Should You Chill or Freeze the Glass for Making Dirty? What's Special About -86°C Dirty? What is Dirty Coffee?

With its striking appearance and a drinking experience unlike conventional methods, Dirty coffee emerged as a standout star during the peak of the "dirty culture" trend and became a rising sensation in the coffee world. Even now in 2025, its popularity has not faded, and it remains a flagship product at many coffee shops.

As we all know, Dirty is typically made in a chilled glass. This is not only because Dirty is an iced coffee drink, but also because a chilled glass effectively prolongs the layered effect between coffee and milk. Dirty's unique drinking method and stunning appearance both come from the repulsion between espresso and milk. Precisely because the espresso does not blend with the milk, we can feel two completely different sensations — hot espresso and cold milk — collide in the mouth with a single sip.

To achieve that, we use various methods to separate the milk and espresso more thoroughly in the glass: increasing milk concentration, reducing the distance when pouring espresso into milk, increasing the temperature difference between them and, as FrontStreet Coffee mentioned above, using a chilled glass to slow the milk's temperature rise so the layered effect lasts longer. Coffee shops typically use refrigerated glasses rather than frozen ones, which has led many friends to wonder, "Since a colder glass makes the layers last longer, would Dirty made with a frozen glass taste better than one made with a refrigerated glass?"

Surprisingly, making Dirty with frozen glasses has indeed become popular recently, with some even using glasses frozen to -86°C. So today FrontStreet Coffee will test whether Dirty made with a frozen glass truly tastes better than one made with a refrigerated glass.

The Experiment

FrontStreet Coffee does not have a freezer that can chill glasses to -86°C, but a regular freezer's effect is actually sufficient (-15°C to -20°C). In this experiment, FrontStreet Coffee made two cups of Dirty using the same milk, the same espresso and the same method. The only difference was the glass — one refrigerated, one frozen. Everything else was identical. (Left: frozen; right: refrigerated.)

First, we took out the chilled and frozen glasses prepared a day in advance, then poured 180ml of refrigerated milk into each. Immediately after, we began extracting espresso using parameters of 20g of coffee grounds to yield 40ml of liquid weight in 30 seconds.

We positioned the glass of milk under the portafilter spout, stopped the extraction when the coffee reached the target volume, and began tasting. (It is not recommended to drink immediately after making, as you might get burned.)

The results were as follows.

Dirty Made with a Refrigerated Glass

The first sip was taken half a minute after the coffee was made. Because of the perfect layering, this sip clearly felt the collision of cold and hot in the mouth. The espresso was rich and mellow, while the milk was silky and sweet — two separate experiences "dancing" on the palate.

FrontStreet Coffee then tasted at 1, 2 and 3 minutes after the coffee was made. Apart from the gradual decrease in espresso presence, there were few other changes. The layering remained distinct, though the temperature contrast diminished as the espresso cooled. By the 5th minute, clear layering was still visible, but the temperature difference was almost gone.

That concludes the experience with the refrigerated-glass Dirty. Next up is the Dirty made with the frozen glass.

Dirty Made with a Frozen Glass

Again, the first sip was taken half a minute after the coffee was made. Compared with the refrigerated-glass Dirty, the frozen-glass version had a greater temperature difference on the first sip because the milk was colder. However, the milk's flavor and texture were somewhat inferior — not as aromatic or smooth.

FrontStreet Coffee then tasted at the same intervals — 1, 2 and 3 minutes after making. The results showed that because the frozen glass was colder, the espresso temperature dropped faster, so the temperature difference between the two shrank rapidly. In addition, the espresso and milk blended faster than in the refrigerated glass, and the layered effect lasted a relatively shorter time.

From this we learn that although a frozen glass can initially increase the contrast between espresso and milk and better showcase Dirty's character, it cannot maintain the layered effect well — because the milk temperature is too low.

When we make Dirty, we use refrigerated milk because its temperature is lower than room-temperature milk. This increases the milk's density, allowing it to support the espresso and create excellent layering.

