Milk is a key ingredient in coffeeshops worldwide. The smooth and shiny foam makes espresso-based milk beverages so attractive for customers. The pairing of silky milk with coffee has helped popularise coffee globally. Let’s have a closer look at milk chemical composition and all changes that occur with heating and foaming.
Milk consists of three main groups of ingredients: proteins, fats, and carbohydrates or, simply, sugars. Each component plays its sufficient role in foam, aroma, and flavour formation.
Proteins are mainly responsible for the formation and the stability of milk foam. However, we should keep in mind that there are two main groups of proteins in dairy milk – whey proteins and caseins, and we are going to focus mainly on «wheys», because caseins due to their micellar structure are more temperature resistant. In contrast, whey proteins, namely -lactoglobulin and -lactalbumin, are present in milk in the form of highly organised globules. Due to their structure, they are sensitive to high temperatures that cause denaturation of whey proteins. Denaturation is a process where a protein loses its initial structure and properties under severe conditions, like heating or the change of pH level.
So, what happens when a barista foams dairy milk? We will start from the farm. Milk used in coffeeshops must comply with safety regulations. For that, manufacturers apply heat treatment to raw milk thus making it microbiologically safe. Pasteurization and ultra-high temperature processing or, simply UHT, are the two main processes that make dairy milk safe for human consumption. However, UHT implies more severe heating that causes major changes in whey protein structure. At this point, a bartender decides what type of milk they would like to use in a coffeeshop considering that pasteurised milk naturally contains more whey proteins available for the stabilization of foam.
Milk whey proteins have an amphiphilic structure. That means that a molecule of a whey protein, e.g., -lactoglobulin, in water, will be at the same time hydrophilic (will “love” water) and hydrophobic (will “hate” water). That happens because molecules of milk whey proteins contain amino acids with both hydrophobic and hydrophilic radicals A radical is a part of a protein molecule that allows amino acids to distinguish one from another. Some of the radicals are soluble in water and some of them are not. Since molecules of proteins contain numerous different amino acids, some of them will be hydrophilic and others – hydrophobic. Due to this property, milk whey proteins can bind on the surface of air bubbles forming a thin film on their surface and thus stabilizing them. In short, whey proteins act as natural surfactant, exactly like chemical molecules in household detergents that create a foam.
Another phenomenon that leads to foam formation and stabilisation is the denaturation of whey proteins. Generally, the term denaturation of proteins denotes a loss of their initial structure and properties. The denaturation might occur under different conditions, i.e., heat treatment, pH change, addition of organic solvents. With coffee, it is most relevant to focus on thermal treatment. So, when milk is being heated (in a factory or later in a coffeeshop), the highly organized tertiary structure of whey proteins starts to unfold. Initially, there are highly reactive thiol groups buried within the protein structure that become exposed on the surface. This results in a formation of new cross-compounds, thus making the denaturation process irreversible. The temperature of 70oC is the point when the denaturation begins, however, even at 60oC -lactoglobulin starts to react with a protein found on the surface of the at globule, thus losing its initial state.
At 60-65oC, the temperature recommended for cappuccino preparation, the denaturation of whey proteins begins, however, it does not cause any serious damage to the molecules. The viscosity of the milk and foam is slightly increased and leads to the formation of a ‘net’ around the air bubbles. Partially denatured proteins are also more surface-active.
At high levels of denaturation, proteins mostly lose their initial structure, resulting in fewer proteins available for creating a stabilizing film around air bubbles. We observed similar behaviour during our experiments.. Foams at temperatures higher than 70oC are dry, thick, tend to drain, and collapse faster. This confirms the recommendation not to exceed 65oC while making a cappuccino.
The industrial heat treatment also affects protein structure, so pasteurized and UHT milks foam differently. Since UHT milk is exposed to higher temperatures, the proteins are not in their initial state in the milk bottle. When a barista foams UHT milk with a steamer, it results in the formation of a less stable foam. Therefore, the less exposure to thermal processing a protein undergoes the easier it will be for a barista to steam micro-foam.
First of all, the quality and composition of milk strongly depends on the season and animal feed. There is also a process called lipolysis. Free fatty acids, monoglycerides and diglycerides are present in milk. These compounds are formed when bacteria or enzymes interact with milk fats (pasteurised milk is not sterile, so non-pathogenic bacteria can be found). If there are many of those compounds present in milk, they will destabilise air bubbles within milk foam since the bubbles are surrounded by fatty films too, not only proteins. This will lead to foam destruction.
Several conditions may lead to lipolysis: cow feed, the phase of their lactation, bad weather conditions or poor storage (before thermal treatment) of milk on the farm.
Lipolysis may also occur because of poor storage of already heated milk; thus, the longer milk is being stored the worse will be its foaming properties. This can be experienced with UHT milk that can be stored for a year for example.
To sum up, milk foams because of amphiphilic nature of milk whey proteins, mainly -lactoglobulin and -lactalbumin and their partial denaturation. Fats and lactose play a secondary role in a foam’s formation. We can successfully foam milk without fats and lactose, but not without proteins. Protein binding (absorption) on the surface of air bubbles is stabilizing. Heat treatment causes numerous reversible and irreversible changes in protein structure. At high temperatures they denature, and the initial structure unfolds. However when milk simply will not hold a stable foam, this can be the result of lipolysis. This process may actually be the reason the Flat white was first invented. As historian Jonathan Morris cites the first documented reference is from Melbourne cafés. When fresh milk was not 'foamable' to create the domed top of the cappuccino, the cafés would put a sign up saying flat whites only.
Bals, A., Kulozik, U. (2003) Effect of pre-heating on the foaming properties of whey protein isolate using a membrane foaming apparatus. International Dairy Journal, 13(11), 903-908
Belitz, H.D. (2009) Food Chemistry. 4th ed. Springer
Huppertz, T. (2010), Foaming properties of milk: A review of the influence of composition and processing. International Journal of Dairy Technology, 63: 477-488
Morris, J. (2019), Coffee: a global history. Reaktion books ltd. Londo
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