MIT Chocolatiers & The Nanotech in Our Food
MIT students perfect their truffle-making skills, while the New York Times examines the use of nanomaterials in common food products. Read more
MIT students perfect their truffle-making skills, while the New York Times examines the use of nanomaterials in common food products. Read more
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
The 2013 Science & Food lineup is here!
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We can’t wait to see you at the 2013 Science & Food lectures!
Primitive X Modern: Cultural Interpretations of Flavors
Featuring Chef Alex Atala
Wednesday, April 17 @ 7:00pm
Moore Hall 100 (map)
Chef Atala will discuss his approach to food and how his cooking has been impacted by science. Atala is renowned for pioneering regional cuisine using indigenous Brazilian ingredients and works closely with anthropologists and scientists to discover and classify new foods from the Amazonian region.
Edible Education
Featuring Chef Alice Waters, Dr. Wendy Slusser, and Chef David Binkle
Thursday, April 25 @ 7:00pm
Royce Hall Auditorium (map)
Chef Alice Waters will be joined by Professor Wendy Slusser and Chef David Binkle to provide and informative discussion on initiating change in how we eat through school lunches, edible gardens, and healthy campuses.
The Science of Pie
Featuring Chef Christina Tosi and Chef Zoe Nathan
Sunday, May 19 @ 2:00pm
Covel Commons Grand Horizon Room (map)
Chefs Christina Tosi and Zoe Nathan will share their perspectives on inventing desserts, with a particular emphasis on pie. Here, the students of the Science & Food undergraduate course will present results from their final projects, including live taste tests of apple pies. Final projects will be judged by a panel of esteemed local chefs, scientists, and food critics including Christina Tosi, Zoe Nathan, Jonathan Gold, and UCLA Professors Andrea Kasko and Sally Krasne.
Learn more about a Harvard microbiologist who studies microbial communities in cheese, and check out these historical cooking practices that have helped shaped public health. Read more
David Chang is the chef and founder of Momofuku and author of the best-selling cookbook of the same name. To follow David on his food adventures, check out an issue of Lucky Peach or watch Mind of a Chef on PBS. You can also watch David’s 2012 Science & Food lecture, “A Microbe in My Ramen?”
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.
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 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.
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.
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.
About 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.