Steam Frothing of Milk

Steam frothing of milk is a critical process when serving a high quality cappuccino or macchiato in a cafe. It’s the job of the barista to get that level of frothiness just right. The creation of froth is not only art but science too. In more formal speak, a milk foam is a very important quality attribute especially when it comes to describing any espresso drink like coffee.

It all depends on whether milk is capable of foaming or not (Levy,  2003; Chandan et al., 2008).

The Chemistry Behind Milk Frothing

Milk frothing, commonly associated with the preparation of beverages like cappuccinos and lattes, involves the creation of a stable foam or froth on the surface of milk. This process relies on the interplay of various chemical and physical factors. The chemistry is covered under these basic concepts.

Proteins

Milk contains proteins, primarily caseins and whey proteins, which play a crucial role in frothing. Caseins, in particular, are amphiphilic molecules, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. When milk is frothed, these proteins help stabilize the foam by forming a network around the air bubbles.

Fat

The fat content in milk contributes to the creaminess and texture of the froth. While too much fat can hinder frothing, a moderate amount can enhance the mouthfeel and stability of the foam.

Lactose

Lactose is the sugar present in milk. It does not directly contribute to frothing, but its presence influences the flavor of the frothed milk.

Water Content

The water in milk is crucial for steam-based frothing methods. Steam is injected into the milk, creating bubbles and heating the proteins, which then denature and contribute to the stability of the foam.

Temperature

Frothing is often done at elevated temperatures. Heating milk causes proteins to denature, and the resulting unfolding of protein molecules facilitates their interaction with air, leading to the creation of stable foam.

Air Incorporation

The process of frothing involves incorporating air into the milk. This can be achieved through steam injection (common in espresso machines) or mechanical frothers that whisk air into the milk.

Milk pH

The pH of milk influences the behavior of proteins. Generally, milk has a slightly acidic pH, and the acidity can affect the stability of the foam.

Type of Milk

• • The type of milk used also affects frothing. Whole milk tends to froth better than skim milk due to its higher fat content. Non-dairy alternatives like soy or almond milk may require different frothing techniques due to variations in protein and fat content.

The frothing process involves transforming liquid milk into a stable foam through the interaction of proteins, fats, and other components. Achieving the desired froth requires understanding the characteristics of the milk being used and employing the appropriate frothing method. Additionally, the type of equipment and technique used, such as steam wands, frothing pitchers, or handheld frothers, can impact the quality of the froth.

Steam-Frothing – The Barista’s Art

Steam frothing of milk occurs through steam injection. It is the act of generating a foam in milk by involving the incorporation of steam that is blown into the milk through a nozzle (Hupertz, 2010).

The barista follows some key steps in creating the froth.

Here’s a step-by-step explanation of the process:

1. Start with cold milk: Cold milk, preferably whole milk, is typically used for steaming and frothing. The amount of milk needed depends on the desired beverage and cup size.
2. Purge the steam wand: Before beginning, it’s important to purge the steam wand by turning it on for a brief moment to clear any condensed water that may be present.
3. Position the steam wand: Submerge the steam wand into the milk, ensuring that the tip is just below the surface and near the side of the pitcher.
4. Activate the steam: Turn on the steam wand, and the pressurized steam will begin to flow into the milk. Initially, you’ll hear a loud, hissing sound as air is introduced into the milk.
5. Texturing the milk: As the milk begins to heat up, gradually lower the pitcher while maintaining the steam wand near the surface. This creates a rolling, swirling motion in the milk, incorporating air and creating microbubbles that contribute to the frothiness.
6. Heating the milk: Continue steaming until the milk reaches the desired temperature. For most coffee beverages, a range between 140°F (60°C) and 160°F (71°C) is recommended, but preferences may vary.
7. Shut off the steam: Once the milk is heated and the desired amount of froth has been achieved, turn off the steam wand. It’s important to remove the wand from the milk before shutting off the steam to avoid any residual milk from being sucked into the machine.
8. Clean the steam wand: Immediately after steaming, wipe the steam wand with a damp cloth to remove any milk residue. This helps prevent clogging and ensures proper hygiene.
9. Swirl and tap: Gently swirl the pitcher to incorporate the milk and froth, and tap the bottom of the pitcher on a countertop to eliminate any large bubbles and create a smoother texture.
10. Pour and serve: Pour the steamed milk and froth into your prepared espresso shot or coffee, holding back the froth with a spoon to control the pour and create latte art if desired.

