Temperature and Humidity

The proper maintenance and regulation of temperature and humidity in the production of quality Bakery Foods is of the utmost importance in regards to the efficient and successful operation and management of a Bakery. Quality ingredients and the handling of the same throughout the various stages of production will not result in a quality product unless the dough temperature, and temperature and humidity of the Bakery are properly regulated. Modern day bakeries utilize special automatic air conditioning equipment to accomplish this. In view of the important part which temperature and humidity plays in the successful operation of a bakery, it is essential that every baker understand in a general way the following points.

  1. What is meant by temperature and humidity.
  2. How temperature and humidity are measured.
  3. What effect does temperature and humidity have on the fermentation and conditioning of doughs. (This information is covered in Part Two, Bread Production Procedures).
  4. The proper dough temperature to be maintained. How determined and calculated. (This information is covered in Part Two also).
  5. The proper temperature and humidity to be maintained in the various parts of the bakery, and how this is controlled.

Temperature and Heat.

Temperature is considered the degree of hotness or coolness of any object. Scientists describe temperature as the intensity of heat.

Ways in which heat is transmitted.


Upon holding a metal rod at one end, the molecules will vibrate and pass the effect on down the line so that the other end becomes hot also. If a copper wire is held in one hand and a glass rod held in the other of the same size and length, and they are both placed over a flame, the heat will pass along the copper wire long before it will pass along the glass rod. The result will be that the copper wire will have to be dropped long before the glass rod has a chance to even feel warm where it is being held. Usually good conductors of heat are good conductors of electricity and vice versa. Liquids and gasses are poorer conductors of heat than metals. Iron conducts heat 100 times better than water and water conducts heat 25 times better than air.


In the case of water, convection is explained by the fact that when water is heated, it expands. This decreases its density and the warm water rises. The movement of the water sets up convection currents. A good example is cake batters such as marble cake. The convection currents cause swirls of batter to rise from either side and go downward in the center causing a certain design of the two types of batter being used, usually yellow cake batter and chocolate came batter.


Radiation is heat sent out in straight lines. Most of the heat in ovens is radiated heat. Light objects reflect heat and dark objects absorb heat. In turn, dark objects radiate more heat than light objects. Iron will heat or bake faster when dark than when shiny. Following are examples which will illustrate this principle:

  1. Take two pieces of sheet metal about 10 inches wide and 18 inches long.
  2. Paint one of the pieces BLACK and one WHITE.
  3. Bend about 6 inches of each piece so that the short end will face horizontal so when it is placed on a table, the vertical end will stand up straight.
  4. Melt the ends of two candles and cause one candle to stick to the dark piece of the metal and the other candle to stick to the white piece of the metal.
  5. Place the two pieces of metal about 4 inches apart with the candles facing outwards.
  6. Place a Bunsen Burner between the two pieces of metal making sure that there is equal distance between the two. Light the Bunsen Burner.
  7. Because the dark metal will absorb heat faster than the white metal, the candle will fall off long before the one on the while metal does

Another example is when baking pastries containing dark and light colored fillings. If one coffee cake is filled with a dark colored filling and one is filled with a light colored filling, and the two are placed on the same baking pan, the coffee cake containing the dark filling will bake sooner than the one with the light colored filling and the crust will be darker.

The same holds true if for example apple pies and cherry pies are being baked at the same time. The cherry pie will bake faster and to a darker degree than will the apple pie.

