2
Heat Transfer
Exercises and Experiments
Exercise 1: Heat Transfer
Imagine that you are baking cookies in an oven and you need to slow down heat transfer, so that the cookies don’t burn on the outside before they are cooked throughout. Explain the reason that each of the following techniques works to decrease heat transfer. As an example, number 1 is completed for you.
1Use a lower oven temperature.
Reason: This is the most direct way to decrease heat transfer, since it reduces the amount of heat radiated from the heat source.
2Use shiny metal sheet pans rather than black matte ones.
Reason:
3Use stainless-steel pans instead of aluminum.
Reason:
4Replace old, stained sheet pans with shiny new ones.
Reason:
5Use thick-gauge pans rather than thin-gauge ones.
Reason:
6Use double sheet pans by placing one sheet pan inside another.
Reason:
7Keep sheet pans away from oven walls.
Reason:
8Place cookies on a silicone pad (Silpat) instead of directly on a sheet pan.
Reason:
9Turn off the fan in a convection oven.
Reason:
Experiment2: Hot Spots in a Conventional Oven
It’s hard to imagine an oven that heats evenly throughout. The next best thing to having the perfect oven is knowing where the hot spots are in your oven. The fastest and easiest way to “map an oven” is to use an infrared thermometer. Aim the thermometer at various surfaces throughout the preheated oven, and you will learn very quickly where uneven baking might occur.
Another way to find the hot spots is to bake actual product in different locations in the oven and observe where differences occur.
Objectives
Determine if, and where, hot spots exist in an oven.
Products Prepared
Cookies baked in different locations of a conventional or deck oven (no convection fans)
Materials and Equipment
•Scale
•Sieve
•Parchment paper
•Mixer with 5-quart mixing bowl
•Flat beater attachment
•Bowl scraper
•Plain drop cookie dough (see Formula) that makes 24 or more cookies if using full sheet pans, or that makes 12 or more cookies if using half sheet pans
•Two full or half sheet pans (depending on the size of the oven), as nearly identical as possible
•Size #30 (1 fl. oz./30 ml) portion-control scoop or equivalent
•Oven thermometer
Formula
Drop Sugar Cookie Dough
Yield: 48 cookies
Ingredient / Pounds / Ounces / Grams / Baker’s PercentageFlour, bread / 8 / 250 / 50
Flour, cake / 8 / 250 / 50
Salt / 0.25 / 8 / 1.6
Baking soda / 0.25 / 8 / 1.6
Shortening, all-purpose / 13 / 410 / 82
Sugar, regular granulated / 18 / 565 / 113
Eggs / 6 / 185 / 37
Total / 3 / 5.5 / 1,676 / 335.2
Method of Preparation
1Preheat oven to 375°F (190°C).
2Allow all ingredients to come to room temperature (temperature of ingredients is important for consistent results).
3Blend flour, salt, and baking soda thoroughly by sifting together three times onto parchment paper.
4Combine shortening and sugar in mixing bowl and mix on low for 1 minute. Stop and scrape bowl as needed.
5Cream shortening-sugar mixture on medium for 2 minutes. Stop and scrape bowl.
6Add eggs slowly while mixing on low for 30seconds. Stop and scrape bowl.
7Add flour to shortening-sugar-egg mixture and mix on low for 1 minute. Stop and scrape bowl.
Procedure
1Prepare cookie dough using the formula given or using any plain drop cookie formula. To minimize experimental error, use shortening instead of butter.
2If necessary, clean sheet pans to remove burned-on food. Line the sheet pans with parchment paper.
3Label parchment paper, indicating which end of pan will be at the front of the oven and sheet pan placement within the oven (top rack, against left wall of oven, etc.).
4Scoop cookie dough onto prepared sheet pans using #30 scoop or equivalent. Space dough evenly on sheet pans. Place six cookies on half sheet pans and twelve cookies on full sheet pans.
5Use an oven thermometer placed in center of oven for an initial reading of oven temperature. Record results here:
6When oven is properly preheated, place both sheet pans in oven and set timer for 19–21 minutes, or according to formula.
7Bake cookies all for same amount of time (do not rotate pans during baking).
8Remove pans from oven and cool cookies directly on sheet pans.
9Check final oven temperature. Record results here:
Results
1With cookies still on sheet pans, evaluate the amount of browning on each cookie. Use a scale of 1 to 5, 1 being lightest color.
2Record evaluations for each sheet pan by filling in the drawings of sheet pans in Figures 2.3 and 2.4. See Figure 2.2 for an example of how to record evaluations.
Back of OvenLeft Wall / 4 / 4 / 4 / 4 / Right Wall
3 / 3 / 3 / 4
2 / 2 / 2 / 3
Front of Oven
Type of Oven: Conventional
Location of pan in oven: Center rack
Figure 2.2 Sample results for experiment: full sheet pan placed horizontally on center rack in conventional oven
Back of OvenLeft Wall / Right Wall
Front of Oven
Type of Oven:
Location of pan in oven:
Figure 2.3
Back of OvenLeft Wall / Right Wall
Front of Oven
Type of Oven:
Location of pan in oven:
Figure 2.4
Sources of Error
List any sources of error that might make it difficult to draw the proper conclusions from your experiment. For this experiment, note in particular any problems with the pans (uneven bottoms, dents, or baked-on food) and the ovens (was oven temperature stable during baking?).
Answer:
State what you could do differently next time to minimize or eliminate each of the sources of error.
Answer:
Conclusions
Select one from the choices in bold or fill in the blanks.
1The difference in color between the cookies nearest the oven walls and those farthest from the walls was smallmoderatelargeno difference. The darker cookies were nearest the oven wallsin center of ovenneither. This is probably because:
Answer:
2The difference in color between the cookies nearer the back of the oven and those nearer the front was smallmoderatelargeno difference. The darker cookies were nearest the back of the ovennearest the frontneither. This is probably because:
Answer:
3What do these results tell you about whether there are hot spots in this oven?
Answer:
If there were any hot spots, what can you do in the future to compensate for them in this oven, so that they are not a significant source of error in future experiments?
Answer:
4Did you notice any other differences in the cookies, or do you have any other comments about the experiment?
Answer: