Have you ever been served a cappuccino that is too hot to handle? Or received one that is lukewarm and unappetising? Or maybe you noticed that the foam on your espresso-based drink collapsed by the time you got it to your table.
The temperature of milk used in a cappuccino affects taste, texture, and stability. Let’s take a closer look at what happens to your milk when it’s heated and what that means for your drink.
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What Happens To Milk During Foaming?
When we foam milk, we force water vapour and air into milk while heating it. Milk is made up of hundreds of chemical compounds and two groups of these are vital to the success of your foam.
The first of these is protein. Milk contains whey and casein proteins. When it is heated, the whey protein structures unravel, or denature, and create spheres around the air. These formations stabilise into bubbles, making the texture we want in our coffee.
The second is fat. Fats actually destabilise milk foam and you will always get better foam with skimmed or fat-free milk. But fats are also responsible for that smooth mouthfeel and the rich body we enjoy, so it’s useful to understand how to work with fat content.
Cow’s milk typically contains about 4% fat, which presents in globules – a mixture of triglycerides (fats) surrounded by a membrane. The membrane protects the fats from any mechanical or chemical damage. In simple terms, the fat is in large spheres that are protected by a layer. Knowing this helps us understand what happens when milk is heated.
A pitcher of milk on an espresso machine. Credit: Hamza Bounaim
The SCA recommends heating milk to 55–65°C (139–149°F) with a maximum temperature of 70°C (158°F) and minimum of 50°C (122°F). This suggestion is backed up by research into the chemistry of heating milk.
The International Dairy Journal reports that although skimmed milk foam is most stable at 45°C, milk fat has a detrimental effect on foam formation and stability of whole milk, especially in the range 15–45°C. It indicates that UHT-processed whole milk foam is most stable at 65°C (149°F).
But what happens if you heat the milk too high or try to foam it when it’s too cold?
An espresso-based beverage with latte art. Credit: Lex Sirikiat
When milk is foamed between 30–40°C (86–104°F), it is unstable. This means that the foam will be thin and you will see different sizes of air bubbles merging together. But why does it do this?
In this low temperature range, whey proteins have only just started to denature and the fats are a mixture of liquid and solid.
Solid fats destroy foam by piercing the thin lipid membrane. This results in partially liquid fats entering the fragile air bubbles formed by the denatured proteins. Partially liquid fats can’t form the elastic layer around air bubbles that is needed for stable foam. Liquid fats can also displace proteins from the surface of air bubbles and cause the bubbles to join together, or coalesce. This also happens because of the low viscosity of milk at lower temperatures.
Three cappuccinos with latte art. Credit: Nate Dumlao
Milk foam gets more stable as the barista increases the temperature. This is because higher temperatures increase denaturation in the whey proteins.
Heating also reduces viscosity – that is, the milk becomes less thick and more watery. This allows the denatured whey proteins to get to the air bubbles and stabilise them. But it is important to quit while you’re ahead and not overheat your milk.
Overheated milk makes it impossible to taste the subtle flavours of your coffee and can burn your mouth. The milk can also develop a sulphurous smell and flavour during prolonged heat treatment, which again detracts from the nuances of your coffee.
When you overheat milk as high as 100°C, lactose reacts with proteins and forms a brown side products and undesirable aroma. Fats become involved in oxidation reactions that create an unpleasant flavour. In short, you get scorched milk.
A steaming hot drink. Credit: Alexandru Stavrica
The Perfect Temperature For Cappuccino Milk
At the recommended temperature of 55–65°C (139–149°F), all of the fats in milk have melted into liquid form and will not destroy the foam.
At this temperature, the amount of whey protein denaturation is ideal for the best adsorption on the surface of air bubbles, so your foam will be stable. And by staying lower than 70°C (158°F), lactose does not have the opportunity to react with the proteins and cause browning and undesirable flavours.
Temperature also affects our overall taste evaluation. We best perceive the sweetness of food and drinks at 60°C (140°F). By keeping an eye on the temperature, you can be sure not to detract from the flavour profile of your coffee in an espresso-based drink.
A barista creates latte art. Credit: Di Bella Coffee
When you heat and foam milk properly, you’re left with fine-textured and stable foam that complements your coffee by emphasising sweetness and providing mouthfeel.
So make sure to use a thermometer and learn how to use your steam wand well. By understanding what happens at different temperatures, you can make your perfect cappuccino.
Enjoyed this? Check out How to Choose The Best Milk Jug for Steaming & Latte Art
Feature photo credit: Chevanon Photography
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