Bakers have also observed that the old Swedish baking pans which were dull black in color, were more efficient than our modern tin-plated pans. A bright new tin-plated pan will not bake bread as well as the same pan after it has been properly burned- in. Burning -in produces a coating of tin oxide which is dark, and therefore it is capable of absorbing more of the radiated heat from the oven. In turn, the radiated heat is transferred to the bread being baked. The burning-in is accomplished as follows:

  1. The new shiny pan is placed in an oven having a temperature of 375 degrees F. to 425 degrees F. for a period of several hours until the surface starts to acquire a bluish tinge. It is recommended that the oven doors be left open to permit free circulation of air throughout the baking chamber.
  2. Caution must be taken not to place the baking pans in oven having a temperature above 425 degrees F. , because tin has a melting point of approximately 439 degrees F. If the tin is allowed to melt, it will peel away from the steel plate, leaving the steel exposed so that it may rust.
  3. While the pans are still warm, grease them lightly with a smooth flowing shortening. The melted fat fills in the tiny pores in the metal which have been opened during the heating process. When the metal cools, the pores are closed, entrapping a small amount of shortening rendering the pan ready for the baking process. Note: If the pans are to be glazed, it will not be necessary to grease the pans. Usually pans are sent to a company which specializes in the glazing of pans. The glaze will last for quite some time, and when the bread starts to stick to the pans, the glazing process must be repeated.


The British Thermal Unit (BTU).

In measuring the amount of heat used in accordance with the British System, a unit termed as British Thermal Unit or B.T.U. is employed. This is the amount of heat required to raise the temperature of 1 pound of water 1 degree F. Thus, the specific heat of water is defined as the amount of heat necessary to raise its temperature 1 degrees F. The specific heat of all other substances that you use every day, have specific heats lower than water. Therefore, 1 BTU will raise one pound of any other common substance more degrees F. than it will raise 1 pound of water.


BTU will raise 1 Pound of. Degrees F.

  • one Water 1
  • one Bread Dough 1-1/2
  • one Cake batter 2
  • one Flour 2-1/2
  • one Aluminum 5
  • one Copper 10
  • one Silver 20
  • one Lead 30

Heat of evaporation of water.

When a container of water is placed on a burner, the water gets hotter and hotter until it boils. Then no matter how much heat is applied to the container, if there is a free outlet for the steam to escape, the temperature remains constant at 212 degrees F. at sea level. The heat energy which seems to be absorbed in the process of boiling the water with out change in temperature is called the Heat of Evaporation or the Latent Heat of Steam. The heat of evaporation is the energy needed to pull the molecules of water from one another and set them free as steam. Experiments have shown that the heat of evaporation of water is about 972 BTU'S. In other words, it takes 972 BTU'S of heat to change 1 pound of water at 212 Degrees F. into steam at 212 degrees F. It takes more than five times as much heat to change 1 pound of water into steam as it does to change 1 pound of water from the freezing point 32 degrees F, to the boiling point of 212 degrees F.

Boiling point of water at High Altitudes.

Degrees F. Altitude in Feet Location.

  • 210, 1000, Phoenix
  • 208, 1,900, Spokane
  • 206, 2,900, Boise, Idaho
  • 204, 4,100, Helena, Mont.
  • 201.5, 5,300, Denver, Col
  • 199, 7,000, Santa Fe, N.M.
  • 186, 14,000, Pikes Peak

Water will boil at lower temperature under vacuum.
Water will also boil at higher temperature under pressure.

Baking Cakes at High Altitudes

As we go higher in altitude, the pressure of the atmosphere becomes less, therefore less leavening action is needed. Up to 2,000 feet no adjustment is needed. After 2,000 feet up to 15 percent less baking powder is needed, and progressively less is needed as we go higher. Past 5,000 feet reduce baking powder 45 percent. Past 6,500 feet reduce baking powder 60 percent and increase water by 20 percent. Water is increased because water boils at a lower temperature and there is an increase in evaporation. (At Pikes peak, a pressure cooker is necessary to cook beans and potatoes, because water boils at 186 degree F rather than 212 Degrees F, at sea level so it doesn't get hot enough to cook them). Additional Adjustments: 1. beginning at 2,500 feet add 2-1/2 percent more eggs or egg whites. Gradually increase eggs until at 7,500 feet 15 percent more eggs are needed. 2. Over 4,500 feet grease baking pans slightly heavier, and increase oven temperature approximately 25 degrees F. 3. Less beating of eggs and egg whites are needed for Angel Food Cakes and Sponge Cakes.

