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• describe influences of energy transfer on food quality.
• describe energy transfer involved in the processing and preparation of a food item.
• define and describe the way the first law of thermodynamics functions in foods.
• summarize the relationships between forms of energy.
• summarize and identify why foods differ in their ability to transfer energy.
• identify and describe specific methods and factors which influence heat transfer.
• interconvert degrees fahrenheit and degrees celsius.
• define, list, and identify the processes of microwave cooking.
• delineate the advantages and disadvantages of microwave cooking.

Why cook?

We cook to bring about changes in flavor, texture and color. The effects of cooking food include:

Increased edibility and palatability.

Improved sanitary quality of a food.

Improved digestibility and nutritive value of a food.

Increased quality or shelf life of some foods.

Creation of new and delectable dishes.

These changes may occur as the food is melted, volatilized,sublimated, solvated, hydrated,gelatinized, coagulated, dispersed, and/or concentrated.

In Foods the forms of energy of interest are:

	heat is the result of energy. I have some trouble deciding whether we can call it an energy form or it is a result of energy. In any case, one needs to be aware that it has major impact on food preparation and processing.
	surface and electrical generally is energy which is converted to heat such as the use of an electrical or gas range.
	chemical energy is that produced through a chemical reaction. Baking powder is likely our best example of such reactions.
	mechanical may be the kneading of bread or, as in this case, the fermentation of yeast has an impact on leavening, increasing the volume, of the bread. The yeast does this in at least two ways. It will actually increase the the volume of the dough during fermentation and proofing through the production of carbon dioxide. During the baking of bread, the alcohol produced by metabolism will also produce the "oven spring" we find in the bread. In addition to kneading, it may include shaking, stirring, pounding, chopping, beating, and crushing.
	biological those of you reading this are using biological energy [ATP, etc] to accomplish it. In foods, biological energy may be the source of food preparation and processing energy [coal, wood] or it may be more directly the source of importance such as in the leavening due to yeast action.
	solar for those of us who go out hiking and/or work in underdeveloped countries with minimal energy resources, this form of power still is a tremendous resource.

The First Law Of Thermodynamics states that energy is neither created nor destroyed only changed from one form to another. This is of more than esoteric interest in foods. We know that there are a number of types of energy. Part of the problem is measuring the result as heat or action versus the source such as electrical, water, muscle power. This picture of the dam at Bonneville in Oregon, the coal mine in Southeast Kansas both indicate a manner where gravity or a coal is changed into electricity which is converted to heat on the burner. There are many other examples of the rather simplest application of the First Law of Thermodynamics. Certainly, there is a tremendous amount of research and investigations trying to minimize leakage of energy during such conversions. That is, as one changes from one form to another, there is an attempt to have no loss of energy during the conversion. Perfect conversions would allow for accounting all energy from beginning to end in an enclosed system. Of pertinence to those of us in the food industry is acknowledgement of these conversions and an attempt to minimize wastage and, thus, increase profits through lowered energy costs.

How Is Heat Transferred?

Conduction Convection Radiation

Heat is developed in foods by friction, chemical change or physical change. Once the heat is formed or developed, it may be transferred by conduction, convection. or radiation. These are methods of transferring heat. The extent of transferring heat is determined by obtaining temperature data.

Conduction Conduction is a method of heat transfer where molecular energy is transferred directly from one molecule to another. Energy transfers from regions of higher to lower temperatures. There is no need for physical movement of the mass from one location to another. The cookies above are a perfect example of conduction;.. While the direct transfer of heat is rapid in the cookie product, in other products such as solid-pack pumpkin, conduction is rather slow.This is partly due to the low thermal diffusivity of the food itself.

Convection

The simplest convection currents are exampled by the beaker of liquid illustrated to the right.
Another classic example is the air currents around the cake layers baking in the oven .
These dynamics are even involved in the hot cup of cocoa you might enjoy on a cold winter's day.

Convection Convection ovens are used in food preparation, particularly at the institutional or food processing plant level. These ovens have a fan or other apparatus to accelerate the movement of air. In addition to convection ovens, some soups and sauces are heated by convection created by an agitator which mechanically stirs the product. Convection heating uses either liquid or air currents to bring about the heat transfer between molecules.

Radiation Two Basic Kinds of Radiation IONIZING NON-IONIZING xrays radiowaves gamma rays microwaves cosmic rays infra-red waves ultraviolet rays light waves To put it simply, ionizing radiation brings about chemical changes and non-ionizing radiation contributes energy in the form of heat. X-rays, gamma rays, and ultraviolet rays are all ionizing-- used to denature certain proteins and preserve foods. One example of this is that ultraviolet rays have been used in meat storage containers and bread sponge dough cabinets. The microwave oven is a common example of non-ionizing radiation. However, radiant energy has been a source of heat transfer for centuries. Broiling foods involves some amount of radiant energy.

