Cooking Chemistry: Not-Your-Traditional Pumpkin Pie
With the holidays right around the corner, many of you are planning meals for the festivities. Pies are a staple of the harvest season, but it is likely that you have not considered the chemistry involved in making one of those delicious pastries. A field of science known as molecular gastronomy examines the physical and chemical transformations that occur during cooking. This year, prepare a pumpkin pie from scratch and explore the complex science behind baking.
- ½ cup Vegetable Shortening
- 1½ cups + 1 tbsp All-Purpose Flour
- 1 tsp Salt
- ½ cup Cold Water
- 1 Sugar Pumpkin
- 2 tbsp Cooking Oil
- 2 Eggs
- 1 cup Packed Light Brown Sugar
- 1 tsp Ground Cinnamon
- ¼ tsp Ground Nutmeg
- ¼ tsp Ground Ginger
- 1⁄8 tsp Ground Cloves
- 1 Can Evaporated Milk (12 fl oz)
- Measuring Cup
- Measuring Spoons
- Mixing Spoon and Fork
- 3 Mixing Bowls (large, medium, and small)
- Rolling Pin
- Pastry Cloth
- Small Knife
- Baking Dish
- Aluminum Foil
- Pie Plate
- Kitchen Oven
Note: Readers are reminded that food must never be prepared and consumed in a laboratory not expressly designed for those purposes. The following instructions assume preparation and consumption occur in a home or school kitchen. Take care around the oven and sharp utensils. Keep an eye on your assistants. Have fun!
- Wash and dry the sugar pumpkin to remove dirt and chemicals. Cut around and then remove its stem, and then cut the pumpkin in half and remove the seeds. The pumpkin’s orange color is due to large amounts of lutein, plus alpha- and beta-carotene. When ingested, these chemicals are converted to vitamin A. Optional: Save the seeds and roast them for a snack packed with magnesium, zinc, and nearly as much protein as a serving of beef.
- Line a baking dish with aluminum foil. Lightly oil the foil. Place the pumpkin halves on the foil cut side down. Bake at 325° F (165° C) for 40 min. Heat causes the cells of the pumpkin’s fleshy endocarp to rupture, creating air pockets that result in a tender flesh. The pumpkin is ready when the flesh easily gives when poked with a fork.
- Cool the pumpkin to room temperature. Use a fork or spoon to scrape the pumpkin flesh from the peel. The peel is rich in nitrogen and can be composted with carbon-rich materials, such as wood chips.
- Mash or puree the pumpkin flesh, small batches at a time, in a blender. This process continues to break down the network of plant cell walls and rupture cells, releasing a rich mixture of fats, proteins, and water. Increase the oven temperature to 450° F (230° C) and set aside the puree.
- Prepare the pie crust by mixing ½ cup shortening, 1½ cups flour, and ½ tsp salt in a medium bowl with a fork until very crumbly. Slowly add cold water until the crust sticks together. The amount of water needed will vary based on the type, age, and stability of the flour. Water causes 2 proteins in flour, glutenin and gliadin, to undergo a hydration reaction. Disulfide bonds join the 2 proteins, forming gluten, an elastic protein that gives the pie crust its structure. See Fig. 1.
Figure 1 When gliadin and glutenin (in flour) are mixed in the presence of water, sulfide bonds form between the 2 proteins, forming a network of gluten.
- Use a fork to lightly mix the dough. Mixing not only encourages uniform gluten networks, but also helps to blend the water-soluble and water-insoluble ingredients. Starch (a polysaccharide) from the flour and shortening (a lipid) are not water soluble. Starch granules begin to swell with water, the first step in gelatinization, an important baking process that helps to hold baked goods together.
- Roll the dough on a floured pastry cloth, forming a circle about an inch larger in diameter than the pie plate. Lift the dough and press it into the plate. Use a small knife to trim the dough to about ½" beyond the plate’s rim. Fold up and pinch the dough along the rim.
- Crack 2 eggs open into a large bowl and slightly beat them. This mechanical process denatures the proteins in the egg yolks and whites, unraveling them into amino acid chains. Add 2 cups of the pumpkin puree and 1 can of evaporated milk (12 fl oz). Stir well after each addition. The pumpkin puree is rich in water, so evaporated milk is substituted for regular milk. Evaporated milk has about 60% less water than regular milk, but still contains lactose (see Fig. 2), a disaccharide that lends a sweet flavor, and butterfat triglycerides (see Fig. 3) that provide a creamy texture.
Figure 2 Lactose molecule
Figure 3 Typical butterfat triglyceride, composed of myristic, palmitic, and oleic acids.
- Mix 1 cup light brown sugar, 1 tbsp flour, ½ tsp salt, 1 tsp ground cinnamon, ¼ tsp ground nutmeg, ¼ tsp ground ginger, and 1⁄8 tsp ground cloves in a small bowl. Light brown sugar contains 3.5% molasses, or cane syrup. Molasses is hygroscopic, attracting and holding water from the environment, which gives brown sugar a soft texture.
The spices contain chemicals that give the pie its characteristic taste and aroma. Cinnamaldehyde, the flavor molecule in cinnamon, is characterized by a phenyl group attached to an unsaturated aldehyde. See Fig. 4. Often, when flavors go well together, there are common aroma molecules. This is the case with nutmeg, cloves, and ginger.
The main ingredient in cloves is eugenol, and in nutmeg, isoeugenol. The chemicals vary only in the placement of a double bond on the hydrocarbon side chain. The active ingredient in ginger is zingerone, which produces a pungent hot flavor. This molecule is very similar to eugenol. These spices result in a release of endorphins in the brain, leading to the pleasurable feeling we experience after indulging in this dessert.
Figure 4 Spice flavor molecules
- Add the dry ingredients in the small bowl to the wet ingredients in the large bowl, slowly, while mixing. By first mixing the wet and dry ingredients separately and then combining them you get a homogenous mixture, which ensures an even texture and taste. Mix the wet and dry ingredients completely.
- Pour the mixture into the unbaked pastry shell. Wrap a strip of aluminum foil around the crust’s edge to prevent it from burning.
- Bake for 10 min at 450° F (230° C). Then reduce the oven temperature to 350° F (175° C) and bake an additional 40 to 50 min, or until a toothpick inserted near the center comes out clean. Remove the strip of foil about 20 min before the pie is done so that the crust’s edge will be a light golden brown.
As the pie is baking, heat provides the catalyst for many processes to occur. The proteins in the eggs continue to denature and uncoil, causing the pie to rise. Salt in the filling strengthens gluten bonds and prevents carbon dioxide bubbles from forming. Sugar and evaporated milk in the filling caramelize as a result of the lactose breaking down into glucose and galactose monosaccharides, causing the pie’s top to brown. The starch in the crust and filling continues the process of gelatinization, swelling and releasing amylose, a polysaccharide that acts as a gel. See Fig. 5.
Figure 5 During baking, the starch granules, swollen with water, release amylose polysaccharide. The amylose begins to form a gel.
- Cool the pie to room temperature before serving. This allows the starch gelatinization process to complete, solidifying the amylose gel and holding the pie together. See Fig. 6.
Figure 6 As the pie cools, the amylose gel sets, holding the starch molecules together.