Does a Frozen Glass Ruin Dirty Coffee Layering?

A refrigerated glass keeps the layers in a Dirty coffee longer than a frozen one. Front Street Coffee ran a side-by-side comparison and found that although a frozen glass widens the initial temperature gap between hot espresso and cold milk, it speeds up the collapse of the layers and slightly dulls the milk's mouthfeel and aroma.

The test compares a refrigerated glass against a frozen glass using the same espresso and milk. To slow the fading layers, shops raise milk concentration, shorten the drop height when pouring, widen the temperature gap, and rely on a chilled glass; most use refrigerated rather than frozen cups. Purified milk is named as the better partner for a frozen glass.

Dirty, with its high aesthetic appeal and the unique experience of hot-cold contrast, has always been a popular product in coffee shops. When making a Dirty, using a chilled cup can extend the layering effect between espresso and milk, but should you use a refrigerated cup or a frozen cup? Through comparative experiments, Front Street Coffee found that although a frozen cup can increase the initial temperature difference, it accelerates the dissipation of the layers, and the milk's mouthfeel and aroma are slightly compromised. The popularity of -86°C Dirty has brought frozen cups into the spotlight again. This article explains in detail the differences between refrigerated and frozen cups, the precautions for making Dirty with a frozen cup, and why purified milk is the best partner for a frozen cup, helping you fully understand the intricacies behind this dirty coffee.

With its outstanding visual presentation and unconventional drinking method, Dirty (dirty coffee) quickly rose to fame in the year the dirty trend emerged and became a hot star product in the coffee world. Even in 2025, its popularity has not diminished, and it remains a signature drink in many coffee shops.

Anyone familiar with the Dirty-making process knows that a chilled cup is almost essential. There are two reasons: first, Dirty itself belongs to the iced coffee category; second, a chilled cup can effectively slow down the fading of the layers between coffee and milk.

The reason Dirty can offer such a unique drinking method and eye-catching appearance lies in the immiscibility between espresso and milk. It is precisely because the two do not mix that the drinker can experience in one sip the strong contrast between hot espresso and cold milk.

Therefore, we use various methods to keep the milk and espresso as separated as possible in the cup. For example, increasing the milk's concentration, shortening the drop height when pouring espresso into milk, widening the temperature gap between the two, and, as Front Street Coffee mentioned earlier, using a chilled cup to slow the milk's warming rate so the layers last longer.

Generally speaking, the chilled cups used in coffee shops are mostly refrigerated cups rather than frozen cups, which leads many customers to wonder: "Since the colder the cup, the longer the layers last, wouldn't a frozen cup make a Dirty taste better than a refrigerated cup?"

Coincidentally, making Dirty with frozen cups has recently become popular, and the cups are frozen to -86°C. So today, Front Street Coffee will put it to the test and see whether a Dirty made with a frozen cup is really better than the refrigerated-cup version.

Experimental Comparison

Although Front Street Coffee does not have freezing equipment that can reach -86°C, an ordinary refrigerator's freezer compartment is sufficient (-15°C to -20°C). In this test, Front Street Coffee will use the same milk, the same espresso, and the same procedure to make two Dirty coffees, with the cup as the only variable. One uses a refrigerated cup, and one uses a frozen cup, with all other conditions completely identical.

(The left is frozen, the right is refrigerated)

Without further ado, let's begin the experiment! First, take out the chilled cups that were refrigerated/frozen a day in advance, and pour 180ml of refrigerated milk into each. Immediately afterward, extract the espresso. The parameters used here at Front Street Coffee are 20g of grounds, extracting 40ml of liquid yield in 30 seconds.

Move the cup containing the milk under the portafilter spout, stop the extraction when the coffee liquid approaches the target amount, and then it's ready to taste!

(It is not recommended to drink it immediately after making it, as it can easily burn your mouth)

The results obtained are as follows:

Dirty made with a refrigerated chilled cup:

The first sip was taken half a minute after the coffee was completed. Because the layers were perfect, the collision of cold and hot was extremely vivid on entry. The espresso was rich and full-bodied, the milk silky and sweet, and the two sensations danced independently in the mouth.

