COMPARING PERCOLATION VS. IMMERSION BREWING

We all know it: the cold coffee category is booming. In the US alone, cold brew is set for an annual CAGR (compound annual growth rate,) of more than 22% over the next five years. By 2027, the industry is expected to be worth 1.35 billion dollars.  

Trends Driving Cold Coffee Consumption

Who's driving this surge in consumption? We're seeing Gen Z and Millennials embrace cold coffee over hot; this research from Mintel shows 57% of Gen Z consumers regularly drink cold coffee beverages compared with hot coffee. 

One trend contributing to the popularity of cold coffees is the "mini-treat culture," and customisation options, as consumers use different flavours, additions, and toppings to create their signature beverages and give themselves a little pick-me-up throughout their day. As the Food Institute writes:

"On TikTok and Instagram, Gen Zers and millennials post about small indulgences such as vanilla matcha lattes and mini chocolate cupcakes as they engage in self-care and escapism."

Social media channels play an essential role here, with 60% of Gen Z consumers saying that TikTok is a source for learning and exploring more about coffee and getting inspired. 

Finding the Right Cold Brewer

As cold beverage consumption extends far beyond the summer months, many operators are looking for the best cold brew equipment for their business.

Alongside the growth in cold brew, the cold brew system market has also grown: the global cold brew coffee makers market size is projected to be $124.28 million in 2031, exhibiting a CAGR of 33.7% from 2021-2031. (Business Research Insights)

In this article, we’ll explore the differences between the two most popular methods of cold brewers: percolation and immersion. We will also compare these methods in terms of extraction efficiency, speed, and taste.

1. What is Immersion?

Immersion brewing is one of the simplest and most traditional methods of making coffee and the most common method of making cold brew coffee. It involves steeping coarsely ground coffee beans in cold water for an extended period, often up to 24 hours.  

2. What is Percolation?

Percolation, on the other hand, is a dynamic method where cold water is continuously passed through a bed of coffee grounds. Unlike immersion, which is a static process, percolation is an active process that can take anywhere from 30 minutes to a couple of hours (for cold brew).  

3. The Difference: Extraction Efficiency

Both percolation and immersion brewing are based on the process of diffusion, or the principle of mass transfer. The below formula outlines the variable factors that impact extraction. 

Rate of Mass Transfer (dm/dt ): : This part of the equation represents how quickly something is moving from one place to another (i.e. the rate of solubles from coffee grinds into water.) 

A = represents the surface area of the coffee  

D =  how easy it is for coffee compounds to move through the water. Some compounds move easily (like caffeine), while others (like certain oils) might move more slowly. 

L =  This could represent the average size or thickness of the coffee grounds. Finer grounds create a shorter distance for the coffee compounds to travel to diffuse into the water, potentially speeding up the extraction but also increasing resistance. 

Csat - Csol = This is the concentration gradient – the difference in concentration of coffee compounds between the coffee grounds and water. A higher gradient (a big difference between the concentration in the grounds and the water) will drive a faster extraction as the compounds move from an area of high concentration (the grounds) to lower concentration (the water). 

Between the two methods, percolation brewing stands out for its extraction efficiency.  

Immersion relies on diffusion of coffee solubles from areas of high concentration (around the ground coffee) to areas of low concentration (in the brewing water).

The rate of extraction is dependent and proportional to this “concentration gradient”, but over time the concentration of coffee solubles in the brewing liquid increases and the rate of extraction decreases, eventually reaching a plateau or equilibrium.

To use our formula above, this means that (Csat-Csol) becomes zero, or close to zero. This means that extraction slows down dramatically. 

Percolation, on the other hand, has a consistent flow of water passing through the coffee bed, meaning that an area of low concentration is constantly being created, and this enables a constant concentration gradient.  

Consequently, this gradient, according to Fick’s laws of diffusion, propels a steady and efficient transfer of solubles from the coffee grounds into the water.  

