Heat Transfer Background

Almost all food processes depend on or are affected by heat being added or removed at some stage in the operation. The efficient and effective utilization of heat results in economic savings, minimum adverse affects on nutrient components, higher quality consumer-ready products, and minimum effect on the many environmental factors associated with food processing. This efficient and effective use of heat depends on knowledge and subsequent application of the heat transfer mechanisms involved in heating, cooling, and changing the state of foods and food products. The rate at which heat is transferred depends on the type of product, the condition of the product, and the type of heat transfer by which heat enters or is removed from the product.

Other than maintaining a food under handling and storage conditions that ensure the highest quality product, the only option involving heat transfer is the control of processing conditions to give the desired rate of heat transfer. This means considering the three basic mechanisms that control the rate of heat transfer and selecting processing conditions and facilities that optimize the process. Hence a thorough knowledge of heat transfer mechanisms and the relationship between heat transfer rates and other physical and chemical factors is probably the most important consideration involved in the processing of foods. This is further emphasized by the fact that the control of mass and energy balances and fluid flow mechanics, the basics for designing, constructing, and operating the machinery, equipment, and facilities involved in food handling, storing, and processing, is dependent on considerations involving the mechanisms and rate of heat transfer.

Heat is transferred from one body to another by three different mechanisms, namely conduction, convection, and radiation. In fact, most processing facilities utilize two or all three of these means of transferring heat from or to a product. Before considering the mathematical relationships that describe heat transfer and allow the control of the amount and rate of transfer, it is well to have a visual understanding of how each functions.

Conduction heat transfer takes place when two bodies at different temperatures are in contact with each other. Through this direct contact, heat energy is transferred from particle to particle between the solid bodies with no bulk movement of material. When one places a hand on an object having a temperature different from body temperature there is a flow of heat. Hence, when the object is a blackboard in the classroom at room temperature, there is a sensation of the blackboard being cold. Conversely, if one touches a warm element on a stove, there is an immediate sensation of heat being transferred to the hand. The difference in temperatures between the two objects, as well as the characteristics of the conducting body, determines the rate of transfer. In the case of the blackboard, the temperature driving force was probably not more than 20°F (11°C), while the driving force between a warm stoveelement and the hand is considerably higher. Hence the sensation of heat will be detected and create considerably more reaction than placing a hand on the blackboard. Other things to consider include the nature of the material to which heat is being transferred. This determines how fast the heat will penetrate as well as the temperature gradient within the material. In general, conduction is highly desired for food being frozen, especially those forms having flat surfaces that can insure maximum contact with plate freezers. In most other cases of freezing irregularly shaped items and heating, cooking, or sterilizing, a combination of conduction and other transfer mechanisms is more efficient and controllable.

Convection heat transfer depends on the bulk movement and mixing of liquids that are initially at different mass temperatures, or on the contact of a solid with a moving liquid stream of a different temperature. These two basic types of convection must be considered together in most heat transfer involved with processing food products. Regardless of whether hot liquids are being mixed in a batch or continuous basis, the temperature of the final combined liquid mixture reaches an equilibrium temperature somewhere between the original temperatures of the two liquids. If a solid is involved, the temperature equilibrium occurs between the bulk of the liquid and the surface of the solid. The velocity of a liquid flowing past a solid affects the rate of transfer between the two materials. Natural or free convection is caused by density gradients (thermal expansion) formed when a liquid is changing in temperature. These can not be controlled and are often a detriment to processing since the rate of transfer is at the mercy of the naturally rising or mixing streams. Forced convection, the pumping or blowing of a liquid or gas over a surface, can be controlled and the heat transfer rates can be predicted. Hence most food processes utilizing convection heat transfer depend on facilities and equipment that use forced convection. This can be demonstrated by considering the wind chill factor, by which a wind causes a person to feel colder than expected at a given outside temperature. For example, in the winter one might bear a weather report in which the outside temperature is given as 0°C and the wind chill factor makes it feel as if the temperature were — 15°C. This is because the wind is a forced convection whereas the 0°C temperature is measured under shielded or still conditions.

Many food processing operations include a combination of conduction and convection heat transfer. This is demonstrated in cooling and freezing curves in which blast freezing or convection is the method of heat transfer (see Figure 9 in Fish and shellfish products) and a plate or conduction heat transfer is combined with blast freezing or convection (see Figure 9 in Fish and shellfish products). The critical period during which heat of fusion is being removed between about 30 to 22°F (- 2 to - 6°C) is 220 min in Figure 10 of Fish and shellfish products. The total time for cooling, freezing, and dropping from 50 to 0°F (10 to - 18°C) is 472 min in Figure 10 and 93 min in Figure 9. As has been discussed in Fish and shellfish products, the more rapid freezing as shown in Figure 9 resulted in high quality frozen fish, similar to the fresh product. Conversely, the slower freezing in Figure 10 resulted in considerable cell degradation that greatly reduced the quality of the end product.

The third type of heat transfer used for heat processing of foods is radiation, the transmission of electromagnetic energy through space. Whereas conduction and convection are dependent on a physical medium through which to transfer heat energy, radiation requires no carrier to transfer wave energy from a surface of one body to another. The amount of energy transmitted is dependent on area and the nature of the exposed surface and the temperature of the body. The amount of radiation absorbed or deflected also depends on the nature of the absorbing body and the body temperature. Hence each body receives radiation from every other body, the amount depending on how much the bodies can "see" of each other and the ability of the body to radiate or deflect and absorb radiation (emissivity or absorptivity).

There are several different types of radiation used in the food industry, each used for a specific reason. The radiation heating from a warm or hot body to a food is akin to that of a person standing in the shade or the sun on a hot day. The ambient air temperature may be the same in both positions; however, a person becomes much warmer in the sun where the direct radiation is being added to the body in addition to the convection heating occurring from the surrounding air. The radiating body for heating a food by radiation can be the hot element in an oven or grill (roasting, baking, broiling). In this case the surface receives energy so rapidly that it can not be conducted into the food fast enough to prevent the desired browning (actually scorching and/or Maillard browning reaction) of the surface.

Other types of radiant energy are used in processing food. Some of these are becoming as prevalent as the conventional concept of cooking or processing by exposure to a hot element or heat source. When an alternating current is passed through a conductor, energy is emitted in the form of waves having a specific wavelength and frequency. Microwave heating is a good example of this process, whereby the energy absorbed by a food is converted into heat due to the friction of moving molecules or atoms.

Ionizing radiation from x rays and gamma rays, while not heating a food during a normal exposure time, can destroy microorganisms and thus accomplish the aim of pasteurizing or sterilizing a food.

The units of each item in SI and English units are shown in Table 1. It is conventional to place a negative sign in front of the equation to signify a positive heat flow from the higher to the lower temperature.

Although the complete mathematical analysis of heat transfer can become extremely complicated when it comes to considering complex three-dimensional flow and integrations over total volumes, most problems involving conduction in food processing systems can be greatly simplified. In most steady-state cases, considering the relatively short range of temperature changes, the uniformity in area over the distance of heat conduction, and the uniformity in thermal conductivity over these temperature ranges, equation 1 can be simplified to q = -kA(t, - U)!Ax - kA — (2)

Equation 2 is typical of many problems in nature that involve a driving force analogy. In the case of heat transfer, the rate of transferring some discrete quantity of energy q is equal to the driving force DF that makes the movement happen divided by the resistance R to this movement, or q = DF /R (3)

A common use of this relationship is in the transfer of electrical energy, where the current flowing (I, amperes) is equal to the driving force (E, volts) divided by the resistance (R, ohms), or

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