Afterward, Front Street Coffee tasted it at 1 minute, 2 minutes, and 3 minutes after the coffee was made. Apart from the proportion of espresso in the mouth gradually decreasing, there was little other change. The layers remained clear; it was just that as the espresso's temperature dropped, the contrast brought by the temperature difference gradually weakened. By the 5th minute, the distinct layers of espresso and milk were still visible in appearance, but the temperature difference had almost disappeared. That concludes the experience of the refrigerated-cup Dirty. Next up is the frozen-cup Dirty~

Dirty made with a frozen chilled cup:

Similarly, the first sip was taken half a minute after the coffee was completed. Compared with the refrigerated-cup Dirty, the frozen-cup Dirty had a greater temperature difference on the first sip because the milk was colder. However, the milk's flavor and mouthfeel were slightly lacking, the aroma was not as full, and the smoothness was somewhat inferior.

Then Front Street Coffee tasted it at the same time points, namely 1 minute, 2 minutes, and 3 minutes, and found that because the frozen cup was colder, the espresso cooled faster, and the temperature difference between the two quickly narrowed. At the same time, the espresso and milk integrated faster than in the refrigerated cup, and the layering effect lasted for a relatively shorter time.

From the above results, it can be seen that although the frozen cup can further widen the contrast between espresso and milk in the initial stage and better showcase Dirty's characteristics, it cannot maintain the layers well. The root cause is that the milk temperature is too low.

The reason we choose refrigerated milk when making Dirty is that refrigerated milk is lower in temperature than room-temperature milk. This increases the milk's density, allowing it to support the espresso and form an excellent layering effect.

However, when the milk temperature drops to a certain point (below 4°C), the density instead gradually decreases. At the same time, the milk's aroma and body also weaken as the temperature drops further. Here, Front Street Coffee additionally took two chilled cups (refrigerated and frozen), poured in milk, and measured the temperature. The results showed that after pouring into the refrigerated cup, the milk temperature was about 5°C, while after pouring into the frozen cup, it dropped to 1°C. It can be seen that an overly cold cup is, to some extent, not a good thing.

But this does not mean frozen cups cannot be used to make Dirty. Just as -86°C Dirty can go viral across the internet, as long as the details are well controlled, a Dirty made with a frozen cup can also be outstanding!

(And without a comparison, the difference from a refrigerated cup is actually hard to taste)

What should you pay attention to when making Dirty with a frozen cup?

First, it is best to use purified milk! Because purified milk has a higher concentration, it can completely ignore the negative effects of extremely low temperature on density and mouthfeel. In fact, low temperature is even beneficial for highly concentrated purified milk!

This is because some purified milk has long received negative reviews such as "too cloying" and "too thick" due to its high concentration, and lower temperature happens to appropriately restrain its taste and mouthfeel, making it rich without being overly assertive. Therefore, when making Dirty with a frozen cup, it is best to pair it with purified milk.

Second, pay attention to drinking speed! Because the cup is colder, the espresso cools faster. As Front Street Coffee's experiment shows, compared with a refrigerated-cup Dirty, a frozen-cup Dirty loses its temperature contrast more quickly, and the espresso and milk integrate faster. Therefore, when drinking a frozen-cup Dirty, it is best to finish it quickly in a short time so that you can enjoy Dirty's ultimate contrasting experience throughout.

If you really can finish it in a short time, you will also discover a small surprise brought by the frozen cup at the bottom—solidified milk. Although it is not very tasty, it is still a different kind of experience~

So that is all for today's share from Front Street Coffee! Finally, let's answer a few questions that friends are sure to be curious about: "Will the cup stick to your lips?"—"No"; "Will Front Street Coffee stores offer it?"—"No" (only the regular Dirty)~~~By the way, when freezing cups, be sure to seal them well and dry them thoroughly, and choose sturdier cups, otherwise they can easily pick up bacteria, and....

you will lose a cup....

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The above content is compiled by CoffeeHunters, a coffee news website.