Why is a chilled cup essential for making a Dirty coffee? What does 'dirty' mean and how is it made?

The Science Behind Chilled Cups for Dirty Coffee

Even in 2024, the popularity of Dirty coffee remains undiminished. Discussions about its preparation method continue to thrive across various platforms. Upon closer examination, its key preparation points boil down to three essential elements: cup temperature, milk concentration, and the distinctiveness of layering!

FrontStreet Coffee has shared extensively about the latter two aspects, but regarding Dirty coffee cups, FrontStreet Coffee has only briefly mentioned them without highlighting their primary purpose. This brings us to the question: Why are the cups used for making Dirty coffee typically chilled by default? Neither room temperature nor frozen—what exactly is the effect of using a chilled cup?

Coffee Experiment

To clarify its purpose, let's conduct a series of experiments to observe the differences between Dirty coffees made with room temperature, chilled, and frozen cups. The Dirty coffee parameters are as follows: 180ml of chilled milk, 40ml of espresso (extracted from 20g of coffee grounds over 30 seconds to yield 40ml of espresso). The preparation method is as follows: First, pour iced milk into the cup, then begin extracting espresso. Place the cup containing milk under the portafilter's spout to catch the espresso, allowing it to float on the surface! Stop extraction once the target liquid weight is reached, remove the cup, and the Dirty coffee is complete!

(If using a bottomless portafilter and the cup is positioned too far from the spout, we can place a spoon to reduce the impact of the espresso, which will result in better layering)

Room Temperature Glass Cup

Because the cup's temperature is higher than the chilled milk, when 6°C iced milk is poured into the cup, its temperature rises to 8°C. The Dirty coffee made with this cup initially shows relatively clear layering! When tasting the first sip, there's a noticeable temperature difference between the milk and coffee. However, starting from the second sip (approximately two minutes after preparation), the temperatures of milk and coffee have become quite similar, with the temperature difference experience gradually decreasing. By the third minute after preparation, the temperature difference experience is virtually gone! At this point, the milk and espresso have reached the same temperature.

Chilled Glass Cup

Since both are in the same refrigerated environment, the cup's temperature is quite close to the milk's temperature. Therefore, when 6°C iced milk is poured into the cup, the milk temperature slightly decreases to around 3°C (due to the cup being colder than the milk), resulting in very distinct layering!

When tasting the first sip, we can distinctly feel the temperature difference between the cold milk and hot coffee. By the second sip, although the coffee and milk have begun to blend slightly, they still maintain a relatively obvious temperature difference, preserving the layers. Although this temperature difference lasts longer, if you finish it within three minutes, you can maintain a relatively comfortable drinking experience throughout.

Frozen Glass Cup

The frozen cup, due to the significant temperature difference with room temperature, begins to condense within tens of seconds after being removed from the freezer. When iced milk is poured into the cup, the temperature drops to around 1°C.

The Dirty coffee made with it also shows distinct layers, allowing us to perceive the different characteristics of coffee and milk both visually and texturally. Its layering duration is similar to that of the chilled cup, with no significant difference. From these experiments, we can conclude that Dirty coffee requires chilled cups because low-temperature cups better maintain the temperature of iced milk, thereby achieving better layering effects! The layering effect refers to both appearance and texture. The choice between chilled and frozen cups leans toward chilled because frozen cups easily condense due to the significant temperature difference with room temperature. Considering production efficiency and practicality, chilled cups are more suitable (chilled cups also condense, but less so)!

Interestingly, when Dirty coffee was first invented, there was no step involving chilled cups! FrontStreet Coffee has shared multiple times that its inventor, Mr. Katsuyuki Tanaka, simply used iced milk paired with specially prepared espresso. Even without specifically chilling the cup, the espresso he prepared had such high concentration that it could maintain layering with iced milk for an extended period. It wasn't until Dirty coffee spread to China and changes were made to espresso preparation that the application of chilled cups was introduced to pursue an even more perfect taste experience!

FrontStreet Coffee
No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou, Guangdong Province

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473