F on
his scale. He later adjusted the freezing point of water from 30°F to 32°F, thus making the
interval between the freezing and boiling points of water an even 180 degrees (and making
body temperature the familiar 98.6°F). The Fahrenheit scale is still commonly used in the
United States.
Celsius..
Anders Celsius (1701-1744) was a Swedish astronomer who is credited with the invention of
the centigrade scale in 1742. Celsius chose the melting point of ice and the boiling point
of water as his two reference temperatures to provide for a simple and consistent method of
thermometer calibration. Celsius divided the difference in temperature between the freezing
and boiling points of water into 100 degrees (thus the name centi, meaning one hundred, and
grade, meaning degrees). After Celsius’ death, the centigrade scale was renamed the Celsius
scale and the freezing point of water was set at 0°C and the boiling point of water at
100°C. The Celsius scale takes precedence over the Fahrenheit scale in scientific research
because it is more compatible with the base ten format of the International System (SI)
of metric
measurement. In addition, the Celsius temperature scale is commonly used in most
countries of the world other than the United States.
Kelvin..
Lord William Kelvin (1824-1907) was a Scottish physicist who devised the Kelvin (K) scale in
1854. The Kelvin scale is based on the idea of absolute zero, the theoretical temperature at
which all molecular motion stops and no discernable
energy can be detected (see the
molecular motion lesson
for more information). In theory, the zero point on the Kelvin scale is the lowest possible
temperature that exists in the universe: -273.15ºC. The Kelvin scale uses the same
unit of division as the Celsius scale, however it resets the zero point to absolute
zero: -273.15ºC. The freezing point of water is therefore 273.15 Kelvins (graduations are
called Kelvins on the scale and neither the term degree nor the symbol º are used) and
373.15 K is the boiling point of water. The Kelvin scale, like the Celsius scale, is a
standard
SI
unit of measurement used commonly in scientific measurements. Since there are no
negative numbers on the Kelvin scale (because theoretically nothing can be colder than
absolute zero), it is very convenient to use Kelvins when measuring extremely low
temperatures in scientific research.
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