There are better, more precise methods of determining the degree of this heating.

Heat penetration is one of the oldest methods used are the temperature profiles that the canning industry uses. These are a number of different derivations obtained from these curves and associated data. Survivor curves is an expression of logarithmic reduction in the concentration of bacterial spores with time for any given lethal temperature. This is often a first-order kinetics reaction of thermal inactivation of bacteria. A logarithmic order of deal is a reaction describing the thermal inactivation of bacterial spores and is described mathematically with a common(base 10) logarithm. A survivor curve is obtained by a semilog plot to again show the logarithmic reduction in the number of surviving spore over time when exposed to a lethal temperature. Another expression to indicate quality of energy input [heating] is thermal death time (TDT) curve which reflects the temperature dependency of elimination of viable spores.

Cooking time. is used by many to determine doness and time of cooking. This may be done on the basis of total time, minutes per pound, or degrees celcius/minute/pound.

Heating and cooling curves have been plotted by many researchers. Such curves to a given endpoint, present a little more data that just cooking time or cooking time/weight.

There are a number of factors that impact the ability to heat a food, the thermal properties. One of them is whether or not there is any direction to the product. In this particular instance the vegetable soup would heat much faster than the cream of mushroom soup. The reason is there can be some convection currents formed. Foods heat differently because they have different thermal properties. That is, the heating of a food is dependent upon the ability of the energy to pass the food barrier, to be conducted within the food and to bring about the change in foods. There are considerable factors that evaluate the food. In the example of the soup above, the vegetable soup heats faster as convection currents can arise. Generally, the higher the water content in foods it heats faster.

Heat and energy transfer is impacted by a number of different factors. These are summarized in the following table.

FACTORS INFLUENCING HEAT TRANSFER

Sources of Heat Whether on an electric range or over the outdoor fire place, whether heat is transferred by conduction, convection, or radiation, all transfer energy.
Utensils UsedGlass, metals, and plastic all transmit heat energy differently. This is important when discussing speed of heating.
Nature of Food Conductivity value is directly related to speed of heating. Cream and broth soups represent this aspect of how the nature of a food will impact its heating rate. Of the two products, the vegetable soup would likely heat much faster than the cream of mushroom. The more liquid a food, the higher its thermal conductivity value. The more viscous, the lower its thermal conductivity value.	This image has the same conductivity issue as the soups above,but there are also a number of other factors which will influence the thermal properties.

Temperature change is used in foods to bring about chemical changes and/or maintain a specific quality. Following are some representative temperatures that recipes sometimes use.

C	F	Description
-18	0	maximum possible frozen temperature with ice/rock salt
0	32	freezing point of pure water
0	32	usual freezing temperature for fruits and vegetables
3-5	38-40	ideal refrigerator temperature
23	72-4	room temperature
37	98	lukewarm temperature (body temperature)
65	150	scalding temperature
85	185	simmering temperature
100	212	boiling point of pure water & boiling slowly
100	212	rapidly boiling water

Notice that these temperatures are given in both degrees Celcius and degrees Fahrenheit. Food scientists should be able to make the conversion necessary. The measurement of these different temperatures can be done with a number of recording devices. In each case, regardless of the type of device, it is vital to measure at eye level, and with the device placed appropriately.

The actual conversion is as follows:

oC = (F - 32) times 5/9

Sample C = 212 - 32 = 180 now multiply by 5/9 9 goes into 180 at 20 times and 5 times that = 100 The instructor always uses this as she never can remember which equation goes to which.

F = (C* 9/5) + 32

The electromagnetic spectrum is a graphical representation of types of waves and their relative frequencies, or radiant energy outputs. Induction heating waves and microwaves are most frequently used to heat foods. For information on the irradiation of foods, check out the Food Resource link. Radiant waves are characterized by wavelength and by frequency. As seen in the diagram below, the more space between the start and end of a wave, the less energy it carries. The closer together waves are, the higher their frequencies and energy outputs are.

As noted there are different wave lengths and frequencies.

The microwave oven itself works by the movement of polar substances back and forth, causing friction to occur. There are a number of points of interest about microwave ovens. For example, not all microwave ovens cause the friction in the same manner. It is critical that there is an understanding of the way the microwave oven works. Microwaves themselves are generated by a magnetron tube which converts electrical energy at 60 cycles/sec into an electromagnetic field with positive and negative charges that change direction billions of times per second. It contains polar and non-polar molecules and functions by switching back and forth. This switching back and forth causes molecular friction by disrupting hydrogen bonds between neighboring water molecules. Thus, in order to have microwave heating of foods there must be a polar substance available. Fortunately, water is such a polar substance, and is a constituent of all foods to at least some degree.

