Dirty is what everyone commonly calls dirty coffee. The fascinating thing about this drink is the clear gradient layer when it is served: dark brown espresso floating above milky white iced milk, like an ink wash painting slowly blending and spreading. Many friends who make coffee at home with a semi-automatic machine or a small home machine have tried to reproduce this effect, sometimes successfully, sometimes the coffee liquid immediately mixes into a single mass with the milk as soon as it goes in. Actually, layering is not some mystery; there is a clear set of physical logic behind it. Once you understand the principle, the success rate will be much higher.

Espresso is denser than milk and also much hotter, which is the basic condition for layering. Hot liquid has lower density and will naturally float upward; at the same time, the freshly extracted espresso is covered with a layer of dense crema, and this layer of crema is like a buffer net that holds up the coffee liquid and lets it temporarily stay on the surface of the milk. If the milk and the cup have both been refrigerated in advance at this point, and the temperature is low enough, the temperature difference as the espresso comes down becomes more obvious, and the layering will also be more stable. Conversely, if the milk is at room temperature, or the cup still carries heat, the espresso cools and spreads rapidly as soon as it touches what is below, and the gradient layer naturally becomes blurry.

In terms of operation, three details are especially critical. First, the milk and the cup must be refrigerated in advance. It is recommended that the milk be around 4°C, and the cup also be put in the refrigerator for more than half an hour, so that the overall temperature comes down. Second, when extracting, the portafilter spout should be as close as possible to the surface of the milk, with the distance controlled within 1 cm, so that the espresso lays softly on the surface of the milk like a thin thread. If the spout is too high above the liquid surface, the impact force of the coffee stream will smash a pit into the milk, the crema buffer layer will be broken through, and the coffee will stab directly into the milk and disperse. At the end, do not add ice cubes. The cold sensation of Dirty depends entirely on the refrigerated milk. Adding ice cubes will dilute the concentration of the milk, and after melting will also disturb the layering, so the loss outweighs the gain.

In terms of parameters, a home semi-automatic machine can use 18 to 22 g of grounds, extracting about 40 g of coffee liquid within 25 to 30 seconds, keeping the grounds-to-liquid ratio at about 1:2. Use 150 ml of refrigerated fresh milk, and control the finished cup volume between 180 and 220 ml. This cup volume just allows the espresso to float on top and form an obvious layer. When drinking, there is no need to stir; take a big sip downward, and the flavor of every sip is changing, transitioning from rich hot coffee to icy sweet milk. This is the experience unique to Dirty.

If you want a beautiful layer, the richness of the crema is the decisive factor. If there is insufficient crema, the lifting force is not enough, and the espresso will easily sink. Qianjie Classic Blend adopts the Mamba approach, with Indonesian Mandheling and Brazilian Red Bourbon as the main body, adding a small amount of Robusta to improve crema performance. The medium-dark roast brings the aroma of butter, nuts and dark chocolate. When made into Dirty, the crema layer is especially thick, and the gradient can be maintained for a longer time. Qianjie Specialty Blend is a combination of Brazilian Cerrado and Colombian Huila. Qianjie has used this recipe for nearly ten years. The flavors of nuts and caramel blend very naturally with the milk, and the crema support is equally reliable. If you prefer 100% Arabica, Qianjie Black Cocoa Blend uses Yunnan washed small beans paired with Brazilian semi-washed beans. The crema is abundant and carries the flavors of creamy chocolate and ripe plums, and the value for money is also very prominent. Qianjie Queen's Estate SOE uses Brazilian Yellow Bourbon. After medium-dark roasting, it presents the smoothness of roasted peanuts and cream. When made into Dirty, it can retain a rounded chocolate finish, with delicate layers.

Qianjie's coffee beans are all shipped very fresh. After receiving them, rest them for four or five days before starting extraction, and the flavor will be more stable. Drinking them within one month allows you to taste the complete aroma changes. When choosing beans, look for medium-dark roast styles with good crema performance. Together with refrigeration and operational details, you can also make a beautifully layered cup of Dirty at home.