Deconstructed Apple Pie

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The Science of Pie – May 19, 2013
Best Tasting Pie
Stephan Phan, Kevin Yang, Amirari Diego (Team Apples to Apples)

Using the technique of spherification, this team applied their knowledge of diffusion and gelation to prepare “reconstituted” apples. They found that optimizing both the calcium chloride concentration and gelation time was key to making a delicious modernist apple pie.

TeamApplestoApples

photos courtesy of Patrick Tran

Calcium promotes the solidification of alginate networks. Alginate is a long, negatively charged molecule called a polysaccharide. Positively charged sodium ions (Na+) dissociate from the alginate when dissolved to create a goopy but liquid solution. Doubly charged calcium ions (Ca2+) can bind two different alginate strands simultaneously, thereby crosslinking and solidifying the solution. Increasing the number of calcium crosslinks by raising the concentration of calcium chloride and/or lengthening the soaking time create a more solid gel.

The Recipe
Deconstructed apple pie with pie crust crumbs and spherified apples

10 g sodium alginate
20 g calcium chloride
1 L 100% organic apple juice*
1 L water**

*Team Apples to Apples recommends using pulp-free organic apple juice. Freshly pressed apple juice tends to have too much pulp, while additives in non-organic apple juice may interfere with the spherification process.

**For the Science of Pie, Team Apples to Apples used 10g of sodium alginate in 1 L of apple juice and 20g of calcium chloride in 1 L of water. This recipe does not require such large volumes, but it is important to maintain these ratios as they affect the gelation time for the apple spheres.

Mix the sodium alginate into the apple juice. We recommend using an immersion blender, but whisking vigorously will also work. Let the solution sit until any foaming subsides; if large amounts of foam formed during mixing, you may also want to skim foam from the surface of the solution. The solution is ready for spherification once it has reached almost an apple sauce viscosity.

Prepare your calcium bath by dissolving the calcium chloride into the water. Mix lightly; the solution is ready once all visible particles have disappeared and the liquid it appears translucent again.

To create each spherified apple, scoop no more than one tablespoon (it becomes increasingly harder with bigger volumes) of apple juice solution using a deep spoon and carefully drop it into the calcium chloride solution. It helps to use a second spoon to scoop the apple solution out of the first spoon. This is mainly technique—you will get the hang of it after a dozen or so attempts!

Let the apple juice solution sit in the calcium chloride solution for approximately 30 seconds. There will not be a noticeable difference if left for an additional 30 seconds, but the apple juice solution will continue to solidify as it sits in the calcium chloride solution and fully solidify after about 10 minutes. Feel free to play around with the timing of this step to achieve the desired spherified apple texture.

To serve, place the spherified apple in an Asian-style soup spoon and garnish with a bed of sugar and graham cracker crust crumbs, a sliver of green apple skin, and a dusting cinnamon.

More information about spherification can be found at Molecular Recipes.

Shortbread Apple Pie

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The Science of Pie – May 19, 2013
Best Overall Pie
Alia Welsh (Team Sablé)

This solo effort explored the vast parameter space of pie, studying the effect of fat content and temperature on the texture of the shortbread crust, as well as the effect of pH on the browning of the streusel topping. The final winning pie had shortbread made with room temperature standard American butter.

TeamSable

photos courtesy of Patrick Tran

Effect of different fats on the shortbread crust. The quality of the shortbread crust was evaluated based on its color and texture. Porousness (“porosity”) was quantified by converting crust image pixels to black or white, with black pixels representing holes in the crust. A higher percentage of black pixels corresponds to a higher porosity and thus a crumblier crust. The extent of browning was quantified by calculating the RGB values of each crust image and comparing to a “deep golden brown” color standard (RGB 184-134-11). Standard American butter created the most desirable crust in terms of both browning and porosity.

The Recipe
Apple pie with shortbread crust and streusel topping

For the filling:
3-4 Granny Smith apples, peeled and cored
3-4 Fuji apples, peeled and cored
3/4 cup granulated sugar
2 tbsp flour
1/2 tsp salt
1 tsp cinnamon
1/4 tsp nutmeg
1/4 tsp allspice

For the crust:
1 1/4 cups all purpose flour
1/3 cup granulated sugar
1/2 tsp salt
2 sticks of standard American butter at room temperature
1 egg white, separated

For the streusel:
3/4 cup rolled oats
1/2 cup chopped walnuts, pecans, or almonds
1/4 tsp salt
7 tbsp flour
6 tbsp brown sugar
4 tbsp melted butter
2 tbsp honey

Preheat oven to 375F.

To prepare the filling, cut the apples into approximately ¼ inch slices. Combine with the remaining filling ingredients and sautée over low heat until the water from the apples forms a sauce and thickens slightly. Set aside to cool.

