March 7, 2022

What is flash brew coffee?


Today, when most people talk about cold coffee, cold brew or iced coffee generally come to mind first. But as far back as the 1960s, people were enjoying an entirely different kind of cold coffee beverage in Japan: Japanese iced coffee, also known as flash brew.

It wasn’t until the late 20th and early 21st century, however, that this beverage started to become popular further afield in the US. Since then, it has played a role in the wider rise of cold coffee.

I spoke to two coffee brewing champions to get a better understanding of how this drink is prepared, how to brew the perfect cup, and how it’s different to cold brew. Read on to learn more.

You might also like our article exploring alternative coffee beverages.

pour over coffee on scale

The fundamentals of flash brew

First things first: with flash brew or Japanese iced coffee, you should start by using a pour over coffee brewer, such as a Hario V60 or Kalita Wave.

The concept is simple. You retain the same ratio of water to coffee, but a proportion of your water weight is frozen as ice. The ice is placed in the server beneath the dripper, while the remaining water is heated to your target brewing temperature and used to brew a kind of pour over coffee concentrate.

This results in a brew that is hot for a brief moment before being quickly chilled, which allows for more nuanced flavours in the final cup. 

As with normal filter brewing, following a specific recipe is important, if not more so because of the addition of ice.

To understand the process more closely, I spoke with 2020 UK Brewers Cup Champion Matteo D’Ottavio. He currently works as a barista at WatchHouse in London. 

“The way to prepare Japanese iced coffee is to create a concentrate with the first part of the brew, and then balance it with the ice,” he explains. “This way, you get the right amount of solubles without losing the strength.”

He adds that most of the coffee’s soluble compounds are extracted during the first 50% of your brew, while the second half is really there to balance the cup. 

Matteo also notes that as well as having a defined recipe, getting your brew ratio right is vital.

“I like to create a beverage with a coffee to water ratio of 1:17,” he says. “Some 65% of the total water should be hot, while the remaining 35% should be ice.”

This is why it’s key to weigh out both the water you’re going to brew with, as well as the ice you’re putting in your cup or server. Make sure it equals your total brew weight, otherwise your brew won’t be properly balanced.

Flash brew recipe

Below you can find an example flash brew recipe using 30g of coffee, which will yield 500g (around enough for two cups):

You’ll need:

  • 30g coffee 
  • 325ml water, heated to between 90°C and 96°C
  • 175g ice
  • Pour over dripper
  • Filter paper
  • Server or jug with a capacity of more than 500ml
  • Kettle
  • Grinder
  • Timer
  • Scales


  1. Weigh out your coffee and grind it, medium-fine. The coffee will need to be finer than usual for pour over, as you are using less water.
  2. Boil your water and load your ice into the server or jug.
  3. Place your filter into your dripper, and rinse it through with hot water. Don’t do this over your server, however, as the ice will melt.
  4. Place your ground coffee into your dripper.
  5. Saturate the coffee bed with your water (60ml to 90ml), and let the coffee bloom for 30 to 45 seconds.
  6. Resume pouring in concentric circles, filling in stages. Pour slowly, aiming to reach your total brew weight by around 2 minutes.
  7. Once the brew water has drained, dispose of the filter and coffee, then serve and enjoy.
flash brew coffee

Finding the sweet spot

Just as with filter coffee, it’s important to experiment with your recipe and tailor all the different brewing variables

Former UK Brewers Cup champion Lisa-Laura Verhoest, who’s currently the coffee manager at GAIL’s Bakery in London, tells me that the true beauty of Japanese-style iced coffee lies in its potential for experimentation.

“You can really tailor the recipe to get the best out of your brew,” she says. “Because you’re brewing with hot water and there’s so much more energy in the water, you’ll get a lot more complexity in your cup, compared to when you steep cold brew over time.”

This is the main reason why the flavour profile of a cold brew coffee is smoother and sweeter, while flash brew retains its acidity and brightness.

“To achieve a good flash brew I’d say it’s best to use a coffee that is quite high-grown, with a good amount of acidity and brightness to it,” she notes.

To get the most out of your brew, she recommends making sure that your ice makes up no more than 30 to 40% of the total water weight, to allow for proper extraction. To compensate for this, she also uses a finer grind, as mentioned in the recipe above.

“We need to remember that we’ve still got the same amount of coffee, but less water to extract all the flavours,” she adds.

On the other hand, Matteo not only adjusts the grind size, but also the water temperature.

“I like to grind my coffee finer than usual and brew with a water temperature below 90°C,” he says. “With this method, you get good extraction, but you won’t need more ice to cool down the coffee.”

pour over coffee

Choosing right coffee

Matteo says that the type of coffee you use will depend on your personal preference.

He says: “For flash brew, I usually prefer fruity or floral coffee with bright acidity, but if you prefer something sweeter with chocolatey and nutty flavours, go for it.” 

Lisa-Laura agrees, saying that you should take the time to find a coffee you like, whether it’s a specific origin or a roast profile. She notes that changing our the actual brewer itself can help to amp up or dial back certain flavour notes.

“You can choose the device that will help you achieve the best flavour for that particular coffee,” she says. “If I’m brewing an Ethiopian coffee, and looking for lots of bergamot and floral notes, I’m going to be using my V60.

“Meanwhile, If I know that I’m going to be brewing a Costa Rican, where I expect a base note of chocolate and some higher notes as well, I’m going to be using my Kalita,” she adds. “I feel like a Kalita is the best way to get all of those flavours out into the brew.”

Generally speaking, it’s a good idea to start with what you have and explore a range of coffees for each new brewing method. Japanese iced coffee is no different.

coffee grinder

Flash brew today

Although flash brew is certainly more well-known today than it was a few decades ago, it’s not as widely offered by specialty coffee shops as you might expect. 

Nonetheless, it can be a great addition to a coffee shop menu and can help encourage coffee drinkers to explore new flavours. 

While it might also sound like a lot more effort than cold brew, flash brew can actually be made with a batch brewer and some ice. 

“It is actually much more efficient and cost effective to use a batch brewer to make Japanese iced coffee than to steep cold brew for 12 to 24 hours,” Lisa-Laura tells me.

It’s also interesting to see forward-thinking coffee professionals experimenting on this classic technique to innovate with cold coffee flavour profiles. For example, Matteo tells me that he was impressed by the work of 2019 Brewers Cup Champion Lewis Maillardet. 

“Lewis’ technique consists of pouring the first part of your coffee in an empty vessel, pouring the rest over ice, and finally adding that first pour to the iced brew,” he explains.

“We noticed that the coffee was brighter with a more vibrant acidity if we avoided the thermal shock that would occur when you poured the whole brew onto ice straight away.”

pour over coffee

Today, flash brew isn’t as prominent among the global coffee market as one might expect, with cold brew having taken the lead. Nonetheless, its unique preparation method means it often has a brighter, cleaner, and more aromatic flavour profile. 

As the Covid-19 pandemic has already forced coffee consumers to experiment with home brewing, however, it will be interesting to see whether or not flash brew coffee becomes more prominent in the months and years to come. 

It is quick and easy to make, and certainly could become a summer coffee favourite if it becomes more popular. Whether or not this will be the case, however, remains to be seen.

Enjoyed this? Then try our article exploring Indonesia’s iced coffee revolution.

Photo credits: Matthew Deyn, Unsplash

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