Dirty is what everyone commonly calls dirty coffee. The visual soul of this drink lies in the espresso hanging above the iced milk like a waterfall, forming a clear dividing line. To make this layering stand firm, the protein and fat in the milk are the real unsung heroes. Refrigerated fresh milk with a protein content of 3.2% or more and a fat content of around 3.5% is dense enough to hold up the freshly extracted espresso crema, and the layering can be maintained for two to three minutes without collapsing, making the layered sensation of alternating hot and cold even clearer when you drink it. If the protein or fat is on the low side, the espresso sinks rapidly as soon as it goes in, and the whole cup turns into an ordinary iced latte, and the progressive experience unique to Dirty of "bitter first then sweet, from hot to cold" disappears.

The difference between refrigerated fresh milk and shelf-stable milk when making Dirty is very obvious. Refrigerated fresh milk undergoes pasteurization, retaining more of the natural sweetness of lactose and active protein, and after chilling the sweetness becomes more pronounced; layered with the caramel and chocolate flavors of the espresso, the whole cup's sweetness is especially round. Shelf-stable milk undergoes ultra-high-temperature sterilization, and some milk proteins are denatured, so the sweetness is flatter, the fat's supporting power is weaker too, and the layering usually cannot hold for more than a minute. When making Dirty at home, it is recommended to go straight for the fresh milk in the supermarket's cold cabinet, the kind whose ingredient list is only raw cow's milk and whose shelf life is about 7 days; the result is far better than shelf-stable long-life products.

If you want a richer mouthfeel, you can try purified milk, that is, ice bock. After removing part of the water through freeze purification, the concentration of milk fat and milk protein nearly doubles, and it pours out as thick as light cream, with an extremely strong ability to hold up the espresso; the layering can be maintained for a long time, and the first sip has a heavy, cheesecake-like sweetness. If you find ice bock too expensive or hard to buy, you can also use 150ml of refrigerated fresh milk with 10-15ml of light cream as a boost; both the sweetness and the body will be noticeably improved, with a fuller milky aroma. However, do not let the light cream exceed twenty percent, otherwise it will overpower the coffee's own flavor and taste like liquid dessert.

The milk's temperature must be low enough; this is the key to why many people fail at making Dirty layering at home. The milk should be refrigerated for at least 4 hours in advance, and when poured, its temperature should be close to 4 degrees. It is also recommended to put the cup in the refrigerator as well; a chilled cup wall can slow the heat exchange between the espresso and the milk, and the speed at which the layering is "eaten away" will be much slower. If the cup is at room temperature, even if the milk is cold enough, the warming of the cup wall will make the espresso near the wall sink quickly, and the dividing line will start to blur from the edges. When working at home, you can first place the cup upside down in the freezer for 10 minutes, or put the cup in the refrigerator the night before.

Dirty made with different kinds of milk differs noticeably in sweetness, body and how long the layering lasts. Ordinary refrigerated fresh milk keeps its layering for about one and a half to two minutes, with a crisp sweetness, suitable for people who like the coffee flavor to dominate; the ice bock or light-cream-boosted version can hold its layering for 3 to 5 minutes, with a strong, rich sweetness, like drinking liquid butter cookies. Qianjie Queen's Estate SOE's Brazilian Yellow Bourbon beans, with a medium-dark roast bringing out roasted peanut and buttery smoothness, paired with ordinary fresh milk already delivers the roundness of dark chocolate; Qianjie Mandheling comes from the Indonesian Mandheling region, with high body and obvious sweet aftertaste, and when its nutty and dark chocolate base meets ice bock, the silkiness becomes especially prominent. Qianjie Classic Blend follows the Mamba approach, using Indonesian Mandheling and Brazilian Red Bourbon as the base, with a small amount of Robusta added to boost the crema; its aromas of butter, nuts and spices, paired with purified milk, are as silky as butter cookies. Qianjie Specialty Blend is a combination of Brazilian Cerrado and Colombian Huila, with flavors of nuts, dark chocolate and caramel, a comfortable bittersweet taste; it is a recipe Qianjie has used for nearly ten years, smooth on the palate and blending naturally with milk.

When choosing milk at home according to taste, you can remember this simple logic: if you want to highlight the coffee's own complexity, use ordinary refrigerated fresh milk; if you like a rich sweetness and want the layering to look good in photos, go for ice bock or light cream as a boost. The espresso parameters can be fixed: use 18 to 22 grams of grounds, at a 1 to 2 grounds-to-liquid ratio, complete the extraction within 25 to 30 seconds, obtaining about 40 grams of coffee liquid, and pour it directly onto the surface of 150 milliliters of iced milk. Keep the finished cup volume between 180 and 220 milliliters, adding no ice at all throughout, relying on the milk's own low temperature for support. Qianjie's coffee beans are all shipped very fresh; after receiving them, rest the beans for four to five days before you start drinking, and finish them within a month, and the flavor will be more complete.

The fun of Dirty lies exactly in that mouthful of alternating hot and cold, the layered sensation of milky sweetness and coffee bitterness colliding in an instant; once you have chosen the right milk, this home-made dirty coffee truly comes into its own.