However, once the milk temperature drops to a certain point (below 4°C), its density actually decreases gradually, and its aroma and richness also diminish as it gets colder. FrontStreet Coffee used two additional glasses (one refrigerated, one frozen) to pour milk and measure temperature. The results showed that after pouring into a refrigerated glass the milk was around 5°C, while after pouring into a frozen glass it dropped to 1°C. So having the glass too cold is not necessarily a good thing.

That said, frozen glasses are not unsuitable for Dirty. Just as the -86° Dirty became wildly popular online, as long as the details are controlled properly, a frozen-glass Dirty can be equally outstanding. (And without direct comparison, the difference from the refrigerated version is actually hard to distinguish.)

What to Watch When Making Dirty with a Frozen Glass

First, it is best to use ultra-filtered milk. Because it is more concentrated, it can ignore the negative effects that extremely low temperatures have on density and texture — in fact, lower temperatures may even suit highly concentrated ultra-filtered milk. Some ultra-filtered milks often draw negative feedback like "too rich" or "too thick" precisely because of their concentration, and a lower temperature can appropriately temper that, making the milk concentrated without being overpowering. So when using frozen glasses for Dirty, pair them with ultra-filtered milk.

The second thing to watch is drinking speed. Because the glass is colder, the espresso cools faster. As FrontStreet Coffee shared above, compared with refrigerated-glass Dirty, the frozen-glass version loses its temperature contrast more quickly while the espresso and milk blend faster. So it is best to drink frozen-glass Dirty quickly within a short time to enjoy Dirty's ultimate contrast from start to finish.

If you truly finish it quickly, you will find a small surprise at the bottom of the frozen glass — frozen milk. It may not taste great, but it is a unique experience in its own right.

So that is all for FrontStreet Coffee's sharing today. Finally, a few questions friends are surely curious about:

"Will the glass stick to your lips?" — "No."
"Will FrontStreet Coffee's stores offer this?" — "No." (Only regular Dirty is available.)

Also, when freezing glasses, be sure to seal them properly, dry them thoroughly and choose sturdy glassware; otherwise they can easily harbor bacteria, and you might lose a glass.

- END -

FrontStreet Coffee Recommendation

Dirty is made from the same materials as an iced latte — milk and espresso — the only difference being that Dirty has no ice, which is why it is often jokingly called an iced latte without the ice. To make a good Dirty, it is best to start with a medium-dark roast blend or an SOE single origin, since these carry richer caramel aromas and a heavier body that contrast nicely with sweet milk for a long finish.

For consistency and value, choose FrontStreet Classic Blend. With soft Brazilian beans, wild Mandheling and a small amount of Robusta, it extracts rich, fine crema with layered aroma, giving a Dirty the rich scent of butter and roasted chestnut. For a single-origin route, FrontStreet PWN Golden Mandheling SOE meets the 18-mesh standard with fewer than 3 defective beans per 300g sample, placing it at the top G1 grade; the beans are dark green, neatly shaped flat beans, sorted by hand three times to remove defects and keep the cup clean. Roasted medium-dark, it shows caramel, dark chocolate and toasted bread aromas that pair well with milk — a Dirty made with it feels like drinking iced creamy cocoa.

Because the first sip of Dirty meets the espresso crema, choosing a sweeter bean improves the experience. FrontStreet Brazil Queen's Estate SOE is a Yellow Bourbon, a mutation of the Bourbon genetics from São Paulo state that ripens to orange-yellow rather than red; naturally processed, it is sweet and clean with tropical fruit aroma. If you want less contrast and a fresher taste, FrontStreet Strawberry Candy Espresso is roasted medium and combines the honey sweetness of Costa Rican coffee, the chocolate and nutty sweetness of Colombian coffee and the floral honey of Panamanian Gesha; served over iced milk, it highlights cranberry cookie and hazelnut ice cream flavors.