by Thom Huppert, NIZO Food Research
We’re always focused on the coffee, but most baristas use milk nearly as frequently every day to make customers’ favorite cappuccinos, lattes, and cortados. More than just nature’s baby food for calves, milk is a chemically complex fluid with unique physical properties. The more we know about how milk “works,” the better we can do uniting it with coffee for the best taste and foamy beauty for our customers.
Milk is used widely in the coffee world, not in the least for its well-known ability to create stable foams for coffee drinks. Several components of milk play an important role in creating stable foams. These same ingredients facilitate the creation of other popular dairy products, such as cheese, yogurt, ice cream and dairy drinks. For creating stable milk foams of desirable texture and stability, milk fat and milk proteins are of crucial importance. Milk proteins stabilize the air bubbles in milk foams. To complicate matters, milk fat destabilizes foams, but is desired for flavor. Creating stable milk foams is thus an intricate interplay between balancing the desirable foaming properties of milk proteins with the destabilizating milk fat.
Cows’ milk typically contains 4-5 percent fat, made up of a complex mixture of types of fats. Fat in milk exists in the form of milk fat globules, which range in size from ~0.1 to 10 µm and consist of a core of triglycerides surrounded by a membrane consisting of proteins, phospholipids, and additional glycerides. This natural milk-fat-globule membrane protects the milk fat against oxidation or degradation by enzymes that can create off-flavors in milk.
Milk contains two classes of proteins, the caseins and whey proteins, which are found on the surface of the fat globules in homogenized milk. Of the 3-4 percent protein in cows’ milk, the caseins represent ~80 percent of total protein and exist primarily in the form of particles of 100-200 nm, called casein micelles. These casein micelles contain tens of thousands of casein molecules, as well as calcium phosphate and water. The whey proteins, on the other hand, represent ~20 percent of total protein in milk and are found primarily as individual proteins or small aggregates in milk. However, extensive aggregation of whey proteins can occur when milk is heated; such effects are limited by commercial pasteurization (e.g., 72°C for 15 sec) but UHT treatment (e.g., 140°C for 5 sec) and sterilization (e.g., 10 min at 115°C) cause extensive aggregation of whey proteins, forming either whey protein aggregates or casein-whey protein aggregates, causing extra viscosity.
Milk is often homogenized to reduce the tendency of milk fat globules to cream (form a cream layer on top during storage). Creaming occurs because of the lower density of milk fat. By reducing the size of fat globules, homogenization slows down the creaming process. During homogenization, milk is typically passed through a small valve at high pressure, as a result of which the fat globules are disrupted. As a result, the total surface area of the fat globules increases, and milk proteins are absorbed by the surface of the fat globules.
Foaming of milk: stabilization by proteins
Just as they stabilize the surface of the milk fat globule after homogenization, milk proteins are also crucial in creating the surface of air bubbles in milk foams, whipped cream, and ice cream. Introducing air into these products by agitation (whipping) or injection (steaming) leads to an unstable interface between air and water, which needs to be stabilized by the adsorption of “surface-active components” made up of proteins. The proteins attach to the surface of the air bubbles and stabilize them. Normal milk contains more than enough protein to stabilize the air bubbles in a milk foam. Even 5 percent of the typical ~3.5 percent protein in cows’ milk is sufficient. Of the milk proteins, caseins are found to preferentially attach to air bubble interfaces. The milk proteins form a stable surface layer, resulting in stable foam. However, foam stability eventually decreases, which is due to liquid draining from the foam, as a result of which air bubbles come in close contact with each other and merge. This leads to larger and larger air bubbles, which eventually collapse.
Milk foams: destabilization by fat
In addition to the normal destabilization of foams, there are a number of factors related to milk quality, composition, and processing that affect the foaming of milk. In many cases, this is related to milk fat. Skim milk is ideal for preparing foams from a physics perspective, but skim milk foams lack flavor and mouth feel, thus necessitating the inclusion of some fat. However, increasing fat content leads to a considerable reduction in the ability of milk to foam, as well as in foam stability. The destabilizing influence of fat is particularly strong at 10-40°C. At this temperature range, milk fat globules are destabilized during foam formation, leading to the leakage of liquid fat from the globules, which destabilizes foams. Increasing milk temperature to >40°C, i.e., where all milk fat has melted, replacing milk fat with a vegetable oil, or decreasing the size and increasing the stability of milk fat globules by homogenization help to overcome the negative effect of natural milk fat globules on foaming of milk.
Influence of milk quality on foaming of milk
A further aspect related to milk fat to be considered is that of the enzymatic degradation of milk fat by the enzyme lipase, i.e., lipolysis. This results in the formation of surface-active mono- and diglycerides that (partially) replace milk proteins on the surface of the air bubbles. As a result, the air bubbles become unstable. Lipolysis can also create a rancid taste. Therefore, preventing lipolysis is of utmost importance. This responsibility involves all aspects of the dairy supply chain, starting with raw milk quality, where damage to fat globules can enable lipase activity. Milk lipase is easily inactivated by normal pasteurization. However, lipases may also be produced by bacteria and these, unlike the bacteria themselves, can be extremely heat-stable and in some cases, survive UHT processing and sterilization. Hence, contamination of milk should be avoided at all costs, as the issue of lipolysis may only be partly rectified through heat treatment.
Influence of heat treatment on milk
While heat treatment has little effect on the foamability and foam stability of milk, it should be considered in terms of product quality and safety, and also from a flavor perspective. Pasteurized, sterilized, and UHT-treated milk all have distinct flavor characteristics, which may be preferred or not in various markets around the world. In areas where fresh or pasteurized milk is the milk of choice, UHT-treated milk and particularly, sterilized milk, are often disliked for their “cooked” flavors, including some sulfur-y off-notes. On the other hand, in areas that have no history of consuming fresh milk, pasteurized milk is often considered bland and lacking in certain desired flavor characteristics; the “cooked” flavor disliked in the traditional dairy nations may be particularly desired here.
As outlined above, it is clear that the selection and production of milk for the creation of milk foams can be an intricate process, requiring careful consideration of certain trade-offs. These include milk quality and stability on the one hand, and foamability and foam stability on the other. However, consumer acceptance relies on texture, flavor and mouth feel, and should be paramount in any consideration of milk in coffee applications.
Thom Huppertz holds an MSc in Dairy Science from Wageningen University, The Netherlands and a PhD in Dairy Science from University College Cork, Ireland. Currently, he is employed as a principal scientist at NIZO food research, Ede, The Netherlands. His research focusses on the physical chemistry of dairy products, with particular emphasis on the protein functionality and product-process interactions. In addition, he is an adjunct professor in dairy science and technology at South Dakota State University and is an editor of the International Dairy Journal.