The Fundamental Divide
Flash brewing is more or less the exact opposite of cold brewing. Flash brewing uses the pour over method. This distinction cuts to the heart of what separates these two approaches to cold coffee. Cold brew coffee is brewed with cold water and steeped for at least 12 hours. Cold brew coffee takes longer to brew because cold water doesn’t extract the oils and flavor from coffee as rapidly as hot water does. In contrast, hot water is poured over ground coffee, which is immediately drained into a container of ice, instantly chilling the coffee in the flash brew method.
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. Despite their shared goal of producing cold coffee, these methods represent opposing philosophies: cold brew relies on time and patience, while flash brew harnesses heat and speed.
Extraction Science and Flavor Development
The scientific differences between these methods produce dramatically different flavor outcomes. 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 in flash brewing. Since hot water does release coffee’s oils and natural sugars, the final brew has coffee’s classic floral notes, acidity and aromas. There’s no way around it, hot water extracts coffee in a way that cold water just can’t.
Cold brew follows a different extraction pathway entirely. Because the time periods for hot brew and cold brew are drastically different, 6 minutes vs. 1440 minutes respectively, the intragranular diffusion may limit the extractable concentration of soluble coffee compounds in the hot brew, as compared to the cold brew.
The end result of cold brewing will be a smooth coffee, but lacking in the acidity and earthiness traditionally associated with coffee. The oils in coffee that cause those earthy flavors are released in hot water, not cold water. Without those grounded flavors, the coffee tastes sweeter overall, but the lack of acidity on the tongue leads the drinker to taste more of a coffee’s general roast flavors and less of the interesting and unique flavor notes that come from specialty coffee.
Technical Brewing Parameters
Flash brew requires precise ratios to account for ice dilution. I like to create a beverage with a coffee to water ratio of 1:17. Some 65% of the total water should be hot, while the remaining 35% should be ice.
Heat your water to between 195 and 205 degrees Fahrenheit for optimal extraction. The brewing process is rapid, with extraction time to be around 2-3 minutes for flash brewed coffee .
Cold brew operates on entirely different parameters. Cold brew coffee is made by steeping coarsely ground beans in cold or room-temperature water for 12–24 hours. This long, slow extraction pulls out natural sweetness, body, and flavor while leaving behind much of the acidity that makes hot coffee bitter.
In cold extraction at room temperature, Fuller and Rao observed that the concentrations of 3-chlorogenic acid (3-CGA) and caffeine increased rapidly over the first 180 minutes before slowing and reaching equilibrium at approximately 400 minutes for different coffee preparations.
Caffeine Content and Strength Considerations
The caffeine profiles of these methods reveal another key difference. Depending on the brewing method used, caffeine content can vary wildly between flash chilled iced coffee and cold brew. Generally, cold brew coffee will have more caffeine than flash chilled coffee because it sits for longer periods. However, this difference becomes nuanced when considering concentration factors.
Cold brew concentrate is often 1:4 to 1:8. It is literally a concentrated coffee drink and is much stronger - and has much more caffeine - than the same amount of drip coffee liquid.
However, almost nobody drinks cold brew concentrate straight. It’s usually cut with more water, ice, or even milk or alt milk, which pushes the caffeine content of a single serving back down to the normal range for a typical cup of coffee. So unless you’re making a strong concentrate and chugging it as is, your glass of cold brew should be about the same as a hot cup of coffee, caffeine-wise.
Coffee Selection and Optimization
The choice of coffee significantly impacts results for each method. For those who prefer light roasted single origin coffees, enjoy acidity and tend to drink coffee black, flash brew methods are going to produce a better cup. For those who want a full bodied, low acid cold coffee that is a great platform for milk or cream, cold brew is the way to go.
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 , as this method will extract more acids than cold brew. This is particularly well suited for light roasted coffees which are selected and roasted for the distinct acidity and delicate aromatics that makes up their origin character. Cold brew, conversely, works exceptionally well with dark roast with lots of character , where the extended extraction time allows fuller body development without excessive bitterness.
Practical Applications and Service
From an operational standpoint, these methods serve different needs. Flash brew produces perfectly-balanced, ready-to-drink iced coffee, in a FLASH! making it ideal for on-demand service. 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.
Cold brew excels in batch preparation and convenience. Cold brewing can be very forgiving of older coffee beans, and it’s easy to prep in large quantities in advance. This creates a refreshing, convenient, and caffeinated drink for busy mornings.
As a concentrate and when refrigerated, cold brew can last a few days to about a week and a half. If you’re brewing your own cold brew, we strongly advise storing it in the fridge after it’s been brewed and filtered.