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With the exception of sugar solutions, cookery in boiling water is done at 212°F (100°C) or less unless a pressurized system is used. However, much higher temperatures can be utilized when fat is the cooking medium, because fat does not boil even when the temperature rises as high as 475°F (246°C). Fat can be maintained at around 375°F (190°C), a desirable temperature for frying. Such hot fat means that foods will reach their desired degree of doneness quite quickly in comparison with the length of time that would be needed if they were to be boiled in water at 212°F (100°C). After all, boiling water is more than 150 F° (83 C°) cooler than the fat used in frying.

The temperatures utilized in the preparation and storage of food generally span a range over almost 500 F° (about 280 C°). People working in food preparation and management need to know what temperatures are appropriate and how to control them. An overview of the various temperatures and their applications is illustrated in Figure 4.12.

colloidal dispersion

system containing protein or other molecules or particles between 1 and 100 millimicrons in size dispersed in a continuous phase, in this case in water.

coarse suspension

Dispersion of particles larger than colloidal size mixed in water or other liquid.

Figure 4.11

A cook near Lake Titicaca in Peru adds the pressure gauge to her pressure cooker to build pressure and help speed the cooking of the vegetables she is preparing in the Andes Mountains at an elevation of 12,500 feet. Courtesy of Plycon Press.

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Such high temperatures highlight the need for being safety conscious in preparing foods. Burns occur all too easily when people fail to appreciate the extremely high temperatures of boiling candies or of the fat being used in deep-fat frying. In fact, kitchen counters with vinyl or formica tops can be damaged simply by placing a pan with a boiling candy solution or hot fat on them.

A related danger exists when water is added to caramelizing sugar or to hot fat. The extreme temperature difference between a caramelizing sugar or hot fat and cold water causes excessive splattering, which may result in burns on the hands, arms, or face—wherever the splatters reach the skin. To reduce splattering, water should be boiling when it is added to caramelizing sugar, and potato slices for French-frying should be blotted as dry as possible with paper towels before being added to the fat.

therMOMeterS

To help ensure good temperature control in food preparation and handling, appropriate thermometers are used: generally, a candy thermometer, a deep-fat frying thermometer, and a meat thermometer (Figure 4.13). The thermometer for candy making will need to have an upper limit of approximately 325°F (163°C) to allow a margin of safety above the probable high of 300°F (149°C) needed for some candies. The deep-fat frying thermometer will have an upper limit of at least 500°F (260°C). A meat thermometer will register up to about 185°F (85°C) and should have a short sensing rod to accommodate to roasting in the oven. A pocket thermometer is convenient, but its plastic cover will melt if used in the oven.

Appliances occasionally may fail to maintain their thermostatic control. A refrigerator thermometer and an oven thermometer are useful checks on appliances to be sure that food is being held at safe temperatures. Food-service professionals in industry find thermometers invaluable in ensuring that foods on the steam table are being held at a high enough temperature and that refrigerated items are being stored at safe temperatures. 220C 190C 425F 375F 300F 240F 234F 212F 211F 180F 150F 100F 40F 32F 150C 116C 112C 100C 82C 65C 40C 5C 0C 99C Baking zone Deep-fat frying Candy making Simmering range Scalding Lukewarm Refrigerator zone Freezing Pressure canning Boiling water Figure 4.12

Temperatures in food preparation. Courtesy of Plycon Press.

Figure 4.13

Instant-read thermometer (left) measures temperatures to 220°F; candy-making and deep- fat frying thermometers measure much higher temperatures. Courtesy of Plycon Press.

factors in food preparation | chapter four

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prINCIpLeS OF heatING FOODS

Conduction

Heat can be transferred by conduction; that is, heat is transferred directly from one molecule to another. In cooking, heat is transferred by conduction from the metal of an electrical coil on a range directly to the metal of the pan in which a food is being heated. This method of heat transfer works slowly, but it is fairly successful when good conductors are used. Aluminum pans, for example, provide quite uniform heating if they are thick. In contrast, stainless steel conducts heat unevenly, causing some pans made with this metal to develop some spots that are too hot, while others may be too cool.

Convection

When water, oil or other fat, or air is heated, currents begin to develop within the system and aid in moving the heat throughout the food. This circulation of heated liquids or air is a process called convection. The convection oven is based on this principle. By forcing the circulation of heated air currents in this type of oven, foods will be heated and baked significantly more quickly than they will be when circulation of heated air is limited.

All ovens maintain some circulation of heated air during operation, but the amount of movement in a regular oven is quite low. If conventional ovens are loaded so that baking pans extend to the sides, front, or back of the oven, the normal flow pattern of the hot air currents will be obstructed, and uneven baking results. It is important to avoid placing one pan directly beneath another or to prevent loading an oven with pans being jammed right against each other and against the oven walls if even baking and browning are to be achieved in a con ventional oven. However, pans can be placed beneath other pans in a convection oven satisfactorily because of the effective circulation of hot air. Convection currents are the key to successful baking in both convection and conventional ovens.

Convection is also a significant part of the heating process when foods are being heated on the range. The pan holding the food will be heated by conduction, but the heat begins to move through the food itself with the aid of warming currents of water or other liquid. Stirring is an added aid in helping to distribute the heat uniformly through the food.

Radiation

Radiation is the direct transfer of energy from the energy source to the food. Broiling is a familiar example of this type of heating. The energy involved in radiation from broiling is in the infrared range (somewhat longer waves than are in the range of visible light).

conduction transfer of heat from one molecule to the next. http://hyperphysics. phy-astr.gsu.edu/hbase/ thermo/heatra. —Information on types of heat transfer. convection transfer of heat throughout a system by movement of currents of heated air, water, or other liquid.

radiation transfer of energy directly from the source to the food being heated.

sCIenCe note