Temperature Fahrenheit and Centigrade Scales.

The measurement of temperature is expressed in terms of degrees.

  1. According to the Fahrenheit System, 32 degrees F. indicates the temperature at which ice melts, and 212 degrees F. is the temperature at which water boils under standard conditions (at sea level).
  2. According to the Centigrade Scale, the freezing point of water is 0 degrees C, and the boiling point is 100 degrees under the same conditions.

Means of Measuring Temperature.

The Necessity of Temperature Measuring Instruments.


Ordinary glass tube thermometers may be used for determining the temperature of the ingredients. However, a standard Dough Thermometer with a long metal base should be available for use in checking the dough temperature. Fahrenheit thermometers ordinarily are used in bakery, but if Centigrade readings must be used, the Fahrenheit readings can be converted easily to Centigrade degrees.

Converting Fahrenheit degrees to Centigrade degrees

Subtract 32 from the Fahrenheit reading. Then multiply the answer by 5/9. This will give the Centigrade degree

Example: Suppose it is desired to express 77 degrees F. in terms of degrees Centigrade, then 77 minus 32 equals 45. 45 times 5/9 equals 25 Degrees Centigrade. The dough thermometer is used to check the temperature of the dough immediately after completion of mixing it has been removed from the mixer.


The ordinary mercury thermometer is perhaps the best known instrument for measuring temperature. The operation of a mercury thermometer is based on the fact that mercury or quicksilver expands when heated and contracts when cooled to a much greater extent than glass. The amount of this expansion or contraction corresponds to the change in temperature and therefore may be used to indicate the existing temperature at any time.

A simple mercury thermometer consists of a heavy glass tube of very narrow bore and has a bulb at one end containing mercury. The other end of the tube is tightly sealed. The thermometer tube is graduated or marked so that it will indicate the correct prevailing temperature to which the thermometer is exposed to. Mercury thermometers can cover ranges of temperatures between 38 degrees below zero Fahrenheit and about 900 degrees above zero Fahrenheit.


The basic principle upon which this type of instrument operates is similar to that described for the mercury thermometer. The bulb tubing is filled with mercury gas. The bulb is placed at the point where temperature is to be determined. A rise in temperature at the location of the bulb increases the internal pressure which is transmitted through the tubing. The mechanism is so arranged that this increase in internal pressure causes a hand to move across a graduated dial. The extent of this movement naturally depends on the temperature at the location of the bulb. Therefore the temperature is indicated on the dial accurately and immediately.


This instrument is sometimes called an electric thermometer, but is different from either the mercury thermometer or the metallic expansion thermometer in it's mode of operation. The electric pyrometer is generally used for high temperature measurements such as that of ovens.


In order to secure a permanent record of any particular location, (fermentation room, proof box, or oven) recording devices have been constructed in connection with various types of temperature-indicating instruments. These recording attachments usually consist of a chart which is gradually moved by clockwork and a pen point which is attached to the temperature indicating needle. This pen point rests on the moving chart and in this way marks down the temperature in the form of a line. Therefore at the end of the day there will be a complete record on the chart of the temperature at all times during the day. Such recording devices are used to good advantage in modern bakeries, and are generally termed Recording Thermometers.

Temperature Regulating Devices.

In bakeries equipped with air conditioning equipment where air is circulated through the dough fermentation room, make-up room or proof box at the desired temperature and humidity, the problem of maintaining proper atmospheric conditions is considerably simplified. Temperature regulating devices are available for doing this automatically. Such regulators are often spoken of as thermostats.


The function of this instrument is to control the temperature of gas or electric baking ovens, or doughnut machines, by opening or closing motor operated valves, or other electrical devices. It can be set for any desired temperature and the control point is quickly and easily changed. The bulb is placed at the point where the temperature is to be controlled and the case containing the indicated dial may be located at any convenient point. The temperature changes at the bulb are transmitted by pressure through the tubing to the mechanism which starts or stops the motor or other controlled device. At the same time the instrument indicates the actual temperature at the location of the bulb.