Another factor in microwave heating is that the positive and negative ions of table salt in foods will interact with the electrical field. By migrating to the oppositely charged regions of the electrical field, they disrupt the hydrogen bonds with water and generate heat.

A number of factors will influence the speed of cooking of a microwave oven. The electrical and physical properties of a particular food determine microwave penetration depth, heat capacity, density, and the conventional heat transfer and heating rate.

Radiant energy is translated into heat in microwave cooking. There are advantages and disadvantages as indicated below.

Advantages of Microwave Cooking	Disadvantages of Microwave Cooking
speed of heating	does not surface brown unless special utensils or a special oven are used
energy savings in comparison to conventional ovens	food characteristics may be time and temperature dependent
oven walls and surrounding air do not become hot.	overcooking of foods is easy (dehydration, charring possible)
special advantages in reheating, precooking, and thawing frozen foods	unevenness of cooking
	microbial safety concerns
	variability in heating times when different foods are heated together

It has become apparent that even microwave cooking will change flavor. Flavor release in microwave cooking is increased by steam distillation. The actual water vapor serves as a mechanism for rapid transfer between the food and air. Another problem is the "bumping" which may occur in microwave cooking. This popping or spattering is the explosion of food particles, not the boiling of liquids. It is the result of local superheating. The container shape does influence the heating pattern of a product and influences bumping.

Although there are considerable problems with microwave cooking, there is constant research to solve them. The use of special packaging with a susceptor. One of the problems with microwave cooked foods is that of browning and of "crisping". In reviewing the many microwave cooked foods in the supermarket, one sees that directions indicate that they should be cooked on the reversible side of the packaging. The susceptors will permit the surface to actually heat up in the microwave oven. The susceptors are the metallic material on the "pan" bottom aimed at improving browning.

GLOSSARY

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blanch boiling point braise calorie conduction convection currents deep-fat frying dielectric properties Fahrenheit heat susceptors heat transfer latent heat latent heat of condensation latent heat of crystallization latent heat of fusion latent heat of sublimation latent heat of vaporization microwave poach polarized molecules radiation rehydrate saute scald specific heat sterilization stew susceptors tempering vapor pressure

REVIEW

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Describe the way energy changed and prepared a food that you ate or drank today.

How is heat measured?

What is heat?

List the forms of energy.

Indicate the energy involved in preparing a cheese souffle until it is served to a family.

The Commander of StarShip Feast (SFC) indicated they need to minimize energy consumption. The SFC saw no reason why they could not just shuffle energy and cooling needs back and forth within the Ship. Is this feasible to meet the diverse needs to the StarShip? Briefly discuss the basis and consideration for your answer. What is the law which the statement "Energy can not be created or destroyed" referring to.

What is the first law of thermodynamics?

Which of the following products is likely dominantly heated by conduction heating? What is the dominant form of heating for each food.

Identify and differentiate the speed of heat transfer of the following four cakes

*one baked in a glass square pan *one baked in a black aluminum square pan *one baked in a shiny aluminum round pan *one baked in a shiny aluminum square pan

List and discuss the advantages and disadvantages of the use of the microwave for the heating of foods.

In the following diagrams indicate the major form of heat transfer at the question location
-center of the chicken broth soup.
-juncture of the burner and the pan.
- surface of the broiled steaks
- surface of the baked cake

What are the implications of latent heat in food processing/preparation? (Why is it important to know something about it?

In discussing energy, heat transfer, temperature, what is a manner in which one can actually record and/or measure this?

What is the basic standards for temperature measurement? What measurement techniques are used? Discuss the principle involved in each.

What is temperature? How is it measured?

One day when beginning to cook the roasts, the chefs indicated they were going to heat the roasts to an endpoint temperature of 165F. Being patriotic, Hillary said they must follow the American dream of conversion to the metric system. How many degrees Celsius should the cooks roast the roasts to?

Describe how heat transfer occurs in microwave cooking. What are the implications of using a frozen foods in microwave cooking? Explain.

Since the magnetron field works as a rapidly changing magnet are there foods which will become magnetized or end up "sticking to the roof of the microwave oven" as they contain metal ions?

Do microwaves heat from the inside out or the outside in?

If one wished to heat food, would they use ionizing or non-ionizing radiation?

Does microwave cooking really heat faster than conventional cooking?

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Updated: Wednesday, October 24, 2007.