To prepare the crust, whisk together flour, sugar, and salt. Cut in the butter with knives or a pastry blender. Pour the mixture into a pie pan and spread evenly with back of a large spoon or measuring cup. The crust should be about 1/2 inch thick.

Bake crust at 375F for about 15 minutes or until the crust is a light golden brown. Allow the crust to cool for 2-3 minutes, then brush with the egg white.

While crust is baking, prepare the streusel. Combine the dry streusel ingredients. Mix in melted butter and honey to form clumps. Set aside.

To assemble the pie, pour filling into the pre-baked, egg-washed crust and sprinkle streusel on top. Bake for about 35 min at 375F. Streusel should be deep golden brown.

Homemade Butter

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ButterBigger

Despite the misconception among certain pop culture icons that butter is a carb, butter, like other fats and oils, is a lipid. Broadly defined, lipids are any molecules that have hydrophobic, or water repelling, characteristics.  In contrast to simple molecules like water (H20) or sugar (C6H12O6), butter does not have one molecular formula; rather, it is a mixture of triglycerides. Here is what a triglyceride looks like [1]:

triglyceride

Triglycerides are molecules made of three fatty acids bound to glycerol, a sugar alcohol. Fatty acids are long hydrophobic chains of hydrogen and carbons that repel water. Triglycerides do not have to be the same three fatty acids, but can be mixed and matched. For example in butter, oleic acid (32%), myristic acid (20%), palmitic acid (15%) and searic acid (15%) make up the greatest percentage of the fatty acids [2].

buttercontent

In addition to all these lipids, surprisingly, butter contains water. While oil and water don’t normally mix, in butter, tiny microscopic water droplets are dispersed within the fat.  This is commonly known as a water-in-oil emulsion. An emulsion is any mixture of two liquids that don’t usually mix. The opposite of a water-in-oil emulsion would be an oil-in-water emulsion in which oil droplets are entrapped within water.

To understand the secret of how butter can be made of two immiscible liquids, we need to delve back into the molecular structure.  Butter is made from the cream, which has a higher fat content (15-25%) than milk (5 – 10%) [3].  In milk and cream, which are oil-in-water emulsions, the fatty triglycerides stay suspended in liquid because they are encapsulated in tiny fatty spheres or globules. Each globule is surrounded by a nanoscopically thin layer of phospolipids and stabilizing proteins. Phospholipds have hydrophobic lipid tails that love to repel water; they also have hydrophilic, or water loving, heads that contain a phosphate group (thus the name, phospho-lipid). Here is a picture of a phospholipid [1]:

phospholipid

The phospholipids organize themselves in a thin layer so that the water repelling hydrophobic portions are aligned with the fatty acid chains while the water loving hydrophilic heads interact with the milk liquid.  This allows the fats to remain dissolved in the milk and float around like little water balloons.

Milk Fat globule. (A) Diagram of the phosopholipid layer surrounding a fat globule [3]. (B) Cryo-electron microscopy image of a fat globule [4]. The scale bars are 0.1 μm.

Now, that we have talked about the structure of butter, how to get from cream to butter?  (Remember: milk and cream are oil-in-water emulsions and butter is a water-in-oil emulsion.) The oil-in-water emulsion of the cream is reversed into a water-in-oil emulsion in butter. During the churning or mixing process of butter making, the fatty globules in the cream break open to release the entrapped fat molecules. The hydrophobic fat molecules clump together and mix to form larger fat globules that coalesce into larger solid fat droplets. This processes pushes out the liquid portion and the solid portion becomes the butter.  Since these types of fat molecules typically melt at temperatures of 30 to 41°C (86 to 106°F), this means that at cool temperatures below approximately 39°F (4°C), the remaining liquid gets trapped within the solid fat matrix and is unable to separate out of the butter [5].

milktobutter

Below is a recipe for making your own homemade butter. You don’t need fancy equipment or churners like your ancestors used; a well-sealed glass jar works wonders.  The shear forces generated by rigorous shaking are sufficient to convert your cream into butter.

Ingredients

Heavy whipping cream (6 cups makes about 1lb of butter)
Salt, to taste
Jar with lid, any size

Procedure

1. Fill the jar about ¾ of the way to the top with the heavy whipping cream and close the lid.

2. Shake the jar for about 4-5 minutes until the cream begins to thicken. Shake longer if you wish for a thicker consistency.

The shaking motion breaks down the fat globules. The membranes surrounding each fat globule break, releasing the hydrophobic triglycerides. The triglycerides clump together and push away the hydrophilic liquid, the buttermilk.

3. Drain off the buttermilk and place butter in a small bowl. Knead the butter under cold running water to remove any remaining buttermilk.

