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The Science of Steamed Milk: Understanding Your Latte Art

Guest post by Christina Jayson

Photo credit: Dan Lacher (journeyscoffee/Flickr)

Photo credit: Dan Lacher (journeyscoffee/Flickr)

Watch a barista at work and you will observe the art of preparing a perfect café au lait, cappuccino, macchiato, or mocha – all of which involve different quantities of steamed milk. Behind the artistic foam hearts and milk mustaches lies a science to steamed milk.

Students of UCLA’s SPINLab (Simulated Planetary Interiors Lab) team developed an app that allows you to “calculate the power output of your steamer” and predict the “steaming time for optimal milk temperature based on amount, type and starting temperature of your milk”. Samuel May of SPINlab explains the calculations the app takes into account that allows it to predict the temperature of milk at a given time. They show that the temperature increase of milk over time is linear, allowing them to make these predictions based on a Linear Heating Model.

But what exactly happens when you steam milk? Steaming involves introducing hot water vapor (T = 250-255 °F) into cold milk (T = 40 °F) until it reaches the ideal temperature for a “perfectly steamed latte.”

While the process sounds simple enough there are a host of variables that need to be considered. Most importantly, different milks require different amounts of steaming time. As SPINLab expert, Sam warned, too high a temperature can scald the milk: scalding kills bacteria and denatures enzymes; this inactivates the enzymes and causes curdling as denatured milk proteins clump together.  Since different types of milk and dairy alternatives have different molecular compositions, this means they have different steaming temperatures. This difference all boils down to the composition of milk.

CJ_steamed milk_2

Figure 1. Milk broken down into its molecular constituents. Modified from Properties of Milk and Its Components. [3]

Milk is composed of three main components: of proteins, carbohydrates, and fat (Figure 1).

Milk is 3.3% total protein, including all nine essential amino acids; the protein content can be broken down into two main types, casein and serum. Serum, or whey proteins, contain the majority of the essential amino acids. Whey proteins can be coagulated by heat and denaturation of some of these proteins with heat; this gives cooked milk a distinct flavor. Caseins form spherical micelles that are dispersed in the water phase of milk [1]. When steaming milk, the injected air bubbles disrupt the micelles. The protein molecules then encompass the air bubbles, protecting them from bursting and leading to the formation of foam. The take away: The different protein content of different milks consequently affects each milk’s ability to maintain that frothy foam decorating your latte [2]. Whole milk results in a thicker, creamier foam and skim milk results in more foam and larger air bubbles, while almond milk is able to hold a light and long-lasting foam [2].

Table 1: Percent of protein in different types of milk and non-dairy alternative [2]

Milk % Protein
Skim milk 3.4
1% milk 3.4
2% milk 3.3
Whole milk 3.2
Soy milk 2.7
Almond milk 0.4

Lactose is the carbohydrate component of milk – a disaccharide composed of D-glucose and D-galactose. There are two forms of lactose present in an equilibrium mixture due to mutarotation, α-lactose and β-lactose. β-lactose is the more stable form, and also the sweeter form of the two [3]. When you steam milk past a temperature of 100 °C, this causes a “browning reaction,” or the Maillard reaction, in which the lactose and milk proteins – mostly caseins – react to form what is know as an Amadori product [4]. The colorless Amadori product is a molecular complex between the lysine residues of protein molecules and the lactose molecules. As the reaction continues with heating, the Amadori product can undergo dehydration and oxidation reactions, or rearrangements that lead to a loss of nutritional value and the formation of unappealing flavor compounds in milk that Sam warned could result from over-steaming.

The last main constituent of milk is the milkfat that exists as globules in the milk. Over 98% of milkfat is made up of fatty acids of different types, including saturated, monounsaturated, and polyunsaturated fatty acids. These fat molecules can also stabilize the formation of foam by surround the air and entrapping it in a bubble. While higher fat content leads to stable foam at temperatures below room temperature, milks with lower fat contents (like skim milk) are better at stabilizing foam at higher temperatures [3]. This could be due to the reduced surface tension of the fat along the air bubble surface that is a result of an increase in fat percentage. Heating up these fat molecules not only affects foam texture; when heated or steamed, the fatty acids also participate in chemical reactions, such as oxidation reactions, that can give rise to an undesirable flavor [5].

For the lactose intolerant and fans of non-dairy alternatives, you may be wondering how lactose free options such as soy or almond milk compare. Their steaming temperatures differ mildly due to their distinct properties – for example, almond milk has a lower protein content (Figure 2). According to the experience and experimentation of expert baristas, certain brands of soy or almond milk can hold a foam better than others; the science underlying this phenomenon still remains to be determined.

Table 2: Ideal steaming temperatures for milk and non-dairy alternatives [6]

Milk Soy Milk Almond Milk Coconut
150 °F 140 °F 130 °F 160 °F

The moral of the story is that each component of milk contributes to its ability to froth and foam, and steaming influences each of these components. With this knowledge, you can wisely choose your milk at Starbucks depending upon your foaming desires, or simply download Sam’s app and perfectly steam your milk at home.

References cited

  1. O’Mahony, F. Milk constituents. Rural dairy technology: Experiences in Ethiopia, Manual No.4; International Livestock Centre for Africa Dairy Technology Unit, 1988.
  2. Blais, C. The Facts About Milk Foam. Ricardo, [Online] November 2014;
  3. Chandan, R. Properties of Milk and Its Components. Dairy-Based Ingredients.; Amer Assn Of Cereal Chemists, 1997; pp 1-10.
  4. van Boekel, M.A.J.S. Effect of heating on Maillard reactions in milk. Food Chemistry. 1998, 62:4, 403-414.
  5. Walstra, P. Dairy Technology: Principles of Milk Properties and Processes; CRC Press, 2013.
  6. Dairy Alternatives – Soy, Almond, Coconut, Hazel, Cashew. Espresso Planet. [Online] April 2013;

Christina Jayson is a recent UCLA Biochemistry graduate about to embark on her Ph.D. journey at Harvard.

Milk: From Breast to Cheese with Dan Drake

Veterinarian and goat cheese expert Dan Drake introduced UCLA students to the science of cheesemaking as part of our 2013 Science and Food course. Did you know that good cheese starts with healthy, happy goats? Check out the highlights:


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.

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Homemade Ice Cream

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

∆T    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

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