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Have you ever wondered why your first sip of espresso tastes different than your last? Have you noticed that some baristas stir an espresso during calibration, or even swirl the decanter before they serve your brew? The simple answer is it changes the taste and improves the experience of the coffee. But here at Pilot the simple answer generally doesn’t satisfy our curiosity. Having observed for ourselves that stirring coffee, whether drip or espresso, changed the drinking experience, we set out to find out exactly why.

First question: does coffee have a natural tendency to stratify?

 Stratification refers to anything that naturally forms or has been deposited in layers. Since stirring changes the taste and balance of the coffee, it strongly suggests that the answer is yes – there are initially layers of flavor compounds stratified in the cup.

Our own experiments confirm what we’ve read; the first third of a brew cycle does all the heavy lifting dissolving the majority of flavour compounds. For the remainder of the brew cycle we are mostly diluting those flavours we have already dissolved. This dynamic of the extraction process has been addressed in an earlier fix article discussing the solubility of coffee as it ages. Read on to find how we tested this with drip, immersion, and espresso methods.

Drip Methods

The graph below is borrowed from the article linked above. The results displayed are of brewing a pour over with a 17-day-old coffee into six separate 50 ml sample cups. These samples are then tested for the amount of coffee flavouring material, or total dissolved solids (TDS), they contain.

You can see the amount of TDS measured in each sample below the numbers 1-6.  This means that the liquid in the bottom third of the coffee has dissolved significantly more flavour compounds than the liquid in the top two-thirds your cup.                                                                                                                                                                                                           Note: using older coffee helps minimize the impact degassing has on the rate of extraction to illustrate this point more concisely.

 

For all the visual learners out there, let’s take a look at these six samples we brewed to illustrate how the process of stratification plays out.  Pictures are worth a thousand words, and you can very clearly see concentration decrease from the first cup brewed on the left to the last cup on the right . 

   

Immersion Methods

Earlier in the year we were developing a protocol for measuring the solubility of our cupping bowls. What we found was a concentration gradient throughout the cup. For the purposes of quality control we needed to identify how far below the surface we would find a representative sample – or in more technical terms, the depth at which we would capture the median TDS in the bowl.  This got us into playing around with immersion brew methods to determine whether they had the same inclination to stratify as drip.

For our experiment, we prepared two cupping bowls and drew samples from the surface of the liquid using a syringe in an attempt to remove one layer at a time.  From the first bowl we removed ten 15 ml samples. Each was stirred, and then measured for TDS. We followed the same procedure for the second bowl, but this time drew three 50 ml samples.  The cupping bowls graph shows that immersion methods produce beverages with gradients of concentrations and flavours as well, just like drip methods.

Espresso

If brews made in both drip and immersion methods stratify, we can pretty safely assume that we’ll find the same process at play when it comes to espresso. Below is a graph produced in the same fashion as the cupping bowls, but in this case removing seven 5 ml samples from the surface of the coffee. In the end, we found very similar results.

 

These experiments clearly showed the presence of layers , or at least increasing concentration levels, as you get towards the bottom of a coffee. So there you have it, stratification in coffee is a thing! But we were still left with a lingering thought, which brings us to:

 

 

 

Second question: why are different flavours found at different levels in the cup?

We’ll defer to a few experts on this point: 

During brewing the total amount of flavouring material in the beverage changes, as does the proportion of each compound. In other words, the flavor changes continuously as time elapses.  As a general rule, the most flavorful compounds are extracted first. The longer coffee particles remain in contact with the water, the greater the quantity of less-flavourful compounds released.
The flavor of coffee pg 1 (SCAA Brewing Handbook)
Some of these solubles evaporate easily and are responsible for the brew’s aroma, while others are not so volatile and are the source of the brew’s taste. Aroma and taste combine to produce coffee’s flavour. The insoluble compounds - those that don’t dissolve - become coffee’s body. … The acceptability of the taste perception is also tied to the brew’s chemical composition, which changes continuously during the brewing cycle. The changes occur because each flavouring compound dissolves at a different rate.
Aroma + Taste = Flavour. pg 21 (SCAA Brewing Handbook)
The solid materials dissolve at different rates, which means that different soluble yields will contain different mixtures of liquids and gases. Each mixture, or unique combination of solubles yield, will exhibit a different flavor.
Extraction (solubles yield) pg 10 (SCAA Brewing Handbook)

 

So, to sum up, the short and sweet answer to our second question is that flavour compounds in coffee dissolve at differing rates.

The dynamic outlined above coupled with stratification explains why, when you’re sipping your coffee, flavours can change so notably throughout the cup.  Not only are they being deposited in a concentration gradient which influences your perception of them, the compounds themselves are being dissolved at different rates.  Each point of depth has a unique combination of flavour compounds. For example, the top of the coffee might have both high acidity and bitterness with a light body, while the bottom of the cup may have much more body and sweetness. We all have our unique palates that make our experience of these flavours personal which is part of the beauty of coffee. Ultimately, unless you stir or swirl your coffee, we can reasonably state that my sip won’t be like yours. Whether that’s something you want to change is entirely up to you.

The composition of the cup can prove mysterious in many ways. We spend our days decoding these mysteries, but there’s always much more to discover and learn. So if you have perspectives and experiences to share, don’t hesitate to get in touch!

 

Yours in coffee,

Pilot Coffee roasters.