4. Salt to taste. Form butter into a ball or log. Serve immediately or refrigerate.

Recipe Adapted From:

Online Resources

  1. General Chemistry Online: What is the chemical structure of butter?
  2. “Overview of the Buttermaking Process” from University of Guelph

References Cited

  1. K562. Overweight & obesity. http://www.indiana.edu/~k562/ob.html
  2. Fatty acids in butter. Percentage composition from Practical Physiological Chemistry, P. B. Hawk, O. Bergeim, Blakiston:Philadelphia, 1943.
  3. Gallier, S. et al. 2012. Structural changes of bovine milk fat globules during in vitro digestion. J Dairy Sci. 95(7): 3579- 3592.
  4. Robenek, H. et al 2006. Butyrophilin controls milk fat globule secretion. PNAS. 103 (27): 10385-10390.
  5. Butter: Some Technology and Chemistry. http://drinc.ucdavis.edu/dfoods1_new.htm

Vince ReyesAbout the author: Vince C Reyes earned his Ph.D. in Civil Engineering at UCLA. Vince loves to explore the deliciousness of all things edible.

Read more by Vince Reyes

Simple Vegan Strawberry Shortcake

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BirthdayCake

With increasing numbers of people embracing dietary-restricted, vegetarian, and vegan lifestyles, birthday parties are becoming more complex. Consider the simple birthday cake. Everyone should be able to partake in its delicious sugary goodness; however, this can prove difficult. Specifically, how can you bake a cake if you cannot use one of the most important components, eggs? Read more

Ceviche

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Through the process of cooking, molecular transformations alter the macroscopic properties of our food. Consider what happens when you fry an egg: the transparent, liquid egg whites become an opaque white solid. These striking changes in the egg’s color and texture are a result of protein denaturation. Read more

Homemade Ice Cream

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Phase transitions—transformations from one state of matter to another—are ubiquitous in food and cooking. Butter’s phase transition from a solid to a liquid results in flaky pie crusts, while water’s phase transition from a liquid to a gas can be used to steam vegetables. There are various ways to manipulate these phase transitions, such as by altering temperature, pressure, or salt content. In this classic home experiment, we will make ice cream by using salt to alter the phase behavior of water.


Objectives

  • Understand how solutes (salt) affect the phase behavior of a solvent (water).
  • Use freezing point depression to make a batch of amazing ice cream.


Materials

  • 1 cup cream
  • 1/2 cup sugar
  • 200 grams ice
  • Kosher salt
  • 1 quart Ziploc bag
  • 1 gallon Ziploc bag
  • Thermometer
  • Scale


Part 1: Use salt to lower the melting point of ice

To successfully freeze ice cream without the help of a freezer, we need a way to efficiently transfer heat out of the ice cream. Liquid water is much better than solid ice at transferring heat, so an ice-water bath will absorb heat from our ice cream better than solid ice. To effectively freeze ice cream, however, we need stable temperatures well below 0˚C.

How is it possible to have a mixture of water and ice at a temperature below 0˚C, water’s freezing point?

When you take ice straight out of the freezer, the ice will be roughly the same temperature as the freezer itself. The temperature in a home freezer is typically between 0˚C and -20˚C. As the ice sits out, it will absorb heat from its surroundings and slowly get warmer until it reaches 0˚C and begins to melt. Adding impurities like salt to ice will lower its melting point.  This means that salted ice will start melting at temperatures below 0˚C. As a result, a salty ice-water bath can stay liquid at temperatures well below 0˚C and efficiently freeze our ice cream. We refer to this phenomenon as “freezing point depression.”

We can use the freezing point depression equation to calculate how much a solvent’s freezing point will drop as a solute is added:

∆Tf = b · Kf  · i

∆Tf      Freezing point depression, defined as Tf of  pure solvent – Tf of solution.
K f        Cryoscopic constant of the solvent. This is an intrinsic property of the solvent.
b          Molar concentration of the solute: the number of moles of solute per kilogram of solvent.
i           Number of ion particles per molecule of solute, also known as the “Van’t Hoff factor”.
Salt is made up of one sodium ion and one chloride ion, so its Van’t Hoff factor is 2.

  1. Use the freezing point depression equation to calculate how much salt (our solute) is needed to decrease the freezing point of water (our solvent) from (a) 0˚C to -5˚C, (b) 0˚C to -10˚C, (c) 0˚C to -15˚C, and (d) 0˚C to -20˚C.
  2. Plot the magnitude of freezing point depression (ΔTf) versus salt concentration (Results from 1a, b, c, and d). Remember to use units!
  3. Based on your answer from 1d, calculate how many grams of salt are required to create a -20˚C freezing point depression for 200g of ice. This is the amount of salt you will use in Part 2.

