The temperature of the milk used in a cappuccino affects the taste, consistency, and stability.
What happens when we steam milk?
Steaming milk involves forcing water vapor and air into it while it is being heated.
Milk is composed of hundreds of chemical elements, and two groups of these are vital for the success of your foam.
The first is protein. Milk contains whey and casein proteins. When heated, the protein structures unravel or denature and create spheres around the air. These formations are responsible for the texture of your milk.
The second are fats (approximately 4% content); these actually destabilize milk foam, and you will always get a better result with skimmed milk, but at the same time, they are responsible for that smooth sensation and rich body that we love to find in cappuccino, so it is useful to understand how to work with the fat content!
The perfect temperature for milk
At the ideal temperature of 55-65 °C, all the fats present in the milk have melted into a liquid form and do not destroy the foam! with a minimum temperature of 50 °C and a maximum of 70 °C
At this temperature, the amount of whey protein denaturation is ideal for the best absorption on the surface of air bubbles, so the foam will be stable. And by staying below 70 °C, lactose does not have the chance to react with proteins and cause browning and undesirable flavors.
Milk not heated enough
When milk is heated between 30-40 °C, it is unstable.
This means that the foam will be thin, and you will see different sizes of air bubbles merging together. In this low-temperature range, whey proteins have just begun to denature, and fats are a mixture of liquid and solid.
Solid fats destroy the foam by perforating the thin lipid membrane; partially liquid fats enter the fragile air bubbles formed by denatured proteins and cannot form an elastic layer around them, which is necessary to obtain a stable foam. Liquid fats can displace proteins from the surface of air bubbles and cause them to merge.
Overheated milk
Milk foam becomes more stable as the barista increases the temperature because higher temperatures increase the denaturation of whey proteins.
Heating also reduces viscosity, meaning the milk becomes less dense and more watery. This allows proteins to reach the air bubbles and stabilize them. But it is important to stop at the exact moment to avoid overheating the milk; when this happens, lactose reacts with proteins and forms a brown byproduct and an undesirable aroma, and fats are involved in oxidation reactions that create an unpleasant taste.
Overheated milk makes the aromatic notes of coffee unidentifiable, develops a sulfuric smell and taste, and risks burning your mouth!