In addition, a separate mechanism, called advection (due to the movement of the brew water through the coffee bed) also contributes to the rate of extraction. During percolation brewing, it is possible to maintain a higher rate of extraction because the brew water with coffee solubles is always exiting the system and being replaced by fresh water, thus we can maintain a higher concentration gradient. 

Simply put, imagine a sponge full of water. When fresh water continuously flows over it, it could pick up more water from the sponge compared to if it just sat in a pool of water. That’s how percolation brewing can extract more in less time. 

The Marco ColdBRU

The Marco ColdBRU uses percolation brewing to achieve a higher extraction than the leading immersion brew competitor. On average, the Marco ColdBRU achieves 19% extraction compared to 15% 

In one test carried out by Marco we used 500 grams of coffee and following the recommended recipe for each brewer (1:6 for ColdBRU and 1:8 for Immersion Brewer.)  

The results were:

• ColdBRU delivered a yield of 2.3L: at 4.68% TDS and 21.72% extraction

• Immersion Brewer delivered 1.7L at 3.15% TDS and 11% extraction 

Why is this important for coffee roasters or retailers? Well for the same amount of coffee, you’re getting a higher, more profitable yield.  

4. The Difference: Speed

The speed of brewing is a clear differentiator between immersion and percolation methods. While immersion can take up to 24 hours, percolation methods dramatically reduce this time, requiring just 30 minutes to 2 hours. This efficiency is not just about saving time; it’s about agility in a commercial setting. A busy coffee shop, suddenly finding itself with an empty cold brew tap, can quickly remedy the situation with percolation brewing. This rapid turn-around means that coffee shops can serve fresh cold brew throughout the day without extensive planning or wait times, keeping up with demand and reducing waste.

5. The Difference: Taste

Generally, taste comparison for cold coffee has focused on the difference between iced coffee and cold brew. With the latter being perceived as sweeter, smoother, and less acidic compared to a coffee that is brewed hot and then chilled.

Of course, taste is entirely subjective and there are a whole host of factors to consider around the sensory profile of a cup. The Specialty Coffee Association explains in their article published in May 2023, TDS, Extraction and filtration can all impact sensory difference. (SCA, 2023.)

So, whilst we cannot say definitively that one brew method delivers superior taste, we carried out an informal taste assessment to compare between the two methods. We asked Q Grader Alin Giriada from Coffee Laboratory to assist with our test.  

For this test, we used a percolation brewer (Marco ColdBRU) and an Immersion Brewer. We tested a light and dark roast, following the recommended recipes for each brewer (Percolation 1:6, Immersion 1:8.) 

TDS was adjusted to 1.7% for all four samples.  

Each time, two samples were tasted blind and assessed for sweetness, mouthfeel and acidity using the SCA Combined Assessment form  

The tasting notes paint a clear picture.

• For the light roast Kenyan coffees the ColdBRU scored 92.8 points compared to the Immersion technique with 77.8 points. The big difference here was the sweetness, which is much more preserved with the ColdBRU method. And where the acidity with the immersion technique is described as dry and grassy, with the ColdBRU it is sweet. 

• With dark roasted coffees the differences are considerably smaller. The immersion technique is a bit sweeter, and the ColdBRU is a bit fresher with a more pleasant aftertaste.

Conclusion 

The growth of cold brew has opened a range of new opportunities for operators. While immersion is the most common and traditional method of making cold brew, it can take up to 24 hours and might not deliver the best yield possible from your coffee. 

For a busy site, the ability to quickly brew a batch of cold brew to meet a rush period is invaluable. The speed, consistency and extraction efficiency of a system like the Marco ColdBRU, means that operators have a system that delivers more from their coffee, in a quicker, more consistent way. This not only improves speed of service and customers experience, but it also drives more profit. 

With the ability to brew at such a higher concentration (average 4% TDS,) the ColdBRU also delivers on flexibility for sites that want to use a coffee concentrate as a base for a range of cold beverages. The added value of using a system like the Marco POUR’D to dispense a shot of coffee concentrate again, means that operators are optimising workflow whilst delivering a range of choice for their customers.