This instrument not only regulates the temperature, but records on a chart, the temperature that has been maintained. The temperature may be quickly changed by inserting a turning key which moves a pointer on the chart to any degree of temperature desired. Such an instrument is accurate and sensitive. It operates by means of compressed air, opening or closing valves in order to regulate the temperature. It may be used in connection with proof boxes to control the temperature or as a double duty system to control both the wet and dry bulb temperatures, thus regulating the relative humidity, and at the same time producing a record of these temperatures on one chart.


Roughly speaking, humidity means the wetness of the atmosphere, or in other words, the amount of moisture or water vapor contained in air. It is a well known fact that on some days the air is dryer or less humid than on other days.


Relative humidity means the relative amount of moisture contained in air at a definite temperature in comparison with the amount of water vapor which air at that temperature is capable of holding. Relative humidity is expressed in terms of percentage. All air naturally available under ordinary conditions contain some moisture, but let us suppose, for the sake of example, that we did have air at a certain temperature which contained no moisture. This air would have a relative humidity of 0.0 percent. If however, this air at the same temperature were saturated with all the water vapor it could hold, then it would have a relative humidity of 100 percent. If this were to contain 70 percent of the maximum amount of all the moisture it could possibly hold at this temperature, it would have a relative humidity of 70 percent. The higher the temperature, the greater the amount of water vapor that can be held by air. Therefore, hot air can hold much more moisture than cooler air, and in the summer time, the air is often more humid than in the winter time. If air with fairly high humidity were chilled, it would soon have a relative humidity of 100 percent and then drops of water would begin to separate out in the form of dew.


The maintenance and control of humidity in all parts of the bakery, dough fermentation room, proof box, oven etc. is of the utmost importance in the efficient production of quality bakery foods. The proper humidity in different parts of the bakery may be secured by the use of automatic air conditioning equipment as explained earlier.


The percentage of relative humidity in the various parts of the bakery is measured by an instrument known as a hygrometer. There are several different types of such instruments which can be used.


The principle, on which the wet and dry bulb thermometer operates so as to indicate the relative humidity of the atmosphere, is based on the fact that when water or any liquid evaporates it has a cooling effect; and that the faster this evaporation, the greater the cooling effect. This point may be demonstrated by wetting the hand with water and then fanning the moistened surface so that the water will evaporate or dry quickly. Everyone is familiar with the cooling sensation produced. The wet and dry bulb thermometer arrangement merely consists of two thermometers identically alike, mounted close together on a frame.

The mercury bulb is one of these thermometers when left exposed to the air. This is called the dry bulb thermometer. The mercury bulb of the other thermometer is tightly covered with a wick or cloth, the other end of which extend into a small vessel or tube of water. The water soaks up through the wick and the mercury bulb is surrounded by a thin layer of water and is therefore always wet. This is called the wet bulb thermometer. Due to the cooling effect of the evaporation of the water, the wet bulb will ordinarily read lower than the dry bulb thermometer. The difference in the readings of these two thermometers at any one time depends upon the rate of the evaporation of the water surrounding the mercury bulb of the wet bulb thermometer.

Since water evaporates more slowly in humid air than it does in drier air, it can be easily seen that there will be less evaporation of the water in the wick where the relative humidity is high than when it is low. Consequently, the lower the relative humidity the greater will be the difference between the readings of the wet and dry bulb thermometers, and vice versa. If, for instance, the air is saturated with moisture, or in other words, if the relative humidity is 100 percent, then there would be no difference at all between the readings of the two thermometers.

In using the wet and dry bulb thermometer arrangement it is necessary that an adequate supply of clean distilled water be kept in the reservoir at all times so that the wick will always be thoroughly wet. Frequent replacement of wicks by new ones is important. The entire wick must also be kept clean and free from dirt or anything which would interfere with the free seepage of water through it. It is also a good idea to fan the wet bulb thermometer slightly before making the actual readings.

