“Osmotic-flow” is one of the pour over method to let water pass through coffee powder’ s filtering layer and induce a convection flow continuously then pull out only pleasant taste component from coffee powder by generating osmotic pressure continuously.
The point of Osmotic-flow is to let water pass through the filtering layer then generate concentration difference between coffee essence and water, and the pleasant taste of coffee is brought out fully into coffee liquid by osmotic pressure. “Once water pass through the filtering layer completely, osmotic pressure becomes zero. At this point, pour water again to generate another osmotic pressure” ; pleasant taste component can be extracted fully by repeating this series of operation.
As you know, water always pass through the filtering layer, so water never stay in a dripper.
“Full immersion” is one of the hand-drip brewing method to soak coffee powder in water for a while; not induce a convection flow, when a time comes, then separate coffee powder and coffee liquid. In this case, all component of coffee powder, besides pleasant taste component but also unpleasant taste component come out into the coffee liquid.
The biggest difference from Osmotic-flow is that the full immersion is to soak coffee powder in water by letting water stay in a dripper, by contrast, Osmotic-flow never let water stay in.
Though coffee itself can be brewed because concentration difference is occurred between water and coffee essence by this full immersion method, but any filtering layer cannot be formed.
What is Osmotic Pressure?
Osmotic Pressure occurs when two solutions, containing different concentration of solute, are separated by a semipermeable membrane. Solvent molecules pass preferentially through the membrane from the lowconcentration solution to the solution with higher concentration. The transfer of solvent molecules will continue until equilibrium is attained.
In case of brewing coffee, when different concentration is occurred between coffee essence and water, coffee essence with higher concentration transfer in water with lower concentration by osmotic pressure, then coffee liquid containing coffee essence comes out.
Pour Over Brewing by Osmotic-Flow method
“Blooming” is the process to gather coffee essence in spaces between coffee powders by pouring water on coffee grounds. In the spaces before pouring water filled with gas, but the gas is pushed out by poured water. The gas is light so it comes up then the grounds are lifted up and expand fully. This time, water flows in the spaces instead of the gas.
Here, thanks to osmotic pressure, coffee essence is seeped out in water then stay in the spaces.
By pouring water again and passing water through the filtering layer at the center after blooming process, osmotic pressure occurs between the water and the coffee essence kept in the spaces. Because the concentration of essence is higher than that of water, the essence is pulled in water then comes out as coffee liquid.
Once water pass through the filtering layer, no more osmotic pressure is occurred. In order to occur the pressure again, repeat to pour water then make again the concentration difference. Repeat this series of operation to brew coffee until it reaches the volume you want.
Osmotic-flow is the brewing method to occur osmotic pressure by letting water pass through the filtering layer continuously.
For the reason, in case you pour water in a large amounts at one time over coffee grounds, then water is stayed and the filtering layer is destroyed. Just at this moment, it is no more “osmotic-flow” even if you use a dripper for the osmotic-flow brewing.
Blooming is for what?
It is for making a passageway for water.
Coffee powder has a porous membrane. When water can seep fully into the porous coffee powder, then it expands fully; that means the small holes on a porous membrane of coffee powder are “opened” fully then water can reach inside of coffee powder.
“Blooming” can be said to be the preparation for brewing.
What happen if water is poured continuously?
- It causes what we call “uneven extraction” .
Water is not circulated by convection inside dripper, then the first water path becomes the easiest path for water to flow. Because of this, many components of coffee are brought out around the first water path but less from the other coffee grounds. Continuous water pouring causes uneven extraction.
○ Proper Blooming
Water spreads evenly over coffee grounds, and the coffee powder swells like a dome.
Several drops of coffee liquid brewed out from the bottom are ideal.
☓ Bad Blooming
When water is poured around the border between coffee powder and paper, water is oozed out from the side, not from the bottom. It results in uneven blooming; it can be said same as the case of too much water pouring.
Mechanism of Osmotic Flow
When well bloomed, start to pour water again. By this operation, soluble solids included coffee powders are brought out in the poured water; this is exudation.
When give words to the mechanism of Osmotic Flow, it can be said “Pour water and filtration is done in parallel”.
So, in case pour too much water at one time then coffee powder is soaked in water (full immersion brewing), the physical force balance in a filter is lost then the filtration speed gets faster by water pressure. The extraction efficiency of this “full immersion” method is worse than that of “Osmotic Flow” brewing.
The important points for
Osmotic-Flow brewing are as below:
Pour water from lower position, and water volume shall be small and thin.
Water volume and speed shall be even.
How to Pour Water
Tip a dripper to make 90°angle between water and surface of coffee grounds. Then move the pot to draw a small circle.
It is important to know well how to control water volume.
The important thing to pour over is to think about “ the center gravity for poured water”. Many people say “to pour water like drawing a circle”, but let us recognize this not as a level, but as a three-dimension.
The coffee powders in a filter form a layer of coffee powders. When the water volume poured from top and that of coffee liquid brought out from bottom is kept same, then pressure in the filter is reached a state of equilibrium. So brewing can be stable.
When water is poured again after blooming, the water and coffee powders in a filter is gone down from the center by filtering operation.
It is important to pour water continuously not to break this balance.
Role of coffee powders in paper filter
The coffee powders in a paper filter play the role of filtering.
- ①Gas is included in coffee powders.
- ②To empty the gas from coffee powders evenly, pour water like drawing a circle. (Much gas is included in big particles. When the gas is pulled out, buoyant force works so the gas is gone up.)
- ③When pour water evenly, the particles without gas go down in heavy order then form a sedimentary layer along the filter wall and bottom.
- ④The sedimentary layer of coffee powders play the role of purification, it blocks the bad taste of coffee powders including the light bubble.
When pour water in batches, the sedimentary layer of coffee powders become thick. The sedimentary layer plays as filtering layer; to block the bad taste of coffee including coffee powders.
When pour water all at once after blooming, the sedimentary of coffee powders become thin. In this case, coffee liquid pass through the thin layer fast, then there is little filtering effect of bad taste of coffee.