an intuitive guide to coffee solubles, extraction and tds - 3

the longer water is in a cell, the more solubles are extracted...

in the coffee world, extraction rate is the term used to quantify how many of the solubles should remain locked up in the coffea arabica prision cell and how many we want to free.
the longer that water is in a cell, the more solubles are able to be dissolved.

the optimal guidelines set by the scaa for extraction are 18 - 22%. so this means that when we take the total weight of our coffee beans, 18-22% of that mass will be dissolved by water and end up in our cup. 
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water can only extract solubles from cells it can touch


now that we know how water interacts with each cell individually we can look at how it will interact with the entire bean. to simplify, we are going to look at the cells as if they were in a 2d or flat environment. in reality the bean is a 3d structure but the concepts transfer easily. in the image above imagine that the entire bean was placed into water. 

the water would only be able to access the cells on the outer surface of the bean, represented by the blue highlight. our goal is to free the solubles throughout the entire bean so we need to find a way for the water to access the inner cells.



grinding the coffee bean increases the number of cells water can access.
we can increase the number of cells that water can access by breaking the bean into smaller pieces. as the pieces get smaller the total number of cells that water can contact increases exponentially ( look at left side picture ).



water travels deeper into each coffee particle over time
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think of the boxes above as a single coffee ground with 30 cells by 30 cells. in the first box water has been in contact with the coffee ground for 6 seconds. in this time water was able to enter the first two cells on the outer edge of the coffee ground and carry out the solubles inside of them.
after 12 seconds the water has made it through 15 cells. if you were to stop the brewing process at this point the solubles that are in cells at the center of the coffee ground would still be trapped there. this would be considered an under extracted coffee ground.
after 30 seconds water has worked its way into all of the cells and the coffee particle is fully extracted.

grind size determines the speed that extraction happens...

now consider what happens when we change the grind size (look at the picture on the left ). the time scales of 6, 12 and 24 seconds are the same, and the total number of cells is the same. 
yet it is now possible to fully extract the same number of cells in 6 seconds as 30 seconds.
grind size doesn't determine what is being extracted, it determines how long it will take for the water to reach all of the cells.
hopefully it is starting to become clear how time and grind size are inversely linked. when we increase one, we must decrease the other.


extraction is a balance between grind size and time...








what if we use different grind sizes but keep a time of 24 seconds for all sizes ?

recall from earlier that it is possible to over extract a cell and release some of the bad solubles if water is left in contact with it for too long. the orange cells in the box on the left represent cells that were over extracted ( look at the picture above ).
the middle represents our optimum extraction of 20%. the time and grind size are in the right balance where water has just enough time to dissolve the good solubles and leave the bad ones locked up.
there wasn't enough time for water to access all of the cells for the box on the right so both of the good and bad solubles remained locked inside the center of the coffee ground.
we can now see why it is important to have a consistent particle size when we grind coffee beans into smaller pieces. if the grind is inconsistent it will cause some of the grounds to be over extracted while others are under extracted.

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