The actual difference in temperatures noted is an indication of the percentage or degree of relative humidity of the atmosphere at the time and place that the readings are made. By referring to a chart which may be secured from the U. S. Weather Bureau, the actual relative humidity can be secured. Such charts are usually supplied with the wet and dry bulb thermometers when purchased along with instructions on how to read the chart.

The wet and dry bulb thermometer is used to register the temperature of the wet bulb and the dry bulb thermometer of the fermentation room, and of the proofing cabinet. This is done so that the percent relative humidity can be determined by referring to the relative humidity.

See step by step procedure for reading the relative humidity table to determine percent relative humidity of the fermentation room and of the proofing cabinet.


At ordinary barometric pressure the relative humidity chart will give the baker the necessary data covering the range of relative humidity’s ordinarily encountered in the different sections of the bakery. In using such a chart proceed as follows:

  1. Note the room temperature by reading the dry bulb thermometer.
  2. At the same time read the wet bulb thermometer.
  3. Refer to the Relative Humidity Table, locating the reading in the left-hand vertical column which corresponds to the existing room temperature.
  4. Follow this line over horizontally until it meets the column headed by the number of degrees representing the depression on the wet bulb thermometer (or in other words the difference between the dry bulb and wet bulb thermometers). The number located represents the existing percentage of relative humidity.

Example: Suppose the reading of the dry bulb thermometer is 80 degrees F. and the corresponding reading of the wet bulb thermometer is 72 degrees F. The difference between these readings is 8 degrees. Therefore the existing relative humidity is 68 percent.


This instrument is a special type of wet and dry bulb thermometer, and the principle of its operation is basically the same as the one discussed above. However, the humidity guide is equipped with a semi-automatic scale located between the individual wet and dry bulb thermometers. This scale can be adjusted by a knob at the top, so as to determine the existing relative humidity as indicated by the difference in temperature of the two thermometers. This instrument indicates relative humidity with unusual accuracy, but at the same time is small and easy to use, and does not require reference to a separate Relative Humidity Table as is the case with the ordinary wet and dry bulb thermometer. The wet bulb, which is kept moist by a wick running into a water container, should be fanned before taking a reading, as with the ordinary wet and dry bulb thermometer.


The hygrodeik is a special form of wet and dry thermometer which is so arranged, that reference to the relative humidity tables is not necessary. The hygrodeik is constructed with a certain chart placed between the two thermometers. This chart is so drawn that curved lines start from all points on each thermometer. Whenever it is necessary to ascertain the percentage of relative humidity, the small sliding pointer is moved to the scale to the left and set at the temperature on the scale corresponding to the reading of the wet bulb thermometer. The index arm of the instrument is then swung to the right until the line meets the curved line originating to the degree on the right hand scale corresponding to the reading of the dry bulb thermometer. When the sliding pointer is directly over the intersection of these two curved lines, the prevailing relative humidity will be shown by the location of the index arm over the scale at the bottom of the instrument. Therefore the percentage of relative humidity may be read directly.

RECORDING HYGRODEIK (wet and dry bulb type).

A familiar form of recording hygrometer is an instrument consisting of a combination of wet bulb thermometer and a dry bulb thermometer. This instrument is constructed so that the respective temperatures registered by each of these instruments will be recorded in the form of lines drawn by pen points on a revolving chart which constitutes the face of the instrument. Therefore, there will be a continuous record of the wet and dry bulb thermometer readings at all times. From these two readings as recorded on the chart, the percentage of relative humidity can be readily secured for any given time. In this way, the baker will have a permanent record of existing relative humidity at the location of the recording hygrometer.


1. A Treatise on Baking, Standard Brands, Inc.
2. Conversion Factors of the Industry, Research Department, Pillsbury Mills, Inc.
3. Notes taken during classes at the American Institute of Baking, and at the Oklahoma Tech School of Baking. American Institute of Baking

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