Can You Drink Bock Milk Directly? Is Making Dirty Coffee with Bock at Home Difficult?

Last week, while visiting cafes, FrontStreet Coffee happened to encounter a barista making ice bock. Through the glass you could see a rich milk liquid dripping drop by drop, and it immediately made me want to try a Dirty made with ice bock. Unfortunately, the barista said the remaining portion was not enough to serve, so FrontStreet Coffee could not have that coffee and has been thinking about it ever since.

Since that is the case, FrontStreet Coffee decided to go all out today and make it at home.

What Is the Principle Behind Ice Bock?

"Ice bock" comes from the German Eisbock, which originally referred to the freeze-distillation method for making beer more concentrated. The principle is not complicated: it uses the different freezing points of various substances. By releasing substances in a specific order as they melt, some of the water can be removed, achieving concentration. Applied to milk, the concentrated liquid that comes out is what baristas call ice bock.

As everyone knows, water makes up the largest share of a bottle of milk, with the rest being solid substances such as protein, fat, and sugars. The freezing point of water is 0 °C, while the other components have lower freezing points. When milk is placed in the freezer, the water freezes first, followed by the other substances, until everything becomes ice.

The melting order reverses the process. Proteins, fats, and other substances melt first, followed by the water, creating a water-milk separation. By removing some of the water, ice bock raises the proportion of the other substances, making the taste thicker and more mellow, with a hint of cheese-like salty aroma.

How to Make Ice Bock

Making ice bock at home is extremely simple. All you need is a refrigerator, a carton of milk, and a container to hold the ice bock, and you can start right away.

First, place the entire carton or bottle of milk in the freezer compartment until it is completely frozen solid. FrontStreet Coffee used Guangming Fresh Pasture 950 ml boxed fresh milk. It went into the freezer last night and had frozen solid by the morning. (FrontStreet Coffee has heard that in northeastern China in winter, you can just leave milk outside for a while.)

Next, open a small opening in the frozen "ice brick," just as you would open milk normally. Then invert the carton over a pre-prepared pot and wrap the exposed part in plastic wrap, letting it melt naturally. This keeps the milk safe from outside contamination, such as insects getting in, and stabilizes the structure so it does not tip.

FrontStreet Coffee finds that leaving it to melt at room temperature is fine and should complete the concentration in a few hours. FrontStreet Coffee spent 3.5 hours and finally got 400 ml of ice bock, an extraction rate of about 42%. Finally, seal it and store it in the refrigerator for later use.

(If you really do not have time to wait and there is enough space in the refrigerator, you can run the process in the upper refrigerated section. The dripping takes longer, but it is more reliable, and the concentrated milk stays at a low temperature throughout.)

How Does an Ice Bock Dirty Taste?

To find the difference between the two, FrontStreet Coffee tested both regular fresh milk and freshly dripped ice bock with a concentration meter. Regular fresh milk had a concentration of 12.11%, with other substances at 14.39%, while ice bock was significantly higher both in concentration, at 16.22%, and in the proportion of components such as fat and sugars, at 18.94%.

FrontStreet Coffee took a sip of the high-purity ice bock straight. The milk aroma was very rich, the lactose sweetness high, and it tasted mellow and viscous. Trying regular fresh milk again afterward, the latter tasted quite bland by comparison. Precisely because ice bock is so concentrated, though, drinking it plain can easily become overwhelming, so it is best enjoyed with espresso. Among combinations, Dirty and ice bock is widely regarded in the coffee community as the best pairing.

There is generally no fixed ratio for making a Dirty; it often depends on the capacity of the cup you are using. For a better visual, FrontStreet Coffee used a 250 ml transparent wide-mouth cup, first pouring in 200 ml of the concentrated milk, then positioning it directly under the portafilter to extract the coffee. (Using today's Warm Sunshine blend parameters for the espresso: 20 g of coffee grounds yielding 36 ml of coffee liquid in 30 seconds.)

Because ice bock is denser than regular fresh milk, the crema easily stays on top, and the layered effect created by the black and white colors becomes more obvious and lasts longer. On tasting, FrontStreet Coffee could feel the rich milk and the bittersweet espresso colliding in the mouth. After swallowing, there was the sweetness of White Rabbit milk candy mixed with hazelnut biscuits. As the two gradually blended, a cheese-like salty aroma emerged, with a creamy, silky texture that made the whole cup far more layered and complex.

FrontStreet Coffee Recommendation

Making a good Dirty starts with choosing the right bean. The ingredients are the same as an iced latte, milk and espresso, and the only difference is that a Dirty skips the ice, so it is often jokingly called an iced latte without ice. For the best result, FrontStreet Coffee recommends a medium-dark blend or a medium-dark SOE, because those carry richer caramel aromas and deeper notes that contrast clearly with sweet milk and leave a long finish. For consistency and value, FrontStreet Classic Blend is the pick: it pairs a soft Brazilian coffee with wild Mandheling and a small amount of Robusta, so the extraction has a rich, fine crema and layered aroma, and a Dirty made with it brings butter and roasted chestnut.

FrontStreet Sumatra PWN Golden Mandheling SOE is another excellent choice. Its spec requires beans of 18 mesh and above with fewer than 3 defective beans per 300 g green coffee sample, the highest G1 grade, and it also goes through 3 rounds of hand sorting to remove defects and keep the cup clean. With its concentrated, wild aroma, solid body, and strong finish, and a medium-dark roast that brings out caramel, dark chocolate, and toasted bread, it pairs beautifully with milk; a Dirty made with it shows the hot espresso on top and cold milk below, like drinking an iced creamy cocoa.

FrontStreet Brazil Queen Farm SOE is a Yellow Bourbon, a variety that arose from a mutation of Bourbon in São Paulo state: the ripe fruit does not turn red but orange-yellow, which is how it got its name. The bean is naturally sweet and clean, and sun-drying adds a tropical fruit note after roasting. Because the first sip of a Dirty hits the crema, a sweeter bean makes the experience better, and Queen Farm fits that role well.

If you want a smaller contrast and a fresher coffee flavor, choose a medium-roast bean, since light roasts are unsuitable for Dirty because they are too acidic and low in crema. FrontStreet Strawberry Candy Espresso is made specifically for espresso machines, combining the sweetness of a honey-processed Costa Rican coffee, the chocolate and nutty aroma of Colombian coffee, and the floral honey of a Panama Geisha. Medium roasted, it extracts into a rich, clear cup with rose, grape, and dried blueberry aromas, and a Dirty made with cold milk highlights cranberry cookie and hazelnut ice cream flavors.