Reflectance spectroscopy of sugar cookie baking


Several years ago, a segment of How It’s Made on the Discovery Channel revealed how potato chips were made (Discovery Channel Series 2-1, Episode 14, Segment C, 2002). While the segment is no longer being aired, it can be found with a quick search on As the chips exit the fryer, an electric camera identifies the chips with brown/black spots or other defects and blows them out of the production line. The video inspired an investigation of the color changes a simple sugar cookie undergoes as it bakes. Sugar cookies of similar size and weight were baked at the same temperature for increasing bake times and the reflectance spectrum was measured for each. 


The sugar cookies were made using the following recipe: 1 lb. margarine, 1 cup sugar, 4 cups flour, 1 teaspoon vanilla, with the recommended baking temperature of 350 oF (177 oC). A very similar weight of dough was pressed into a Wilton Industries® molded cookie sheet with circular indentations to maintain uniform shape and size (see Cookie Data Table).

The reflectance data was taken with an Ocean Optics Spectrometer with integration time of 8 sec, boxcar smoothing of 10, between 350-750 nm (2048 pts.). An Ocean Optics diffuse reflectance standard (PTFE) was used for calibration with a UV-Vis balanced spectral light source.

As the cookies bake, they lose moisture and begin to brown, and if left in the oven long enough, burn and char via many complex organic reactions. The most important reaction being the loss of water with the complicated reactions between the sugar, flour and margarine to form the solid cookie. Baking times for the optimum golden sugar cookie will vary with the recipe and the oven used. As a result, each recipe’s author offers the optimum baking time relative to their own experience and oven used. 

The reflectance spectra of the cookie at the same temperature, but at various baking times, shows an interesting trend. At 0 minutes the cookie has a pale yellow-white color with about 12% reflectance relative to the reflected light from the white standard. As the cookies bake, water loss ensues along with numerous organic chemical reactions causing the reflectance to rise to about 28%. The maximum overall reflectance of light correlates to the optimum baking time suggested by the recipe of 10-12 minutes with a 7-8% weight loss. At 15 minutes, enough water loss has occurred that the rest of the cookie’s ingredients continue to (react) burn, until a charred sugar cookie results at 60 minutes, and gives 5% or less reflection of light relative to the white standard. 

Reflectance spectroscopy of a sugar cookie baking six cookies on gradually starting while going to a darker brownIf a reflective probe could be designed for home ovens1 to measure the reflective spectrum of the cookies as they bake, the best baking time2 could be more accurately determined. The uniform cookie dough amounts with similar size, shape and dough thickness would allow for a more accurate time recommendation, and along with monitoring the cookies’ reflective spectra, would assure the baker not only of a completely baked cookie, but more importantly, when the cookie is about to burn. 

The recipe used in this investigation recommended a baking time of 10-12 minutes. This corresponded well to the prized golden sugar cookie. For this investigator’s taste and particular oven, 11 minutes was the ideal time for the perfect sugar cookie. 


  1. The fiber optic cables would need to be protected from the heat.
  2. Baking time depends on having similar amounts of material and uniform dimensions of the dough. These variables were controlled in this experiment.
Dough weight  (g) Diameter (cm) Thickness (mm) Bake time (min) Weight after bake (g) Percent weight change
60.2 8.5 8.0 0 60.2 0.0%
60.3 8.5 8.1 5 59.4 1.5%
60.8 8.7 7.7 10 57.2 5.9%
60.2 8.3 7.7 12 56.2 6.6%
60.2 8.2 8.2 15 54.8 9.0%
60.2 8.2 8.3 30 51.6 14.3%
60.2 8.5 8.2 40 50.4 16.3%
60.2 8.5 8.3 50 50.0 16.9%
60.5 7.9 7.8 60 49.2 18.7%

Table: Cookie data

A graph showing different lines of percent reflection verse wavelength for cookies baked to different times - 10 lines are shown with the top line is 12 minutes, the bottom line is 60 minutes

 Questions for students:

  1. Assuming the weight loss is due to mostly water loss, calculate the number of moles of water lost from the cookie dough at 5, 10, 12, 15, 30, 40, 50 and 60 min. 
  2. Plot the moles of water lost versus baking time. Should the overall shape of the graph be linear?
  3. Now plot the grams of weight loss versus baking times. How does the graph in Q2 compare to this graph? Why?
  4. Plot the baking time versus the percent reflection (estimate from the graph) for the cookie at 500 nm, 560 nm and 640 nm. How do the graphs compare? What is their overall shape?
  5. Describe what is happening to the overall amount of light reflected from the cookies from 0-12 min? What happens to the overall reflected light after 12 minutes?

Selected answers  

Graph of moles of water loss of a cookie at different baking times - showing a positive trend upwards and leveling off

Graph of percent reflection at 500 nm at different baking times showing a downward parabola with the maximum at 12 minutes