Heat

When two bodies are at different temperatures, there is a transfer of heat energy from the body having the higher temperature t1 to the body having the lower temperature t2. Hence, the state of energy of the colder body is increased and that of the warmer body decreased. This situation of unsteady-state heat transfer continues with the driving force t1 — t2, or St, decreasing until both bodies are at the same temperature and there is no unbalanced state to give a driving force. Such is the case when a hot or warm food is placed in a container of cold water to decrease the temperature of the food.

Steady state heat transfer occurs when the temperature driving force At remains constant and the rate of heat transfer between two bodies is constant. An example of this type of steady-state transfer is boiling of water on a hot element of a stove. The element temperature is kept constant by a continuous flow of electrical energy being converted to heat and the boiling water stays constant at the boiling point of water (100°C at standard atmospheric pressure). On the other hand, if the heating element is kept constant and a food is being heated below the boiling point, there is an unsteady-state heat transfer, which results in a temperature rise of the food.

The last two examples also illustrate that there are different types of heat within a substance. Sensible heat is the amount of heat that can be added or removed from a given mass of product between two temperatures of the product (ij to t2) without changing the state of the body (eg, heating or cooling a food by cooking or refrigerating). Latent heat refers to the amount of heat necessary to change a given mass from one state to another (eg, boiling water, freezing food products). Sensible heat results in a temperature rise within a body, whereas latent heat is the heat (at constant temperature) necessary to change the state.

There are numerous systems used in the world to quantify mass and energy, including the English (Imperial) system and several systems using metric units. This can be well demonstrated by the measurement of sensible heat. Each food or material requires a different amount of heat, called the specific heat (heat capacity), to raise or lower a given mass to a given temperature. Through laboratory research, specific heats have been determined and given in different unit systems. A British thermal unit (English system) is defined as the amount of heat required to raise the temperature of 1 lb of water 1°F. A kilocalorie in the metric system is the amount of heat required to raise 1 kg of water 1°C.

Several international organizations have attempted to standardize the unit systems, symbols, and quantities to prevent the confusion that often exists (1,2). The result is the Systeme International d'Unites, or the SI system, in which base metric units have been defined. Table 1 gives the SI system unit definitions along with factors for converting from the Imperial system to SI units.

In the SI system, the basic unit of force is the newton (N), the force that gives a mass of 1 kg an acceleration of 1 m/s. The basic unit of energy is the joule (J), the work done when the point of application of 1 N is displaced by a distance of 1 m. Hence specific heat c is expressed in the three principal systems as:

The mathematical relationship between heat, temperature, and specific heat in a food can be expressed in SI units as:

Q = M |2 cdt where Q = heat (heat gained or lost in KJ), M = mass (kg), c = specific heat (kJ/kg°C), and dt = temperature change (°C).

If the specific heat is at constant pressure it is designated as cp. If the process is carried out at constant volume (eg, a container of compressed gas), it is designated as cv. For liquid and solid food, the difference between cp and c\, is negligible. Because over the range of temperatures for food processes the cp is essentially constant, heat in a system is normally calculated as:

Thus if 100 kg of apples with a specific heat of 3.6 kJ/kg/ K) are cooled from tree temperature of 18°C to 5°C, the amount of heat removed would be

As a point of reference, 1 Btu equals 1.055 kJ; therefore, this would be equal to 4,436 Btu.

When learning the SI system, which will eventually be the world standard, it is useful to remember some basic approximate conversion factors to visualize the relationships between values. This is especially true for the United States where the English system has become so well entrenched. It is helpful to remember that 1 ft is approximately 0.3 m, 1 Btu is approximately 1 kJ, 1 lbf is approximately 4.5 N, 1 Btu/h is approximately equal to 0.3 W, and 1 Btu/lb is approximately 2.33 kJ/kg. If the equivalent English and SI values are visualized when working a problem in SI units, it will become easier to think in both systems.

Table 1. Base Units of SI System (Metric) and Conversion from Imperial to SI units

Conversion factor

Measurable quantity SI base unit SI symbol Imperial base unit Imperial base symbol (imperial X factor = SI)

Table 1. Base Units of SI System (Metric) and Conversion from Imperial to SI units

Conversion factor

Measurable quantity SI base unit SI symbol Imperial base unit Imperial base symbol (imperial X factor = SI)

Length

Meter

m

Foot

ft

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