Some useful values:
Freezing point (Tf) for pure water: 0˚C.
Cryoscopic constant (Kf) for water: 1.853 ˚C*kg/mol.
Molecular weight of salt (NaCl): 58.44 g/mol.

Click here to check your answers.


Part 2: Use freezing point depression to make ice cream

  1. Combine cream and sugar in the quart-size bag and mix well. Place this bag inside the gallon bag.
  2. Record the initial temperatures of the ice and the cream mixture.
  3. In the gallon bag, pack the ice around the quart-size bag, and then sprinkle the calculated amount of salt over the ice. Be careful that the salt does not fall into the cream mixture.
  4. Gently shake the bag until the cream mixture solidifies into ice cream.
  5. Record the final temperatures of the ice-salt-water mixture and the ice cream.
  6. Enjoy your homemade ice cream!


Questions

  • What was the final temperature of the ice cream? Did it end up below 0˚C? How does its temperature compare to the temperature of the salt-ice-water mixture?
  • What was the final temperature of the ice-salt-water mixture? Is warmer or colder than the ice you started with? How does the temperature compare to the freezing point depression you calculated in Part 1?


Discussion

In this experiment, we used salt to lower the freezing point of water. By adding salt to ice, we were able to achieve a salt-ice-water mixture that was able to freeze our ice cream.

Why does ice cream need temperatures colder than the freezing point of water in order to freeze?

When water freezes, it forms a well-ordered crystalline structure (an ice cube). This unique crystalline structure is what gives solid water a slightly lighter density. Although ice cream is a combination of  cream, sugar, and flavorings, it is still approximately 60% water. The remaining 40% is a mixture of sugar molecules, fat globules, and milk proteins [1]. This liquid mixture is emulsified: the water molecules are dispersed among sugar molecules, milk protein complexes, and large clusters of fat globules.. When this mixture is brought to the freezing temperature of water, the fats, proteins, and sugars hamper the freezing process by interrupting the formation of ordered crystal water structures. The ice cream mixture thus remains a liquid, requiring even colder temperatures below 0˚C to successfully solidify [2].

Structure of ice cream. (A) an electron micrograph of ice cream showing air bubbles, ice crystals, and the sugar solution [3]. Fat globules and milk proteins are not visible at this resolution. (B) Diagram of ice cream structure adapted from University of Guelph.

How did the salt in our experiment create a salt-ice-water mixture below 0˚C?

At 0˚C, ice and water are “at equilibrium” with each other. The total amount of water and ice remains relatively constant, but individual water molecules are constantly switching states: as some water molecules melt and become liquid, other water molecules freeze and become solid. Adding a solute like salt shifts this equilibrium. Solutes essentially “trap” water molecules in the liquid state, preventing them from readily switching back to the solid state. On a macroscopic scale, salt causes solid ice to melt faster and at temperatures below 0˚C, resulting in a salt-ice-water mixture below 0˚C. To get a better feel for how this process works at the molecular level, check out this interactive demonstration of how temperature and solutes affect the water-ice equilibrium.

Contrary to popular belief, the addition of salt to ice does not actually make the ice any colder!

The temperature that you recorded for the salt-ice-water mixture was probably colder than the temperature of the pure ice you started with. How is this possible? When you take the temperature of solid ice, you are not really measuring the temperature of the ice itself—you are measuring the average temperature of the ice, the air around the ice, and any water that has formed from the ice melting. The true temperature of the ice depends on the temperature  freezer it came from (typically between 0˚C and -20˚C) and the length of time the ice has spent out of the freezer.


Online Resources

  1. Interactive explanation of how temperature and solutes affect water-ice equilibrium
  2. “Ice Cream Structure” from University of Guelph

More from On Food and Cooking

  • McGee, Harold. On Food and Cooking. Scribner, 2004. (39–44).

References Cited

  1. Goff HD (1997) Colloidal aspects of ice cream—A review. International Dairy Journal 7: 363–373. doi:10.1016/S0958-6946(97)00040-X.
  2. Hartel RW (1996) Ice crystallization during the manufacture of ice cream. Trends in Food Science & Technology 7: 315–321. doi:10.1016/0924-2244(96)10033-9.
  3. Clarke C (2003) The physics of ice cream. Physics Education 38: 248–253. doi:10.1088/0031-9120/38/3/308.

Liz Roth-JohnsonAbout the author: Liz Roth-Johnson is a Ph.D. candidate in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.

Read more by Liz Roth-Johnson

Ricotta Cheese

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Protein networks are responsible for the structure and mechanical properties of many foods such as eggs and meat. Even bread gets its chewy texture from the formation of springy gluten protein networks. As we will see in this recipe, protein network formation is vital for the successful production of cheese. Read more