Steam frothing is an alternative to mechanical agitation.

Steam injection does not have a significant effect on initial bubble hold-up but it does increase general foam stability. The increase in foam stability is probably due to a greater extent of protein denaturation.

Issues with Milk Frothing

For many baristas, being able to foam milk is not straightforward and in a number of cases doesn’t happen at all. The explanations are not forthcoming!

The case of producing either a poor foam or none at all when milk is injected with steam was reported back in the 60s (Buchanan, 1965; Kitchen and Cranston, 1969) but goes back to observation on antifoam performance even earlier to the 50s (Brunner, 1950).

The composition of milk is an important factor, especially the quantity and quality of the free fatty acids. These are substrates for milk lipoprotein lipase. Lipolysis catalysed by this enzyme produces surface active monoglycerides, diglycerides and free fatty acids (FFA).

Milk lipoprotein lipase is an enzyme found in milk that plays a role in the digestion and metabolism of fats. Lipoprotein lipase breaks down triglycerides, a type of fat, into fatty acids (FFAs) and glycerol, which can then be absorbed and utilized by the body. This enzyme is important for the efficient utilization of dietary fats and is found in the milk of lactating animals, including humans.

Free fatty acids destabilise milk foam. Their levels can be as low as 1.5 microequivalents/ml (Deeth & Smith, 1983). Knowing the level of FFAs in milk is a good indicator of both the foaming potential of the milk and the degree of rancidity.

Long-chain FFAs produced by lipolysis reduce the surfac tension of milk

The FFA content of milk is analysed using a variety of methods (Deeth & Fitzgerald, 2006).

References

Augustin, M. A., & Clarke, P. T. (2008). Skim milk powders with enhanced foaming and steam-frothing properties. Dairy Science and Technology, 88(1), 149-161

Brunner, J. R. (1950). The effectiveness of some antifoaming agents in the
condensing of skim milk and whey. Journal of Dairy Science, 33(10), pp. 741–746.

Buchanan, R. A. (1965). Lipolysis and the frothing of milk. The Australian Journal of Dairy Technology, 20, pp. 62–66

Chandan, C. R., Kilara, A., and Shah, P. N., (2008). Dairy Processing and Quality Assurance. New Delhi : John Wiley and
Sons, Inc, 2008.

Deeth, H. C., & Fitz-Gerald, C. H. (2006). Lipolytic enzymes and hydrolytic rancidity. In P. F. Fox & P. L. H. McSweeney (Eds.), Advanced dairy chemistry: Lipids (3rd ed., pp. 481–556). New York: Springer.

Deeth, H. C., Fitzgerald, C. H., & Wood, A. F. (1975). A convenient method for determining the extent of lipolysis in milk. Australian Journal of Dairy Technology, 30, pp. 109–111.

Deeth, H., Smith, R., ( 2016). Lipolysis and Factors affecting the
Steam Frothing Capacity of Milk. The Australian Journal of Dairy Technology, Vol. 38, pp. 50-62

Kamath, S., Wulandewi, A., & Deeth, H. (2008). Relationship between surface tension, free fatty acid concentration and foaming properties of milk. Food Research International, 41(6), pp. 623-629.

Kinsella, J. E., (1981). Functional Properties of Proteins: Possible
relationships between structure and function in foams., Food
Chemistry , pp. 273-288.Elsevier. https://doi.org/10.1016/0308-
8146(81)90033-9

Kitchen, B. J., & Cranston, K. (1969). Lipase activation in farm milk supplies. Australian Journal of Dairy Technology, 24(3), 107–112

Levy, M. (2003). The effects of composition and processing of milk on foam characteristics as measured by steam frothing. Louisiana State University and Agricultural & Mechanical College. USA

Münchow, M., Jørgensen, L., Amigo, J. M., Sørensen, K., & Ipsen, R. (2015). Steam-frothing of milk for coffee: Evaluation for foam properties using video analysis and feature extraction. International Dairy Journal, 51, 84-91.

Silva, S., Espiga, A., Niranjan, K., Livings, S., Gumy, J.C., Sher, A., (2008). Formation and stability of milk foams. In: Campbell, G.M., Scanlon, M.G., Pyle, D.L. (Eds.), Bubbles in Food 2: Novelty, Health and Luxury. AACC International, St. Paul, Minnesota, pp. 153–161