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 unique drinking experience different from conventional methods, Dirty coffee successfully emerged as a standout star during the peak of the "dirty culture" trend, becoming a rising sensation in the coffee world! Even today in 2025, its popularity hasn't waned, remaining a flagship product at many coffee shops.

As we all know, when making Dirty coffee, we typically use chilled glasses. This isn't just because Dirty is an iced coffee beverage, but also because using chilled glasses effectively prolongs the layered effect between coffee and milk. You should know that Dirty's unique drinking method and stunning appearance both stem from the repulsion between espresso and milk. It's precisely because the espresso doesn't blend with the milk that we can experience the collision of two completely different sensations—hot espresso and cold milk—in our mouths with one sip.

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

Surprisingly, using frozen glasses to make Dirty has indeed become quite popular recently, with some even using glasses frozen to -86°C. So today, FrontStreet Coffee will experience whether Dirty made with frozen glasses truly tastes better than those made with refrigerated glasses.

Experimental Comparison

Although FrontStreet Coffee doesn't have a freezer that can chill glasses to -86°C, the regular freezer's chilling effect is actually sufficient enough (-15°C to -20°C). In this experiment, FrontStreet Coffee will make two cups of Dirty using the same milk, the same espresso, and the same method. The only difference between them is the glass used—one with a refrigerated glass, the other with a frozen glass. All other aspects remain identical. (Left: frozen, right: refrigerated)

Without further ado, let's begin our experiment! We first took out the chilled/frozen glasses that had been prepared in the refrigerator/freezer a day in advance, then poured 180ml of refrigerated milk into each. Immediately after, we began extracting espresso. Here, FrontStreet Coffee used parameters of 20g of coffee grounds to extract 40ml of liquid weight in 30 seconds.

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

The results were as follows:

Dirty Made with Refrigerated Glass

The first sip was taken half a minute after the coffee was made. Due to 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" in the mouth.

FrontStreet Coffee then tasted at 1 minute, 2 minutes, and 3 minutes after the coffee was made. Apart from the gradual decrease in espresso presence in the mouth, there weren't many other changes. The layering remained distinct, though the temperature contrast experience gradually diminished as the espresso temperature dropped. By the 5th minute, you could still see clear layering between espresso and milk visually, but the temperature difference was almost gone.

So that concludes the experience with the refrigerated glass Dirty. Next up is the Dirty made with the frozen glass~

Dirty Made with Frozen Glass

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

FrontStreet Coffee then tasted at the same intervals—1 minute, 2 minutes, and 3 minutes after the coffee was made. The results showed that because the frozen glass temperature was lower, the espresso temperature dropped faster, causing the temperature difference between the two to shrink rapidly. Additionally, the espresso and milk blended faster than in the refrigerated glass, with the layered effect lasting for a relatively shorter time.

From the above, we can learn that although using a frozen glass can initially increase the contrast between espresso and milk, better showcasing Dirty's characteristics, it cannot maintain the layered effect well. The reason for this is that 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, when the milk temperature drops to a certain point (below 4°C), its density actually gradually decreases. Meanwhile, the milk's aroma and richness both diminish with further temperature drops. Here, 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 temperature was around 5°C, while after pouring into a frozen glass, it dropped to 1°C. This shows that having the glass too cold isn't necessarily a good thing.

However, this doesn't mean frozen glasses aren't suitable for making Dirty. Just as the -86° Dirty became wildly popular online. As long as we control the details properly, Dirty made with frozen glasses can be equally outstanding! (And the difference between it and the refrigerated glass version is actually hard to distinguish without direct comparison)

What to Pay Attention to When Making Dirty with Frozen Glasses?

First, it's best to use ultra-filtered milk! Because ultra-filtered milk has a higher concentration, it can completely ignore the negative effects that extremely low temperatures have on density and texture. In fact, this might even be a good thing for highly concentrated ultra-filtered milk! Some ultra-filtered milk products have often received negative feedback like "too rich" or "too thick" due to their high concentration, and lower temperatures can appropriately temper their flavor and texture, making them concentrated without being overly prominent. Therefore, when using frozen glasses to make Dirty, it's best to pair them with ultra-filtered milk.

The second thing to note is the drinking speed! Because the glass temperature is lower, this causes the espresso to cool down faster. As FrontStreet Coffee shared in the experiment above, compared to refrigerated glass Dirty, frozen glass Dirty loses its temperature contrast more quickly, while the espresso and milk blend faster. Therefore, it's best to drink frozen glass Dirty quickly within a short time to ensure you can enjoy Dirty's ultimate contrast experience throughout.

If you can truly finish it quickly within a short time, you'll discover a small surprise brought by the frozen glass at the bottom—frozen milk. Although it doesn't taste great, isn't this also a unique experience~

So, that's all for FrontStreet Coffee's sharing today! Finally, let's answer a few questions that friends must be very curious about:

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

Oh right, when freezing glasses, make sure to seal them properly, dry them thoroughly, and choose sturdy glasses, otherwise they might easily harbor bacteria, and... you might lose a glass....

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Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473